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This building block solution presents an innovative Building with Nature solution for coastal development through the application of feeder beaches: a spatially concentrated nourishment which is placed at a specific location with the aim to gradually feed the surrounding coast. Wind, waves and currents will spread the nourished sediments along the coast thereby contributing to the coastal safety on the longer term while creating more opportunities for nature and recreation.
In the past, erosive coasts were protected by man-made, hard structures like dikes and dams. It was soon realised that these defences often induce erosion down-drift and so softer engineering options were introduced. Structures were built that worked with the natural depositional processes occurring at the coast. Groins and breakwaters were seen as the preferred option to build up beach and dune levels in order to offer a natural barrier to the sea. In the last 15 years alternative but nowadays quite common solutions of soft nourishments were applied. Sand was placed at the location where it was directly needed, and slowly dispersing in time, while interacting with the hard structures.
Artificial coastal sand nourishments are applied for various reasons:
In the case of structural erosion, the nourishment will have to be repeated from time to time as the erosion is an ongoing process. The interval between successive nourishments depends on the rate of erosion and on the equipment mobilization costs. In addition, the timing of the nourishment should be carefully chosen with respect to breeding season of species (seals, birds, etc.) and nursery habitats of fish and shrimps. Generally, an interval of five years is considered acceptable (for Dutch cases). Compared to hard solutions, nourishments must be repeated from time to time, hence it is flexible, while costs are spread over a longer time. For conditions along the Dutch coast this makes the soft nourishments more economical than the hard solutions with the added advantage that no lee-side erosion occurs (typical for hard solutions).
Sand nourishments can be carried out at various locations in the profile, such as the shoreface or foreshore (underwater), beach & surf zone and in the dune zone. In alongshore direction, the nourishment can be distributed over a large distance or placed more locally (concentrated). In the latter case, the goal of the nourishment could be to feed the adjacent coast gradually. Tide, winds and waves transport the sediments and spread it along the coast. This distribution of sand occurs gradually at rates that the adaptation of nature and species can keep up with. This enables nature and species to adapt. In case of an emerged nourishment, where part of the nourishment reaches above high water level, one can speak of a concentrated nourishment feeding the surrounding coastal area: e.g. the new beach provides new services such as recreation. A very large submerged foreshore nourishment does not provide such services; thus, for additional service for coastal development it is important that part of the nourishment is emerged and accessible (either by foot or boat). In case of an emerged concentrated nourishment the length of the shoreline increases due to the nourishment providing more space for recreation, water sports and beach lovers. In addition, new dunes and vegetation could develop increasing nature value.
Nourishments for coastal development attracts new services and hence new beach visitors (human and animals), which require a different management of the coast at the nourishment and its surroundings. In case of an unprotected nourishment, the shape and bathymetry of the nourishment will change continuously under the forcing of tide, wind and waves. It is therefore important that an adaptive management strategy is adopted to anticipate on unforeseen and poorly predictable developments.
Shallow areas in front of hydraulic infrastructure act as a buffer for safety by dissipating wave energy. Due to the extra segment of water that is created by the rise in sea-level, a sediment shortage on many of such shoals will be created leading to a decrease in area. Nourishments can be an effective measure to fulfill the sediment demand of the shoals and maintain a dynamic equilibrium between erosion and sedimentation.
How to use
To determine the feasibility and design of a concentrated nourishment for coastal development, guidance is presented below. First, the feasibility of a concentrated nourishment for coastal development is assessed using a rule-of-thumb method. Subsequently, a preliminary design of the concentrated nourishment is determined using simple (interactive design) tools including parameterizations. For appropriate detailing and a geometric design more advanced tools (like process-based models) are available to be applied.
Guidelines for feasibility of concentrated nourishments
To be able to determine the feasibility of a concentrated nourishment for coastal development the following systematic approach is recommended.
Step 1: Project initiation
In the initiation phase of the project the next items need to be considered:
Step 2: Analyse present situation
Step 3: Asses impact of desired intervention
To assess the impact of the intervention and to investigate whether impacts are acceptable or desirable, or that certain impacts need to be mended, it is advised to:
Designing tools for concentrated nourishments
Step 4: First order designing (conceptual)
A first design step is to optimise the location of the concentrated nourishment w.r.t. goal, long-term strategies, stakeholder involvement. For this purpose a design tool has been developed:
Step 5: Appropriate detailing
Once the location and general size and contours of the concentrated nourishment have been determined, further detailing of the nourishment and its development over time, has to be made. For this purpose several tools are made available increasing the level of detail and accuracy.
It is noted that other well reputed knowledge institutes offer comparable model suites.
Step 6: Construction and Monitoring
When a suitable design has demonstrated to be feasible, a dedicated construction and monitoring program needs to be initiated and maintained.
On the right a couple of cases is presented providing some application experiences that can be used as inspiration for future designs
Below some characteristic data on the cases displayed on the right are summarized.