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THE SCHEME
Up to fourteen options were considered. Each contained variations for the 8 Defence Sections along the eaplanade, piers, harbour and east Beach. The best elements from these schemes were brought together to form the Preferred Option.
REPLACEMENT OF THE WEST PIER - THE PREFERRED OPTION
The preferred scheme (sorry, file not currently available)
Benefits of the preferred option:
Wave Heights
The realignment of West Pier would result in a significant reduction in wave heights, both in the entrance channel and in the Harbour Basin; this will have the following benefits: - ·
Significantly reduced wave loadings on the Piers ·
Significantly reduced overtopping of the Harbour walls ·
Significant reduced damage to vessels moored in the Harbour Basin
Maintaining West Pier on the same alignment would not address the problems of the high wave loading, overtopping of the harbour walls, and damage to moored craft.
Focusing on Wave Energy at the Root of West Pier/Mound
The realignment of West Pier will significantly reduce the focusing of wave energy at the root of West Pier/Mound. This focusing of energy has been the main cause of the many structural failures of the coastal defences in this area.
Maintaining the existing alignment of West Pier and strengthening the outside face with additional rock armour would reduce this problem. However, the reduction will not be as significant as the reduction achieved through the realignment of West Pier.
Entrance Channel
A succession of repairs to the Piers has reduced the width of the channel leading to the Harbour Basin. Any further reduction would seriously increase the navigational problems.
Realignment of West Pier would significantly reduce the wave heights experienced between the Piers, thus allowing safe passage of vessels along the entrance channel, even during extreme storm conditions. This would be a significant maritime safety benefit that could not be achieved by maintaining the Piers on their current alignment.
Construction difficulties
Strengthening the Piers and maintaining the same alignment would have the following problems: - ·
The Harbour would have to be closed for a period of at least 9 months, possibly over two summer periods in order to remove the existing steel sheet piling and remove the concrete infill between the piling and the original masonry face. ·
Future repairs would still require the Harbour entrance to be closed whilst the repairs were undertaken. Realignment of the West Pier would only result in the minimum of disruption and allow future repairs to be undertaken without the need to close the entrance channel.
Environmental Issues
The series of coastal defence emergency repairs that have taken place over the last 40 years, have transformed the Mound area into an environmental eyesore. Scheme option 1 would further detract from the environmental value of this area through the use of additional rock armour.
The realignment of West Pier provides the opportunity to enhance this area through a scheme that fulfils the coastal defence objectives, and is sympathetic and compatible with the surrounding natural and built environment.
Conclusion
The present orientation of the two parallel-aligned piers is responsible for most of the coastal defence and harbour operation problems. Previous coastal defence schemes have only repaired structural failures, or strengthened the existing defence structures; they do not address the fundamental cause of the problems.
A realigned West Pier, appropriately designed, would provide a structure that could accommodate high wave loadings and require only low cost maintenance.
The realignment of West Pier would address the harbours operational problems and avoid the need to transfer commercial interest, such as fishing, to neighbouring ports whilst strengthening works were being undertaken.
The realignment of West Pier would provide the opportunity to enhance the Mound area.
THE PREFERRED SCHEME
PHYSICAL MODEL
Introduction
The realignment of West Pier evolved through a series of Project Review Meetings held between WDDC and HR Wallingford. With the design being based on engineering judgement, there was a need to undertake physical model testing to determine how effective the realignment would be in reducing wave heights and loadings.
Test Results
A model was built and tested for a series of water levels and storm events with different return periods. The test confirms that the realignment of West Pier will result in wave height reductions of between 50 - 75%.
BEACH MANAGEMENT PLANS
An integral part of this scheme is the implementation of separate beach management plans for West Beach and East Beach. Their purpose is to prevent the beach falling below its design standard of protection, and to allow timely and efficient planned maintenance. A design standard of protection of 1 in 200 years has been adopted for West Bay. Posford Duvivier were commissioned to produce the Beach Management Plan in collaboration with HR Wallingford. The beach response and wave data produced by HR Wallingford was based on a series of mathematical and physical model studies.
Key Stages
The beach management plans comprise of the following stages: -
Design of Beach Profile
The beach profiles have been designed to prevent critical conditions, such as flooding, breaching and sea wall failure, occurring as a result of a storm event that would be expected to occur once every 200 years.
Pre- construction Data Collection
Before construction work commences a detailed beach survey will be undertaken. This will provide the data that will allow a comparison to be made between the pre and post construction performance of the beaches.
Monitoring Programme
Beach monitoring will be undertaken at regular intervals to determine if any actions are required to prevent conditions occurring that would result in the beaches failing to provided their design standard of protection.
Data Appraisal
The data collected from the monitoring programme will be analysed to: - ·
Identify and understand past and present problems ·
Identify future work necessary to ensure the standard of protection is maintained ·
Identify improvements that can be made to the Beach Management Plan
Action Plans
This consists of a series of procedures that will be followed once a beach has reduced in volume to a predetermined trigger volume. The trigger volume signals the need to undertake works to reduce the risk of failure occurring.
Annual Beach Status Report
The purpose of this report is to correlate all the information relating to beach response and sea conditions in order to assess what actions are needed in order to maintain a 1 in 200 year standard of defence for the following year.
5 Year Review
The Beach Management Plan will be reviewed at intervals of 5 years to identify any amendments necessary to ensure the Plan provides an effective, economic, and environmentally acceptable plan for the management of the beaches. |
