Evaluation of a tidal flat sediment nourishment as estuarine management measure
The tidal flats in the Eastern Scheldt tidal basin (The Netherlands) are eroding as a result of the construction of the storm surge barrier. These intertidal areas are important foraging grounds for birds and therefore it is important to mitigate the negative effects of erosion. As a pilot, a small part (20 ha) of the Galgeplaat tidal flat in the middle of the Eastern Scheldt was nourished with 130,000 m3 of sediment in 2008. This paper investigates this tidal flat nourishment using a large set of morphological and ecological monitoring data from 2008 to 2012. The nourishment proved effective in mitigating the negative effects of tidal flat erosion. The elevated tidal flat provides a foraging area for birds that is available for a longer period. The nourished area was relatively stable, giving time for the recovery of benthic macrofauna such that birds can benefit from the longer exposure time. Therefore, we conclude that sediment nourishments are an effective management measure to counteract the negative consequences of tidal flat erosion in the Eastern Scheldt, and have potential for other estuaries worldwide.
Practical implications of tidal flat shape
Arising from high precision empirical measurements of whole shore shape involving determination of incremental areas in narrow height bands undertaken during the 1980s, now confirmed by physical and mathematical models and theoretical studies, it has been established that accretion-dominated mudshores tend to be high and convex in cross-sectional shape, whereas their erosion-dominated counterparts tend to be low and concave. High and convex mudshores are preferable in coastal defence, port engineering and conservation terms to low and concave ones. In this paper the practical implications of these contrasted shapes are drawn out. The natural shapes of mudshores contrast with what might be anticipated from the well-established Bruun Rule for sandy shores. Accordingly, a new mudshore conceptual model, the Mehby Rule, is proposed which embraces all aspects of mudshore behaviour and will stand alongside the Bruun Rule. A new mudshore management tool, a top-down model, called S-factor analysis, is proposed and developed to a semi-quantitative state by way of a demonstration of its possibilities. It can be applied to quantifying the stability of mudshores. Mudshores with an S close to zero are stable in shape and approaching one type of equilibrium condition. As S departs progressively from zero in the negative direction shores become more accretion-dominated and, similarly, as they depart from zero in the positive direction they become more erosion dominated. A new mudshore management technology, tidal flat regeneration, is also proposed. The Mehby Rule shows that, to be stable and self-sustaining, whole shore profile manipulation is required, whilst S-factor analysis indicates these will have an S close to zero. By simultaneously placing a minimal wave attenuator and feeding with dense muddy dredge material, a low and concave mudshore can be converted into a more desirable high and convex one. The technology is proved from physical model tests and by analogy with the natural response of low and concave mudshores to a sudden change in the degree of wave protection. Thus, the new method seeks deliberately to copy accidentally induced shape change. The next stage will be to undertake full-scale field trials.
Impact of Beach Restoration on the Deep-burrowing Ghost Shrimp, Callichirus islagrande
Abstract Restorations of barrier islands can be beneficial to coastal marshes, but may also have detrimental effects on intertidal macrobenthic communities. Two barrier islands of the Isles Dernieres chain of Louisiana were recently renourished with dredged sediments taken from the adjacent estuary. This study addresses the impact of beach restoration on intertidal populations of the burrowing ghost shrimp, Callichirus islagrande. We compared sediment characteristics of the restored sites and neighboring sites (both with and without shrimp) and examined the effect of sediment alteration on recolonization rates. Beach height was increased by an average of 2.4 m. Differentiation among sites with respect to silt/clay, sand, and gravel fractions was significant (P<0.001). East Island, the primary site of restoration, had the largest proportion of silt/clay sediments (40 % of total) whereas Trinity Island had the largest proportion of gravel (12 %). Minimal population recovery (total = 3 individuals) was evident 2 years post-restoration. High levels of silt/clay loading at East Island appear to have slowed population recovery.