topographic, bathymetric & tidal surveys at ......topographic, bathymetric & tidal surveys...

Woodbrook Coastal Erosion Study- Dun Laoghaire Rathdown County Council

Hughes Hydro Surveys www.hugheshydrosurveys.com

HUGHES HYDRO SURVEYS

TOPOGRAPHIC, BATHYMETRIC & TIDAL SURVEYS AT

WOODBROOK CO DUBLIN


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Project Description:

Hughes Hydro Surveys was commissioned by Dun Laoghaire Rathdown County Council to undertake a variety of surveys

to allow Malachy Walsh & Partners to assess ongoing coastal erosion and thereafter to design a coastal protection

scheme. The scope of the project was to carry out a detailed topographic, bathymetric and tidal flow surveys at

Woodbrook Co Dublin to facilitate the assessment and design of coastal protection works.

Location:

The site is located south of Woodbrook Golf Course and approximately 100m north of Bray Harbor, the main area of

interest/erosion is a 300m long area of foreshore that was formerly a landfill site which consists of gravels mixed in with

landfill material which overlays glacial till as can been in the image below, Plate1.

Plate 1.


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Specifications & Scope:

Scope:

• Toe of cliff face.

• Crest of cliff face.

• Elevation of the cliff face for the 300m stretch of the landfill site to identify different layers.

• Ground levels at 5m gird intervals.

• Seabed levels at 10m grid interval for the full site.

• Structures on site.

• Current metering at one location for a full tidal cycle.

• 3 Sea bed samples and associated testing.

Topographic:

The topographic survey was carried out to 50m west of the cliff face where possible and to extend to the low water line

at a 5m grid interval. At the northern extent of the study limits the survey was extended north by 400m and only 10 m

west of the cliff face.

Bathymetry:

The bathymetric survey was carried out form the cliff face and extending 400m east of the face, in addition the northern

and southern limits extended 400m north and south of the 300m site limits at a grid density of 10m.

Current Metering:

Tidal velocity and bearing recordings were observed mid depth at one location for a full tidal cycle at 30-minute

intervals.

Grab Samples:

There grab samples were obtained and sent for testing to IGSL to comply with BS EN ISO 14668-1&2 to determine

particle size distribution and specific gravity.

Given the nature of the site, access limitations and restrictions, health and safety concerns, tides, environmental and

weather issues we utilized many different survey techniques to capture the required data. These included,


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• Multi beam bathymetry.

• Laser scanning.

• Traditional survey techniques for both topography and marine observations.

Project Control and Equipment:

The survey is referenced to Irish Transverse Mercator IRNET95 system with all levels relative to Malin Head Datum, using

the OSGM15_Malin geoid model. In order for us to be able to capture the necessary information we required near ideal

weather conditions- flat sea state, during a spring tidal cycle so that we could observe maximum tidal velocities and so

that we could physically get as close to the shoreline as is practical and safe.

Equipment Used:

• Trimble R8s and R10 VRS RTK

• Trimble TX8 Laser scanner

• Reson Seabat T50-R multi beam

• Van Veen grab

• Valeport 308 current meter

• Valeport Swift SVP

• Software included, Trimble Business Center, LSS, AutoCad, Teledyne PDS and Microsoft Office.

Results:

Topographic Survey:

The foreshore intertidal area was surveyed using a combination of methods, the original plan was to undertake the

bathemetry during a spring high tidal cycle to gain maximum coverage, as was the case once on site. Thereafter the plan

was to laser scan the foreshore and cliff face during a spring low water tidal cycle, however this did not happen as we

lost daylight hours with the changeover to GMT. As a result, we had no other option but to survey this area during a

neap low water tidal cycle which meant that we could not get an overlap with the multi beam data set. This narrow

band between the two data sets running the full length of the foreshore was surveyed using Trimble RTK. The foreshore

area is mostly composed of mixed stones/small boulders and sand, see Plate2. There a number of structures located

along the foreshore, these include


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Plate 2.

