Inter-comparison between five devices for determining erodability of intertidal sediments

Abstract

Five erosion devices were compared using five intertidal estuarine sites covering a range of sediment stability from newly settled mud to very cohesive mud at the margins of a saltmarsh. The erosion devices use different methods of fluid shearing from horizontal currents/bed shear stresses to vertical water jets, and have different ‘footprint’ areas. The devices included: (1) the annular flumes (AFs—diameter 64 cm; footprint area 0.17 m 2) of the Plymouth Marine Laboratory (PML); (2) PML's mini-annular flume (MAF—diameter 19 cm; area 0.026 m 2); (3) the annular mini-flume (AMF—diameter 30.5 cm; area 0.032 m 2) of the National Oceanography Centre Southampton (NOC); (4) NOC's Cohesive Strength Meter (CSM—diameter 3 cm; area 0.0007 m 2); (5) NOC's EROMES (ER—diameter 10 cm; area 0.0079 m 2). The quantification of threshold shear stress for bed erosion ( τ e) and sediment erosion rate was complemented by the measurement of physical, chemical and biological properties of the sediment (grain size, bulk density, water content, organic content, chlorophyll a, carbohydrates, macrofauna). The results demonstrated a significant correlation ( r 2=0.98) between the PML AF (laboratory measurement of undisturbed cored sediment) and PML MAF ( in situ) for measurement of erosion thresholds for bed sediment. However, there were no significant correlations between AFs, the CSM and EROMES. There were no consistent correlations with physical or biological sediment properties due to the spatially unrelated sites and the marked differences in benthic assemblages. The sources of differences and the lack of correlations between erosion devices were due to several factors, including operational procedures (e.g., sediment resuspension during filling with water), definition of erosion threshold, the nature of the force applied to the bed, and method of calibration. In contrast to the CSM and EROMES, both types of AFs were able to record significant differences in the erodability of soft sediments from four sites. This indicates that the CSM and EROMES may not be very effective at measuring the differences in erosion thresholds of soft estuarine sediments.