chapter 10 hydrology - mayo national roads design office · ireland, section 50, sub-section 2 and...

Roughan & O’Donovan - FaberMaunsell Alliance N5 Westport to Turlough Road Project Consulting Engineers Environmental Impact Statement

Ref: (07.522) September 2013 0/1

Chapter 10 Hydrology

10.1 Introduction Work Brief Hydro Environmental Ltd. was commissioned by Roughan & O’Donovan on behalf of Mayo County Council to carry out a hydrological assessment for the proposed N5 Westport to Turlough Road Project’s Environmental Impact Statement, relating to approximately 27km of mainline carriageway. The proposed road runs from just north of Westport, traversing south of Castlebar and finishing just east of Turlough tying into the existing N5 road. This chapter of the EIS presents the hydrological assessment of the proposed N5 Westport to Turlough Road Project. The chapter in accordance to EPA EIS Guidelines (EPA 2002, 2003) sets out the methodology used in the assessment (Section 10.2), describes the existing hydrological environment (Section 10.3), details the likely significant hydrological impacts associated with the construction and operational phase of the road scheme, (Section 10.4), describes measures to mitigate identified significant impacts (Section 10.5) and details residual impacts post mitigation (Section 10.6). The principal potential hydrological impacts to the character of the receiving waters are associated with the proposed road crossing points and the potential for sediment loading and associated road drainage pollutants entering such watercourses during both constructional and operational phases. There is also potential for hydrological and hydrogeological impacts to the complex karst drainage system identified in the Balloor / Shanvally area of the proposed scheme. The potential impacts assessed specific to this road scheme are described in detail in Section 10.4. The assessed potential impacts include: • Surface watercourses crossed by the proposed road involving culvert and

bridge structures and associated realignment of the watercourse channel; • Surface watercourses discharged to via proposed road drainage outfalls and

downstream impacts; • Potential alteration to flood risk at and upstream of proposed crossing points

and downstream impacts from storm outfall locations; • Sites of aquatic ecological importance in proximity to surface watercourses

(namely Black Lough, Lough Lannagh, Shanvally wetland and River Moy SAC downstream of Turlough Village);

• Surface Water Abstraction in proximity to surface water crossings and downstream of storm outfalls (i.e. Lough Clogher Abstraction Source located 1.5km downstream).

10.2 Methodology Data Sources This chapter is prepared having regard to the requirements of the 1993 Roads Act of Ireland, Section 50, sub-section 2 and 3. It has also been prepared having due regard to the following guidance documents: • EPA Guidelines on the Information to be contained in Environmental Impact

Statements, March 2002;


Ref: (07.522) September 2013 Page 10/2

• EPA Advice Notes on Current Practice in the preparation of Environmental Impact Statements, September 2003;

• Surface water and drainage guidance in the National Roads Authority Design Manual for Roads and Bridges;

• NRA Guidelines on Procedures for the Assessment and Treatment of Geology, Hydrology and Hydrogeology for National Road Schemes;

• NRA Environmental Impact Assessment of National Roads Schemes – A Practical Guide, November 2008;

• DoEHLG (Nov 2009) Flood Risk Management and the Planning System Guidance document.

The Hydrological Impact Assessment Methodology is in general agreement with the guidance outlined in Section 5.6 of the NRA Guidelines pertaining to the treatment of Hydrology. The Impact category, duration and nature of impact have been taken into account in this assessment as per the guidelines. The range criteria for assessing the importance of hydrological features within the study area and the criteria for quantifying the magnitude of impacts follow the NRA guidelines. The hydrological assessment has been prepared by expanding the desk study work carried out for the Constraints Study and Route Corridor Selection Reports. It includes an assessment of published literature available from various sources including a web based search for relevant material. Site specific topographical, aerial photography has been reviewed to locate any potential features of hydrological interest, and these have been investigated on the ground by walkover surveys in order to assess the significance of any likely environmental impacts on them. Available topographical and hydrometric work carried out for the early EIS stage have been used to perform hydrological impact assessments of all culvert crossings and proposed outfall locations. All watercourses and water bodies which could be affected directly (i.e. crossed or realigned/ diverted) or indirectly (i.e. generally lie within 250m of the road development boundary or would receive storm runoff from the proposed road) were assessed through a series of initial walkover visits followed up by a more detailed survey and hydrological assessments. Due to the nature of the hydrological environment it is necessary to consider the larger river catchment environments that the proposed road development traverses. Specific Flood Risk Assessments were carried out where the proposed road encroaches on floodplain areas, namely Lough Lannagh, Balloor flood area and the Castlebar River at Turlough. The following list of data sources were the main information sources reviewed as part of this Environmental Impact Assessment report section: Ordnance Survey • Discovery Series Mapping (1:50,000) • Six Inch Raster Maps (1:10,560) • Six inch and 25inch OS Vector OS mapping • Orthographic Aerial Mapping Environmental Protection Agency (EPA) • Teagasc Subsoil Classification Mapping • Water Quality Monitoring Database and Reports



• Water Framework Directive Classification • EPA Hydrometric Data System OPW • Arterial Drainage scheme land benefitting Mapping for Ireland • OPW and Drainage District arterial Drainage Channels and maintained

channels • OPW hydrometric Data WEB Site • Floodmaps Site Mayo County Council • Mayo County Development Plan 2008 – 2014 • Mayo County Development Plan 2008 – 2014, Strategic Environmental

Assessment • Castlebar Town & Environs Development Plan 2008-2014 • Westport Town & Environs Development Plan 2010-2016 • Planning Register • Water Services – Abstractions, Discharges & Supply Schemes National Parks and Wildlife Service (NPWS) • Designated Areas Mapping • Site Synopsis Reports Other sources • Final River Basin Management Plan for the Western River Basin District in

Ireland (2009 – 2015) • Aerial survey photography (flown in 2004, 2006 and 2011) • Bing Aerial mapping (2012) Consultation with regulatory and other bodies Consultation was made with all relevant regulatory bodies including various departments of Mayo County Council, and the OPW and Inland Fisheries Ireland. Field surveys Field surveys and walkover assessments were carried out to assess the hydrological aspects of the proposed road alignment. Detailed site walkovers and stream surveys were made at any key areas of concern. Specifically all culvert and bridge crossing locations, proposed outfall locations and ecological sensitive areas were visited and field measurements carried out along with walkover of flood risk areas. Flow measurements in selected outfall streams were conducted.

10.3 Existing Environment Regional Overview of Hydrology The proposed route of the N5 Westport to Turlough Road Project crosses numerous small watercourses, which are in:



• the River Moy Catchment system via the Islandeady, Saleen and Lannagh Loughs, the Castlebar River and upstream tributaries of the Manulla River;

• the Moyour River Catchment via Doo and Clogher Lough; • and the Deerpark Stream (Westport) Catchment. The Deerpark Stream and Moyour River Catchments are in Hydrometric Area No. 32, while the Castlebar River and Manulla River Catchments are in Hydrometric Area No. 34 which is part of the Moy River System. Watercourses intercepted by the proposed route from the scheme start to Ch 10,000 (Section A) flow west to Westport and ultimately outfall into the Clew Bay Complex and are referred to in this report as being in the Deerpark Stream catchment. The watercourses crossed by the proposed route between Ch 10,000 to Ch 14,500 (Section B) flow to Clogher Lough, which in turn flows to the Moyour River which ultimately outfalls northwest into Clew Bay at Rosmindle and are referred to in this report as being in the Moyour River catchment. Between Ch 14,500 and Ch 34,800 (Section C) the watercourses crossed by the proposed route outfall to the Castlebar River Catchment upstream of Castlebar, and are divided into the following sub-sections: • between Ch 14,500 and 19,200 (Section C1) flows to Islandeady Lough; • between Ch 19,200 and Ch 30,400 (Section C2) flows to Lannagh Lough; • between Ch 30,400 and Ch 34,800 (Section C3) flows to Saleen Lough. Both Islandeady and Saleen Loughs discharge into Lannagh Lough which in turn is drained by the Castlebar River which flows eastward to join the Manulla River 2.5km east of Turlough Village and which eventually enters the Moy River. Between Ch 34,800 and Ch 37,100 (Section C4, 2.3km of route) the watercourses crossed by the proposed route outfall to the Manulla River Catchment. Thereafter from Ch 37,100 to the proposed scheme end at Turlough (Section D, 7.0km of route) the watercourses crossed by the road scheme discharge to the Castlebar River downstream of Castlebar. Plates 10.7.01 to 10.7.12 showing catchment sections and subsections to the various watercourse crossings and outfalls are included at the end of this chapter. There are 43 No. watercourse crossings and 20 No. surface water outfall discharges proposed along the route and these are summarised below in Table 10.1. In the following sections a general description of the principal river catchments will be given, followed by a more detailed description of individual watercourses and their respective sub-catchments that are crossed by or act as receiving waters to the proposed road scheme.



Table 10.1: Road Section and Associated Catchments

Section No.

Section Name Road Scheme Chainage:

Section Length,

km

No. of Watercourse

Crossings

No. of Outfalls

From To A Deerpark N59 tie in 4+305 1.8 8 (See Note 1) 4 B Moyour 10+000 14+500 4.5 12 3 C1 Islandeady Lough 14+500 19+200 4.7 9 3 C2 Lough Lannagh 19+200 30+400 1.2 4 2 C3 Saleen Lough 30+400 34+850 4.45 7 3 C4 Manulla 34+850 37+450 2.6 1 2 D Castlebar River 37+450 44+600 7.15 2(See Note 2) 3 Total 26.4 43 20

Note 1: This number includes a proposed bridge crossing of the Deerpark River at Ch26+650 (N59) which was included in the N59-Westport to Mulranny Road Project (MWC-01.0, MO-C-17 Deerpark East Culvert) Note 2: Excluding the existing Gortnafolla and N5 Bridges. The former has been upgraded by Mayo Co. Co. and will be re-used as part of this road scheme. Principal River Basin Catchments River Moy Catchment The River Moy rises at the foot of Knocknashee in the Ox Mountains in County Sligo with a mainstream length of c. 100km. For the greater part of its length, the Moy flows south-westward, entering County Mayo and passing not far from Swinford before turning north near Ballylahan and heading northwards towards Ballina, where it enters the Atlantic Ocean at Killala Bay. Major Lakes contributing to the Moy are Lough Conn and Lough Cullin (combined area of 57.3km2). The main River Moy tributaries include the Deel and Addergoole Rivers that flow into Lough Conn, the Clydagh that flows into Cuillin, the Eignagh, Owengarve, Gweestion River and Strade Rivers that flow into the Moy upstream of Foxford and the Yellow and Brusna that outfall between Foxford and Killala Bay. The Castlebar and Manulla Rivers, which lie within the N5-Westport to Turlough study area, are principal tributaries of the Clydagh River. The River Moy has a catchment area to Ballina and the Moy Estuary of some 1911km2. This is a gauged river having a hydrometric gauging station (ref 34001) located only 1km upstream of the Moy Estuary at Rahans. The Rahans (Rahins) gauging station has an A2 Rating Classification (good reliability) for its flood flow-stage relationship. The record period available for the station is from 1968 to 2008. Arterial Drainage works were carried out by the OPW on the Moy River System from 1960 to 1971. This arterial drainage included all its tributaries including sections of the Castlebar and Manulla Rivers. Based on the www.OPW.ie/hydro database the River Moy to Ballina has an annual average flow rate 55.4 cumec, a 95-percentile low flow of 7.9cumec (data derived for the period 1969 to 2004) and mean annual maximum flood flow rate of 172.6 cumec ( 0.09cumec per km2) for the period 1968 to 2011 with the maximum recorded flood flow rate of 287 cumec occurring in 1989.In the EPA report entitled ‘Hydrometric Data for 2009’ it was reported that the average annual flow at the gauge site to December 2009 was 63 cumec. This variation on the 1969 to 2004 average may be representative of a generally increase in river flows associated with climate change or may be associated with a particular wet year.

http://en.wikipedia.org/wiki/Knocknashee

http://en.wikipedia.org/wiki/Ox_Mountains

http://en.wikipedia.org/wiki/County_Sligo

http://en.wikipedia.org/wiki/County_Mayo

http://en.wikipedia.org/wiki/Swinford

http://en.wikipedia.org/wiki/Ballina,_County_Mayo

http://en.wikipedia.org/wiki/Atlantic_Ocean

http://en.wikipedia.org/wiki/Killala_Bay

http://www.opw.ie/hydro



Castlebar River (Moy System) The Castlebar River is a tributary of the Moy Catchment and originates as the outflow from Lough Lannagh and flows eastwards through Castlebar town and eventually collects the Manulla River becoming the Toormore River and then joins the Clydagh River before discharging to Lough Cuillin. The river has a catchment area of some 99 km2 upstream of its confluence with the Manulla River. The river is gauged at the village of Turlough 6km downstream of Castlebar Town (gauge Reference 34018). This gauging site is classified as having A1 flood flow rating (i.e. A1 represents a very reliable flood flow rating) based on an OPW review of hydrometric gauging stations as part of the flood studies update. Manulla River The Manulla River, a downstream tributary of the Castlebar / Toormore River, is gauged at Gneeve Bridge Station 34011 and has an A2 flood flow rating Classification (which is considered to be reasonably reliable for Flood estimation). The river has a catchment area of some 172 km2 upstream of its confluence with the Castlebar River. There are no other sources of gauged flood flow information for the Castlebar River. Castlebar River Loughs The Castlebar River catchment upstream of Castlebar includes a number of relatively large lakes, including Lough Lannagh, Islandeady Lough, Mallard Lough and Saleen Lough. In general, all of the above lakes discharge to Lough Lannagh with Saleen Lough discharging close to Lough Lannagh outlet. Under winter flood conditions both Islandeady and Lough Lannagh Lakes are practically at the same water level representing hydraulically a single lake system. Lough Lannagh had a continuous lake level recorder which was managed by the EPA from 1976 to 1990 (gauge Reference 34073). This recorder was damaged in 1990 and was subsequently not replaced. Discussions with the hydrometric section of the EPA (McCarthaigh per comms 2009) provided information on the highest observed flood level for the lake during the gauged period 4th Mar 1976 to 12th Dec 1990 which was 33.74m O.D. Poolbeg (i.e. 31.03m O.D. Malin) occurring on the 29 October 1989. This was based on surveyed debris marks by the EPA as the recorder station had been overtopped. The Lough Lannagh 34073 Station does not have a flood flow rating relationship. Debris marks after the recent November 2009 Flood event indicate that the Lake levels reached approximately 30.5m O.D. Malin. The October 1989 flood event represented a localised extreme flood event for West and South Mayo, with the River Deel through Crossmolina producing devastating flooding of the town centre (worst flooding ever observed). The contributing catchment area of Lough Lannagh to its outfall is shown in Plate 10.1 below and represents an area of 66km2. The catchment area to OPW Gauging Site 34018 located 8km downstream of Lough Lannagh is 95.4km2. An annual maxima series of flood flows, from 1976 to 2009, is currently available for this gauge from the Hydrometric Web Site (www.opw.ie/hydro).

http://www.opw.ie/hydro



Plate 10.1 Lough Lannagh Catchment including Islandeady and Saleen

Loughs Lough Lannagh is an elongated narrow lake having a permanent lake surface area of 87ha with a longitudinal dimension of 5.2km and average width of 167m. The summer lake levels are generally at 28.75 to 29 m OD Malin and typical winter flood levels ranging from 29.5 to 30.5m O.D. Malin. FSR Catchment Characteristics for the Lough Lannagh Gauge (near its outlet) (34073) and the Castlebar River Gauge (34018) are presented in Table 10.2 below. A number of large lakes outfall to Lannagh Lough including Islandeady Lough, Saleen Lough and Lough Mallard while the Dambaduff and Drumeen Loughs outfall to Islandeady Lough. Plate 10.2 below presents the relative location of these lakes. Table 10.2 FSR Catchment Characteristics

Catchment Characteristic Lough Lannagh (34073) (not including Saleen

Lough)

Castlebar River (34018)

AREA (km2) 65.73 81.78 Annual Rainfall SAAR (mm) 1320 1320 Winter Rainfall Acceptance potential SOIL Index

0.4 (Soil Class 4 – low WRAP)

0.43

Channel Flood Slope S1085 m/km

5.47 3.06

Stream Density Function jns/km2

7.87 5.84

LAKE fraction of catchment contributing to lakes

100% 84%

URBAN – fraction of catchment 0% 8%

WESTPORT

NEWPORT

CASTLEBAR

TURLOUGH

Lough Lannagh

Islandeady L.

