managing urban drainage: a theoretical case study of suds design

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Managing urban drainage: a theoretical case study of SUDS design

Copyright of LabSearch Ltd, a working title of Dr Malcolm Sutherland, 2013

MANAGING URBAN DRAINAGE:A theoretical case study of SUDS design

Malcolm Sutherland (0204783)

A coursework report submitted in fulfilment of the requirements for the MSc module in Sustainable

Urban Drainage Systems (WW545) at the University of Abertay Dundee, April 2003

REVISED May 2013


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CONTENTS

SECTION 1: the changing hydrology of a river catchment

Introduction

1.1: 19501970

1.2: 19701990

1.3: 1990 - 2010

SECTION 2: installation of a combined sewer overflow

2.1: Purposes and problems of a combined sewer over-flows

2.2: Determination of Formula A

2.3: CSO arrangements for discharge into two contrasting rivers

REFERENCES (websites may no longer be available)


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SECTION 1: THE CHANGING HYDROLOGY OF A RIVER CATCHMENT

This chapter concerns the hydrological and environmental changes caused by developments

along a section of a theoretical river. As shown in Figure 1, this section includes a small

tributary, which joins upstream from where a New Town has-been established. Two damshave been raised to create boating and Fishery lakes, and Hood barriers have been raised

downstream of the town, in order to protect agricultural land.

Figure 1: layout of the hydrological catchment

At the gauge point, the following changes in river depth and flow were noted for are being

predicted) (Table 1):

Table 1: records at the gauge point and historical information

Year Flow (m3/s) Water depth (m) Developments

1950 12.5 2.1 New Town established in 1952

New flood banks raised in 1965

Boating lake constructed in 1995

Fishery opened in 2000

1970 13.0 2.3

1990 13.7 2.2

2010 13.0 2.6

The following sub-sections address the possible causes of these changes in flow and depth,

how they correlate with the developments described above.

1.1: 1950-1970

The river flow increases from 12.5 m3/s to 13.0 m

3/s, and the water depth rises from 2.1m

to 2.3m.

HYDROLOG1CAL CHANGES

Both the river flow rate and the depth increased during this period. Two main events lake

place alongside the river, which are likely to have caused this to happen: (i) urban

development; and, (ii) the construction of flood banks where the gauge point stands.


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Urban development increases the area of impermeable surface cover, and it requires

drainage systems. This reduces the amount of water passing into the soil, and being

absorbed into vegetation, or evaporating. As a consequence, urbanisation can increase the

flow rate of receiving rivers by between 200% and 600% (Kiely, 1997).

This may drastically change the hydrologic cycle section (from rain hitting The ground until

water enters the river), and drain pipe networks with result in the direct and shortened

discharge of drainage into the river (CIRIA, 2000; Herricks, 1995). The rise in water depth

may owe to the construction of buildings and bridges, which reduce channel capacity

(Figure 2) (Bennett et al, 1997):

Figure 2:the encroachment of development upon a natural river, resulting in rising levels

MORPHOLOGICAL CHANGES

The increase in flow will lead to more erosion of the riverbanks and the riverbed. This

pattern was researched by Derricks (1995), who describes how increased runoff from

urbanised catchments contributes to faster and deeper riverbank erosion and widened

unstable river channels downstream. Straightened river channels (usually in urban settings)

are out of phase with the water and sediment discharge regime, resulting in increased bank

erosion (Bennett, 1997).

A SEPA report on the Weinflu Case Study (Kennedy et al (see References: websites),

described the effects of urbanisation and flood control along this river passing through

Vienna. These include changes to the river pathway, along with disruptions to sediment

transport and flow regime. Regular bank re-enforcement is a necessary precaution due to

increased erosion.

WATER POLLUTION CHANGES

Increased water pollution is likely to be the result of changes occurring at this period-

Typically, the flora and fauna within urbanised river channels deteriorate due to decreasing

water quality and damage lo habitats (Herricks, 1995). Turbidity will inevitably increase with

effluent/storm water discharges and increased erosion. There are several reports of changes

to water pH, BOD and DO levels, and increased levels of heavy metals, oil, pesticides,

nitrates, coliform bacteria, and dissolved ammonia. These are often directly attributed to

wastewater, storm drainage and soil erosion (Herricks, 1995).


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Construction areas and parking lots are an important source of suspended solids and heavy

metals. River sediment may be characterised by high BOD levels, along with contamination

by pesticides and hydrocarbons. Possible causes tor this include eroded (organic-rich) soil,

garden/field pesticides (Manahan, 2001), spilled substances such as fuel oil, or

trade/sewage discharges (Herricks, 1995; Radojevic et al, 1998).

Furthermore, the excavation of soil and sediment required for the construction of flood

banks will necessitate the use of heavy machinery, which contributes to soil erosion, and

increased turbidity (Dept. of Natural Resources & Environment (Australia), 2001).

1.2: 1970-1990

The flow rate increases from 13.0m3/s to 13.7m

3/s, and the water depth decreases

marginally, from 2.3m to 2.2m.

