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Knowledge - Biogeomorphological effects of nourishment, NL

 

Abstract: Sand nourishment has effects on the benthos on the beach and the foreshore through, among others, burial of benthic species and changes the structure of the beach and foreshore morphology.

Technology Readiness Level: 5 (technology validated in relevant environment)
Environment: Sandy shores
Keywords: Nourishment, biogeomorphology, ecology

 

 

 

 

The sandy shores of The Netherlands are maintained by supplying sand to the beaches and the upper shoreface. To enable predicting future states of beach ecosystems, and to contribute to the development of effective and sustainable nourishment practices, knowledge on ecological and morphological processes is essential. As these two aspects of the environment are mutually dependent, understanding how they influence each other is an absolute necessity. A literature review has been conducted, with focus on the macrozoobenthos in and on the sediment on the shoreface and in the surf zone. Results are described in Baptist et al. (2008) from the integrated perspective of the disciplines geomorphology, ecology and biogeomorphology. The latter is the study of the interaction between geomorphological processes and biota (Baptist, 2005).


Geomorphology

The majority of the Dutch coast consists of beaches and dunes and can be divided into three parts with their own characteristics:

 

  1. The Southwestern Delta coast from Zeeuws-Vlaanderen to Hoek van Holland. This coastal system is shaped by former islands in between the former estuaries of the rivers Scheldt, Rhine and Meuse and is subject to large anthropogenic influences such as the closure of most of the tidal inlets after the storm surge of 1953 and the construction of the Maasvlakte.
  2. The 120 km long closed Holland coast from Hoek van Holland to Den Helder. This part has a number of harbours and navigation channels affecting the net longshore transport of sediment. Across shore typically two or three breaker bars are present.
  3. The Wadden coast from Den Helder to the Dutch-German border. This coastal system is still affected by the construction of the Afsluitdijk (1932) and the closure of the Lauwerszee (1963).

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Ecology

The Dutch coast is generally described as mesotidal, barred, dissipative, and moderately exposed (Janssen & Mulder 2005). Based on tidal range and fall velocity of sand, the Holland coast and Southwest coast are considered barred intermediate and the Wadden coast ultra-dissipative. The beach exposure, i.e. dynamics, is of great influence on the occurrence of species (Janssen & Mulder 2004). As tide range or wave energy increases, or sand particle size decreases, beaches become wider, flatter and more dissipative. Faunal communities increase linearly in species richness and exponentially in abundance over this range of beach types (McLachlan 1996). The beaches of the Holland coast and Southwest coast are generally more exposed than the beaches of the Wadden coast.


The macrozoobenthos of the beaches and foreshore is a well-investigated group and consitsts of molluscs (bivalves and snails); worms; spiny-skinned animals (Echinodermata); and shrimp-like animals (crustaceans). Different ecological zones can be distinguished on the Dutch sandy coast. Generally, the number of species increases with increasing depth and also correlates with grain size, bed slope and bed morphology.

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Biogeomorphology

Wave energy associated with sediment particle size and tidal range has been emphasized as a major structuring force for beach morphology, and for the infaunal communities of these habitats (e.g. Brown & McLachlan 1990; Menn 2002). Thus, shore morphodynamics may considerably influence the biotic beach system which, in turn, can influence the physical nature of the beach (Menn 2002). Benthic (sea-bed) organisms may be classed as ‘ecosystem engineers’ or ‘bio-engineers’ in that their activity has a profound effect on their environment, resulting in a significant alteration of the sediment properties (Mazik et al. 2008).

 


Schematic model of interactions between wave

energy, beach morphodynamics and biotic

components on the sandy shore (Menn 2002)


 

 

 

 

 

 

 

 

 

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Future considerations

The influence of grain size on the ecological effects of nourishments is important to consider in future nourishment projects. Several studies indicate that the nourished sediment characteristics must be similar as closely as possible to the original sediment to prevent large ecological effects. Another important aspect to consider in predicting the ecological effect of future sand nourishment is the spatial (i.e. ecological zones) and temporal distribution of nourished sand in relation to ecological recovery times. Furthermore, bio-engineers can significantly influence the morphodynamics of a nourishment. In order to predict this stability, the site-specific species composition should therefore be known, including the response of the species composition to a change in physical parameters. Field experiments should be conducted to get real insight into the biogeomorphological interactions which determine the stability of nourishments. During these field experiments it is recommended to monitor the recovery of benthos, the change in physical parameters (and corresponding habitat) and the behaviour of the nourishment itself. Ecologically relevant abiotic parameters within nourishment projects are grain size, layer thickness, oxygen level, geochemical effects, turbidity/SPM (Suspended Particulate Matter), depth and distance to the shore (see Baptist et al. 2008).

 

References

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