Estimation of critical shear stress from cohesive strength meter-derived erosion thresholds

Robert C. Grabowski, Ian G. Droppo, and Geraldene Wharton

Limnol. Oceanogr. Methods 8:678-685 (2010) | DOI: 10.4319/lom.2010.8.0678

ABSTRACT: Measures of cohesive sediment erodibility, such as erosion thresholds and rates, are essential for the development of accurate sediment transport models. In situ devices for erodibility quantification are increasingly being used to capture the high spatial and temporal variability in erodibility found in natural cohesive sediments. The cohesive strength meter (CSM) is a small, hand-held, commercially available device that has been used extensively in recent years to measure in situ erosion thresholds. However, the device is primarily used for relative measures of erosion thresholds, due to a difficulty in comparing the vertical forces generated by the CSM with horizontal bed shear stress. In this article, we describe the development of a methodology for the estimation of horizontal bed shear stress from CSM-derived erosion thresholds. The approach used cohesive sediment mixtures with varying clay contents to create sediments with a range of erodibilities and tested them using the CSM and a laboratory annular flume. A calibration is proposed based on a comparison of mean erosion thresholds from each device per clay treatment. This study is the first successful inter-comparison between CSM- and flume-derived erosion thresholds. Significant differences in erosion thresholds were noted between CSM routines, so it is recommended that the empirical calibration be applied only to erosion thresholds estimated using the Sand 1 routine, and ranging from 40-90 Pa stagnation pressure. The calibration allows the conversion of CSM-derived erosion thresholds into critical shear stress, a form that permits incorporation into sediment transport models.