In Situ Erosion Flume (ISEF): determination of bed-shear stress and erosion of a kaolinite bed

The strength of a cohesive sediment bed is difficult to determine and must be found experimentally from laboratory tests or through in situ field tests. A new in situ erosion instrument, the In situ Erosion Flume (ISEF), has been developed which is a circulating flow system in the vertical plane. The erosion process of sediment particles of the bed in the test section of the ISEF is related to the prevailing bed-shear stress induced by a unidirectional current. The bed-shear stress can be determined from the measured velocity profile in the test section assuming a logarithmic distribution in a vertical direction. The bed-shear stress equation has been calibrated under various conditions characterised by initiation of motion of sand and gravel particles at a flat bed where the bed-shear stresses mobilising the particles are known from the Shields curve. Three reproducibility tests were carried out under laboratory conditions. The bed consisted of kaolinite and was formed by sedimentation in still fresh water. The results of the three tests showed similar values. The erosion of a kaolinite sediment bed is more precisely determined under laboratory conditions by means of the ISEF. The results are compared with data from the literature. The ISEF is a relatively simple instrument for the determination of the strength of (cohesive) sediment beds. The results based on the ISEF measurements represent the minimum shear stress exerted at initiation of erosion of the bed, which will lead to the maximum shear strength of the top layer of the (cohesive) sediment bed.