HYDROLOG1CAL CHANGES

The water depth falls slightly during this period, in spite of increasing flow-rate. The initial

impacts of raising flood embankments include increased water depth and riverbed erosion

(ESCAP Publications). Flood banks only re-locate floods to either end of the levied stream.

During storm events, the restrictive effect of the banks produces increased flow rates. The

same effect leads to more concentrated sedimentation within the protected channel, which

may explain the decrease in depth over this time (Bennett, 1997).


The establishment of the flood banks will lead to an increase in the flow-rate, due to these

preventing the dissipation of floodwaters onto the surrounding land. However, the water

depth will be lower, as the banks were raised, using dredged material from the old river bed

banks, leading to higher flow velocity, and riverbed erosion in the new cutting (Figure 3):

Figure 3: morphological river bed changes. Sediment is dredged up from the old river bed (bold brown line), to

be used for building the flood barriers (dotted brown line). The blue bold line is the old water level; the dotted

blue line shows the new river water level (Bennett, 1997).

As mentioned, scouring of the embankments and the riverbed will occur within this

modified river course, and increased flow conditions can exacerbate riverbank erosion

(SUDS Working Party, 2000). The new town will still continue to expand during this time,


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leading to further culverts being constructed. River modifications within the town and

alongside the flood banks can lead to increased sedimentation and a changing river form

downstream (Natural Resources and Environment, 2001).

WATER POLLUTION

As the New Town will still be expanding, there will be further increases in surface drainage

and treated sewage/industrial discharges into the river. The effects of this have already

been discussed, and (if this was in Scotland) these may not yet have been addressed under

any environmental legislation. (As recently as 1996, sewage effluent discharges were the

main causes of river pollution in Scotland (SUDS Working Party).) Since the fishery has not

been considered yet, there may be no economic driver behind imposing water quality

improvements as yet.

The flood hanks will have reduced the amount of sediment and soil being washed into the

river, along with diffuse agricultural pollution such as pesticides, fertilisers or manure. (NSWGovernment). This would result in reduced nitrogen and phosphorus, pathogens, turbidity,

pesticides and hydrocarbons (e.g. from spilled oil) within the water column.

1.3: 1990-2010

It is predicted, that the flow rate will decrease from 13.7m3/s, to 13.0m

3/s, and that the

water depth will increase from 2.2m to 2.6m.

HYDROLOGICAL CHANGES

The creation of the fishery and boating lakes will require damming the river upstream and

downstream of the town and embankment. This may lead to reduced flows downstream

from these artificial lakes. These may also be weirs behind the lakes, and the water depth at

the gauge point will have risen, if this is a short distance upstream from the fishery lake.

The effects of a dam on river flows downstream are more significant during storm events, as

these are mitigated with more steady flows passing out An average decrease in flow rate

also requires a significant effort being made to control the diffuse inputs from the new town

and rural land upstream of the gauge point (Bennett, 1997.)


The construction of a dam results in sediment being trapped behind this barrier under

tranquil reservoir conditions. The resulting reduction in the river's sediment load

downstream may result in abrasive, sediment-free water causing rapid channel incision. A

study of the Stony Creek river in California revealed that this impact, resulted in the river

changing from a braided pattern to a single-channelled meandering stream, which migrated

laterally (across lad), and restricting land-use in that area (Bennett et al, 1997). This is

illustrated in Figure 4over-page:


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Figure 4:fluvial changes downstream of a dam along the Stony Creak river (these are not accurate maps)

The construction of a fishery will raise the altitude of the water surface level, downstream of

the gauge point. This may serve to raise the water depth as this retained water may extend

upstream past this point, and increased sedimentation will occur within the reservoir.

WATER POLLUTION

The implementation of sustainable urban drainage systems under SEPA has been prolific;

since 1996, over 760 SUDS systems have been developed across Scotland (CIRIA, 2000).

Improvements would have to be made, if the river passing into the fishery can support the

aquatic population therein. The UWTD (Urban Wastewater Treatment Directive) requires all

sewage being discharged to rivers, sensitive waters, and those used for commercial

purposes, to be treated to a very high standard of effluent purity. The instalment of SUDS

within the new town would serve to reduce levels of turbidity, BOD, heavy metals,

pesticides, floating solids (e.g. litter), oils and other hydrocarbons within the water column,

and passing into the river sediment (CIRIA, 2000).


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SECTION 2: INSTALLATION OF A COMBINED SEWER OVER-FLOW

2.1: PURPOSES AND PROBLEMS OF COMBINED SEWER OVER-FLOWS

Combined sewer systems are single-pipe sewer networks, conveying bothdomestic/industrial effluent, and storm-water run-off, to an STW. During periods of intense

rainfall however, these can be overwhelmed, and excess wastewater may have to be

discharged straight into receiving waters, in order to prevent flooding at the STW.

This emergency procedure is called the combined sewer overflow (CSO), and it has been

permitted on the basis that the large volume of wastewater will be dilute enough, for some

of it to be discharged into a river/lake, without causing serious environmental damage.

Nevetheless, in practice this has not always been successfully achieved; the environmental

damage to rivers in the UK by CSOs has been a major cause of urban river pollution for many

years (Herricks, 1995).

