A coastal buffer zone, or "inlaag” in Dutch, is a predominantly salt/brackish zone along the coast between two dikes. Historically these areas arose when people built a spare inner dike parallel to the coastal defence when there was a threat of dike failure. Because of the presence of a secondary dike, the primary coastal defence can suffice with lower safety standards as for example reduced height. Limited overflow during extreme high water conditions is acceptable because the secondary dike will stop the water from inundating the land behind the secondary dike. This is interesting from a cost and dike maintenance point of view especially with respect to sea level rise. The area between the dike is not suitable for high quality land use as housing, but can serve as nature and recreation area and also has potential for aquaculture.
The principle of a coastal defence zone is already practised. An example is the project Ellewoutsdijk where two parallel dikes were strengthened and together form a flood protection zone. The Coastal Laboratory that is initiated by Zeeuwse Landschap is another example of a coastal buffer zone (www.kustlaboratorium.nl). Here, potential synergy between coastal safety, aquaculture innovation, nature and recreation is investigated.
Inlagen of the Oosterschelde
Historically an “inlaag” was used in the Netherlands as buffer zone between the sea and the inhabited area behind the dikes. If an area seemed no longer safe to inhabit because of a progressing sea a secondary inner dike was built. This dike was meant to provide protection in the unfortunate case the primary dike would fail. Nowadays “inlagen” are still preserved or reconstructed along the coast of Schouwen-Duivenland and Noord-Beveland in the Southwest Delta in the Netherlands. The area between the dikes is usually unattractive for agriculture due to high salinity concentrations in the soil (seepage). Due to the historic character an “inlaag” also holds high cultural values.
Related pages
Comparable applications are described in the Building Solutions Managed realignment and Inland shores .
Required skills
With this Building Solution description, anyone with a basic level of knowledge and/or working experience on coastal systems can make a first order assessment.
BwN interest
The added value of this Building Solution within BwN-type projects is that it enables the creation of coastal protection systems in a multi-functional way, combining nature, recreation and aquaculture. This Building Solution is mainly applicable in the planning and design phase.
A coastal buffer zone enables zonation of flood risk. The flood risk in the buffer zone can be higher than normal standards. The advantage is that the primary dike does not have to be raised when sea level rises and will save costs. In fact, limited overflow can be permitted. When allowing water and higher groundwater levels in the area between the dikes this can also reduce land subsidence processes. To enable overflow of the crest of the dike, the landward side of the primary dike might have to be strengthened. Depending on the frequency of overflowing, the area between the dikes is subject to occasional flooding which puts limitations on land use. By choosing low-lying, coastal areas with extensive land use (nature areas), the adverse effects on land use practices are brought to a minimum.
Boundary conditions
Site suitability
Areas that are interesting for application of a coastal buffer zone are mostly low-lying areas where a secondary dike is already present. In areas where land has been reclaimed in the past, former sea dikes are often present inland. Some of these areas are dominated by salt water seepage which makes the land less valuable for agriculture or housing. These areas are very suitable for the application of a coastal buffer zone.
Dikes
Some adjustments need to be made to ensure dike stability:
- The primary dike should be able to overflow, therefore the inner slope of the dike should be checked for overflow resistance and if necessary should be strengthened and hardened to prevent erosion during overflow.
- The secondary dike should be strong enough to withstand water flowing over the primary dike and should be adjusted if necessary.
Safety standards
The coastal buffer zone should be suited for regular flooding. Where flood risk standard for inhabited areas are set to 1/4000 year or even 1/10.000 year, a standard of 1/100 or even 1/50 is allowable in the buffer zone. This will considerably reduce costs on dike maintenance and strengthening.
Design & Construction
Preferably a coastal buffer zone is applied where two parallel dikes (one shore side, one inland dike) are already present. This will save the expense of constructing a second dike. Based on the ComCoast pilot at Ellewoutsdijk a concrete example of a coastal buffer zone is summed up below:
- The crest of the primary dike is strengthened with asphalt.
- Flood risk 1/50 is allowed. Once every 50 years on average, a few decimeters of water will collect between the two dikes, and once every 100 to 250 years, 1 m or 2 m of water will collect. This will not pose a threat to the safety of the land behind the secondary dike. An even lower flood risk could be allowed to benefit certain goals as nature development and at the same time still guarantee a sufficient safety level.
- The landward side of the old dike is strengthened with open asphalt covered with soil allowing grass to grow on.
- The seaward side of the new dike is strengthened with a layer of open asphalt. This layer is covered with soil allowing grass to grow on. These areas are also suitable pastures for live stock to graze upon.
Land use practices
Areas that are applied as coastal buffer zones are usually low-lying areas along the coast, subject to salt water seepage unsuitable for cultivation of crops and livestock. Most of these areas are therefore nature area. Because of the variation in salinity of fresh, brackish to saline, vegetation is diverse. The brackish water area with shallow pools provides a valuable foraging, roosting and nesting area for birds. Especially with high water birds will move here to find their food. This coastal nature enclosed by dikes also provide opportunities for recreation like walking and cycling along the dikes along the area. It is also a good place for bird watchers for many different types of birds to be spotted.
Aquaculture and saline crops
The (near) availability of salt water and relatively controlled conditions that make a coastal buffer zone have potential for aquaculture and saline crops. The table below describes several forms of saline land use with a potentially high market value, based on De Mesel et al. (2013) and related studies.
| Type | Species | Method | Yield | Reference |
|---|
Shellfish | Blue mussel - Mytilus edulus | Mussel growth on poles ("Bouchot") in the intertidal zone | 41920 €/ha/year | www.fao.org |
| Pacific Oyster – Crassostrea gigas | Oyster growth on “tables” in the intertidal zone | 25000 €/ha/year | www.fao.org |
| European flat Oyster – Ostrea edulis | Oyster growth on “tables” in the intertidal zone | 20900 €/ha/year | (Van der Hiele et al., 2008) and related references |
Worms | Ragworms - Nereis virens | Extensive digging for rag worms at low tide | ? | ? |
Sea weeds | Tough Laver - Porphyra umbilicalis | Marcoalgae attached to floating devices | 651 | (Lobban &Wyne, 1981) |
| Hudson - Gracilaria verrucosa | Marcoalgae attached to floating devices | 435 | (Xin, 1989) |
Saline vegetables | Common glasswort – Salicornia europaea | Harvest from wild growth on salt marshes | 84000 €/ha/year | (Van de Voort et al., 2005) and related references |
| Sea aster – Aster tripolium | Harvest from wild growth on salt marshes | 35000 €/ha/year | (Goosen, 1999) |
The principle of a coastal buffer zone is already being practised at several locations.
