Lahinch Coastal Protection Foreshore & Design Report · The document is the copyright of JBA Consulting Engineers & Scientists Ltd and Cronin Millar Consulting Engineers. 1.1 Introduction

Lahinch Coastal Protection

Foreshore & Design Report

October 2016

Prepared for:

_______________________________________________________________________________________________________ Foreshore & Design Report Page 2

Contents

1 INTRODUCTION ............................................................................................................ 3

1.1 Introduction .............................................................................................................. 3

1.2 Ownership and Operation .......................................................................................... 3

2 EXISTING INFRASTRUCTURE ........................................................................................... 4

2.1 Coastal Defences ....................................................................................................... 4

2.1.1 Rock Armour ......................................................................................................... 4

2.1.2 Recurve Wall ......................................................................................................... 4

2.1.3 Sea Wall ................................................................................................................ 5

2.2 Defects ..................................................................................................................... 5

3 ENVIRONMENTAL DATA ................................................................................................ 8

3.1 Tidal data ................................................................................................................. 8

3.2 Extreme Water Levels ................................................................................................ 8

3.3 Extreme Wave Heights ............................................................................................... 8

3.4 Effects of Climate Change ........................................................................................... 9

4 PROPOSED WORKS ..................................................................................................... 10

4.1 Rock Armour Revetment .......................................................................................... 10

4.2 Repairs to Existing Sea Wall ...................................................................................... 10

4.3 Steps to Beach ........................................................................................................ 10

4.4 Drainage ................................................................................................................. 10

5 OTHER ....................................................................................................................... 11

5.1 Foreshore Lease Area .............................................................................................. 11

5.2 Section 10 Approval Area ......................................................................................... 11

5.3 Foreshore Licence Area ............................................................................................ 11

5.4 Outfall Area ............................................................................................................ 11

5.5 Appropriate Assessment Screening ........................................................................... 11

5.6 Environmental Issues ............................................................................................... 11

Shane McCarthy C.Eng., MIEI

Chartered Engineer

Project Ref: 2015S3069 / C905

Revision Control Table & Document History Record

Rev. Date Description & Reason for Issue Orig. Chkd. App.

0 27th July 2016 Draft SMC AC AC

1 25th October 2016 Draft for Client Review SMC AC AC

No part of this document may be re-produced or transmitted in any form or stored in any retrieval system of

any nature, without written permission of Cronin Millar Consulting Engineers, as copyright holder, except as

agreed for use on this specific project.


1 INTRODUCTION This report has been prepared by JBA Consulting / Cronin Millar Consulting Engineers (JBA/CMCE) for the proposed Coastal Protection Works at Lahinch Co. Clare. The document is the copyright of JBA Consulting Engineers & Scientists Ltd and Cronin Millar Consulting Engineers. 1.1 Introduction Lahinch is a coastal community located in the heart of Liscannor Bay and the town relies heavily on the coastline to protect the community and infrastructure on its west facing coastline. Over the years, the existing sea wall and rock armour protection along Lahinch’s coast have become heavily dilapidated and in need of urgent repairs. During larger storm events, such as the December 2013 storm, areas of the town and carpark suffered from severe flooding. Temporary repairs have been undertaken over the years to the dilapidated structures, however the condition continues to deteriorate resulting in wave overtopping along the frontage, damage to infrastructure and a public safety risk. JBA/CMCE were commissioned to develop a detailed design to permanently address the condition of the existing defences and reduce levels of wave overtopping along the coastal frontage. 1.2 Ownership and Operation Clare County Council is the local authority with the responsibility for the provision and maintenance of coastal defences in the town.


2 EXISTING INFRASTRUCTURE This section provides a brief description of the findings of a visual condition assessment of the existing coastal defences fronting Lahinch. The town of Lahinch is located adjacent to Lahinch beach in west County Clare. The town is currently protected by two forms of coastal defence: a concrete sea wall and rock armour revetment. The concrete sea wall was constructed in varying forms over several decades. It extends from the public carpark in the north, to the coastal cliffs at the south. A rock armour revetment extends from the carpark to the location of the start of a concrete recurve wall. The coastal defences provide some protection from extreme wave conditions to the town. Assets in the location include private property, a public car park, a public promenade and a public road; however, the defences do not provide adequate protection to the assets. JBA/CMCE have been tasked with developing a detailed design to reduce wave overtopping and coastal erosion to approximately 350m of coastline fronting Lahinch.

