Beach drainage: verschil tussen versies
Versie van 21 jan 2007 om 21:07
Beach Drainage is a ‘soft’ shore protection method. A drainage system is installed under the beach face and parallel to the coastline to enhance beach accretion by artificially lowering the ground water table.
The transport of sediment across the beach face is performed by wave uprush and backwash. The upwash moves sand on-shore while the backwash transports it offshore. The wave motion also interacts with the beach groundwater flow. Seawater may infiltrate into the sand at the upper part of the beach (around the shoreline) during swash wave motion if the beach groundwater table is relatively low. In contrast, groundwater exfiltration may occur across the beach with a high water table. Such interactions have a considerable impact on the swash sediment transport.
Two mechanisms are expected to be important in altering the uprush and backwash sediment transport: sediment stabilisation and boundary layer thinning due to infiltration on the uprush, and sediment destabilisation and boundary layer thickening due to exfiltration on the backwash.
Seawater infiltration under a low water table was found to enhance on-shore sediment transport, whereas groundwater exfiltration under a high water table promotes offshore sediment transport. Thus it is expected that an artificially lowering the ground water table, with a drainage system, advances accretion of sediments for accretive wave conditions, and retards beach erosion for erosive waves. The above conclusion is confirmed by field and laboratory measurements.
The pipes of a beach drainage system are buried in the beach parallel to the coastline and drain the seawater away to a collector sump and pumping station. The collected seawater may be discharged back to sea but can also be used to various applications (marinas oxygenation, desalination plants, swimming pools…).
The system includes minimal environmental impact compared with the hard protection methods.
More than 30 Beach Drainage systems have been installed in Denmark, USA, UK, Japan, Spain, Sweden, France, Italy and Malaysia.