Natural shoreline mechanisms
Uprush and backwash
The transport of sediment across the beach face is performed by wave uprush and backwash. The uprush 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 (see definitions of coastal terms) are relevant with respect to beach drainage. These mechanisms directly affect the resulting sediment transport. Given a certain groundwater table in the beach profile and consider 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
- less water retuns to sea; however, still rather high velocities due to gravity effects; sediment particles are transported in seaward direction,
- destabilisation and boundary layer thickening due to exfiltration of groundwater.
Under accretive (wave) conditions the landward directed sediment transport processes apparently over-class the seaward directed processes. Under erosive (wave) conditions it is the other way around.
Working and application of beach drainage
Active beach drainage
When an active drainage system is installed under the beach face and parallel to the coastline, the aforementioned mechanisms will alter:
- during uprush: seawater infiltration under an artificially lowered water table was found to enhance, but transport of particles in landward direction hardly change,
- during backwash:
- less water returns to sea (smaller transport of particles in seaward direction),
- groundwater ex filtration is reduced.
Consequently it is expected that an artificially lowering of the groundwater table, with a drainage system, changes the coastal processes. In case of accretive conditions an increase of the accretion is expected. In case of erosive conditions a decrease of the beach erosion results. The above conclusion is confirmed by field and laboratory measurements. Figure 1 illustrates the lowering of the groundwater level due active beach drainage.
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 various hard protection methods.
More than 30 Beach drainage systems have been installed in Denmark, USA, UK, Japan, Spain, Sweden, France, Italy and Malaysia.
Please note that others may also have edited the contents of this article.