2.1 Coastal Defences JBA/CMCE carried out a condition asset survey of the existing coastal defences in the applicable study area. The coastal defences within the Clare County Council defined study area are described as follows. 2.1.1 Rock Armour In the north of the frontage, the sea wall is protected by varying profiles of rock armour. A full rock armour profile, without a clearly defined crest, is noted in the far north, which reduces in size towards the mid to low extents of the defence. These smaller volumes of rock function as a form of toe and wall protection, rather than functioning as the primary defence to reduce overtopping. The rock armour revetment stretches approximately 100m, is sloped at a shallow angle of 1 in 5, and consists of a wide range of rock sizes, estimated to be in the order of 1t - 10t. The smaller quantities of rock armour in the south, currently providing toe or wall protection, are also similar in grading to the revetment rock armour. The form of this defence is non-uniform in design, and it does not specifically conform to any one particular purpose, but rather provides protection against scour to the sea wall foundations and reduces damage to the sea wall face.

Summary of rock armour protection

Purpose Primary defence, reduces overtopping

Rock armour size 1-10t

Slope 1 in 2 to 1 in 5

Beach material Sand

Table 2-1: Summary of Rock Armour Protection Form

2.1.2 Recurve Wall At the south of the frontage, a curved concrete wall is found. This wall is in poor condition and requires full remedial works and the installation of rock armour protection. Surface water runoff from the adjacent public road currently causes erosion of the embankment above the recurve wall.


2.1.3 Sea Wall A sea wall is located behind the existing rock armour defence. The sea wall is very exposed in locations and is in a fair to poor condition and requires repairs. The form and type of construction varies between reinforced concrete, mass concrete and masonry. 2.2 Defects Several defects were noted during the asset inspection. They are summarised as follows:

Undersized existing rock armour;

Inconsistent existing rock armour profile;

Subsidence of existing revetment;

Insufficient rock armour;

Poor interlocking of rock armour units;

Poor to fair condition of existing sea wall behind rock armour;

Erosion of embankment toe at recurve wall;

Poor condition of recurve wall.

A number of these defects are displayed in the following images:

Figure 2-1: Existing revetment with large range of rock armour unit sizes. Expelled armour units present on beach.


Figure 2-2: Example of existing revetment with a shallow slope (approx. 1 in 5)

Figure 2-3: Exposed section of sea wall at risk of high levels of overtopping


Figure 2-4: Poor condition of wall

Figure 2-5: Poor condition recurve wall with no rock armour protection


3 ENVIRONMENTAL DATA 3.1 Tidal data The following tide levels will be used at Lahinch. The data has been sourced from Malachy Walsh and Partners, 2009. 1

Location Height (mODMalin, 2007)

Mean High Water Springs (MHWS) 2.05

Mean High Water Neaps (MHWN) 0.85

Mean Low Water Neaps (MLWN) -1.00

Mean Low Water Springs (MLWS) -2.30

Table 3-1: Lahinch Tide Levels

3.2 Extreme Water Levels Data for the extreme water levels have been sourced from the Irish Coastal Wave & Water Level Modelling Study (ICWWS), produced by RPS for the Office of Public Works. This data has been made available by Clare County Council and is considered to be the most readily available and effective data set for the proposed site.

The data has a range of expected still water levels and wave heights that incorporate a range of storm events. The data has considered the dependence between still water level and wave heights so that the storms can be thought of as a joint probability return event.

It is proposed that the worst case still water level from each return period is selected as the extreme design water level for this design.

The still water level and wave height data are available for the 1 in 2, 5, 10, 20, 50, 100, 200, and 1000-year events with and without climate change factored in (explained below). The worst case still water levels from the ICWWS joint probability analysis are shown below.

Return Period (1 in X)

Present day water level (mODMalin)

Future water level (mODMalin)

2 2.66 3.16

5 2.79 3.29

10 2.88 3.38

20 2.97 3.47

50 3.09 3.59

100 3.18 3.68

200 3.27 3.77

Table 3-2: Present and Future Extreme Water Levels at Lahinch

3.3 Extreme Wave Heights A review of the joint probability of the water levels and wave heights in the ICWWS data, showed wave heights in the order of 1.8 - 2.2m for the extreme return period events. It is believed that these represent an underestimation of the likely wave heights given the exposed westerly location of Lahinch.

In order to account for these expected larger waves, it is proposed that depth limited wave heights are calculated using the extreme water levels shown above. The selected water depth to depth limit the waves will vary dependent on the intended purpose of the wave height.