Saleen L.



Plate 10.2 Principal Lakes on the Castlebar River upstream of Castlebar Islandeady, Mallard and Saleen Loughs have permanent lake surface areas of 138.5ha, 24.2ha and 15.0ha respectively. Islandeady Lough water quality has been monitored by the EPA in 2003 and the lake was reported to have a Chlorophyll Maximum of 11.2 and a Trophic Status of Mesotrophic. Both Lannagh and Islandeady Loughs are known for game fishing and other water sports. A smaller lake, Black Lough, to the south, discharges to Islandeady Lough with the proposed road footprint crossing its outlet channel and its winter lake basin. Black Lough has been identified in the Section 7.1 of this EIS as a natural lake with well developed wetland vegetation and whilst small is considered to be of local importance (higher value).

Plate 10.3 Hydrometric Gauge Locations between Castlebar and Turlough



Moyour River Catchment The Moyour River, which has an estimated catchment area of 44km2, extends from townland of Mace North to the southeast of Westport and from Slinaunroe to the north east of Westport to Rosmindle to the north of Westport, forming, in general, a triangular shaped catchment. The catchment, which is entirely rural, drains a landscape dominated by drumlins and a number of large lakes including Ballin Lough, Clogher Lough, Doo Lough, Gorteen Lough and Cogaula Lough. The Moyour River Catchment is an ungauged catchment. Clogher, Doo and Cogaula Loughs (Moyour) The proposed N5 route passes through the southern area of the river catchment intercepting 12 No. watercourses. These watercourses ultimately pass through Doo Lough, Cogaula Lough and Clogher Lough which in turn discharges to the main Moyour River channel at Clogher. Lough Clogher has an estimated catchment area of 16.2km2 (37% of the overall Moyour River catchment). The Winter Rainfall Acceptance Potential (WRAP) soil class for the Lough Clogher catchment is Type 3 (moderate winter runoff). Lough Clogher, and its adjacent upstream lakes of Doo and Cogaula Loughs, is the principal water source for the Clogher Group Water Scheme. Table 10.3: Catchment Characteristics of the Moyour River System for

estimating return Period flood flows Flood Study Report (FSR) Catchment Characteristic

Moyour River Clogher Lough

AREA – catchment area (km2) 44.3 16.2 Annual Rainfall SAAR (mm) 1300 1300 Winter Rainfall Acceptance potential SOIL Index

0.46 (Soil Class 4 – low WRAP)

0.4 (Soil Class 3 – medium WRAP)

Channel Flood Slope S1085 m/km 2.88 3.71 Stream Density Function jns/km2 3.31 4.77 LAKE fraction of catchment contributing to lakes

40% 100%

URBAN – fraction of catchment 0% 0% Ballin, Clogher, Doo and Cogaula Loughs have permanent lake surface areas of 38.6ha, 34.0ha, 16.2ha and 4.5ha respectively. Clogher Lough is the Drinking Water Source to the Fahy / Kilmaclasser Group Water Scheme.


Ref: (07.522) September 2013 /10

Plate 10.4 Principal Lakes in the Moyour River Catchment Deerpark Stream Catchment The Deerpark Stream, which has an estimated catchment area of 4.4km2, extends, relative to Westport town, from Knockranny and Sheeroe to the south east and from Corratowick to the north east to Westport Demesne to the north west before out falling to Clew Bay. The catchment, which is semi-rural and suburban, drains the lands to the east and north of Westport Town including the business park area at Carrowbeg. There are no lakes within the catchment.



Table 10.4: Catchment Characteristics of the Deerpark Stream Catchment Characteristic Deerpark Stream AREA (km2) 4.4 Annual Rainfall SAAR (mm) 1700 Winter Rainfall Acceptance potential SOIL Index 0.4 (Soil Class 3 – medium WRAP) Channel Flood Slope S1085 m/km 8.4 Stream Density Function jns/km2 4.9 LAKE fraction of catchment contributing to lakes 0% Flood Risk Assessment A flood risk assessment has been undertaken for the N5-Road Scheme with respect to the proposed vertical alignment relative to adjacent water courses / bodies, proposed surface water drainage outfalls and the proposed watercourse crossings. The vertical alignment assessment is subdivided in seven sections relative to the primary receiving water as set out earlier and listed below: • Section A: Deerpark • Section B: Moyour River • Section C1: Islandeady Lough • Section C2: Lough Lannagh • Section C3: Saleen Lough • Section C4: Shanvally Stream in the Manulla System • Section D: Castlebar River It is understood, for this assessment, that all watercourse crossing culverts and bridges will be sized appropriately to convey the associated design flows with negligible upstream afflux (i.e. 100year design flow with Climate Change as per NRA Guidelines and OPW Section 50 requirements) and upstream afflux < 5cm. The flood risk is scored as low, medium or high with no further mitigations proposed for low, minor mitigation for medium and re-design recommended for high risk. All of the Proposed Culvert Crossings were assessed and found to have a low residual flood risk being generously sized for flood flows and culvert soffit freeboard clearance.



Table 10.5: Flood Risk Assessment Summary: Road Vertical Alignment

Section Chainage Comment on Flood Risk Areas

Overall Flood Risk

Mitigation

A: Deerpark N59 tie in to 2+764and Ch3+000 to 4+305

No flood risk areas. Low Existing structures to be assessed.

B: Moyour River

Ch10+000 to 14+500

open drain at chainage 10+220. Very small Catchment

Low None.

C1: Islandeady Lough

Ch14+500 to 19+200

No flood risk areas. Low. None

C2: Lough Lannagh

Ch19+100 to 30+400

Lough Lannagh flood plain between chainage 19+500 and 20+050

Medium Provide a minimum of 300mm freeboard for the road above the design flood level of 31.9m O.D. Malin

C3: Saleen Lough

Ch30+400 to 34+850

cut section at Milebush between 34+400 and 34+860

Medium Separate filter drain system to intercept groundwater flows.

Balloor Flood area

Ch34+850 to 36+000

Balloor flood prone area surface and groundwater

Medium (to adjacent lands)

Rock fill porous embankment.

C4: Manulla River System

Ch 36+000 to 37+450

low point within a cut section at chainage 37+010

Medium Separate filter drain system to intercept groundwater flows.

D: Castlebar River

E1 Ch 37+450 to 39+400

Cut sections through karst areas

Low Separate filter drain system to intercept groundwater flows and discharge to Shanvally Stream.

E2 Ch 39+400 to 44+600

Existing Crossings over the Castlebar River

Low No impact on flooding.

Surface Water Quality Rivers EPA Monitoring River Programme The EPA carries out water quality assessments of river water quality as part of a nationwide monitoring programme. Data is collected from physico-chemical and biological surveys, sampling both river water and the benthic substrate (sediment) in contact with the water. Water sampling is carried out throughout the year with the main parameters that are usually analysed for including: conductivity, pH, colour, alkalinity, hardness, dissolved oxygen, biochemical oxygen demand (BOD), ammonia, chloride, ortho-phosphate, oxidised nitrogen and temperature. Biological surveys are normally carried out between the months of June and October. These look at the relationship between water quality and the relative abundance and composition of the macro-invertebrate communities in the sediment of rivers and streams. The macro-invertebrates include the aquatic stages of insects, shrimps, snails and bivalves, worms and leeches. It is generally found that the greater the diversity the better the water quality is.



The collated information relating the water quality and macro-invertebrate community composition is condensed to a numerical scale of Q-values or Biotic Index. The indices are grouped into four classes based on river’s suitability for beneficial uses such as water abstraction, fishery potential, amenity value, etc (Table 10.6). Table 10.6: Biological river water quality classification system Biotic Index (Q value) Quality Status Quality Class Condition

Q5, Q4-5, Q4 Unpolluted Class A Satisfactory

Q3-4 Slightly Polluted / Eutrophic Class B Transitional

Q3, Q2-3 Moderately Polluted Class C Unsatisfactory

Q2, Q1-2, Q1 Seriously Polluted Class D Unsatisfactory

The rivers that traverse the proposed road alignment and that are monitored varied in quality from being moderately polluted (Q3) to unpolluted (Q4-5). Generally the river reaches downstream of urban areas have a lower quality status than those in more rural areas. There are 11 No. monitoring stations that are of relevance to the alignment (refer to Table 10.7 for monitoring results). Table 10.7: EPA monitored river water quality within or near study area

Water shed

River Name and

Code Stations

Code 1993 1999 2001 2002 2005 2006 2007 2009

Clew Bay

Deerpark East 32_3789,

No data

Clew Bay

Moyour 32_1447

32M010700 4 4 3-4 4

32C0070600 4 4

32M010400 3 4 3

32M010100 3 3-4 3 4

River Moy

Castlebar River 34_1762

34C010040 4

Castlebar River 34_3953

34C010180 3

34C010400 4 3-4 3

34C010300 3 3-4 3

34C010100 4

Saleen River 34_619 34S080100

3

Manulla River 32-1447 34M010300

4 4 3-4

The WFD ‘Water Matters’ website mapping section provides details on the assessments, with a total of 9 water bodies / sub catchments being traversed by the proposed road alignment (Table 10.8).



Table 10.8: WFD classification of river waters traversed from Westport to Turlough

Waterbody name Code Status Objective Risk

Killaghoor Tributary of Carrowbeg IE_WE_32_4312 Poor Restore 1a - At Risk

Carrownaclea Tributary of Moyour IE_WE_32_3732 Poor Restore 1a - At Risk

Doon Tributary of Moyour IE_WE_32_3732 Moderate Restore 1a - At Risk

Drumneen Tributary of Moy IE_WE_34_1421 Good Protect 1a - At Risk

Kilfea Tributary of Moy IE_WE_34_1762 Poor Restore 1a - At Risk

Islandeady Lough Tributary of Moy IE_WE_34_309 Moderate Restore 1a - At Risk

Liscromwell Tributary of Moy IE_WE_34_3953 Poor Restore 1a - At Risk

Cloondeash Tributary of Moy IE_WE_34_619 Poor Restore 1a - At Risk

Manulla Tributary of Moy IE_WE_34_1731 Moderate Restore 1a - At Risk

Lakes As part of a national water quality monitoring programme a number of lakes throughout the country are sampled and the trophic status assessed. Lake water quality is most commonly assessed by reference to a scheme proposed by the Organisation for Economic Cooperation and Development (OECD) (OECD, 1982). This scheme defines the traditional trophic categories by setting boundaries for the annual average values for total phosphorus, chlorophyll and water transparency, and for the maximum and minimum vales of the latter two parameters. As insufficient data is available to allow a full assessment based on these criteria, a modified version is used in which annual maximum chlorophyll A concentration is the only parameter used. This has been further subdivided into six water quality categories by reference to the maximum levels of planktonic algae measured during the period (Table 10.9). Indicators relating to water quality and the probability of pollution are also shown. Table 10.9: Trophic classification scheme for lake waters

Classification Scheme Category Description Lake Trophic Category Annual

Maximum Chlorophyll

(mg/m3)

Algal Growth

Degree of Deoxygenation in Hypolimnion

Level of Pollution

Impairment of Use of

Lake

Oligotrophic (O) <8 Low Low Very low Probably none

Mesotrophic (M) 8 – 25 Moderate Moderate Low Very little Moderately (m-E) 25 – 35 Substantial May be high Significant May be

appreciable

Eutrophic Strongly (s-E) 35 – 55 High High Strong Appreciable

Highly (h-E) 55 – 75 High Probably total High High Hypertrophic (H) >75 Very high Probably total Very high Very high



The trophic status provides an indication as to what degree the lake is enriched by the presence of nutrients such as phosphorus and to a lesser extent nitrogen in the form of nitrate. Along the proposed road alignment there is only 1 No. Lake station currently monitored as part of the EPA water quality reporting. This is Castlebar Lough (Lough Lannagh) and is classified as Oligotrophic to Mesotrophic. Under the WFD classification for surface water bodies, there are 9 lakes that are listed close to the proposed road alignment (Table 10.10). Table 10.10: WFD classification of lake waters within study area Waterbody name Code Status Objective Risk

Doo Lough (Clogher) IE_WE_32_463

Cogaula Lough (Clogher) IE_WE_32_438

Clogher Lough IE_WE_32_450

Drummeen Loughs (Islandeady)

IE_WE_34_328, 329, 360

Islandeady Lough IE_WE_34_376 Moderate Restore 2a - Probably Not At Risk

Black Lough (Islandeady) IE_WE_34_311

Castlebar / Lannagh Lough

IE_WE_34_373, 403,465 Moderate Restore 1b - Probably At Risk

Cloondeash Lough (Saleen) IE_WE_34_372

Saleen Lough IE_WE_34_385 Ecological Receptors A number of ecological receptors adjacent to the road have been identified by the ecologist and discussed in detail in the ecology section. These sites are summarised below in Table 10.11. Table 10.11 Ecological Receptors: Summary