CSOs may contain raised levels of suspended solids, BOD, oils, floating debris, pathogens,

heavy metals, suspended particles and other potentially toxic compounds. These pollutants

pose a threat to aquatic species and human health, and may exceed water quality

standards. The ecological and chemical changes downstream from a CSO or a sewage

effluent discharge are well known. Increases in BOD, pathogens (including protozoa and

bacteria) and suspended solids, are accompanied by depletion in DO, and clean water fauna

(Figure 5) (Harrison, 2001):

Figure 5: ecological and chemical changes in a river downstream from a CSO


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DETERMINATION OF FORMULA A

The maximum combined sewer flow into an STW is conventionally predicted using this

equation. The calculated volume was believed to be dilute enough for excess amounts of

wastewater to be discharged to receiving waters, without any adverse effects. The equation

is reproduced (Read and Vickridge, 1997), and following calculations are provided below:

Formula A(l/s)= DWF + 130P + 2E = (PG + I + E) + 1360P + 2E

DWF: wastewater produced by the population (daily water flow)

P: Population (persons)

I: Infiltration

E: Industrial flow

For a new CSO being constructed at the lowest point of a local sewer system, the following quantitiesof wastewater need to be considered:

P = 4600 persons I = 0.005m3/s E = 0.012 m

3/s

DWF => (4600 0.225m3/day) + (0.005 86400 seconds/day) + (0.012 86400seconds/day)

1035 + 432 + 1036.8= 2503.8 m

3/day

Formula A => 2503.8 + (1.36 4600) + (2 1036.8)

2503.8 + 6256 + 2073.8 10833.4 m3/d or ~10.8Ml/day 10,800,000 litres per day 86400 seconds/day

= 125.4 l/s

SECTION 2.3:

CSO ARRANGEMENTS FOR DISCHARGE INTO TWO CONTRASTING RIVERS

The composition and environmental impacts of combined sewer overflow discharges have

been discussed. The nature of these aspects will vary considerably, depending on the

receiving water. The quantity of the receiving water is one important parameter for

determining the effects of CSOs (Table 2) (Moffa, 1997):

Table 2: general qualitative effects of CSOs on rivers

Significance of impact on

receiving water

D.O. Nutrients S.S. Toxics Pathogens Turbidity Sanitary

debris

Small river High Low Moderate Moderate High Low High

Large river Moderate Low Moderate Moderate High Moderate High


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Under the UWWTD, any discharge must not have any deleterious effects on water flora or

fauna, and must not have any aesthetic effects such as oily layers, discoloration, etc (SEPA,

2003). Incorporating a 6mm or 10mm mesh to remove screenings is now essential for all

CSOs, and any incompliant CSOs must be disabled under the UWWTD.

DISCHARGE INTO A LARGE RIVER PASSING THROUGH AN INDUSTRIAL AREA

It is unlikely that the river passing through an industrial area will have any amenity value

within the vicinity. For discharging into large low-amenity rivers, Read and Vickridgc (1997)

prescribed basic suspended solids removal techniques, including high-sided weirs, stilling

ponds and vortex separation devices. That was in 1997. SEPA now recommend (2003) that

where a discharge entering a river is diluted by a factor greater than 8 (>8:1 dilution), the

CSO may not require storm water storage facilities. In addition to a sewer of Formula A

capacity, a screening mesh of max. 10mm is essential for all CSOs.

Nevertheless, wastewater being released into this river may need to be screened to astandard, which does not compromise water abstraction by industrial firms downstream (if

appropriate). Industrial water uses include washing, processing and cooling processes.

Problems with using poor quality water include scaling (caused by calcium salts); metallic

corrosion (due to dissolved solids and metals); bacterial growth (which poses a health risk to

workers); or, the fouling of pipes (inorganic and microbial slime deposits) (MetCalf and

Eddy, 2003). Suspended sediment in abstracted water can lead to machinery damage and

oilier operational problems.

DISCHARGE INTO A SMALL RIVER PASSING THROUGH A RECREATION AREA

The CSO must attain a high performance standard, in terms of its aesthetic effects on a

receiving river, which is used for recreation. For a new CSO discharging into EU-designated

waters (i.e. bathing waters and sensitive waters), there should not be more than one

spillage per annum or season. This implies that a storm water tank volume of around 820m3

is required. This would be above the 652m3threshold volume, which would meet the SSD

standard (for a dilution of 1:1 or less).

The permitted number of discharges does not reflect the damaging effects, which may arise

from these intermittent events. Since the wastewater is being discharged to a smaller riveras well, it may be advisable to choose a larger storm tank volume than the 800m

3capacity,

or to increase the flow to the STW.

These would incur considerable capital costs, and so the use of SUDS is another option to be

considered, if the CSO discharges lead to any legitimate complaints (e.g. by anyone using

the waters for recreational purposes). Secondly, a finer screen (6mm or less) may be

necessary to prevent large particle-sized matter passing into the river and harming the

aquatic environment.


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Copyright of LabSearch Ltd a working title of Dr Malcolm Sutherland 2013

managing urban drainage: a theoretical case study of suds design

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