In site versus laboratory analysis of sediment stability from intertidal mudflats

Measurements of intertidal mudflat erosion thresholds from in situ and laboratory erosion devices were contrasted. Alteration of the critical erosion shear stresses was potentially brought about either by physical disturbance (vibration, compaction and water loss), ongoing biological activity or changes in the behaviour of infaunal organisms during transport and resting of the excavated cores. In an initial experiment, box cores were collected from the Humber estuary Skeffling mudflats (April 1995) and transported back to the laboratory for measurement in a linear flume. These cores suffered visible disturbance during transport to the laboratory and their erosion thresholds were considerably higher than in situ data obtained by the Sea Carousel erosion device. In the main study, cylindrical cores collected in the Sylt-Rømø Bight (June 1998) were collected and transported in a manner that minimised disturbance. The stability of these cores was measured with the EROMES laboratory erosion device and compared to near-by in situ measurements taken with the cohesive strength meter (CSM) erosion device. These devices use different criterion to calculate the erosion threshold (erosion rate and attenuation threshold, respectively), resulting in differences in the calculated erosion threshold. However, when an attenuation threshold was used for both devices the erosion thresholds were comparable. When disturbance of cores was minimised, in situ and lab erosion thresholds were comparable. However, user bias in site selection can influence results where there is spatial variation in sediment properties. Stability measurements should therefore be made on randomly selected sediment areas.