Flow and sediment properties influencing erosion of fine-grained marine sediments : sea floor and laboratory experiments

Abstract

Erosion processes involving fine-grained marine sediments were studied by using an in situ flume to erode undisturbed bottom sediments on the sea floor in Buzzards Bay, a shallow marine embayment off the Massachusetts coast. The muddy sea floor in that area is characterized by a deposit-feeding infauna that reworks the sediments.Observations made with the in situ flume suggest that erosion resistance of compacted bottom sediments is up to twice as great as the erosion resistance of biogenically reworked sediments. Estimates of erosional bed shear stress from the in situ flume experiments are similar to estimates made during this study of bed shear stress developed in near-bottom tidal currents. It is inferred that erosion by the in situ flume produces reasonable estimates of bed shear stress necessary to erode undisturbed bottom sediments on the sea floor.Buzzards Bay muds were redeposited in a laboratory flume and eroded after various periods of reworking by the deposit-feeding organisms contained in them, Other Buzzards Bay mud samples were treated to remove organic matter, and the erosion resistance of flat beds of these sediments was also investigated in a laboratory flume.The surface of a biogenically reworked bed after two months was covered with mounds, burrows, trails, and aggregates composed of sediments and organic material. This bed was similar in appearance to many of the beds eroded by the in situ flume. The two month bed eroded at an erosional shear stress similar to the erosional shear stress necessary to erode the in situ Buzzards Bay muds (0.8 dynes/cm 2 ). Beds biogeiically reworked for shorter periods had high values or erosional shear stress, up to twice that of the two month bed.The bed shear stress necessary to erode flat beds of Buzzards Bay sediments increased as the concentration of organic matter in the sediments increased. Depositfeeders were absent in these beds, and the mode of deposition was kept uniform, so the increase of erosion resistance with increase in organic content is considered a reliable indication of sediment behavior, and not an artifact of experimental conditions. During the in situ experiments, lee drifts were created behind resistant roughness elements on the sea floor. A brief study of lee drift formation in the laboratory suggests that the formation of lee drifts from fine-grained sediments can be predicted to take place when the body Reynolds number of the resistant roughness elements is below a critical value. ACKNOWLEDGMENTS

Soil structure effect on soil erosion potential

“…The soil structure, including particle arrangement and inter-particle bonding, developed in situ during depositional and post-depositional processes, can significantly differ from that of laboratory-prepared soils [7]. Young et al (1975) conducted a comparison between laboratory and in situ flume erosion testing results, revealing that critical shear stress obtained in the laboratory could be overestimated by a factor of two [8]. However, limited research has been undertaken to investigate the effects of soil structure on soil erosion.…”

Use of an In Situ Erosion Flume for Measuring Stability of Sediment Deposits in Hamilton Harbour, Canada

“…Daborn et al, 1993;Holland, Zingmark, & Dean, 1974;Lau & Droppo, 2000;Young, 1975). As the laboratory flumes may not be able to mimic the exact physical, chemical and biological sediment structure and operation, it is prudent to use in situ erosion flumes for the quantification of the erosion parameters.…”