Regulated tidal exchange
Chichester Harbour (Sussex)
Flood Risk Management
Climate Change (Sea Level Rise) Mitigation
Improve Flood Protection
Provide a Recreational Facility
In 2012, the Environment Agency upgraded an existing coastal defence at West Wittering (Chichester Harbour).
The defence that was in place included a damaged culvert with a flap valve that had become fixed in a slightly open position.
This sluice had therefore not only been facilitating freshwater drainage off the site into the harbour (as it was designed to do) it had been allowing modest amounts of tidal water to flow across the hinterland. This situation had existed for several years previously.
Due to this small tidal exchange, the hinterland had achieved high ecological value. It was part of a designated Special Area of Conservation (SAC) due to the shallow coastal lagoons that had been created. It was also included within a Special Protection Area (SPA) due to the large numbers of roosting and feeding waterbirds it supported.
For the sea defence upgrade works, a new culvert was designed and installed that functioned in the same way as the damaged one. This was done to ensure that a controlled tidal exchange would be maintained and ensure that the lagoons and hinterland habitats would not be adversely affected.
The culvert was also designed to allow for adaptation in the future in keeping with the ‘adaptive management’ coastal defence policy for this part of the coast. To achieve this, a new 60cm diameter pipe was installed and was fronted by an adaptable culvert gate. This gate (flap valve) had planks within it that could be removed to change the tidal aperture and the tidal exchange as desired.
The site was monitored to describe how the lagoons functioned under baseline conditions and then modelled to ensure that an effective RTE function would be achieved. The site was then monitored after the defence works were completed to confirm that it was functioning effectively.
It was necessary during the early stages of the site’s development to adjust the flap vale and open the aperture to match the baseline conditions. This early adjustment was helpful because it demonstrated the viability of the adaptive design.