No. Location/Name Chainage Area of Receptor*

Main habitat(s) Conservation Rating

1 Knockbrack Bog 12+400- 13+500

15.3 Cutover bog PB4 Wet grassland GS4

Local (higher value)

2 Kilbree Lower Bog

14+200 – 14+700

11.5 Cutover bog PB4 Scrub WS1

Local (lower value)

3 Drumneen Bog 14+900 – 15+600

28.0 Cutover bog PB4 Wet grassland GS4 Humic grass GS3 Scrub WS1


4 Black Lough 18+800 - 19+000

2.4 Reed swamp FS1 Birch woodland Lake MS4




No. Location/Name Chainage Area of Receptor*

Main habitat(s) Conservation Rating

5 Lough Lannagh 19+250 – 20+000

19.1 Lake MS4 Reed swamp FS1 Marsh GM1 Wet grassland GS4 Birch woodland

County

6 Derrylea Bog 30+200 – 30+800

21.0 Cutover bog PB4 Wet grassland GS4


7 Aghadrinagh Wetland

32+600 – 33+100

22.5 Rich fen/flush PF1 Wet grassland GS4 Marsh GM1


8 Shanvally Wetland and Wood

36+100 – 36+750

19.7 Rich fen/flush PF1 Cutover bog PB4 Wet grassland GS4 Woodland WN2 Scrub WS1

County

9 Castlebar River Corridor

43+600 – 44+600

20.0 Lowland river FW2 Wet grassland GS4 Reed/large sedge swamp FS1 Riparian woodland WN5


*Areas of Receptors are approximate and receptor may be larger than area surveyed as part of this study. Areas for Lough Lannagh and Castlebar River Corridor are much larger in extent than that included in assessment and shown above. SACs and SPA There are no listed sites traversed by the proposed road alignment, however the eastern tie-in terminates approximately 30 metres from the border of the River Moy SAC (Site Code 002298). The Castlebar River draining the land along the eastern part of the scheme drains into the SAC. It is not anticipated that there will be any impact to the site other than from the potential inflow of contaminated surface runoff flowing into the Castlebar River and then into the Manulla River which is part of the River Moy SAC. (See Chapter 7.2 of the EIS for full details and mitigation measures) The coastal designated Clew Bay Complex SAC (Site Code 001482) is located 1.3km downgradient of the western tie-in to the west of Westport town. River Moy SAC (Code 002298) This site comprises almost the entire freshwater element of the Moy and its tributaries including both Loughs Conn and Cullin. The system drains a catchment area of 805km2. Most of the site is in Co. Mayo though parts are in west Sligo and north Roscommon. Apart from the Moy itself, there are many other rivers included within the site including the Manulla River 320m east of the proposed road alignment eastern tie-in. (See Figure 7.35 in Volume 3) The site is a SAC selected for alluvial wet woodlands and raised bog, both priority habitats on Annex I of the E.U. Habitats Directive. The site is also a SAC selected for old oak woodlands, degraded raised bog and Rhynchosporion, and all habitats listed on Annex I of the E.U. Habitats Directive. (See Appendix7.1.1)



Relative to the proposed road alignment an ecological area (Manulla River) of interest has been delineated immediately eastward of the Proposed Eastern Tie in with the existing N5 at Turlough (Ch 44+600). The main habitats identified locally are lowland river (FW2), wet grassland (GS4) and wet woodland (WN6). NHAs This is the basic designation for wildlife, and is an area considered important for the habitats present or which holds species of plants and animals whose habitat needs protection. Listed sites that were published on a non-statutory basis in 1995, but have not since been statutorily proposed or designated are regarded as proposed NHA i.e. pNHA. All NHAs are legally protected from damage from the date they are formally proposed for designation. No NHAs or pNHAs are traversed by the proposed road alignment (See Figure 7.35 in Volume 3). There are 3 proposed designed sites located within 2km distance of the scheme. Coolbarren Lough (Site Code 000481) is located 1.7km northwest of the western tie-in and is not likely to be impacted. Ardogommon Wood (Site Code 001470) is located 1.8km southeast of the tie-in to the N5 at Knockranny, and 2.05km from main alignment. This is upgradient of proposed alignment and is not likely to be impacted. The third site is Dambaduff Lough (Site Code 001491) that is located 1.8km north of the alignment (Chainage 17+500) in Islandeady. This is not likely to be impacted as Islandeady Lough lies between the site and proposed road. Table 10.12 Summary of SAC, SPA, NHA and pNHA Sites adjacent to the

Road Scheme

Site 1km buffer

5km buffer 10km buffer 15km buffer

SAC River Moy

River Moy, Clew Bay Complex

River Moy, Ballinafad, Clew Bay Complex, Brackloon Woods

River Moy, Moore Hall (Lough Carra), Balla Turlough, Ballinafad, Towerhill House, Moore Hall (Lough Carra),

Lough Carra/Mask Complex, Newport River, Clew Bay Complex, Brackloon

Woods SPA Lough Conn and Lough

Cullin, Lough Carra Lough Conn and Lough Cullin, Lough Carra, Lough Mask, Owenduff/Nephin

Complex

NHA Cunnagher More Bog NHA, Croaghmoyle

Mountain NHA, Lough Greney Bog NHA

Cunnagher More Bog NHA, Croaghmoyle Mountain NHA, Lough

Greney Bog NHA

pNHA Carrowmore Lough Shore,

Dambaduff Lough,

Ardogommon Wood, Kinlooey

Lough, Coolbarreen

Lough, Knappagh

Woods, Clew Bay Complex

Carrowmore Lough Shore, Ballinafad, Moy

Valley, Lough Beg, Carrowmore, Dambaduff

Lough, Ardogommon Wood, Kinlooey Lough,

Clew Bay Complex, Coolbarreen Lough,

Croagh Patrick, Knappagh Woods,

Brackloon Woods, Lough Carra/Mask Complex

Mountpleasant School Turlough, Carrowmore Lough Shore, Balla Turlough, Ballinafad, Slishmeen

Turlough, Moy Valley, Carrowmore Lough Shore, Lough Manan, Lough

Beg, Carrowmore, Cloonboorhy Lough, Lough Carra/Mask Complex, Towerhill

Lake, Moore Hall (Lough Carra), Dambaduff Lough, Ardogommon Wood,

Kinlooey Lough, Clew Bay Complex, Coolbarreen Lough, Croagh Patrick, Knappagh Woods, Brackloon Woods



Water Supply Sources Regional Water Supply There are 3 regional supply schemes in County Mayo, Ballina (Lough Conn), Lough Mask and Westport schemes. The latter two supply water along the proposed road alignment. There are over 22 other smaller public schemes in the region. Water quality is monitored by Mayo County Council to ensure that the parametric values from the European Communities (Drinking Water) No. 2 Regulations, 2007 are complied with. Both check and audit monitoring is carried out. Westport Public Water Supply Scheme The principal source of this supply is Moher Lake, located approximately 8km south of Westport. The scheme serves Westport town and surrounding area, including the Ballinlough Group Water Scheme. The scheme is supplied from the Moher Lake WTW on the eastern shore of the lake. Water is treated by flocculation, horizontal or radial flow clarification, filtration and chlorination. It is then passed by gravity to Westport reservoir. Lough Mask Regional Water Supply Scheme The Lough Mask RWSS is the largest scheme in the County Mayo. Water is abstracted from Lough Mask at the western shore of the northern tip of the lough to serve the towns of Ballindine, Ballinrobe, Castlebar and Claremorris and surrounding areas. The scheme is supplied from the Tourmakeady WTW approximately two kilometres west of the intake. There is generous source capacity and the quality of the raw water is, in general, satisfactory (WS Atkins, 2000). Water is treated by rapid gravity sand filtration and chlorination, and passed by gravity to the Castlebar reservoirs, Ballinrobe water towers and Brookhill reservoir via Ballinrobe. Group Water Supply Schemes The following is a list of the Group Water Supply Schemes traversed (west to east) by proposed Westport to Turlough Road Project. i) Ballinlough GWS

The Ballinlough GWS is located to the north of Westport. It is supplied from the Westport RWS at Moher Lake approximately 8km south of Westport.

ii) Drummindoo GWS The Drummindoo GWS has amalgamated with the adjacent Fahy / Kilmaclasser GWS. (See section below).

iii) Glenisland / Islandeady GWS The Glenisland / Islandeady GWS have been taken over by Mayo County Council and is now part of the Lough Mask Regional WSS distribution system.

iv) Cloonkeen / Curraghmore GWS The Cloonkeen / Curraghmore GWS originally sourced surface water from Lannagh or Castlebar Lough, abstracting from Ballymacran. It has been taken over by Mayo County Council and is now part of the Lough Mask Regional WSS.



v) Ballyheane GWS The Ballyheane GWS has been taken over by Mayo County Council and is now part of the Lough Mask Regional WSS distribution system.

vi) Milebush GWS The Milebush GWS has been taken over by Mayo County Council and is now part of the Lough Mask Regional WSS.

vii) Breaghwy / Castlebar GWS The Breaghwy / Castlebar GWS have been taken over by Mayo County Council and is now part of the Lough Mask Regional WSS.

viii) Turlough / Laghtavary GWS The Turlough / Laghtavary GWS have been taken over by Mayo County Council and is now part of the Lough Mask Regional WSS distribution system.

Fahy/ Kilmaclasser GWS A new treatment plant has been constructed and commissioned for the amalgamated schemes of Drummindoo GWS & Fahy / Kilmaclasser GWS with a surface water abstraction source at Clogher Lough (Grid Reference 103479, 287704). This lake was the original abstraction point for the Drummindoo GWS. Doo Lough discharges a short distance northwards into the larger Clogher Lough. The combined GWS supplies circa 300 No. households. The proposed road alignment will have 12 No. watercourse crossings and 3 No. outfalls upstream of Clogher Lake source as described earlier in this section.

10.4 Impact Assessment Introduction Road projects given their scale and nature have significant potential for causing impact to the hydrological environment both during their construction and on-going operation and consequently require careful planning and detailed assessment to ensure the best solution is attained. Methodology The assessment of hydrological impacts for the proposed road scheme has been based on the analysis and interpretation of the data acquired during the Constraints Study and Route Corridor Selection phases, as well as site specific investigations undertaken as part of the EIA, including the ecological study, intrusive site investigation, agricultural survey, topographical survey and hydrological walkover and surveys. The procedure follows guidelines established by the NRA in its ‘Guidelines on Procedures for Assessment and Treatment of Geology, Hydrology and Hydrogeology for National Road Schemes’. Key hydrological attributes have been identified along the proposed road alignment including: • Regionally important watercourses such as Lough Lannagh and the Castlebar

River which are considered to be of county important due to fishery amenity in respect to Lough Lannagh and a salmonid river and located immediately upstream of the designated River Moy SAC;

• Designated drinking water supply abstraction sources including Clogher Lough and Castlebar Lough;



• Ecologically sensitive surface water features and catchment systems, fishery streams either locally or downstream, Fens, flushes and wetlands etc.;

• Flood Risk Areas; and • Karstic Areas. The individual importance of these attributes has been then assessed with respect to their quality, extent / scale and rarity (Table 10.13). Table 10.13: Criteria for rating site attributes Importance Criteria

Extremely High Attribute has a high quality or value on an international scale

Very High Attribute has a high quality or value on a regional or national scale

High Attribute has a high quality or value on a local scale

Medium Attribute has a medium quality or value on a local scale

Low Attribute has a low quality or value on a local scale

Types of Hydrological Impact These fall into two broad categories of quantitative and qualitative impacts. Quantitative Impacts Hydraulic structures such as bridges, culverts, channel diversions and outfalls can if not appropriately designed can impact negatively on upstream water levels and downstream stream flows. If a bridge or culvert opening is too narrow or a diversion channel undersized it may impede flow during times of floods thus causing water levels upstream of the structure to be raised above what would occur in the absence of the structure. In the road scheme design the adequacy of culvert sizes for local drainage areas and small river catchments is based on providing conveyance for the 100 year return period flood event with recommended climate change allowance. Blockage potential and maintenance requirements are also considered and are often the overriding design factor for small stream crossings. In this respect the design flow used is based on gauged flow data, where available, or the upstream catchment characteristics of the crossing including the catchment area, annual average rainfall for the catchment, the mean channel slope (S1085), the relevant soil type (mostly Type 3 medium winter rainfall acceptance potential (WRAP) and type 5 low WRAP for this scheme), a Flood Studies Report (FSR)100year flood growth factor of 1.96, the standard factorial error for the related estimation method (Institute of Hydrology Report No. 124 3-variable equation (IH-124) =1.65, Flood Studies Report 6 variable equation (FSR) (Ireland) =1.47), climate change allowance of 1.2 and where the channel is a maintained channel under drainage district or arterial drainage scheme an arterial drainage factor of 1.6 is also included. In this respect 100year design flows for small ungauged catchments (< 25km2) are of the order of 4cumec per km2 for maintained drainage channels and 2.5cumec per km2 for non-drainage scheme channels (Deerpark, Roe channels). Such flow magnitudes are considered to be extremely conservative (over prediction) ensuring a degree of overdesign. There are 43 No. crossings proposed for the road scheme. Surface water drainage from the carriageway, grassed margins and embankment slopes can lead to localised increased flows and flooding in the receiving streams.



The proposed road drainage system is a combination of closed piped drains, filter drains and over the side drainage to open drains/ swales which convey storm runoff to one of the nineteen surface outfall locations along the 26.4km of mainline road. Qualitative Impacts Depending on the hydrological and ecological sensitivities of the proposed outfall receiving waters, treatment of the storm water via online or offline detention / water quality improvement ponds may be required upstream of the outfall to protect the water quality particularly from spillage and first flush events. The potential contaminant load and accidental spillage risk for a single outfall and sub-catchment area is a function of the Design Traffic Volume and road paved area/length. General Hydrological Impacts Operational General Impacts: • Permanent interference with river, streams and floodplains at road bridge /

culvert crossing points. These structures can if not appropriately designed create an obstacle to flow, particularly under flood conditions resulting in increased flood risk and damage as a result of afflux by such structures. Such structures can locally alter bed levels and channel dimension resulting in changes in flow velocity and water depth which can during low flow periods represent a barrier to fish passage. These structures can result in localised bed and bank erosion resulting in long-term changes to the morphology of the stream channel.

• Removal of Flood storage as a result of the Road footprint encroaching the Flood plain area. Than can result in slight to moderate reduction in the flood attenuating function of a floodplain.

• Potential diversion of water between drainage catchments as a result of the road alignment and associated drainage and outfalls. At some locations, the creation of the proposed road perpendicular to the natural drainage path may lead to the interception of overland flow into the road drainage system (surface drainage or toe drainage / cut off drains) that will convey it to the nearest associated outfall. This may lead in some cases to permanent diversion of flow resulting in an increase in the rate and volume of flow in one watercourse and a corresponding reduction in the other, with potential implications for flood risk and water quality/ dilution.

