Sedimentologische en biologische processen in de erosiegevoeligheid van cohesieve sedimenten op enkele slikken in de Beneden-Zeeschelde

This paper describes the results of two field campaigns to measure critical shear stress for erosion of muddy sediments in intertidal mudflats of the Scheldt estuary in relation to both abiotic and biotic parameters.The critical erosion shear stress was measured in situ in three intertidal mudflats (Groot Buitenschoor; Galgenschoor and Paardenschor) situated in the Belgian part of the Scheldt estuary; using the HR Wallingford instrument SedErode. The tidal flats are situated in the mesohaline and the turbidity maximum zone of the estuary. On every intertidal flat three sampling locations were chosen. In every location three measurements of erosion stress were carried out and samples of the top layer were taken for sediment and benthos analysis. The sampling stations differed in topographical elevation: the average time of flooding during a spring tide varied between 2 and 9 hours.The pigments chlorophyll a and fucoxanthin were determined as an indication of the presence of microphytobenthos. Large differences in chlorophyll a content were found between both campaigns, with maxima in April, being ten times higher than in September.The number per m2 of all benthic species was measured. A total of 13 different species was found. The highest diversity occurred during September: The benthos density was substantially higher during September than April.Very low pigment content and macrofauna density occurred on the Galgenschoor intertidal flat. The sample locations on this tidal flat are situated low in the intertidal zone and are flooded during a major part of the tidal cycle.The measured critical shear stress for erosion was up to 65% higher during April than September: The value of the critical shear stress is situated between 0.26 and 0.43 Pa (April) and between 0.24 and 0.29 Pa (September). The critical erosion shear stresses measured on the Galgenschoor, where the biological activity was very low, showed little variation between both campaigns. The data do not permit to establish a clear correlation between 050 or mud content and the critical shear stress or density. Some data, however suggest a relation between shear stress and increasing sand content or density. This can be explained by the presence of sand in a muddy bed, which will compact the sediment and increase the erosion resistance. The difference in erosion resistance between both campaigns is for a major part caused by biological parameters.