Beach drains comprise perforated land drain pipes buried below the upper beach surface, and connected to a pump and discharge. The concept is based on the principle that sand will tend to accrete if the beach surface is permeable due to an artificially lowered water table. The system is largely buried and therefore has no visual impact.
Function
Mild upper beach and dune erosion can be controlled by beach drains. The system actively lowers the water table in the swash zone, thereby enhancing the wave absorption capacity of the beach, reducing sand fluidisation and encouraging sand deposition. The deposited sand forms an upper beach berm that protects the dune face during storm events that might otherwise cause erosion.
Benefits are greatest on micro-tidal (<2m range), high value amenity beaches where landscape issues preclude the use of other management approaches. Important backshore assets should not rely on drainage systems for erosion protection during storms, even as a temporary measure.
Active drainage systems have been found to enhance sand deposition at sites in the USA and Europe. Trial sites in the UK have had a mixed reception, partly as a result of poor site selection, inadequate design and lack of management. Installation costs can be relatively low, but maintenance and management commitments are high. Although the drains should increase upper beach volume during low to moderate wave conditions they will not be significantly effective during storms, with the result that beach draw down may lead to exposure of the pipework and system failure.
Methods
Beach drainage systems are relatively simple to install but should always be designed by a competent hydraulic consultant. Little information is available to guide designs, with the exception of that provided by organisations providing a patented installation service. In simple terms, perforated plastic drain pipes with a geotextile sleeve are laid into excavated trenches within the high tide swash zone. The pipes are connected to a pump and discharge system. Pipework must be laid at least 1m below the expected minimum beach level, but not so deep as to be ineffective at draining the surface layer of the beach. The geotextile sleeve serves to filter sand from the seawater collected in the drains. A surround of gravel has sometimes been used to improve flows. Pumping facilities must be appropriately housed and represent the major system installation and maintenance cost. Discharge of collected water should be designed to minimise any interference with natural beach processes.
Consideration must be given to the real time control of the pumping facilities. The system will only operate efficiently while the pipework is within the swash zone: at lower tide levels no wave driven sand will reach the system’s area of influence, and once the still water level has risen over the drains the pumps will be overwhelmed by the total saturation of the beach. A water level sensitive pump control is therefore required, although it would also be possible to produce a time switch system based on predicted tide levels. Regular beach monitoring and system management are required to obtain optimum results from a drainage scheme.
The system will be most effective in areas with a low tidal range (less than 2m) in which the pump would be active for a large part of each tidal cycle. Most of the Scottish coast has a tide range of at least 3m, with a 4 - 5m range being common, so drains installed on or above high water line of the beach will be redundant for at least 75% of each year. Drainage systems will also be most effective over relatively short lengths of shoreline. A single system should be capable of achieving some benefits over lengths of 100m to 400m. Small embayments or discrete lengths of longer frontages could be appropriate sites.
Beach drains laid in the upper beach
Associated vegetation transplanting and fencing/thatching will enhance dune growth. Beach recycling may also be beneficial to the initial installation or to restoring the system if pipework is exposed by storm erosion.
Costs for drainage schemes are dependent on the number of lines laid, the problems of providing appropriate pumping and outfall facilities and the amount of maintenance required to keep the system active.
To date no information is available on which to assess life expectancy. With regular maintenance and repairs to storm damage, a system could remain active for at least 10 years before requiring a major rebuild. Sand clogging within the pipework is likely to be a limiting factor on life expectancy, assuming storm damage can be avoided.