• Interference with local drainage, relocation, discontinuation and combination of existing land drains as a result of the road footprint and its associated drainage system including toe drains and attenuation/detention drainage ponds. This can lead to local changes in the hydrological regime and can lead to a concentration of flows where a number of smaller drains are discontinued / diverted. This can lead potentially to a deterioration of the hydraulic capacity and exacerbation of flood risk. In the event of realignment of watercourses this will effectively remove a section of channel reach including its channel and bank-side ecology.

• Increased runoff to watercourses at proposed Storm outfalls due to the road hardpaved (impervious area), increased transmission time and point loading associated with the road and drainage system. This can particularly in the smaller receiving watercourses/drains lead to increased flood flow magnitudes and increased frequency of flooding.

• Water quality impact on receiving watercourses at storm outfalls from routine road runoff (generally sediment associated contaminants, heavy metals, hydrocarbons and suspended solids, de-icing agents (salt and grit) and to a



lesser extent nutrients, organics, and coliforms. A wide range of heavy metals are known to occur in road drainage waters, the primary metals of concern are cadmium (Cb), Lead (Pb), copper (Cu) and Zinc(ZU). All of these metals are included in the EU substances Directive (76/464/EEC) and the proposed EU priority Contaminating Substances Directive. In particular Cadmium is included in the EU Blacklist of dangerous substances. The main sources of road pollutant constituents is presented in Table 10.14 below.

The road drainage and associated storm outfalls provide a direct pathway for contaminant from accidental spillages associated with HGV’s (agricultural, oil/chemical spillages, bulk liquid, cement, etc.) to gain rapid unattenuated access to receiving watercourses. Salt and grit applications to road surfaces to mitigate icy conditions, will result in an increased salinity, pH, conductivity and total dissolved solids concentrations to receiving aquatic system. Increased salinity of watercourses can alter the ecological balance of the aquatic system and increase the bioavailability of chemical contaminants. Table 10.14: Sources of Road Pollution Constituents (Sansalone et al., 1997)

Table 10.15: Typical Pollutant Build-up Rates (kg/ha/a) Traffic Flow

Two Way AADT

Total Solids (Kg)

COD (Kg O2)

NH4 – N (Kg N)

Total Copper

(Kg)

Soluble Copper

(Kg)

Total Zinc (Kg)

Soluble Zinc (Kg)

< 5000 2500 250 4.0 0.4 0.2 0.4 0.2 5000 - 15000 5000 400 4.0 0.7 0.3 1.0 0.5 15000 - 30000 7000 550 4.0 1.0 0.4 2.0 1.0

> 30000 10000 700 4.0 3.0 1.2 5.0 2.5 Construction General Impacts Constructional activities pose a significant risk to watercourses particularly contaminated surface water runoff from construction activities entering nearby water courses.

Brakes Tires Frame Fuels Concrete Asphalt De-Icing Litter & Body & oil Pavement Pavement Salts

CadmiumChromium

CopperIronLeadNickel

VanadiumZinc

ChloridesOrganic SoilidsInorganic Solids

PAHsPhenols

Legend Primary source Secondary Source

Tyres



Constructional activities within and alongside surface waters associated with bridge and culvert construction, outfalls and channel diversions can contribute to the deterioration of water quality and can physically alter the stream/river bed and bank morphology with the potential to alter erosion and deposition rates locally and downstream. Activities within or close to the water course channels can lead to increased turbidity through re-suspension of bed sediments and release of new sediments from earthworks. Consequently instream works can potentially represent a severe disruption to aquatic ecology. The main contaminants arising from construction runoff include: • Elevated silt/sediment loading in construction site runoff. Elevated silt loading

can lead to long-term damage to aquatic ecosystems by smothering spawning grounds and gravel beds and clogging the gills of fish. Increased Silt load in receiving watercourses stunts aquatic plant growth, limits dissolved oxygen capacity and overall reduces the ecological quality with the most critical period associated with low flow conditions. Chemical contaminants in the watercourse can bind to silt which can lead to increased bioavailability of these contaminants.

• Spillage of concrete, grout and other cement based products. These cement based products are highly alkaline (releasing fine highly alkaline silt) and extremely corrosive and can result in significant impact to watercourses altering the pH, smothering the stream bed and physically damaging fish through burning and clogging by the fine silt of gills.

• Accidental Spillage of hydrocarbons from construction plant and at Storage depots / construction compounds.

• Faecal contamination arising from inadequate treatment of on-site toilets and washing facilities.

Impact of Hydraulic Structures and Interference with Drainage paths This sub-section considers the hydraulic impact of the proposed watercourse culvert crossings along the proposed N5 Westport to Turlough Road Project. The preliminary drainage design has identified that a large number of small drains/watercourses are intercepted by the road based on mapping and field visit verification. A large number of these smaller field drains (i.e. from a hydraulic and fisheries perspective) can be truncated and the upstream portion diverted either to another existing drain close by or connected into the road embankment drainage itself. The minimum size of pipe culvert to be used for any watercourse/drain is a 900mm diameter pipe which is in accordance with OPW Section 50 requirements. Culvert Crossings Table 10.16 below presents a summary of the primary culvert crossings including upstream contributing catchment area and Table 10.17 presents the proposed culvert sizes.



Table 10.16: Proposed Road Culvert Crossings

Culvert No. Location Chainage

Drainage District channel

Stream Name

Catchment / River System

Ecological Evaluation

Catchment Area, km2

CA-01 Mainline 2010 NO Deerpark East Westport Harbour 5 2.07

CA-01a Mainline 2100 NO Deerpark East Westport Harbour 5 0.03

CA-01b N59 North N59-26+650 NO Deerpark

East Westport Harbour 5 3.6

CA-02 Allergan Road 3000 NO Deerpark





CA-05a

Existing N5 at

Knockranny junction south

10000 NO Deerpark East Westport Harbour 5 0.54

CA-05b Knockranny Link 10000 NO Deerpark


CA-05c L-1805 Lodge Rd 3800 NO Deerpark


CB-01 Mainline 10200 NO Monamore Westport Harbour 5 0.13

CB-02 Mainline 10430 NO Monamore Westport Harbour 5 0.34

CB-02a Mainline 10690 NO Monamore Westport Harbour 5 -

CB-03 Mainline 10990 NO Drummindoo Westport Harbour 4 0.53

CB-04 Mainline 11220 NO Sheean Trib Inflow trib to Doo Lough 5 0.17

CB-05 Mainline 11780 NO Sheean Stream

Inflow trib to Doo Lough 4 0.65

CB-05a Access Road 11780 NO Sheean

Stream Inflow trib to Doo

Lough 4 0.65

CB-05b Access Road 11780 NO Sheean

Stream Inflow trib to Doo

Lough 4 0.65

CB-06 Mainline 12700 NO Knockbrack Inflow trib to

Cogaula Lough - Castlebar System

5 0.49



5 0.10

CB-07a Mainline 13050 NO Knockbrack trib

Inflow trib to Cogaula Lough - Castlebar System

5 -



5 0.11





Stream Name



Catchment Area, km2

CB-09 Mainline 13920 YES Kilbree Inflow trib to


5 0.17

CB-09a Access Road 13890 YES Kilbree

Inflow trib to Cogaula Lough - Castlebar System

5 0.17

CB-10 Mainline 14160 YES Kilbree

Trib to outflow channel from


4 7.29

CB-11 Mainline 15380 YES Keeloges Stream

Inflow Trib to Drumneen Lough - Castlebar System

4 4.46

CB-12 Mainline 16120 YES Dooleague Trib

Inflow trib to Black Lough / Drumneen

Loughs 4 0.66

CB-13 Mainline 16380 YES Dooleague trib


Loughs 5 0.29

CB-13a Access 16380 YES Dooleague trib


Loughs 5 0.29

CB-14 Mainline 16980 YES Cloonan Trib Trib to Islandeady Lough n/a 2.27

CB-14a Islandeady Road 17300 YES Cloonan

Stream Trib to Islandeady

Lough n/a 14.09

CB-15 Mainline 17520 NO Claggarnagh East

Trib to Islandeady Lough n/a 0.21

CB-16 Mainline 17870 YES Claggarnagh Stream

Trib to Islandeady Lough 4 6.46

CB-16a Mainline 18300 YES Local Drain Trib to Islandeady Lough n/a -

CB-16b Access Road 18400 YES Local Drain Trib to Islandeady

Lough n/a -

CB-17 Mainline 18630 NO minor field drain


CB-17a Access 18630 NO minor field drain


CB-18 Mainline 18890 YES Black Lough outlet

Trib to Islandeady Lough 4 0.62

CB-18b Mainline 19270 YES Local Drain Trib to Lough Lannagh n/a -

CB-19 Mainline 19600 NO Lough

Lannagh channel

Lough Lannagh 3 0.54

CB-19a Annagh Road 19600 NO

Lough Lannagh channel


CB-20 Mainline 19900 NO Lough

Lannagh floodplain


CC-001 Mainline 30290 NO Local Drain Lough Lannagh n/a -





Stream Name



Catchment Area, km2

CC-002 Mainline 30700 NO Local Drain Trib of Black Lough

/ Cloondeash Lough

n/a -

CC-01 Mainline 31050 YES Derrycosh Stream

Trib of Cloondeash Lough 5 2.21

CC-01a Access Road 30990 YES Derrycosh

Stream Trib of Cloondeash

Lough 5 2.21

CC-02 Mainline 31270 YES Derinlevaun Trib of Cloondeash Lough 5 1.16

CC-03 Mainline 31750 YES Clondeash Trib of Cloondeash Lough 5 0.36

CC-04 Mainline 32330 YES Clondeash Trib of Cloondeash Lough 5 0.4

CC-04a Access Road 32330 YES Clondeash Trib of Cloondeash

Lough 5 0.4

CC-04b Access Road 32330 YES Clondeash Trib of Cloondeash

Lough 5 0.4

CC-05 Mainline 32590 YES Creeragh Trib of Saleen Lough 5 1.42

CC-05b Access Road 32590 YES Creeragh Trib of Saleen

Lough 5 1.42

CC-05c Access Road 32590 YES Creeragh Trib of Saleen

Lough 5 1.42

CC-05a Mainline 33050 NO minor field drain

Trib of Saleen Lough n/a 0.2

CC-06 Mainline 36415 YES Shanvally Stream

Trib of Manulla River n/a 0.91

CC-07 Mainline 38630 NO Liscromwell Stream

Trib of Castlebar River n/a 1.56

CC-07a Access Road 38600 NO minor field

drain Trib of Castlebar

River n/a -

CC-07b Access Road 38600 NO minor field

drain Trib of Castlebar

River n/a -

CC-08

Existing N5 at Castlebar

East Junction

40000 NO minor field drain

Trib of Castlebar River n/a 0.20

As can be seen from above Table 10.16 the majority of the streams intercepted are relatively small in respect to catchment area and the recommended dimensions provided below support the existing stream channel dimensions and will not result in a contraction of the flow at the crossing point. These sizes ensure that the design flow barrel velocity is of the order of 1m/s to 1.5m/s and thus potential upstream afflux is minimised.



Table 10.17: Preliminary Sizing of Water-Crossing Culverts

Culvert No. Location Chainage Length (m)

Size (Pipe Dia)

(metres)

Size (Box Culvert)

Mammal Ledge Single/ Double

Fisheries Requirement

W (m) H (m)

CA-01 Mainline 2,010 52 3.6 2.1 - - CA-01a Mainline 2,100 27 1.50 Single - CA-01b N59 North 26,575 34 3.9 2.1 Single CA-02 Allergan Road 3,000 38 1.2 1.2 Single Box Culvert CA-03 Mainline 3,090 37 1.5 Single - CA-04 Mainline 3,615 33 1.2 - - CA-05 Mainline 3,910 51 1.5 Single -

CA-05a Existing N5 at Knockranny Junction south

10,000 24 1.2 - -

CA-05b Knockranny Link Road 10,000 42 1.2 - -

CA-05c L-1805 Lodge Rd 3,800 45 2.4 1.5 - -

CB-01 Mainline 10,200 32 1.5 Single -


CB-02a Mainline 10,690 79 1.2 - -

CB-03 Mainline 10,990 35 1.2 1.5 Single Box Culvert Silt Trap d/s


CB-05 Mainline 11,780 35 1.2 1.5 Single

Box Culvert Silt Trap d/s Replace Gravel

CB-05a Access Road 11,780 8 1.2 1.5 Single


CB-05b Access Road 11,780 6 1.2 1.5 Single -


CB-06 Mainline 12,700 36 1.8 Single - CB-07 Mainline 12,870 43 1.5 Single CB-07a Mainline 13,050 35 1.2 - - CB-08 Mainline 13,480 42 1.5 Single - CB-09 Mainline 13,920 34 1.5 Single - CB-09a Access 13,890 13 1.2 - -







Culvert No. Location Chainage Length (m)

Size (Pipe Dia)

(metres)

Size (Box Culvert)

Mammal Ledge Single/ Double

Fisheries Requirement

W (m) H (m)


CB-13 Mainline 16,380 55 1.5 - - CB-13a Access 16,380 15 1.5 - - CB-14 Mainline 16,980 45 3.3 2.7 Single -

CB-14a Islandeady Road 17,020 35 3.9 2.4 - -




CB-16a Mainline 18,300 55 1.2 - - CB-16b Access Road 18,400 8 1.2 - - CB-17 Mainline 18,630 65 1.2 - - CB-17a Access 18,630 8 1.2 - -


CB-18b Mainline 19,270 30 1.2 - - CB-19 Mainline 19,600 60 2.1 1.8 Single CB-19a Annagh Road 19,600 20 1.2 - - CB-20 Mainline 19,900 44 2.4 1.5 Single - CC-001 Mainline 30,290 35 1.2 - - CC-002 Mainline 30,700 60 1.2 - - CC-01 Mainline 31,050 70 3 3.3 Single CC-01a Access Road 30,990 8 3 2.7 - - CC-02 Mainline 31,270 50 2.7 2.1 Single - CC-03 Mainline 31,750 37 1.8 Single - CC-04 Mainline 32,330 31 1.8 Single - CC-04a Access Road 32,330 8 1.5 - - CC-04b Access Road 32,330 15 1.5 - - CC-05 Mainline 32,590 30 3 2.4 Single CC-05b Access Road 32,590 8 3 2.1 - - CC-05c Access Road 32,590 8 3 2.1 - - CC-05a Mainline 33,050 35 1.2 - - CC-06 Mainline 36,415 62 2.4 2.1 Single - CC-07 Mainline 38,630 54 1.8 1.5 - - CC-07a Access Road 38,590 10 1.2 - - CC-07b Access Road 38,600 8 1.2 - - CC-08 Existing N5 at