1 Malachy Walsh & Partners (2009) Coastal Protection Assessment, Lehinch Promenade, Lehinch, Co. Clare


3.4 Effects of Climate Change For a coastal defence, it is important to understand the predicted effects of climate change and assess the adaptability of the design. Typically adapting an existing rock armour coastal defences in the future is a costly process due to the scale of plant and materials mobilisation required. From optimisation analysis conducted on the design, the savings made in designing including for climate change is more cost effective when compared to a design to a present-day standard of protection. It is proposed that the design will incorporate an allowance for climate change for the following:

Still water levels; and

Wind and swell driven waves.

Estimates for sea level rise are provided in the ICWWS report, under Mid-Range Future Scenario (MRFS) and High End Future Scenario (HEFS). For this study, it is proposed that the MRFS is considered. This is intended to represent a 'likely' future scenario, based on a wide range of predictions available. The MRFS gives projected sea level rise of 500mm over a 100-year time horizon for Lahinch.

The industry widely acknowledges the difficulty in predicting changes in wave heights over the next 100 years and concludes that there could be a negligible increase in wave height due to wind speed alone. However, the greater depths of water associated with higher still water levels allow larger waves to travel inshore before breaking. The increases in wave height will therefore be accounted for in the larger water depths at the defence.


4 PROPOSED WORKS

4.1 Rock Armour Revetment JBA/CMCE carried out a detailed design to reduce wave overtopping and coastal erosion to the site within the Clare County Council defined study area. The coastal engineering design proposed for Lahinch was developed based on technical feasibility, environmental impact, cost and consideration of the long term vision and key criteria determined by the project stakeholders. The design principles adopted represent the industry standard methods and assumptions for the majority of coastal defences across the Ireland and the UK. The performance standard of this revetment will be measured against its ability to reduce wave overtopping. The design of the revetment will limit the rate of wave overtopping to a value that is deemed acceptable for design. The following tolerable discharge thresholds have therefore been proposed for the revetment design in Lahinch (Source EurOtop2):

In front of promenade - <10/s/m

In front of road - <50l/s/m These tolerable discharges are such that all structures will be considered safe for pedestrian access during the more regular storm event, while vehicular and emergency staff will be safe to inspect defences during the less frequent, higher magnitude storm. The design calculations resulted in a rock armour profile as described in the drawings. The crest of the revetment varies between +3.7m OD Malin and +7.00m OD Malin. The crest width of the revetment varies between 6m and 11m. The rock armour material will be interlocking double layers of 6-10tonne mass. 4.2 Repairs to Existing Sea Wall Before the revetment is constructed, repair works will be required to the existing sea wall. These repair works will comprise concrete pumping and grouting works, and applying sprayed concrete to the face of the sea walls.

4.3 Steps to Beach Access to the foreshore through the existing access steps will be maintained. The beach access will be protected from dominant wave action by a small breakwater.

4.4 Drainage New drainage will be constructed on the N67 public road as part of these works. The road currently drains directly from the roadside which is causing erosion on the cliff face. It is proposed to collect the storm water from the N67 in this location via gulleys, and convey to a new outfall through the rock armour revetment.

2 Pullen, T., Allsop, W., Bruce, T., Kortenhaus, A., Schuttrumpf, H & van der Meer, J (2007) 'Wave overtopping of sea defences and

related structure: Assessment manual'. Accessed from www.overtopping-manual.com


5 OTHER

5.1 Foreshore Lease Area The Foreshore Lease Area is indicated on drawing 2015s3069 - 511. 5.2 Section 10 Approval Area The Section 10 Approval Area is indicated on drawing 2015s3069 - 512. This area comprises construction on Clare County Council owned foreshore. 5.3 Foreshore Licence Area The Foreshore Licence Area is indicated on drawing 2015s3069 - 513. This area comprises the area of foreshore which will be temporarily required during the construction stage as a haulage route to the revetment. 5.4 Outfall Area The Section 10 Approval Area is indicated on drawing 2015s3069 - 514. The area comprises the proposed new outfall location for the road drainage. 5.5 Appropriate Assessment Screening An appropriate assessment screening report was carried out for the site and is included in the Foreshore Application Documentation.

5.6 Environmental Issues The flood defences proposed have been developed with careful consideration to ensure minimal impacts on the environment. None of the options are considered to have any significant long term impact on the environment, and any impact will largely be short term during the construction stage. An EIA Screening report has been completed based on the preferred option proposed and is appended to this report.

Documents

Lahinch Coastal Protection Foreshore & Design Report · The document is the copyright of JBA Consulting Engineers & Scientists Ltd and Cronin Millar Consulting Engineers. 1.1 Introduction