• An old wall in poor state of repair, possibly part of the original railway line as some of the original structure can

be clearly identified and can be seen on OS maps. The original line was completed in 1854 and if this structure

is part of that railway line then there is clear evidence of erosion in the order of 30m at the southern end and

45m at the northern site limits, a rate of erosion of 0.3m per year. In some places the wall is standing to a

height 0.5m above the beach level while in other places it is flush with the beach or nonexistent. The foreshore

is littered with debris form the wall and in a number of places there are a large number of detached sections of

wall, see Plate 3.


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Plate 3.

• At the northern end of the study area there is evidence of an old building which is derelict and several outfalls, 3

in total. The most northern outfall which is approximately 15m south of the Woodbrook Golf Course fence line

runs in an easterly direction form a manhole positioned above the high-water line for a distance of 47.40m into

the sea. We could not be confident whether this outfall is active or not, ideally the manhole cover would have

to be lifted to confirm this. Cover Level 3.54m, invert Level at outfall is -1.52, the dimensions could not be

accurately determined due to high water/tide levels.

Immediately north of the derelict structure is a large exposed concrete pipe outfall, upon inspection there is

evidence of a small trickle of water exiting the pipe, a visual of the pipe internals indicates that it is severely

blocked with stones and debris. The invert level of this pipe is 1.42m with a diameter of 1.26m.

Immediately south of the derelict structure is a buried concrete pipe, this pipe is completely blocked with

debris/stones however there is evidence of running water, probably surface water. We surveyed an invert level

of where the water was exiting, level is 1.51m the true invert could not be determined as the pipe is buried, in

addition the diameter could not be ascertained, see Plate4.


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Plate 4.

• The upper reclaimed area and the golf course were surveyed using RTK. Most of the reclamation site is freely

accessed however their areas of very dense high brambles which prevented data acquisition. As you progress

north on the upper level the topography changes form relatively flat area to high mounds and deep valleys

running in an east west direction, in addition this end of the site is very densely vegetated with high (1.8m)

brambles and scrub/trees which had a very negative impact on survey operations. In addition, underfoot

conditions were very poor once you venture off the well-trodden track as there is unmistakable evidence of

dumped domestic and commercial waste, see Plate5.


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• To assist/aid the overlay and comparison with historical data we surveyed in the southern edge of the northern

arm of the harbor at Bray as this structure is evident on historical maps.

Bathymetry:

The intertidal zone extending out to the low water line is composed of mixed ground, stones, boulders, sand, isolated

rocks and rock/boulder outcrops which is most evident at the most northern end of the survey. Below this line the

seabed topography is mainly flat sand with minor undulations. Levels range form -1.3m at the inshore end of the survey

extents to -7.4m at the north-east corner of the survey. Conditions throughout the survey were ideal.

Currents:

An anchor was deployed at the intended current meter location, thereafter we either stayed permanently moored up or

cycled back and forth to the location to observe current speed and direction readings. In general, the bearing observed

during the ebb tide was between 20° and 300° with an average of 340°. This pattern was apparent until approximately

mid flood tide when the bearing changed to a mean off 150° and remained consistent thereafter. The velocities in


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general on both flood and ebb tides were consistent with an average of 0.14m/s. However, a decrease in velocity was

noticed at two thirds flood tide where a change in the bearing readings was also recorded. Thereafter the velocities

increased to a maximum of 0.29m/s at high water, similar patterns were noticed at the top of the water column also.

110

280

330350 350

20

320 320

20

300320 330

310

240220

180160 160 160 150 160 160 160

130

0

50

100

150

200

250

300

350

400

13:00:00 14:00:00 15:00:00 16:00:00 17:00:00 18:00:00 19:00:00 20:00:00 21:00:00 22:00:00 23:15:00 00:15:00

Bea

rin

g °

Time

Woodbrook Current Bearing Plot


10

0.12

0.16

0.120.09

0.14 0.14 0.15 0.150.13

0.15

0.120.13

0.050.07 0.07

0.13

0.23

0.270.29

0.16

0.20

0.23

0.160.13

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Spee

d m

/s

Time

Woodbrook Current Speed Plot

Seabed Samples:

In total three samples were acquired and sent for testing. Upon visual inspection all three samples are of very similar

type, primarily very fine dense sand with insignificant amounts of broken shell and very fine clay, there is no evidence of

any stone, rock of gravel present. Please see below laboratory results.