Castlebar East Junction

40,000 40 1.5 Single -



Notes: 1) The following culverts are to be constructed to an invert level 500mm below that of the existing

channel: CA-01, CA-01b, CB-03, CB-05, CB-10, CB-11, CB-12, CB-16, CB-18 2) All culverts are to be constructed to an invert level minimum 300mm below that of the existing

channel 3) Unless noted in schedule as ‘Replace Gravel’; culverts will not be backfilled but will be allowed to

silt up naturally 4) The sizes indicated above are full sizes inclusive of any increases required to accommodate

depressed inverts or mammal ledges 5) Mammal ledges are assumed to be 300mm wide, the minimum culvert size proposed for installation

of a single mammal ledge is 1500mm diameter and for double mammal ledge is 1800mm. The above sizes allow for pipe culverts and box section inverts to be buried beneath the existing bed level by depths of 150mm / 300mm in respect to pipes and 500mm in respect to the box sections. All of the above box sections if required by fisheries can be fitted with a low flow channel to allow for fishery friendly conditions during dry weather flows. All other drains to be intercepted are minor in flow requirements and therefore can be culverted using a standard nominal 900mm diameter concrete pipe or equivalent. Under the 1945 Arterial Drainage Act culverting of streams by either new, upgraded or extended culverts/bridges require Section 50 approval from the OPW. This enables the OPW who are responsible for Flood Risk Management and Arterial Drainage to assess the implications of the proposed works. The minimum culvert size to be used in relation to the natural drainage is a 900mm diameter pipe culvert. From a hydraulic capacity, blockage potential and maintenance point of view this minimum culvert size is acceptable and meets the OPW requirement. Full section 50 applications for all culvert and diversion arrangements will be made to the OPW for their consent as part of the final design process for the road scheme. The proposed culverting of the 43 No. Watercourses will have a slight to imperceptible local impact on flooding and flood risk. Stream Diversions Associated with Road Alignment and proposed culverts The construction of watercourse crossings through the proposed road embankment will necessitate the diversion of the existing watercourse in order to: a) Allow construction of culverts to be undertaken outside of the watercourses; b) Facilitate the construction culverts at right angles, where possible, to the

carriageway to minimise lengths; c) Relocate the watercourses away from the embankment construction footprint. The principal impact on a watercourse by a diversion is the change in the watercourse morphology i.e. increase in length with resulting slacker gradients, shorter lengths resulting in steeper gradients, sharp bends and change in direction, flowing through possible cut areas where no natural overbank flow / flood plain exists. The general impacts are summarised as follows: • Slacker gradients: Slower flow velocities with resulting increased flow area and

disposition, siltation promoting vegetation and weeds to grow in channels during periods of low flow;

• Steeper gradients: Faster flow velocities, increase erosion, shallower low flow depth;

• Sharp bends and change in direction: Erosion and deposition;



• Lack of natural flood plains: Increase in upstream flood levels.

Other impacts of watercourse diversion include: • Change to natural low flow channels: Impact on fisheries and other animals; • Change to existing foliage and vegetation: Impact on fisheries and other

animals. The design and construction of watercourse diversion channels must mitigate the above changes in morphology and may include erosion control, low flow channels, baffles / weirs / pools, engineered meanders, riverside planting and construction of compensator flood plain areas. Consultation with the Fisheries board and National Parks and Wildlife Service is required before the diversion design are finalised. Watercourse diversions have been identified along the proposed alignment involving, in general, either diversion in parallel with or at right angles to the existing channels. The following table summarises the proposed watercourse diversion locations and mitigation. Mitigation such as low flow channels, planting, baffles / weirs etc. for fisheries / fauna will be in accordance with the Inland Fisheries Ireland and NPWS requirements (refer to Aquatic Ecology Section 7.2 of this EIS).



Table 10.18: Watercourse Diversions Impact Assessment and Mitigation

Drainage Area Chainage Structure Description Flood Flow

Magnitude Diversion Type Impact on

watercourse morphology

Mitigations

Deerpark Stream

N59 26+650 MO-C_17 / CA-01b Deerpark East Culvert Moderate Parallel Minimal Erosion control at bends

2010 CA-01 Deerpark Stream for culvert Moderate Parallel Minimal Erosion control at bends

2100 CA-01A Open Drain for culvert Minor Right angles Negligible None

175 CA-02 Deerpark Stream tributary for culvert Minor Right angles Minimal Erosion control at bends

3090 CA-03 Deerpark Stream tributary for culvert Minor Right angles Minimal Erosion control at bends

3625 CA-04 Open Drain for culvert Minor Parallel Negligible None 3910 CA-05 Open Drain for culvert Minor Offline Minimal Erosion control at bends

Existing N5 CA-05a Deerpark Stream for culvert Minor Offline Minimal

Knockranny Link Road CA-05b Deerpark Stream for

culvert Minor Right angles Minimal Erosion control at bends

3700 CA-05c Deerpark Stream for culvert Minor Offline Moderate Erosion control at bends, improve

downstream channel

Clogher Lough / Moyour

10200 CB-01 Open Drain for culvert Minor Parallel Negligible None

10430 CB-02 Moanamore Stream for culvert Minor Right angles Minimal Erosion control at bends

10690 CB-02a Open Drain for culvert Minor Parallel Negligible None

10990 CB-03 Drummindoo Stream for Culvert Minor Right angles Minimal Erosion control at bends

11220 CB-04 minor stream for Culvert Minor Parallel Negligible None

11780 CB-05 Sheean stream for culvert Minor Parallel Minimal Erosion control at bends

Access Road CB-05a Sheean stream for culvert Minor Parallel Minimal Erosion control at bends






Mitigations

Clogher Lough / Moyour

Access Road CB-05b Sheean stream for culvert Minor Parallel Minimal Erosion control at bends

12700 CB-06 Knockbrack west stream for culvert Minor Parallel Minimal Erosion control at bends

12870 CB-07 Knockbrack middle stream for culvert Minor Right angles Negligible None

13050 CB-07A Knockbrack east stream / open drain for culvert Minor Right angles Negligible None

13480 CB-08 Kilbree Trib for culvert Minor Right angles Negligible None 13920 CB-09 Kilbree Trib for culvert Minor Right angles Negligible None Access Road CB-09a Kilbree Trib for culvert Minor Parallel Negligible None

14160 CB-10 Kilbree Lower for culvert Major Right angles Moderate Erosion control at bends, extend culvert under farm access road, construct culvert in parallel

Islandeady Lough

15380 CB-11 Keeloges Stream for culvert Major Right angles Moderate Erosion control at bends, construct culvert in

parallel

16120 CB-12 Dooleague Stream West for culvert Moderate Parallel Minimal Erosion control at bends

16380 CB-13 Dooleague Stream East for culvert Minor Parallel Minimal Erosion control at bends

16380 CB-13a Dooleague Stream East for culvert Minor Parallel Minimal Erosion control at bends

16980 CB-14 Cloonan River Trib for culvert Moderate Right angles Moderate Erosion control at bends, construct culvert in

parallel on left bank Islandeady Road CB-14a Cloonan River for culvert Major Right angles Moderate Erosion control at bends,

17520 CB-15 Claggarnagh West open drain fro culvert Minor Right angles Moderate Erosion control, construct in parallel on right

bank

17870 CB-16 Claggarnagh River for culvert Major Parallel Moderate Erosion control, lengthen transitions

Islandeady 18300 CB-16a Open Drain for culvert Minor Parallel Negligible None






Mitigations

Lough Access Road CB-16b Open Drain for culvert Minor Parallel Negligible None 18630 CB-17 Open Drain for culvert Minor Parallel Negligible None

18900 CB-18 Open Drains and Large drainage channel (Black Lough) for culvert

Minor Right angles, parallel Negligible None (ecology considerations)

Lough Lannagh

19270 CB-18b Open Drain for culvert Minor Parallel Negligible None

19600 CB-19 large drainage channel between lakes (d/s of existing causeway)

Minor Inline Negligible

None (ecology considerations). Existing Causeway Culverts to be retained and repaired. Existing inverts and sizes to be maintained.

Annagh Road CB-19a Open Drain for culvert Minor Parallel Negligible None

19900 CB-20 Pheasanthill stream away from road Moderate Right angles Minimal Erosion control at bends

30290 CC-001 Open Drain for culvert Minor Right angles Negligible None

Saleen Lough

30700 CC-002 Open Drain for culvert Minor Parallel Negligible None

31050 CC-01 Derrinlevaun Stream West for culvert Moderate Right angles Moderate Erosion control at bends

Access Road CC-01a Derrinlevaun Stream West for culvert Moderate Right angles Moderate Erosion control at bends

31270 CC-02 Derrinlevaun Stream Middle for culvert Moderate Parallel Moderate Erosion control at bends

31750 CC-03 Derrinlevaun Stream East for culvert Minor Right angles Minimal Erosion control at bends

32330 CC-04 Open drain at Cloondeash for culvert Minor Right angles Negligible None

32590 CC-05 Cloondeash Stream for culvert Moderate Right angles Moderate Erosion control at bends

Access Road CC-05b Cloondeash Stream for culvert Moderate Right angles Moderate Erosion control at bends






Mitigations

Saleen Lough

Access Road CC-05c Cloondeash Stream for culvert Moderate Right angles Moderate Erosion control at bends

33050 CC-05a open drains at Aghadrinagh for culvert Minor Right angles Negligible None

Manulla System 36420 CC-06 Shanvally Stream for

Culvert Moderate Parallel Minimal Erosion control at bends

Castlebar River

38600 CC-07 Liscromwell Stream for Culvert Moderate Right angles Minimal Erosion control at bends

Access Road CC-07a Liscromwell Stream for Culvert Moderate Right angles Minimal Erosion control at bends

Access Road CC-07b Open Drain for culvert Minor Parallel Negligible None Existing N5 CC-08 Open Drain for culvert Minor Parallel Negligible None



Bridge Crossings – Castlebar River at Turlough There are no proposed new river bridge crossings of the Castlebar River. The existing masonry arch bridge at Gortnafolla (L-5779) has been upgraded by Mayo Co. Co. and will be used for the road project. The proposed widening of the existing road bridge crossing of the Castlebar River at Chainage 43+825 will have no noticeable impact on flooding locally upstream or downstream. The span of the extended bridge is 14.65m. The estimated design flood level at this crossing point is 13.87m O.D. Malin and the soffit level for the bridge is 14.8m O.D. Malin which provides just over 900mm clearance above the design flood level. The hydrological impact of the widening to the existing bridge will have a negligible impact on flooding in terms of flood levels and flow velocities either locally, upstream or downstream of the sites. Storm Outfalls The proposed road scheme has 20 separate storm outfall discharges along its 26.4km mainline length. All of these outfall discharge to surface waters. These outfalls have a potential to adversely impact water quality in the receiving watercourse from routine contaminants that are contained in road drainage waters. The water quality and ecological status of the receiving water course is also threatened by contamination from large liquid spillages as a result of an accident. These impacts are assessed by using the guidelines provided in the DMRB manual (Volume II Section 3 Part 10 HA216/06, 2006). These storm outfalls also have the potential to impact the flood and morphological regime of a receiving water by increasing the volume of charge during storm events. The morphology of the stream is significantly influence by ambient flow and flooding conditions in the stream. The potential increase in flow arises from increased impervious area by the road pavement area, the provision of road and embankment drainage with a direct pathway to the receiving watercourse and potential interception of groundwater and diversion of drainage waters that would not otherwise have reached the outfall point. Predicted impact of Storm Discharge on Flooding / Morphology The 20 Outfall discharges and the magnitude of impact to the receiving watercourses have been assessed using flood flow estimation methods, stream channel capacity assessment and evaluation of the importance of the attribute. The potential Impact magnitude is presented in Table 10.23. Generally it is found that the flood impact of the road storm discharge is classified as slight to moderate adverse permanent impact where the receiving watercourse catchment is small i.e. area < 10km2. The impact of the storm outfalls at Lough Lannagh (2 No.) and Castle bar River (3 No.) are considered to be imperceptible. The largest potential impact is the proposed discharge from the Balloor section of the road between to the Ch 34+000 and 36+000 to Knockrawer Flood area as this represents a potential diversion of storm water from an area of c6.0ha (road way and cut slopes). The Knockrawer flood area is water table influenced flooding each winter and has a high level over flow northwards to an inflow channel to Saleen Lough. The impact of c6.0 ha on flooding in Knockrawer has been evaluated as representing a moderate permanent adverse impact. All slight to moderate and moderate impacts will be mitigated for through use of attenuation ponds. Predicted Impact on surface water quality The UK NRA (DMRB, 2006) recommends that a 5 day build-up of contaminants on the road surface discharging to the 95 percentile low flow is used to assess the impact on receiving streams from road drainage discharges. The first flush rainfall



event is taken as 10.4mm representing 30% of the 1 year 24hour duration rainfall. The impact to groundwater and standing waters is computed on the basis of an annual loading rate, the mean annual rainfall and an appropriate recharge coefficient to the aquifer system (depends on the road drainage collection system – French drains, swales, closed pipes, etc.). In general, the most likely impact of untreated road runoff from the proposed N5 Road is the increased total and suspended solids loading to receiving waters and associated trace amounts of heavy metals (Cu, Zn) and hydrocarbons. The projected traffic volumes (AADT) for design year 2033 are presented in the Table 10.19 below: The maximum percentage of HGV’s associated with this AADT is projected at 4.3%. For this assessment the HGV percentage is conservatively universally taken as 5% and the paved road width is taken as 16.5m.

Table 10.19: 2033 Projected 2-way, 24-hr, Traffic Volumes (ADDT) at various

Sections of the proposed N5 Road

Road Chainage AADT (2033) N59 Tie-in to 4+305 4,600 10+000 to 20+300 10,600 30+000 to 33+600 8,000 33+600 to 37+100 7,900 37+100 to 39+400 7,000 40+000 to 44+700 14,700

Water Quality Impact - Accidental Spillage Risk Assessment The risk of serious pollution to both surface and groundwater resulting from accidental spillage is a major issue with road infrastructure projects. Trying to predict the occurrence of a spill with a degree of certainty is difficult. One can conclude that the risk is influenced by the type of roadway (generally Type 2 – Dual Carriageway (16.5m wide paved surface including edge strips) ), length of road, the traffic volume, and proportion and type of heavy goods vehicles (HGV’s). A Spillage risk Assessment of the proposed road was carried out in accordance with the NRA road design Manual. The spillage assessment shows the proposed road scheme to have very low magnitude of risk for individual or grouped catchment outfalls and shows the overall spillage risk for the entire scheme to be < 0.6%. This very low spillage risk consequently does not require specific mitigation measures to reduce the risk with the overall impact classified as Slight Adverse impact. In fact, the improved road alignment and design is anticipated to reduce the number of accidents and will therefore reduce the fuel spillage risk associated with accidents. However given the sensitivity of Clogher Lough as a moderate to large group after abstraction source it is recommended that spillage containment be provided on its upstream storm outfalls.



