Innovative measures to mitigate overtopping of coastal structures

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Overtopping of coastal structures which can be attributed to sea level rise and increased storminess is a major design issue. It is likely that many existing structures are undersized based on future predicted climate changes and will not be able protect shores from future coastal forces. Current structures may also become damaged due to the undermining of the structure as a result of regular overtopping. This can have disastrous effects on coastal areas and communities such as damage to properties, infrastructure, agriculture and loss of life. Ongoing research at the Hydraulics and Maritime Research Centre of University College Cork seeks to explore innovative ways of redesigning or modifying coastal structures such that to future protect them without incurring excessive cost or increasing crest levels beyond acceptable levels. Physical model testing has been undertaken in a 25m long wave flume using coastal structure models with a scale of 1:30. These tests examine two separate wave overtopping protection measures: 1. Changes to the crest detail of rubble mound coastal defence structures by incorporating a reservoir system is being researched as it is expected that this would reduce crest elevation but provide the same level of safety. This may be necessary due to site limitations or for aesthetic reasons. The rate of overtopping q is measured through the use of weir boxes at the back of each reservoir with each weir being of compound composition with a v-notch weir for lower flow rates and a trapezoidal weir for higher discharge rates. Calculation of the inflow discharge rate is based on the outflow discharge rate over the weir into an overflow tank. This is achieved through accurately measuring the water level in the weir boxes using twin wire conductance probes. There are numerous factors being examined such as different reservoir designs, slope of the coastal defence structure, water level and use of a geotextile within the structure. This research of these various parameters will create an overtopping equation which is specific to the structure and can be used for future design purposes. It also incorporates the possibility of extracting energy from the power of waves which enter the reservoirs. Initial indications are that the reservoir system reduces the crest elevations as compared to a standard designed structure. 2. Reduction of overtopping of existing structures by floating a wave absorbent geotextile matting seaward of the structure. It is envisaged that this material would mitigate overtopping by reducing the power of the incoming waves. The matting system will float at water level and will adjust depending on water level. It is attached to the structure by an anchoring system. There are many varying parameters that will be examined such as the geotextile material, porosity and strength. These parameters can have significant effects on the amount of overtopping that occurs and it is through this research and testing that the optimum and most efficient system can be developed. This is a conceptual idea and together with the reservoir system forms part of a research programme which is due for completion in September 2010.