Feeder beaches

 

This building block 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. 

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Artificial coastal sand nourishments are applied for various reasons:

  • to compensate losses due to (structural) erosion, and hence maintaining biodiversity
  • to enhance the safety of the hinterland against flooding
  • to broaden a beach or create new beaches e.g. for recreational purposes
  • to reclaim new areas such as peninsulas or artificial islands for urban or industrial development
  • to feed the surrounding coast with sand at a rate governed by nature

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.

Advantages

  • Concentrated nourishments feeding the surrounding coastal area can facilitate more simultaneous (eco-system) services, like safety against flooding, nature development and recreation.
  • The area to be nourished will be disturbed once in a long period while nature does the work, this can have ecological advantages.
  • Tide, winds and waves transport the sediments and spread it along the coast. This distribution of sand occurs gradually at rates that nature can keep up with. This enables nature and species to adapt.
  • A larger fresh watervolume can be held in the dune area.
  • The price of the large volume of sand for a feeder beach might be lower than the costs of the smaller volumes of sand summed up for regular nourishments.
  • Mobilisation of the dredging equipment is required only once instead of regularly in the case of disposing smaller volumes regularly. This allows for optimization in construction methods.

Disadvantages

  • The evolution of the nourishment depends on the tides, wind and waves and can therefore not be fully predicted. This may lead to unforeseen or poorly predictable situations which should be managed in an adaptive manner.
  • The initial sand volume of the nourishment of a feeder beach will be much larger than for a regular nourishment. Therefore, benthic species will be buried under a larger amount of sand
  • Eventually the nourishment should be repeated. 

 

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.

This Building Block is mainly applicable in the initiation and planning and design phase.

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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:

  • determine the need for nourishment
  • determine the goal of the nourishment: safety, nature, recreation, or other, or a combination
  • determine possible locations of the concentrated nourishment; the location does not have to be the optimal location from coastal management perspective, but can be a location which balances well all other considerations
  • determine the timing of the nourishment in the year: the period of constructing the nourishment should be carefully chosen with respect to breeding season of species (seals, birds, etc.) and nursery habitats of fish and shrimps

Step 2: Analyse present situation

  • determine the characteristics of the area such as hydrodynamics, wind, waves, sediment characteristics (grain size, bed composition, cross-shore profile shape), ecology, annual sediment transports, morpho-dynamics, etc.
  • make inventory of stakeholders within project related area

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:

  • assess the temporal impact of the desired intervention to its surrounding environment; involve experts (like consultants and knowledge institutes) to predict and evaluate the temporal impact. Besides the standard design aspects it is suggested to consider the BwN design aspects listed in the table below:

    Standard design aspects

    Extra BwN design aspects

    Hydrodynamic climate

    Sustainability

    Annual transports

    Dune development

    Profile shape

    Habitat development

    Sand composition

    Recreation

    Sand losses

    Groundwater

    Protection structures

    Diversity in ecological conditions

    Recurrence interval

    Local burial of benthic species

    Costs

    Indirect impact on foreshore 
    as proxy for juvenile fish

    Functionality

     

  • In this step the typical design parameters can be varied, such as the nourishment volume, alongshore length, nourishment frequency, and for some case sediment grain size. If the intervention is very different from previous cases, predictive shoreline modelling is required. The same holds for a first nourishment along an unstudied coast. 

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:

Interactive design tool 
This design tool can be used for 1D shoreline modelling for long-term coastal evolution (up to ~50 yrs). It includes development of coastlines and a parameterized development of dunes and habitat factors related to the coastline dynamics. Interventions can be soft (nourishments) or hard (e.g. groynes) or a combination. Multiple interventions can be evaluated (including interaction). The design variables for nourishments are locations, volumes, and frequency. After a scenario, a scorecard is generated including all kind of relevant (temporal) information including costs. Other information can be displayed (bathymetry, topographic data, etc). The Interactive Design Tool Holland Coast facilitates a user to explore different strategies, climate change scenarios, and optimizing the nourishment location(s). This design tool is a dedicated tool, with an attached calculation module applying the Unibest model.

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. 

Delft Dashboard 
Delft Dashboard is a standalone Matlab based graphical user interface developed by Deltares supporting modellers to quickly set-up new or existing coastal models anywhere in the world. The large number of coupled toolboxes allow for fast and easy model input generation. With Delft Dashboard setting-up a (coastal) model wherever on earth is a matter of minutes, which used to take days or weeks before!  

Delft Dashboard can be used to rapidly assess the impact of a nourishment on the local hydrodynamics e.g. currents along the coast.

Delft3D 
Detailed assessments of the processes around a nourishment can be carried out using the process-based, open source model Delft3D. The state-of-the-art model can combine hydrodynamics (2D or 3D) with waves, sediment transport, morphology, interaction ecology and morphology. 

This model can be used to study the temporal evolution of the nourishment, sediment losses (long- and cross-shore), operational system for predicting swimming conditions, etc.

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.

  • During and after construction
  • Bathymetry, currents, waves, vegetation, topography, benthos, seals, fish, dune development and birds
  • Predicting swimming conditions

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Practical applications

On the right a couple of cases is presented providing some application experiences that can be used as inspiration for future designs

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Below some characteristic data on the cases displayed on the right are summarized.

Location 

Size 

Volume per m 

Alongshore shape 

Measured period 

Available Data 

Nourishments cases 

 

 

 

 

 

Vlugtenburg 

2.7 Mm³

1,500 m³ / m 

uniform 

2 years 

27 monthly surveys 

Sand groynes Delfland  

1 Mm³ 

1,000 m³ / m

3 humps 100 x 50m size 

4 months 

7 surveys (2-5 weeks apart) 

Surfing beach Scheveningen 

1.5 Mm³

500 m³ / m

straight edge (70m) 

4 months 

surveys & aerial photographs 

Sand Engine Delfland

18.5 Mm³ 

4,000 m³ / m

Peninsula 

 

monthly surveys & aerial photographs 
and in future ARGUS video 

Sand Engines IJsselmeer

2* 25,000 m³

100 m³ / m

Peninsula / longshore reef

 

regular surveys & Lidar images & fibre optic measurements
and in future ARGUS video 

Natural development

 

 

 

 

 

Bornrif

20 Mm³

+4,000 m³ / m+

Spit formation 

> 10 years 

Yearly cross-shore profile measurements & bathy surveys  every 5 years 

 

References

Literature

  • Achete, F. (2011). Morphodynamics of the Ameland Bornrif: An analogue for the Sand Engine. MSc Thesis, TU Delft.
  • Achete, F. and Luijendijk, A.P. (2012). Morphodynamics of the Ameland Bornrif: An analogue for the Sand Engine, ICCE 2012. Conference proceedings.
  • Huisman, B. and Luijendijk, A.P. (2010). Approach for eco-morphological modelling of mega-nourishments along the Holland coast.Assessment of tools and approach for multi-scale modelling. BwN Report HK4.1.
  • Huisman, B. and Luijendijk, A.P. (2011). Evaluation of nourishment strategies Holland Coast. BwN Report HK4.1.
  • Van Rijn, L. (1998). Principles of Coastal Morphology. Aqua publications.