Table 10.20: Serious Spillage Risk Assessment at Proposed Outfalls and for catchments

Outfall Outfall

ref Outfall

Chainage Main line

Length km AADT Outfall Risk (%) Catchment

Combined Risk (%)

River System Comment

1 OA-01b N59-tie-in 0.53 4600 0.019% 0.108% Deerpark Stream

Low Risk

2 OA-01 2+020 1.28 4600 0.025% 0.108% Deerpark Stream

Low Risk

3 OA-01a 3+780 1.85 4600 0.016% 0.108% Deerpark Stream

Low Risk

4 OA-04 3+880 1.235 4600 0.048% 0.108% Deerpark Stream

Low Risk

5 OB-03 11+000 0.58 10600 0.011% 0.081% Clougher Lough Low Risk



8 OB-08 15+400 1.17 10600 0.023% 0.077% Islandeady

Lough Low Risk

9 OB-10 16+980 2.12 10600 0.042% 0.077% Islandeady

Lough Low Risk

10 OB-12 17+900 0.63 10600 0.012% 0.077% Islandeady

Lough Low Risk

11 OB-15 19+600 1.4 10600 0.028% 0.055% Lough Lannagh Low Risk

12 OB-16 19+900 1.38 10600 0.027% 0.055% Lough Lannagh Low Risk

13 OC-01 31+300 0.725 8000 0.011% 0.047% Saleen Lough Low Risk



16 OC-04 35+080 2.04 7900 0.030% 0.030% Balloor Lake Low Risk

17 OC-05 36+800 2.555 7900 0.033% 0.033% Shanvalley

Stream Low Risk

18 OD-01 40+100 1.45 14700 0.040% 0.120% Castlebar River Low Risk



Total

0.55%

Low risk <1%

Note: The total is given for indicative purposes only.

Impact of Routine Road Runoff on receiving waters Research has found that a broad band of potential pollutants are associated with routine runoff from road schemes arising from road traffic and road maintenance. These contaminants are generally associated with the particulate phase and are principally heavy metals, hydrocarbons and suspended solids and de-icing agents (salt and grit) and to a lesser extent nutrients, organics and faecal coliforms. In terms of potential impact to receiving watercourses research has found the first flush runoff (10 to 15mm) can produce elevated concentrations locally in the receiving water. The impact of contaminants within routine road runoff depends on the loading (associated with traffic numbers) and the available dilution in the receiving watercourse.



There are a total of 20 outfall locations over a 26.4km road length. This density of discharge points disperses and reduces the pollutant point load from road drainage waters. The design traffic volume in conjunction with the relatively small contributing road areas will not give rise to significant hydraulic or pollutant loads on the receiving waters. The potential impact of routine runoff represents a slight to moderate local impact on water quality in the receiving environment. The available dilutions and the evaluated individual outfall water quality impacts are presented in Table 10.21 and 10.22 respectively. The overall loading of heavy metals, sediment and hydrocarbons on the receiving waters will be significantly reduced through the provision of filter drains and storm attenuation/water quality improvement ponds. Impact to Ecological Receptors A number of ecological receptors adjacent to the road have been identified and discussed in detail in the Section 7.1 of this EIS. The potential hydrological impacts to these ecological sites of interest are presented in Table 10.24.



Table 10.21: Dilution characteristics of receiving surface watercourses

Outfall Outfall Ref:

Outfall Chainage

Road Section

Road Pavement

Area ha

Total Impervious

Area ha

Catchment Area (km2)

Lower 95%

DWF l/s Dilution

Characteristics Receiving Water

details Comment

1 OA-01b N59-tie-in N59 tie in 0.424 0.590 3.69 6.0 Low summer dilution available Deerpark Stream Minor poor quality stream

2 OA-01 2+020 2000 - 3280 1.02 1.418 2.1 3.5 Low summer

dilution available Deerpark Stream Minor poor quality stream

3 OA-01a 3+780 3280 - 3850 0.456 0.775 0.85 1.5 Low summer

dilution available

Carrowbeg Stream trib of Deerpark East

Stream Minor poor quality stream

4 OA-04 3+880 3850 - 10430 1.39 1.99 0.85 1.5 Low summer

dilution available

Carrowbeg Stream trib of Deerpark East

Stream Minor poor quality stream

5 OB-01 11+000 10430 - 11050 0.87 1.14 0.53 0.8

Small Stream Low summer

dilution available

Monamore Stream trib of Doo Lough and

Clogher Lough

Outfall 3.1km u/s of Doo lough and 4.8 km u/s of Clogher

Lough GWS WTW

6 OB-05 11+780 11050 - 13020 2.97 4.221 0.65 1.1

Small Stream Low summer

dilution available

Sheean Stream trib of Doo Lough and Clogher Lough

Outfall 1.0km u/s of Doo lough and 2.7km u/s of Clogher

Lough GWS WTW

7 OB-07 14+150 13020 - 14550 2.325 2.999 7.29 14 moderate to low

dilution available

Kilbree Lower trib of Cogaula Lough and

Clogher Lough

Outfall 1.6km u/s of Cogula Lough and 4.4km u/s of

Clogher Lough GWS WTW

8 OB-08 15+400 14550 - 15750 1.77 2.743 4.46 10 moderate to low

dilution available

Keeloges Stream Trib of Drumneen Lough

and Islandeady Lough

Outfall 1km u/s of Drumneen Lough and 4.0km u/s of

Islandeady Lough

9 OB-10 16+980 15750 - 17890 3.18 5.586 2.27 23 Low summer

dilution available

Trib of Cloonaun Bridge Stream and Islandeady

Lough

Outfall 0.6km u/s of Islandeady Lough

10 OB-12 17+900 17880 - 18500 0.945 1.01 6.46 7 Low summer

dilution available

Claggarnagh Stream trib of Islandeady

Lough





Outfall Chainage

Road Section

Road Pavement

Area ha

Total Impervious

Area ha


Lower 95%

DWF l/s Dilution


details Comment

11 OB-15 19+600 18500 - 19900 2.1 2.27 - - Direct discharge

to lake basin Lough Lannagh Lower

lake

Outfalling Directly to Lough Lannagh

Lough Lannagh summer extent 86.9ha

Lough Lannagh Winter extent 151ha

12 OB-16 19+900 19900 - 30880 2.07 2.213 - - Direct discharge

to lake basin Lough Lannagh Lower

Lake

Outfalling Directly to Lough Lannagh

Lough Lannagh summer extent 86.9ha

Lough Lannagh Winter extent 151ha

13 OC-01 31+300 30890 - 31615 1.088 1.17 1.16 2 Low summer

dilution available

Trib of Cloondeash Lough and Saleen

Lough

Outfall 0.4km u/s of Cloondeash Lough and 3.6km

u/s of Saleen Lough

14 OC-02 31+810 31615 - 32330 1.073 1.44 0.36 0.6 Low summer

dilution

Trib of Cloondeash Lough and Saleen

Lough

Outfall 0.4km u/s of Cloondeash Lough and 3.4km

u/s of Saleen Lough

15 OC-03 32+600 32330 - 34025 2.543 2.77 1.42 2.4 Low summer

dilution Trib of Saleen Lough Outfalls 2km u/s of Saleen Lough

16 OC-04 35+080 34025 –36060 3.053 4.014 - -

Discharge to Knockrawer

Flood area high winter and low

summer dilutions available

Road drainage to be piped to Knockrawer Lake (Balloor Pond)

This flood area has maximum winter extents of and has a flood overflow channel that connects to Saleen Lake.

This flood area drains away with the receding groundwater table with potential karst flow connection to

the Shanvally Stream

17 OC-05 36+800 36060 - 38605 3.8175 5.646 1.4 6 Low summer

dilution

Shanvally Stream trib of the Manulla River

System

Discharge to OPW maintained drain

18 OD-01 40+100 38605 - 41680 4.512 5.849 89.6 151 Good summer

dilution Castlebar River Discharge to Castlebar River –

sensitive River upstream of MOY SAC




Outfall Chainage

Road Section

Road Pavement

Area ha

Total Impervious

Area ha


Lower 95%

DWF l/s Dilution


details Comment

19 OD-02 43+780 41680 - 43800 3.18 3.839 96.8 160 Good summer



20 OD-03 43+900 43800 - 44620 1.23 1.415 96.8 160 Good summer





Table 10.22 Water Quality Impact Assessment


Outfall Chainage

Dilution Characteristics Receiving Water details Water Quality Impact Comment

1 OA-01b N59-tie-in Low summer dilution available Deerpark Stream

minor Permanent Local

Minor poor quality stream

2 OA-01 2+020 Low summer dilution available Deerpark Stream



3 OA-01a 3+780 Low summer dilution available

Carrowbeg Stream trib of Deerpark East Stream



4 OA-04 3+880 Low summer dilution available

Carrowbeg Stream trib of Deerpark East Stream



5 OB-01 11+000 Small Stream Low summer dilution

available

Monamore Stream trib of Doo Lough and Clogher

Lough

Slight to moderate Permanent Local

Outfall 3.1km u/s of Doo lough and 4.8 km u/s of Clogher Lough GWS WTW

6 OB-05 11+780 Small Stream Low summer dilution

available

Sheean Stream trib of Doo Lough and Clogher Lough


Outfall 1.0km u/s of Doo lough and 2.7km u/s of Clogher Lough GWS WTW

7 OB-07 14+150 moderate to low dilution available

Kilbree Lower trib of Cogaula Lough and

Clogher Lough


Outfall 1.6km u/s of Cogula Lough and 4.4km u/s of Clogher Lough GWS WTW

8 OB-08 15+400 moderate to low dilution available

Keeloges Stream Trib of Drumneen Lough and

Islandeady Lough


Outfall 1km u/s of Drumneen Lough and 4.0km u/s of Islandeady Lough

9 OB-10 16+980 Low summer dilution available

Trib of Cloonaun Bridge Stream and Islandeady

Lough

Slight Permanent Local


10 OB-12 17+900 Low summer dilution available

Claggarnagh Stream trib of Islandeady Lough



11 OB-15 19+600 Direct discharge to lake basin Lough Lannagh Lower lake


Outfalling Directly to Lough Lannagh Lough Lannagh summer extent 86.9ha Lough Lannagh Winter extent 151ha

12 OB-16 19+900 Direct discharge to lake basin

Lough Lannagh Lower Lake


Outfalling Directly to Lough Lannagh Lough Lannagh summer extent 86.9ha Lough Lannagh Winter extent 151ha




Outfall Chainage

Dilution Characteristics Receiving Water details Water Quality Impact Comment

13 OC-01 31+300 Low summer dilution available

Trib of Cloondeash Lough and Saleen Lough


Outfall 0.4km u/s of Cloondeash Lough and 3.6km u/s of Saleen Lough

14 OC-02 31+810 Low summer dilution Trib of Cloondeash Lough and Saleen Lough


Outfall 0.4km u/s of Cloondeash Lough and 3.4km u/s of Saleen Lough

15 OC-03 32+600 Low summer dilution Trib of Saleen Lough Slight

Permanent Local Outfalls 2km u/s of Saleen Lough

16 OC-03a OC-04

OC-04a 35+080 Low Summer Dilution

Road drainage to be piped to Knockrawer winter Lake

(Balloor Pond)

Moderate Permanent Local

This flood area has maximum winter extents of and has a flood overflow channel that connects to Saleen Lake. This flood area drains away with the

receding groundwater table with potential karst flow connection to the Shanvally Stream

17 OC-05 36+800 Low summer dilution Shanvally Stream trib of the Manulla River System


18 OD-01 40+100 Good summer dilution Castlebar River Imperceptible to Slight

Permanent Local Large Dilution

19 OD-02 43+780 Good summer dilution Castlebar River Imperceptible to slight

Permanent Local Large Dilution

20 OD-03 43+900 Good summer dilution Castlebar River Imperceptible to slight Permanent Local Large Dilution



Table 10.23 Impact Assessment of Storm Drainage on Receiving Waters in respect to flooding and morphological changes

Outfall Outfall ref

Outfall Chainage

Road Length km

Road Impervious

Area, ha

Receiving Water

Catchment Area ha

Channel / lake Capacity 100yr 1hr storm

event Runoff rate (l/s)

100yr Greenfield Flood runoff rate

in receiving stream (l/s)

Potential Impact

1 OA-01b N59-tie-in 0.53 0.424 0.590

Sluggish vegetated channel limited by downstream road

culverts

65 4680 Slight to moderate local adverse

2 OA-01 2+020 1.28 1.02 1.418


culverts

157 2467.4 Slight to moderate local adverse

3 OA-04 3+780 1.85 0.456 0.775


culverts


4 OA-04 3+880 1.235 1.39 1.99


culverts


5 OB-01 11+000 0.58 0.87 1.14 Moderate capacity 126 760.9 Slight local adverse

6 OB-05 11+780 1.99 2.97 4.221 Low to moderate channel capacity 467 868.9 Slight to moderate local

adverse

7 OB-07 14+150 1.56 2.325 2.999 Moderate to Good

capacity maintained channel

332 963.4 Slight local adverse












Outfall Outfall ref

Outfall Chainage

Road Length km

Road Impervious

Area, ha

Receiving Water

Catchment Area ha

Channel / lake Capacity 100yr 1hr storm

event Runoff rate (l/s)

100yr Greenfield Flood runoff rate

in receiving stream (l/s)

Potential Impact

11 OB-15 19+600 1.4 2.1 2.27 Discharge to lake basin downstream of road 251 Lake imperceptible impact on

lake levels

12 OB-16 19+900 1.38 2.07 2.213 Discharge to lake basin downstream of road 245 Lake imperceptible impact on

lake levels

13 OC-01 31+300 0.725 1.088 1.17 Sluggish overgrown

channel of low to moderate capacity


14 OC-02 31+810 0.715 1.073 1.44 Large deep cut channel

moderate to good capacity


15 OC-03 32+600 1.695 2.543 2.77 Well maintained Channel

of moderate to good capacity


16 OC-04 35+080 3.053 4.014 3.053 Discharge to Winter flood area at Knockrawer 621

moderate adverse discharge to winter flood area

17 OC-05 36+800 2.12 3.8175 5.646 Well maintained OPW channel of moderate to

good capacity 625 1720.0 Slight to moderate

adverse

18 OD-01 40+100 0.8 4.512 5.849 Large River Channel of good capacity 647 21886 imperceptible impact





Table 10.24: Rating of potential hydrological impacts to identified ecological receptors

Attribute Impact Receptor No.

