The Influences of Biological Activity and Consolidation Time on Laboratory Cohesive Beds

摘要

A small annular laboratory flume (Mini Flume) was used to investigate the changes in erosion threshold and erosion rate on natural mud taken from an intertidal mud flat surface in Southampton Water, U.K. Mud samples were wet sieved at 2 mm to remove shells and large macrofauna and macroflora and then mixed into a turbid slurry with local estuarine water. A mud bed was formed in the flume by letting the sediment deposit in still water at room temperature (20°C). The bed was subjected to steady flows of increasing magnitude in a series of steps. The flume was subjected to illumination on a dark-light cycle of 12 hours to simulate the solar cycle. Subsequent erosion experiments were carried out after periods of bed consolidation of 2, 6, 12, 24, 48, and 96 hours in order to examine changes in bed stability with time. This series of experiments was considered the baseline situation in which algal and bacterial binding were present. At the end of each experiment, the bed was fully resuspended, mixed for a period of 5 minutes, and then allowed to redeposit. A second series of experiments was undertaken using the same sediment, but in this case the bed was continuously held in darkness (no photosynthesis of microphytobenthos). A third series of experiments was carried out in darkness, but antibiotics were added to the flume to remove the effects of bacterial activity. A final series of experiments was carried out with the addition of 50 ml l–1 of sodium hypochlorite to remove all biological activity in order to examine the effects of electrochemical binding (the control). The results reveal that under the normal conditions of light (day-night) an increase in sediment stability (stabilization index, BI) of circa 3 was found over that of electrochemical bonding. The bed treated with antibiotics suffered an overall stabilization of 1.5 relative to the control.