Sample Number Easting Northing

BR1 726853 719557

BR2 726890 719691

BR3 726775 719809


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Sample BR1.

particle % Contract No: 20561 Report No. R83480

size passing Contract: Bray

75 100 BH/TP : BR1 Sample 1

63 100 Sample No. N/A Lab. Sample No. A17/5273

50 100 Sample Type: B

37.5 100 Depth (m) N/A Customer: Hughes Hydro Surveys

28 100 Date Received 08-11-17 Date Testing started 13-11-17

20 100 Description:

14 100

10 100 Remarks Note: Clause 9.2 and Clause 9.5 of BS1377:Part 2:1990 have been superseded by ISO17892-4:2016

6.3 100

5 100

3.35 100

2 99

1.18 99

0.6 98

0.425 97

0.3 96

0.15 38

0.063 3

Approved by: Date: Page no:

23-11-17 1 of 1IGSL Ltd Materials Laboratory

Persons authorised to approve report: J Barrett (Quality Manager) H Byrne (Laboratory Manager)

SILT/CLAY

SAND

TEST REPORT

Determination of Particle Size Distribution Tested in accordance with: BS1377:Part2:1990 , clause 9.2 & 9.5

(note: Sedimentation stage not accredited)

Brown slightly clayey/silty, slightly gravelly, SAND

GRAVEL

COBBLES

75

63

50

37.5

28

20

14

10

6.3

53.3

5

21.1

8

0.6

0.4

25

0.3

0.1

5

0.0

63

0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1 10 100

CLAY SILT Sieve size (mm) SAND GRAVEL

Perc

enta

ge p

assin

g (

%)

IGSL Ltd, M7 Business Park, Newhall, Naas, Co Kildare








20 100 Description:

14 100


6.3 100

5 100

3.35 100

2 100

1.18 99

0.6 98

0.425 98

0.3 97

0.15 26

0.063 1


23-11-17 1 of 1

SAND

TEST REPORT



Brown slightly clayey/silty, SAND

GRAVEL

COBBLES

SILT/CLAY

IGSL Ltd Materials Laboratory


75

63

50

37.5

28

20

14

10

6.3

53.3

5

21.1

8

0.6

0.4

25

0.3

0.1

5

0.0

63

0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1 10 100


Perc

enta

ge p

assin

g (

%)









20 100 Description:

14 100


6.3 100

5 100

3.35 100

2 100

1.18 100

0.6 98

0.425 94

0.3 84

0.15 28

0.063 0


23-11-17 1 of 1

SAND

TEST REPORT



Brown SAND

GRAVEL

COBBLES

SILT/CLAY

IGSL Ltd Materials Laboratory


75

63

50

37.5

28

20

14

10

6.3

53.3

5

21.1

8

0.6

0.4

25

0.3

0.1

5

0.0

63

0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1 10 100


Perc

enta

ge p

assin

g (

%)


IGSL LtdMaterials LaboratoryUnit J5, M7 Business ParkNewhall, NaasCo. Kildare045 846176

Report No. R83647 Contract No. 20561 Contract Name:

Customer Hughes Hydro Surveys

Samples Received: 08-11-17 Date Tested: 17-11-17

BH/TP Sample No. Depth (m) Lab. Ref Sample Particle DescriptionType Density (Mgm³)

BR1 1 N/A A17/5273 B 2.65

BR2 2 N/A A17/5274 B 2.69

BR3 3 N/A A17/5275 B 2.66

Notes: Sample Type: B - Bulk Disturbed Remarks:

U - Undisturbed

NOTE: *Clause 3.2 of BS1377 is a "withdrawn" standard due to publication of ISO17892-1:2014

Opinions and interpretations are outside the scope of accreditation.

The results relate to the specimens tested. Any remaining material will be retained for one month.

Persons authorized to approve reports Approved by Date Page

23-11-17

Brown slightly clayey/silty, slightly gravelly, SAND

Brown slightly clayey/silty, SAND

Brown SAND

H Byrne (Laboratory Manager)IGSL Ltd Materials Laboratory

Test Report

Determination of Particle Density

Tested in accordance with BS1377:Part 2:1990, clauses 8.3/8.4

Bray

1 of 1

R83647 .Pd Soils .xls Tmp: Pl.ll Rev 02/10

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