Receptor Name Importance Stage Description Category Level Magnitude

1 Knockbrack Bog Low

Construction

Disturbance due to construction machinery Pollution from machinery fuel spills, from sediments / materials during construction of culverts and other road drainage

Direct, Potential, Temporary

slight Negligible

Operational Accidental Fuel Spills from Road, Diversion of surface runoff /interruption of flow paths , diversion of existing channels

Direct, Residual, Permanent

Negligible Slight

Small Adverse

2 Kilbree Lower Bog Low

Construction

Disturbance due to construction machinery Pollution from machinery fuel spills, from sediments / materials during construction of culvert and other road drainage


Slight Negligible

Operational Accidental Fuel Spills from Road, Routine Pollution from Outfall, Significant channel diversion


Negligible Slight

Small Adverse

3 Drumneen Bog Low

Construction

Disturbance due to construction machinery Pollution from machinery fuel spills, from sediments / materials during construction of culvert and other road drainage


Imperceptible Negligible

Operational Accidental Fuel Spills from Road, Routine Pollution from Outfall, Significant channel diversion


Negligible Slight

Small Adverse

4 Black Lough

Medium to High

Construction

Major Disturbance to flood plain due to construction machinery, pollution from machinery fuel spills, from imported fill material, from sediments / materials during construction of culvert and other road drainage


Significant Large Adverse

Operational Loss in flood plain area, significant diversion of channel

Direct, Cumulative, Permanent

Significant Large Adverse



Attribute Impact Site Name Importance Stage Description Category Level Magnitude

5 Lough Lannagh Very High

Construction

Major Disturbance to flood plain due to construction machinery, pollution from machinery fuel spills, from imported fill material, from sediments / materials during construction of culvert and other road drainage


Slight to Moderate

Small to moderate Adverse

Operational

Accidental Fuel Spills from Road, Routine Pollution from Outfalls, local significant (minor regional) Loss in flood plain area due to encroachment of road embankment. Minor Flood storage loss and minor loss of amenity

Direct, Cumulative, Permanent

Negligible Slight

Small Adverse

6 Derrylea High

Construction

Disturbance due to construction machinery Pollution from machinery fuel spills, from sediments / materials during construction of culverts and other road drainage


Slight Small Adverse

Operational Accidental Fuel Spills, Diversion of open drains, interruption to flow paths


Slight / Imperceptible

Small Adverse / Negligible

7 Aghadrinagh Wetland High

Construction Pollution from machinery fuel spills, from sediments / materials during construction of culverts and other road drainage

Indirect, Potential, Temporary

Imperceptible Negligible

Operational Accidental Fuel Spills from Road, Routine Pollution from Outfall

Indirect, Residual, Permanent


8 Shanvally Wetland &

Wood

High (to very High)

Construction

Disturbance due to construction machinery adjacent to wetland, Pollution from machinery fuel spills, from sediments / materials during construction of culvert and other road drainage



Operational

Accidental Fuel Spills from Road and routine runoff pollution Potential Interference with surface and groundwater supply to wetland – Road in Fill


Slight Small to moderate

Moderate Adverse



Attribute Impact Receptor No.

Receptor Name Importance Stage Description Category Level Magnitude

9 Castlebar

River Corridor

High

Construction

Pollution from machinery fuel spills, from sediments / materials during construction of the bridge widening over the Castlebar River and other road drainage



Operational Accidental Fuel Spills from Road, Routine Pollution from Outfalls, local significant (minor regional) Loss in flood plain area, significant diversion of channels


Slight Negligible to Small adverse



10.5 Mitigation Measures Overview of Mitigation Measures Mitigation measures follow the principles of avoidance, reduction and remedy. The most effective measure of avoidance is ideally dealt with during the Constraints Study and / or Route Corridor Selection stage, by either moving the proposed alignments either laterally or vertically, to ensure that it does not traverse or come in close proximity to any sensitive hydrological attributes. Where avoidance has not been possible, then consideration is given to trying to locally modify the proposed road alignment option to reduce / minimise the extent of the impact and / or the exposure to human contact e.g. via groundwater supply usage. If any modifications are proposed to reduce hydrological impacts it is necessary to also consider any associated impacts to the hydrogeological and ecological regimes. Constructional Mitigation As is normal practice with large road infrastructure projects an Environmental Operating Plan will need to be prepared for the scheme and the following will be implemented as part of the EOP: • Prepare an Emergency Response Plan detailing the procedures to be

undertaken in the event of serious spillage of chemical, fuel or other hazardous wastes, non-compliance incident with any permit of license or other such risks that could lead to a pollution incident, including flood risks.

• Water quality management plan to ensure compliance with environmental quality standards Specified in the relevant legislation (i.e. surface water regulations and Salmonid Regulations 1988). This plan should included details and method statements for the control, treatment and disposal of potentially contaminated surface water.

• Obtain all necessary permits and licenses for in stream construction work and OPW Section 50 consent for all culverts and bridges including new and widening of existing structures.

• Inform Inland Fisheries Ireland. Construction operation will be required to take cognisance of the following guidance documents for construction work on, over or near water. • Eastern Regional Fisheries Board for use by all Regional Fisheries Board –

Requirements for the protection of fisheries habitat during Construction and development works at river Sites.

• Central Fisheries Board Channels and Challenges – The enhancement of Salmonid Rivers.

• CIRIA C532 Control of Water Pollution from Construction Sites Guidance for Consultants and contractors,

• CIRIA C648 Control of Water Pollution from Constructional Sites. • Guidelines for the Crossing of Watercourses during the Construction of

National Road schemes (NRA, 2006). Based on the above guidance documents concerning control of constructional impacts on the water environment, the following outline the principal mitigation measures that will be prescribed for the construction phase to protect all the catchments, watercourses and ecologically protected areas:



• All constructional compound areas will be required to be located on dry land and set back from river and stream channels and out of potential floodplain areas.

• Surface water flowing onto the construction area will be minimised through the provision of berms and diversion channels.

• Management of excess material stockpiles to prevent siltation of watercourse systems through runoff during rainstorms will be undertaken. This may involve allowing the establishment of vegetation on the exposed soil and surrounding stockpiles with cut-off ditches to contain runoff.

• Where constructional works are carried out along side stream and river channels and particularly the Moyour Basin (Clogher Lough and tributaries) and the Moy Basin (Castlebar River and tributaries) protection of such rivers from silt load should be carried out through use of Grassed buffer area, timber fencing with Silt curtains or earthen berms so as to prevent direct runoff constructional site runoff waters to the Watercourses.

• Use of settlement ponds, silt traps and bunds and minimising construction within watercourses. Where pumping of water is to be carried out, filters will be used at intake points and discharge will be through a sediment trap.

• All watercourses that occur in areas of land that will be used for site compound/storage facilities will be fenced off at a minimum distance of 5 m. In addition, measures will be implemented to ensure that silt laden or contaminated surface water runoff from the compound does not discharge directly to the watercourse. Compounds shall not be constructed in lands designated as Flood Zone A or B in accordance with the OPW Flood Risk Management Guidelines (November 2009). Compounds will not be permitted in or within a 100metres of a SAC.

• Protection measures will be put in place to ensure that all hydrocarbons used during the construction phase are appropriately handled, stored and disposed of in accordance with recognised standards as lay out by the EPA. All chemical and fuel filling locations will be contained within bunded areas.

• Foul drainage from all site offices and construction facilities will be contained and disposed of in an appropriate manner to prevent pollution of rivers and local.

• The quality of surface water discharge from the site will meet water quality targets specified to protect riparian ecosystems and protected species. Appropriate Environmental Quality Standards, namely the Surface Water Regulations 2009 will be utilised to determine specific water quality targets.

• Riparian vegetation will be fenced off to provide a buffer zone for its protection and will be specified in consultation and agreement with the Inland Fisheries Ireland and NPWS.

• Any surface water abstracted from a river for use during construction shall be through a pump fitted with a filter to prevent intake of fish.

• The use and management of concrete in or close to watercourses must be carefully controlled to avoid spillage which as stated earlier has a deleterious effect on water chemistry and aquatic habitats and species. Alternate construction methods are encouraged for example, use of pre-cast or permanent formwork will reduce the amount of in-situ concreting required. Where on-site batching is proposed this activity should be carried out well away from watercourses. Washout from such mixing plant will be carried out only in a designated contained impermeable area.



For further details on mitigation measures required to protect ecology please refer to Section 7.1 of this EIS. Operational Mitigation Water Quality Impact Mitigation All road pavement runoff water will be collected in a conventional road drainage system and discharged to receiving surface waters at specified outfall sites (20 No. outfalls sites over the 26.4km scheme length). Spillage containment in excess of 50m3 and pre-treatment in terms of silt traps will be provided upstream of all outfalls. These treatment and spillage containment facilities are proposed to be provided within the storm attenuation ponds for all outfalls except for Lough Lannagh outfalls where attenuation ponds are not proposed (i.e. storm attenuation not required). In respect to the two storm outfalls to the Lough Lannagh system, 50m3 spillage storage and conventional silt trap is proposed. The proposed drainage system incorporates a range of appropriate pollution control features to limit the water quality impact to receiving waters. These include the use of filter drains, closed drainage systems and the use of a vegetated wetland system upstream of all road drainage outfalls which will be sized to cater for at least 10% of the peak 5-year storm flow with further detention storage provided within the attenuation pond system for settlement of suspended pollutants. The vegetated system will allow for the take up of nutrients in the drainage water. These treatment systems will be provided upstream of all proposed outfalls. To facilitate emergency response to serious spillages all pond and storage systems will be fitted with a manual penstock so as to close off the outfall and contain the spillage water within the pond/storage system for pumping out and appropriate treatment and disposal. Storm Runoff Mitigation All outfalls and respective discharges are designed to prevent impact to river morphology and surface water flow hydraulics. In order to minimise local flooding and associated channel morphological impacts all outfall storm discharges to watercourses with the exception of Lough Lannagh and the Castlebar River outfalls are to undergo storm attenuation reducing outflow to the natural Greenfield runoff rate and attenuating the 100 year storm event within the Pond Storage area. The engineering design also proposes to provide storm attenuation for the Castlebar River outfalls which is not necessary given the relative scale of the River and the timing of the flood and storm peaks. This attenuation storage will provide protection in respect to accidental spillage and water quality impacts from the storm Runoff. This attenuation storage ponds for each of the outfall sites will not be located in Floodplain Areas in order to avoid any storage loss to the receiving river / stream. Culverts and Bridges All culverts and bridges are to be designed to prevent permanent impact to the river morphology. This will be achieved by ensuring the river width is not exceeded or contracted by the proposed culvert and that reasonable transitions to and from the culvert is provided where approach and exit channels are skewed to the culvert alignment. The culvert will be embedded into the channel to a depth of 500mm for



box sections and a minimum of 150mm / 300mm for pipe culverts (depending on size). All culverts and bridges are also designed to allow for both aquatic (Trout Brook Lampery, etc.) and mammalian (otter) species migration and to maintain the existing river bed as far as possible, in accordance with Guidelines for the crossing of Watercourses during the Construction of National Road Schemes, NRA 2008. Culverts and bridges. River Diversions The proposed stream and drain diversions have been assessed in Table 10.18. Localised mitigation measures have been identified in respect to preventing bank erosion at sites of bends which were found often to coincide with the proposed road culvert. This protection may be in the form of large boulders or rip-rap along the outer bank. All diversion channels will include fishery friendly requirements where they are identified as having fishery potential. The flood capacity should be enhanced while preserving the low flow channel characteristics. Loss of Flood Storage Flood storage loss as a result of encroachment of the Road Scheme at Black Lough, Lough Lannagh andBalloor was identified. For all of these cases except for the Balloor flood area the loss of flood storage was found to have a negligible impact on flooding and flood risk and thus not considered warranting compensation storage. In respect to Balloor flood area full compensation storage is required so as to maintain the existing flood levels. Specific Mitigation Measures Balloor Flood Area Operational Provide full flood storage compensation in respect to the flood area and flood storage volume lost by the road embankment in the Balloor flood area by providing designated compensation storage lands (lowering of these lands) immediately adjacent to the Balloor Flood Zone. Divert all road drainage runoff (c6.5 ha) northwest to Knockrawer flood area via a gravity storm outflow pipe. Ensure road embankment within Balloor Flood Area is constructed using clean rock fill of suitable porosity to ensure connectivity between separated flood areas to the northwest and southeast and to maintain vertical percolation through the bedrock under the road footprint. To enhance connectivity provide a number of pipe culverts through the road embankment. All drainage pipes including the outflow storm pipe to Knockrawer will be sealed. Provide for a flood relief overflow culvert to Knockrawer in the event that the natural groundwater outlet for the Balloor area deteriorates at some stage in the future. No embankment toe drains will be excavated within the Balloor Area so as to avoid any potential importation of drainage waters that would otherwise have percolated to the deeper groundwater table.



