A beach dewatering system or beach drain, is a shore protection system working on the basis of a drain in the beach. The drain runs parallel to the shoreline in the wave up-rush zone. The beach drain increases the level of the beach near the installation line, thus also increasing the width of the beach.
This article describes the working and application of a beach drainage system. Beach drainage is an example of a soft shoreline protection solution.
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, see Swash zone dynamics.
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, see Figure 1. 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 2 illustrates the lowering of the groundwater level due active beach drainage.
The drain consists of a permeable plastic pipe installed 1.0 to 2.0 m below the beach surface in the wave up-rush zone parallel to the coastline. If there is a significant tide, the drain must be installed close to the MHWS line, i.e. near the shoreline. The only visible part of the drain installation is the pumping well and a small control house. The pipes of a beach drainage system 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 conditions influencing the function of the drain are summarised in the following:
- The site must have a sandy beach. The beach sediments must be sand, preferably with a mean grain diameter in the range of 0.1 mm < d50 < 1.0 mm and preferably sorted to well sorted (Cu = d60/d10 < 3.5). These conditions give the permeability that provides optimal functionality of the beach drain.
- The beach drain works by locally lowering the groundwater table in the uprush zone, which decreases the strength of the down-rush as a higher fraction of the water percolates into the beach. Furthermore, the physical strength properties of the beach sand is increased remarkably by the lowering of the water table in the wave up-rush zone thereby making the beach more resistant against erosion. The groundwater table in the beach is a function of several factors, the most important of which are: a) the groundwater table conditions in the coast and the hinterland, b) the groundwater table caused by tide and storm surge, and c) the groundwater table caused by waves.
- A high groundwater table in the coast and the hinterland influences beach stability and beach formation. The hinterland-based groundwater table saturates a large portion of the beach, causing groundwater seepage through the foreshore. This seepage tends to destabilise (fluidise) the foreshore. The beach drain locally lowers the groundwater table to the level of the drain and counteracts the destabilisation.
- The beach drain works well at locations with relatively high tide because the tide generates an elevated groundwater table in the beach, which can be lowered considerably by the drain. It can therefore be stated that the presence of high tide at a location enhances the functionality of the drain.
- The presence of high storm surges will affect the functionality of the drain by moving the uprush zone landwards away from the drain. The function of the drain during high surge conditions will mainly be indirect; the previously accumulated sand will act as a buffer for the erosion during the storm. When the storm surge falls, the elevated groundwater-level in the beach will increase beach erosion if there is no beach drain to prevent it.
- Waves on a beach increase the height of the local groundwater table in the beach, partly due to the wave run-up on the foreshore and partly due to the locally elevated water-level in the uprush zone called wave set-up. Once again, the beach drain counteracts this.
- The beach drain requires some wave activity on the beach as the drain works by manipulating the downrush conditions on the foreshore. Too small and too high waves make the beach drain inefficient. It works best on moderately exposed coasts.
- As the beach drain system functions only on the foreshore in the uprush zone, it does not directly protect the entire active profile against erosion. Consequently, it is best suited at locations with seasonal beach fluctuations or where the objective is a wider beach at an otherwise stable section of the shoreline. For locations that experience on-going recession of the entire active coastal profile, the beach drain is probably only suitable combined with other measures. The long-term capability of the beach drain under such circumstances remains to be tested.
The beach drain is best suited for the management of beaches with the following characteristics:
- Sandy beaches
- Moderately exposed to waves
- Exposed to tide
- Suffering from high groundwater table on the coast and on the beach
- Exposed to seasonal fluctuations of the shoreline
- Exposed to minor long-term beach erosion
- Locations with a narrow beach, where a wider beach is desired
The beach drain is, however, not recommended as a primary shore or coastal protection at locations with the following characteristics:
- Severely exposed locations
- Protected locations
- Locations exposed to severe long-term shore erosion and coast erosion
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. The beach drain method is patented world wide by GEO, Denmark.
- Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
- Karambas Th. V. (2003). Modelling of infiltration – exfiltration effects of cross-shore sediment transport in the swash zone, Coastal Engineering Journal, 45, no 1: 63-82.
- Law A. W-K., Lim S-Y, Liu B-Y (2002). A note on transient beach evolution with artificial seepage in the swash zone, Journal of Coastal Research, 18 (2): 379-387.
- Sato M., Fukushima T., Nishi R. Fukunaga M. (1996), On the change of velocity field in nearshore zone due to coastal drain and the consequent beach transformation, Proc. 25th International Conference on Coastal Engineering 1996, ASCE, pp. 2666-2676.
- Sato M., Nishi R., Nakamura K., Sasaki T., (2003). Short-term field experiments on beach transformation under the operation of a coastal drain system, Soft Shore Protection, Kluwer Academic Publishers, pp 171-182.
- Ioannidis D. and Th. V. Karambas (2007): "‘Soft’ shore protection methods: Beach drain system", 10th Int. Conf. on Environmental Science and Technology, CEST2007, Kos Island, GREECE, A-528-535
Please note that others may also have edited the contents of this article.
Please note that others may also have edited the contents of this article.