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).
The majority of the Dutch coast consists of beaches and dunes and can be divided into three parts with their own characteristics:
Division of the Dutch coast in 'Wadden sea coast',
'Holland coast' and 'Delta coast' (Picture S.D. IJff)
- 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.
- 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.
- 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).
At present, about 12 million m3 of sand is nourished to the Dutch coastal system, of which about 60% by means of relatively large shoreface nourishments and 40% by means of relatively small beach nourishments. The largest part (49%) of the 12 million m3 of nourished sand is put onto the central Dutch coast (between Hoek van Holland and Den Helder); the Wadden Sea coast and the southwestern coastal system receive 28% and 23%, respectively. The shoreface nourishments typically have a volume of 1-3 million m3 (400-600 m3/m) and are usually placed seaward from the outer breaker bar, at a water depth of 4-8 m.
Most knowledge on the geomorphological behaviour of shoreface and beach nourishments originates from data-analysis studies. Numerical modelling tools have been used successfully in hindcasting behaviour of nourishments, but do not yet have the predictive power to reliably forecast long-term effects.
A shoreface nourishment has the following two effects on the coastal system:
- The lee effect. The artificial sand bar increases wave dissipation, by which the wave height and the alongshore current onshore of the nourishment decrease. As a result, the alongshore sand transport capacity decreases here and therefore sediment accumulates upstream and erodes downstream of the nourishment. When the waves approach the shore perpendicularly, the leeside of the nourishment possibly erodes as a result of divergence of alongshore currents induced by alongshore differences in wave set-up.
- The feeder effect. This refers to the feeding of coastal system onshore of the nourishment with nourished sediment due to cross-shore sand transport processes. The net sand transport in onshore direction is enhanced by the nourishments, because, i) seaward suspended load decreases because the additional wave dissipation by the nourishments reduces the offshore-directed undertow and the wave-induced sediment suspension, and ii) onshore bed- and suspended load increase at the nourishment due to additional wave skewness related to the lower water depth compared to the no-nourished case.
Model of the 'feeder effect' , where a foreshore nourishment leads to sand transport to the beach due to changed
wave climate, followed by wind-driven transport from the beach to the dunes (Van der Valk et al. 2013)
The shoreface nourishment affects the autonomous behaviour of the breaker bars. The autonomous behaviour of the breaker bars is periodically and consists of the following phases:
1) Generation near the beach,
2) net migration in seaward direction through the surf zone, and
3) de-generation at the edge of the surf zone.
The latter phase triggers the generation of a new breaker bar (phase 1) and the seaward migration of the now outer breaker bar (phase 2). This cycle is a cross-shore distribution of sand without a significant loss in offshore direction. The number of breaker bars (between 0 and 4) and the duration of this cycle (between 0 and 15 years) vary along the Dutch coast and are, among other things dependent on the steepness of the coastal profile.
The nourishment, placed against the outer breaker, generally re-shapes itself relatively quickly (within a few months) into a bar with a landward trough. As a result of this, the offshore migration of the original breaker bars is halted; sometimes they even temporarily migrate in the onshore direction. During this stop of offshore bar migration, the bars keep their pre-nourished dimensions.
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.
The most common species on the beaches of The Netherlands is the bristle worm Scolelepis squamata, an important food source for the Sanderling Calidris alba, which is protected under the EU Birds Directive. Two other characteristic species that are commonly reported are Bathyporeia pilosa – the sand digger shrimp, and Euridice pulchra, the speckled sea louse. The beaches of the Wadden Sea islands are generally characterized by relatively fine sand with low carbonate content and large variation in grain size and a high number of species and densities compared to the mainland coast.
The surf zone and shoreface
The surf zone consists of along shore oriented breaker bars. The number if species is generally low in the surf zone, but total biomass is high (dominated by a few species). In this zone the Polychaetes (worms) are dominant. The crests of the breaker bars have lower species densities probably due to higher wave energy. The troughs between the breaker bars primarily have large numbers of the sand mason. These worms presumably play an important role in holding on to and stabilising the sediment and thus the coast (Janssen & Mulder 2004, Janssen & Mulder 2005). The shoreface, seawards from the outer breaker bank, hosts a lot more species. They represent primarily the main groups Amphipoda, Bivalvia and Echinoïdea. The non-native American razor clam (Ensis directus) is the most abundant species on the Dutch coast (Goudswaard et al. 2008).
Number of species as function of the distance to the coast, based on McLachlan et al. (1990)
Monitoring ecological effects
Possible ecosystem effects of nourishments can be divided into direct and indirect effects. Direct effects are mostly related to burial of benthic species. Indirect effects are caused by change in habitat through the introduction of ‘exotic’ sediment (i.e. sediment from another location with different properties). Altered sediment properties affect the habitat suitability for benthos, such as: level of the seabed; penetrability; organic matter content; grain size; and silt content. For example, impacts at nourished sites in the southeastern United States were observed during monitoring as a result of nearshore turbidity, direct burial of organisms and extreme habitat alterations (Lankford & Baca 1989).
Since the 1980s, several monitoring studies have been conducted on macrobenthos and how it is affected by sand nourishments in the Dutch coastal zone. Some of these involved site-specific projects and some are part of yearly monitoring programs. Baptist et al. (2008) give an overview. Most studies assess short term impacts in and near nourishment sites, quantifying the elimination and early recovery of fauna but few studies cover longer periods. Recovery has only been studied on a limited number of (opportunistic) species. No information is available on the degree of recovery on the level of biological communities.