Constructional Cordon off the work area through the Balloor Flood Area minimising the land-take used for construction activities. In particular, cordon off and protect the swallow-hole area near CH 35+450. Provide double silt fences along the cordoned work area to contain silt and sediment runoff. Earthworks activities along this section will be carried out only during low flow summer periods to avoid flood periods making the collection treatment and disposal of construction runoff waters more manageable. Stockpiling, temporary or otherwise, of excess material or topsoil in the Balloor Area will be avoided in order to minimise construction footprint and sources of sediment runoff. No compound area temporary or otherwise to be located within or adjacent to the Balloor Flood area. Black Lough Ecological Receptor Operational The main mitigation measure proposed to reduce the direct impact of the road embankment footprint on the Black Lough Basin is piling through the peat to support the road embankment. The use of geotextile liner and a rock formation layer above the piles is proposed. Above the rock formation layer normal fill activities can take place. This rock layer is likely to form at about 1m below existing ground level. This approach avoids the major excavation works and a doubling on the constructional footprint. This approach will reduce the direct impact of forming an embankment from approximately 1ha (i.e. 40% of the winter flood area) through normal excavation of peat and rock fill to c. 0.5ha (i.e. 20% of the winter flood area). Normal construction measures would involve excavating out the peat layer which has been reported in the site investigation to be of the order of 6m deep at Black Lough resulting in a much greater fill footprint area across the lake basin. This approach will allow piling and filling to progress almost simultaneously across the lake basin and minimises the constructional footprint and potential disturbance of peat and the resultant peat laden runoff. In order to mitigate potential dewatering of the Lough by the rock capping layer that is supported on top of the piles at and slightly below existing ground level the impermeable geotextile along the base should be extended up along the face of the embankment both upstream and downstream faces. No embankment toe drains will be excavated within the Lough Basin itself and any approaching toe drains will discharge to nearby local drainage channels. The twin 300mm diameter land drain observed on the western bank of the Lough should be continued under the road embankment using an equivalent pipe culvert size so as to maintain its outfall with the existing drainage channel. The proposed 3.6m wide by 2.1m high box culvert at Black Lough is sufficiently sized not to impact on the flood regime in the Lough. The upstream invert of this culvert should retain existing levels. It is recommended that this is achieved by embedding the culvert 500mm and replacing /fill using granular material and placing Stop logs or



a concrete weir at the upstream face so as to retain the appropriate invert level. Monitoring of Lake levels both pre and post construction is important for calibration and fine tuning of the weir so as to ensure that lake levels are maintained. Constructional The proposed embankment construction will involve retaining the peat in place by concrete piling and the subsequent placement of a geotextile liner and Rock Fill on top of piles. The concrete piles are likely to be at reasonably close spacing of the order of 1.5 to 2m apart over the 0.5ha foot print area of the lake basin. This piling option reduces peat handling and the subsequent loss and suspension of peat fines and sediments in the Lake water. It is recommended that embankment construction across the lough in terms of piling and placing of the embankment should be timed for the summer period when the Lough has retreated southwards to its permanent lake body. (Note: Based on the Islandeady Lough EPA gauge Ref No. 34072 data from the period October 1976 to May 1996, the minimum lake levels in Islandeady Lough occur during the months May to September, likely corresponding to the decreased long term rainfall (required to raise the lake level) and the increase evapotranspiration). A double silt fence will be placed around the works area at Black Lough including the embankment path across Black Lough. The silt fence will reduce the potential direct runoff from the works area to the Lough. The double silt fence will be continued upstream and downstream of the works area along the bank of the Lough Basin and along its outlet drainage channels (two outlet channels) for a distance of 50 to 100m. Surface water flowing onto the construction area surrounding the Black Lough Site will be minimised through the provision of berms and diversion channels so as to reduce the volume of constructional runoff. No diversion channels will be connected directly to the Lough. All constructional runoff water will be directed through a silt pond and subsequently dispersed by overland sheet flow across a grassed vegetated buffer area before entering Black Lough of its connecting land drains. Where pumping of water is to be carried out, filters will be used at intake points and discharge will be through a sediment trap / Sediment pond. Management of excess material stockpiles to prevent siltation of Black Lough and its associated drains through runoff during rainstorms will be undertaken. This will involve allowing the establishment of vegetation on the exposed soil and surrounding stockpiles with cut-off ditches to contain runoff. Riparian vegetation will be fenced off to provide a buffer zone for its protection and will be specified in consultation and agreement with the IFI and NPWS. Any surface water abstracted from a river for use during construction shall be through a pump fitted with a filter to prevent intake of fish. The use and management of concrete in or close to watercourses will be carefully controlled to avoid spillage which has a deleterious effect on water chemistry and aquatic habitats and species. Where on-site batching is proposed this activity will be carried out well away from watercourses and their associated flood plains. Washout from such mixing plant will be carried out only in a designated contained impermeable area.



Shanvally Wetland ecological Receptor The wetland feature located in Shanvally will be protected, in particular during the construction phase when it will be most at risk from site traffic, soiled water runoff and spillages of hydrocarbons. There is also a risk that shallow conduits within the bedrock that supply the wetland may be compacted up-gradient, restricting or stopping the necessary groundwater recharge. During the construction phase it will be necessary to ensure that the retained wetland area is avoided by all groundwork’s and is protected against construction runoff. A small berm feature and silt curtain along the boundary with the retained wetland area will be constructed so as to intercept and divert any surface water runoff from the working area into the small stream to the north, via suitable measures to remove suspended solids and potential contaminants (i.e. large settlement pond area). The protection of the Balloor Swallow from construction works is necessary to prevent any potential impact should the Shanvally Wetland be connected. The design of this stretch of road will include suitable drainage systems to prevent any operational phase surface runoff entering the wetland area via overland flow or vertically into the underlying aquifer from whence it may discharge to the wetland. No direct surface outfall to the wetland shall be permitted. Discharge shall be to the existing drainage channel downstream of the wetland. Given that the underlying vulnerability of the bedrock aquifer in this section of roadway a sealed pipe drainage system will be provided and road drainage will pass through a wetland treatment system prior to outfall. The road embankment adjacent to the wetland will be constructed on a granular layer so as to maintain existing pathways for overland and interflow from the west and southwest to continue discharging to the wetland. It may be necessary to strip back overburden layer to shallow bedrock so as to ensure that the drainage layer functions appropriately. This granular layer will be wrapped in a geotextile to avoid infiltrating fines reducing its porosity over time. Importantly this granular layer shall be isolated from any existing or constructed surface drains so that it does not have the potential to drain the wetland as opposed to supply it. The use of toe drains along this section of high road embankment (36+060 to 36+360) will be avoided and will be replaced by a stone filter field that promotes the local infiltration of the surface runoff from the road embankment and avoids capture and discharge either to the wetland or the nearby stream channel.

10.6 Residual Impacts The residual hydrological impacts across the proposed road scheme Study Area and adjacent receiving catchments can be assessed in terms of the following hydrological parameters: • Flood Risk • Water Quality and • Channel Morphology Flood Risk Road Runoff There is a potential to increase peak flow rates and runoff volumes due to the increased impermeable area associated with the proposed road scheme and the



collecting drainage system which discharges at outfall points. The implementation of SUDs through use of engineered attenuation ponds and controlled discharges set at the greenfield flood runoff rate on all road drainage outfalls to watercourses is anticipated to mitigate negative impacts on flood risk in the receiving streams from road runoff. Attenuation storage sized to accommodate the 100year storm event which represents a higher design standard to 50year recommended in the NRA Guidelines. Diversion of Runoff between catchments and sub-catchments At some locations the creation of the proposed road scheme and its associated road drainage system has led to the interception of overland flow into the road drainage system and its subsequent discharge to nearby watercourses. This may lead in some cases to diversion of overland flow that would otherwise have discharged to a different watercourse. Attenuation storage is provided for potential interception of groundwater flow and interflow from the road cut section at Doogary / Carheens which will be discharged to the OPW managed Shanvally Stream. The relative scale of catchments involved and the length of reaches affected has led to this effect being overall classified as an imperceptible impact. The proposed drainage measures for the Balloor section of road (i.e. 6.0ha) involve the diversion of storm runoff after attenuation to the Knockrawer flood area. The impact assessment of this diversion on flooding in the Knockrawer and Saleen area was found to represent a small adverse residual permanent impact as the area is serviced by a flood overflow channel that limits the level and extent of the Knockrawer flood area by discharging into Saleen Lough. Flood Conveyance In general, no negative residual impacts on flood risk due to loss of conveyance are anticipated at river and stream crossings. All culvert design flows proposed include large factors for uncertainty associated with flood estimation in small ungauged catchments and thus the proposed culvert sizes are considered to be conservatively large and in all cases substantially exceed the existing culvert sizes on such streams. The proposed road alignment at Black Lough and Lough Lannagh and the Castlebar river bridge widening at Turlough will only result in a slight localised negative residual impacts to flood risk due to loss of conveyance for design flood events of 100year return period. In rarer events than this, there may be some moderate adverse impacts on flood risk to upstream locations due to loss of conveyance. Floodplain Storage Potential loss of floodplain storage has been identified at Turlough Village associated with the widening of the existing N5 Castlebar river Bridge and the northern leg of the compact grade separated junction. This loss of floodplain storage is minor relative to the catchment flood flows and will result in a negligible impact on flood and flood risk at these locations. At Balloor to the south of Castlebar the proposed mainline road crosses through the winter flood area and has the potential to displace flood waters onto the surrounding lands particularly as there is no natural surface outfall drain for this area with flood waters receding slowly through infiltration to groundwater via potential karst underground connections. To avoid a moderate local impact on flood risk, compensation storage will be provided to mitigate the loss of flood storage by the road footprint. Therefore the residual impact of encroachment into the Balloor and



associated works and diversions is considered to represent a small permanent adverse impact. Water Quality The proposed road drainage will be collected and discharged to watercourses resulting in localised water quality impact at the outfall sites. This impact will be minimised through the use of filter drains, closed drainage systems and the use of a vegetated wetland system upstream of all road drainage outfalls which will be sized to cater for at least 10% of the peak 5 year storm flow with further detention storage provided within the attenuation pond system for settlement of suspended pollutants. It is anticipated that the proposed road drainage outfalls will give rise to an overall positive impact on water quality of the receiving watercourses as it will replace the existing situation of untreated storm drainage from the existing N5 discharging directly to these watercourses without any treatment. On the catchment scale such mitigation meets the objectives of the River Basin Management Plan of protecting and improving the water quality status. Accidental Spillage All pollution control facilities and attenuation areas will be fitted with a penstock or similar restriction at the outfall to the receiving channel. The overall risk assessment to quantify the likelihood of a serious accidental spillage indicates a cumulative risk for the entire 26.4km road length to be very small. With individual outfalls having a considerably lower risk (DMRB Volume II Section 3 Part 10). The impact from accidental spillages on stream outfalls will be reduced by the use of treatment forebays incorporated within the attenuation pond upstream of the outfall. Water Abstraction No negative impact to Regional Supplies surface water abstraction sources is anticipated; however there a potential for impact upon the Fahy/ Kilmaclasser GWS abstraction source water quality located at Clogher Lake which is located downstream of 12 No. watercourse crossings and 3 No. outfalls stream from the proposed road. The overall residual impact on the Clogher abstraction is considered negligible particularly as the existing road impacts on these streams. Morphology No Negative residual impacts to surface water feature morphology are anticipated, as all practicable mitigation measures for drainage, bridges and culverts and channel realignments as stated in the mitigation section are to be implemented in the proposed road scheme.

10.7 Conclusion In this section of the Environmental Impact Statement the proposed alignment in relation to hydrology has been assessed and evaluated based on desk studies and detailed site walkovers and field surveys. The proposed N5-alignment between Westport and Turlough crosses numerous watercourses which lie in the Deerpark Stream, Moyour River, Castlebar River and Manulla River catchments. The road alignment passes close to Islandeady Lough and Lough Lannagh which drain to the Castlebar River.



A flood risk assessment has been undertaken for the road alignment and four areas of medium flood risk have been identified. At the proposed Lough Lannagh flood plain crossing it is proposed to raise the minimum road level to provide a 0.3m freeboard above the lough design flood level. At Milebush and between Balloor and Doogary, in proposed deep cut areas, it is proposed to install separate filter drains to intercept potential groundwater flows. The impact of the road scheme on watercourses at proposed crossing points has been undertaken. The proposed culverts and bridges have been design in accordance to the requirements of the OPW under Section 50 of the arterial drainage act 1945 and also include, where necessary, provisions for wild life and fisheries. The impact of the proposed culverts and their associated channel realignments have been assessed to having, in general, negligible to minimal impact of the watercourses morphology. Areas of potential moderate impact were identified and appropriate mitigations, including erosion control at bends, improvement to culvert alignments have been proposed. At the Lough Lannagh flood plain crossing the existing causeway culverts are to be repaired and retained. It is proposed to undertake widening works to the existing bridge over the Castlebar River at Ch 43+825. The hydrological impact of the widening to the existing bridge will be negligible in terms of flood levels and flow velocities either locally, upstream or downstream of the sites. The 20 Outfall discharges and the magnitude of impact to the receiving watercourses have been assessed using flood flow estimation methods, stream channel capacity assessment and evaluation of the importance of the attribute. Generally it is found that the flood impact of the road storm discharge is classified as slight to moderate adverse permanent impact where the receiving watercourse catchment is small i.e. area < 10km2. The impact of the storm outfalls at Lough Lannagh (2 No.) and Castle bar River (3 No.) are considered to be imperceptible. The largest potential impact is the proposed discharge from the Balloor section of the road between to the Ch 34+000 and 36+000 to Knockrawer Flood area as this represents a potential diversion of storm water from area of 6.0ha. The Knockrawer Flood area is water table influenced flooding each winter and has a high level over flow northwards to an inflow channel to Saleen Lough. The impact of 6.0 on flooding in Knockrawer has been evaluated as representing a moderate permanent adverse impact. All slight to moderate and moderate impacts will be mitigated for through use of attenuation ponds. An accidental spillage assessment undertaken for the proposed road scheme concluded that would be a very low magnitude of risk for individual or grouped catchment outfalls and shows the overall spillage risk for the entire scheme to be < 0.3%. This very low spillage risk consequently does not require specific mitigation measures to reduce the risk with the overall impact classified as Slight Adverse impact. It is recommended, in particular, that spillage containment be placed on the outfalls upstream of Clogher Lough (water supply scheme source). The potential impact of routine road runoff from the entire scheme represents a slight to moderate local impact on water quality in the receiving environment. The overall loading of heavy metals, sediment and hydrocarbons on the receiving waters will be significantly reduced through the provision of filter drains and storm attenuation/water quality improvement ponds. As assessment has been undertaken of the potential impact of the road scheme on identified ecological receptors in the study area. The main construction impacts are potential disturbance due to construction machinery, pollution from machinery fuel spills, from sediments / materials during construction of culverts and other road drainage. The main operational impacts are accidental fuel spills, diversion of open drains, interruption to flow paths, loss in flood plain area due to encroachment of road



embankment and potential Interference with surface and groundwater supply to wetland. A list of general operational and constructional mitigations have been given which will reduce the impact scheme and its attributes on the study area’s hydrology including water quality mitigations, storm runoff mitigations, watercourse crossings and diversion mitigations. Specific mitigations have been given for the Balloor Flood Area and the Black Lough and Shanvally Wetland Ecological Receptors. Residual impacts associated with flood risk, water quality and channel morphology have been identified. The proposed drainage measures for the Balloor section of road involve the diversion of storm runoff after attenuation to the Knockrawer flood area. The impact assessment of this diversion on flooding in the Knockrawer and Saleen area was found to represent a small adverse residual permanent impact as the area is serviced by a flood overflow channel that discharges into Saleen Lough. It is anticipated that the proposed road outfalls will give rise to a localised slight permanent negative impact on water quality of the receiving waters. On the catchment scale such mitigated impacts will be imperceptible. There a potential for impact upon the GWS abstraction source water quality located at Clogher Lake from road schemes outfalls and watercourse crossings. The overall residual impact on the Clogher abstraction is considered negligible particularly as the existing road impacts on these streams. Further to implementation of the practicable mitigation measures there will no negative residual impacts to surface water feature morphology are anticipated due to the road scheme.

chapter 10 hydrology - mayo national roads design office · ireland, section 50, sub-section 2 and...

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