Difference between revisions of "Beach drainage"

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(Beach drainage)
(Beach drainage)
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The transport of sediment across the beach face is performed by wave uprush and backwash. The upwash moves sand onshore while the backwash transports it offshore.
 
The transport of sediment across the beach face is performed by wave uprush and backwash. The upwash moves sand onshore 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 sediment transport in the swash zone.  
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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 sediment transport in the swash zone.  
  
Two mechanisms are expected to be important in altering the sediment transport because of uprush and backwash:
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Three mechanisms related to the uprush and backwash processes are relevant with respect to beach drainage. These mechanisms directly affect the resulting sediment transport. Given a certain groundwater table in the beach profile in a situation without active beach drainage:
* sediment stabilisation and boundary layer thinning due to infiltration in the uprush phase,
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* during uprush: sediment stabilisation and boundary layer thinning due to infiltration of water; the mass of water which has to return to sea diminishes; sediment particles are transported in landward direction,
* sediment destabilisation and boundary layer thickening due to exfiltration in the backwash phase.   
+
* during backwash 1): less water retuns to sea; however, still rather high velocities due to gravity effects; sediment particles are transported in seaward direction,
 +
* during backwash 2): destabilisation and boundary layer thickening due to exfiltration of groundwater.   
  
 
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.
 
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.

Revision as of 10:40, 31 May 2007

Beach drainage

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 groundwater table.

The transport of sediment across the beach face is performed by wave uprush and backwash. The upwash moves sand onshore 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 sediment transport in the swash zone.

Three mechanisms related to the uprush and backwash processes are relevant with respect to beach drainage. These mechanisms directly affect the resulting sediment transport. Given a certain groundwater table in the beach profile in a situation without active beach drainage:

  • during uprush: sediment stabilisation and boundary layer thinning due to infiltration of water; the mass of water which has to return to sea diminishes; sediment particles are transported in landward direction,
  • during backwash 1): less water retuns to sea; however, still rather high velocities due to gravity effects; sediment particles are transported in seaward direction,
  • during backwash 2): destabilisation and boundary layer thickening due to exfiltration of groundwater.

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.

        Bds.jpg

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.