Design and application of a recirculating flume for the assessment of geotechnical and geophysical indicators of cohesive sediment erodibility
A laboratory seawater flume was designed and constructed for experimental studies of the erosion characteristics of cohesive sediments. The upright orientation of the flume and the use of an impeller drive system reduced the amount of cross stream flow over the test bed area and allowed the flow to be accurately controlled and replicated on separate flume runs. The flume was designed to use a relatively small volume of seawater so that suspended sediment concentrations could be accurately measured. The flume was used to explore the relationship between threshold shear stress ("tCh) and physical bed properties measured using non-destructive geophysical techniques. The flume was instrumented to measure flow velocity, bed shear stress, and resuspended sediment concentration. The two flow parameters were measured using hot film probes. Calibration of the bed stress probe was carried out in a purpose built flume which could generate stress values measured by manometer tubes. Suspended sediment concentration was measured using infra red optical backscatter sensors which were calibrated in situ by gravimetric analysis of filtered samples. Three physical bed properties were measured: moisture content using standard soil mechanics techniques; acoustic shear wave velocity (Vs), using pizo-electric bender elements; and electrical formation factor (FF), using resistivity probes. Bed porosity was derived from measured formation factor using calibrations obtained in a modified oedometer cell. Rigidity modulus (ll) was obtained using the measured shear wave velocity and calculated bed density (derived from porosity). Flume samples were remoulded estuarine muds. Moisture content was varied by adding water to samples prior to installation in the flume; moisture contents of 110-165% were thus obtained. These samples had FF values of 1.05-1.75%, and Vs values of 0.5-4.0 ms- 1 • Experiments identified significant relationships between l:ch and the measured bed properties. The most important relationships were with Vs and p. These relationships can be described by power relationships such that "tch oc llo.2 and l:ch oc VSO. 2 • These experiments indicate that the erosion potential of a cohesive bed can be predicted by geophysical measurement of bed properties.