The stability of a mudflat in the Humber estuary, South Yorkshire, UK

Abstract

Abstract The stability of a mudflat in the Humber estuary, South Yorkshire, was investigated at seven sites along a shore-normal transect during early spring. This was carried out using the benthic flume Sea Carousel, and from the investigation of surface samples in a laboratory equivalent to the Sea Carousel, the Lab Carousel. A clear trend in erosion threshold [τ c (0)] was evident, showing two maxima: the greatest on the inner mudflat (0.78 Pa); and a second on the central mudflat (0.75 Pa). We ascribe these maxima to two causes: (1) desiccation of the inner mudflats; and (2) biostabilization by algae of the central flats. Our results suggest that of these factors biostabilization was the dominant one at the time of the study. Most of the variation in erosion rate is explained through the sediment bulk density (ρ b ) and colloidal carbohydrate (DCHO): [τ c (0)] = 0.5[1.93log 10 (DCHO + 2.5 × 10 −4 ρ b ] + 1.7; r 2 = 0.63. Thus variations in DCHO had O(10 4 ) greater impact on erosion threshold than equivalent variations ρ b . The relationship between mean erosion rate ( E m ) and current speed ( U y ) was similar for all sites and is defined by the exponential function: E m = 2.47 × 10 −6 · 10 (3.749 U y ) kgm −2 s −1 . In situ mean still-water settling rates ( W s ) were up to 2.46 × 10 −3 ms −1 , which is up to an order of magnitude faster than was measured in other estuaries at similar suspended sediment concentrations, S . The decay constant, k , for still-water settling appeared to be a linear function of S , and compared favourably with values derived from five Canadian coastal mudflats. Results from Lab Carousel showed that the erosion thresholds were the same as those measured in situ , using Sea Carousel. Furthermore the spatial trends in erosion were the same in both studies with the exception of the innermost sites (A and A/B). Differences are explained by consolidation and desiccation caused by solar radiation which were not simulated in the lab. Mean erosion rates showed a similar exponential relationship with current speed to that determined from Sea Carousel. Lab-derived values of k were much higher than the Sea Carousel ones and indicated much faster sediment settling under laboratory conditions. k was a weak inverse function of applied stress. Values of τ d derived from Krone’s (1962) method yielded threshold values of between 0.03 and 0.32 Pa. These values were 38 ± 16% of the erosion threshold τ c (0).