Survival, migration and recruitment may all contribute to the recovery after disturbance (Van Dalfsen & Essink 2001). The recovery after nourishment therewith depends on many factors, such as the application method/location, the sediment characteristics (influencing both chance of survival and recruitment), the species resistance and resilience and the season of application. Recovery can sometimes be fairly rapid (e.g. some months to <1 year, because of the quick dispersal of sediments and/or the intrinsic tolerance of the assemblages) but can quite often be long-lasting, particularly when the sediments alter the native habitat characteristics, or have high organic loads and/or are highly polluted (Colosio et al. 2007). As the sand used for nourishment of the Dutch coast is not polluted or organically enriched, benthos should be able to recover relatively fast, assuming other sediment characteristics (i.e. grain size and mud content) are fairly similar to the original sediment.
Following shoreface nourishment in Dutch coastal waters a short-term opportunistic response of the benthic community was observed followed by an almost complete recovery of community composition and structure after four years (Van Dalfsen & Essink 2001). Opportunistic species will quickly recolonise the affected site, but long-living bivalve species or some sea urchins (such as the sea potato, Echinocardium cordatum) do not reproduce each year. In general, soft bottom benthic communities show partial recovery in one year and full recovery from 18 to 24 months (Allen & Hardy 1980) up to a maximum of 5 years (Mulder et al. 2005).
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)
Many studies have indicated that biological communities can alter the geotechnical properties of marine sediments, and can therefore impact on the geomorphology of the resulting bedforms. Several implications following this relationship are described (Murray et al. 2002):
- The importance of community ecology in sedimentological and geomorphological processes. For example, if within a community, one species is replaced by another species and these species differ in their geomorphological impact, the large-scale outcome will be radically different. Some species known to stabilise sediment are the mud-burrowing crustacean Corophium volutator, the burrowing polychaete worm Nereis diversicolor and the polychaete worm Lanice conchilega producing organic cement in tubes;
- The energetics of sedimentary and geomorphological processes. Geomorphological processes are dependent on the balance between driving forces and resisting forces acting on sediment. By incorporating the biomechanical activity of fauna into such processes, it naturally follows that the chemical energy stored in the tissues of organisms becomes available to do sedimentological or geomorphological work.
Some benthic species, such as Lanice conchilega, Echinocardium cordatum and Tellina fabula, may influence the sediment properties (i.e. they are considered to be bio-engineers). Each species has different effects on the sediment through different mechanisms (Borsje, 2012). The tube building worm L. conchilega protrudes several centimeters from the sediment in the water column, and thereby influences the near-bottom flow. For dense tube assemblages the near-bottom flow reduces, fine sediment will deposit and consequently lower ripples will form. Due to the digging and feeding activities of the bivalve T. fabula up to 10 cm deep in the sediment, the properties of the surficial sediment are modified and the sediment is more prone to erosion. The sea urchin E. cordatum lives in the top 20 centimeters of the bed and mixes sediment in vertical direction resulting in relatively coarser sediment in the top layer of the bed.
It has been concluded that bio-engineers may influence the morphodynamics of a nourishment significantly. Biogemorhological research has offered a parameterization in which biological activity is expressed in physical parameters. This gives us a first insight in the potential impacts of benthos on the stability of a nourishment as well as the feedback from the nourishment to the recovery of benthos.
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).
- Allen KD, Hardy JW (1980) Impacts of navigational dredging on fish and wildlife: a literature review.
- Baptist, M.J. (2005). Biogeomorphology. In: Schwartz, M. (ed.). Encyclopaedia of Coastal Science, pp. 192-193. ISBN 1-4020-1903-3. http://www.springerreference.com/docs/html/chapterdbid/29136.html
- Baptist, M.J. (Ed.), J.E. Tamis, B.W. Borsje, J.J. van der Werf (2008). Review of the geomorphological, benthic ecological and biogeomorphological effects of nourishments on the shoreface and surf zone of the Dutch coast. Report IMARES C113/08, Deltares Z4582.50.
- Borsje, B.W. (2012). Biogeomorphology of coastal seas; how benthic organisms, hydrodynamics and sediment dynamics shape tidal sand waves. PhD-thesis Twente University.
- Brown AC, McLachlan A (1990) Ecology of sandy shores. Ecology of sandy shores.
- Colosio F, Abbiati M, Airoldi L (2007). Effects of beach nourishment on sediments and benthic assemblages. Marine Pollution Bulletin 54:1197-1206
- Goudswaard PC, Kesteloo JJ, Perdon KJ, Jansen JM (2008). Mesheften (Ensis directus), halfgeknotte strandschelpen (Spisula subtruncata), kokkels (Cerastoderma edule) en otterschelpen (Lutraria lutraria) in de Nederlandse kustwateren in 2008, Wageningen IMARES (in Dutch).
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- Mazik K, Curtis N, Fagan MJ, Taft S, Elliott M (2008). Accurate quantification of the influence of benthic macro and meio-fauna on the geometric properties of estuarine muds by micro computer tomography. Journal of Experimental Marine Biology and Ecology 354:192-201.
- McLachlan, A. en A. C. Brown (1990). The Ecology of sandy shores, Elsevier.
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- Menn I (2002). Ecological comparison of two sandy shores with different wave energy and morphodynamics in the North Sea. Ber Polarforsch Meeresforsch 417.
- Mulder S, Raadschelders EW, Cleveringa J (2005) Een verkenning van de natuurbeschermingswet-geving in relatie tot Kustlijnzorg. De effecten van zandsuppleties op de ecologie van strand en onderwateroever, RWS RIKZ.
- Van Dalfsen JA, Essink K (2001). Benthic community response to sand dredging and shoreface nourishment in Dutch coastal waters. Senckenbergiana Maritima 31:329-332.
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