Latest Research News on Bottom Sediment : Mar 2022

Mechanics of bottom sediment movement due to wave action

Up to the present time, very few experimental or analytical studies have been made on the effect of the action of surface waves on the moverrent of sediment at the bottom. Some of these studies have been limited to a few experinental observations, while others are expressed only in general descriptive terms. In the present study, as a result of an analytical and experimental investigation, it is found that, because of the action of surface waves, there are initial and general movements of sediment; and initiation, various stages of development, and complete disappearance, of bed undulations or ripples. The movement of sediment takes place in a boundary layer that is developed at the bottom from the effects of viscosity of the fluid. The initial and general motion of small sizes of sediment occur in a laminar boundary layer and are caused by laminar shear, while similar motions of large sizes of sediment are caused by lift forces in a turbulent boundary layer. Ripples, in general, are not formed unless the now is turbulent in the boundary layer. All motion in turbulent now and the various stages of development of ripples are found to be functions of a dimensionless function representing intensity of flow of the fluid near the bottom. Flow conditions in the boundary layer at the interface are distinguished by the ratio of the thickness of the boundary layer to the size of the sediment. These phenomena of initiation of turbulence and motion of sediment in the boundary layer at the bottom are expressed in terms of the characteristics of the surface waves.[1]



Bedform-generated convective transport in bottom sediment

Convective flow within bed sediment is an important mechanism enhancing the mobility of chemicals, both natural and anthropogenic, and thermal energy in this region of aquatic environments1,2. Experimental observations indicated that significant in-bed convection currents can be generated by water flowing over small obstructions on the surface of a porous bed. Significant porewater flow is induced by imbalances in pressure over distance, generated by differences in temperature, density and hydrostatic head3. We demonstrate here by laboratory simulation and a vignette model that flow over bedforms induces additional pressure imbalances which generate significant and complex convection currents within porous bed sediment. A model is proposed for estimating Peclet numbers for this effect/The results have particular application to chemical transport in the upper sediment layer that is often the recipient of high levels of chemical contamination. Although our analysis reflects river conditions, the results may have wider applications and include submarine currents moving over dune-like mega ripples on the ocean floor.[2]


Survival of Escherichia coli in lake bottom sediment

The survival of Escherichia coli in bottom sediment (Lake Onalaska, navigation pool no. 7, Mississippi River) was studied by using in situ dialysis culture of sterile (autoclaved) and unsterile sediment samples. Bags made from dialysis tubing were filled with either course sand sediment (28.8% fine) or organic, silty clay sediment (77.2% fine) and placed at the sediment-water interface. Bags representing sterile controls, unsterile uninoculated controls, autoclaved inoculated sediment, and unsterile inoculated sediment were studied during a 5-day period for each sediment type. Daily most-probable-number determinations indicated that E. coli populations in unsterile inoculated sediment fluctuated between 5.3 X 10(2) and 2.2 X 10(3) bacteria per g of silty clay and between 3.0 X 10(3) and 1.4 X 10(4) bacteria per g of sand. Autoclaved silty clay sediment inoculated with 1.0 X 10(6) bacteria per g increased to 2.2 X 10(8) bacteria per g in 3 days. During the same period, autoclaved sand sediment inoculated with 1.2 X 10(5) cells per g increased to 5.4 X 10(7) bacteria per g. By day 5, populations in both cultures had decreased by 1 log. The ability of E. coli to survive for several days in aquatic sediment in situ suggests that fecal coliforms in water may not always indicate recent fecal contamination of that water but rather resuspension of viable sediment-bound bacteria.[3]


Bottom sediment sampling

Tests with sampling equipment during 1967 and 1968 in Lake Ontario showed that some of the bottom samples were badly disturbed. As a result of these tests, further extensive trials of bottom-sampling equipment were conducted in Georgian Bay in 1968. These were designed to test the disturbance of the bottom sediments caused by ship movement and to test the efficiency and suitability of various types of sampling devices. It was found that under typical sampling conditions, bottom disturbance was induced by the ship when the total water depth was less than about 6 m. Also it was found that the ship, during typical sampling procedures in deeper water, induces disturbance in the water column, beneath it, to a depth of at least 14 m. Tests were conducted with 3 types of Gravity corer, 2 multiple corers, and 6 Grab samplers. These tests were designed to show their usefulness for both biological and geological sampling. All 3 gravity corers proved efficient for particular tasks. The most successful bottom grab samplers were found to be the Ponar grab and the Shipek bucket sampler.[4]


Impact of marine fish farming on water quality and bottom sediment: A case study in the sub-tropical environment

Field studies were carried out to determine and compare the impact of marine fish farming activities on the water quality and bottom sediment at four fish culture sites with different hydrographic and culture conditions in a sub-tropical environment where trash fish is used as feed. The major impact identified was on the sea bottom, resulting in the development of reducing and anoxic sediments, high sediment oxygen demand, production of hydrogen sulphide and elimination/decrease in benthos. The impact on water quality was less conspicuous. A decrease in dissolved oxygen was observed at all sites while increases in ammonia, inorganic P, nitrate and nitrite were observed only at sites with poor tidal flushing and high stocking density. However, no significant changes in total suspended solids, light extinction coefficient, chlorophyll a, phaeopigment and E. coli were found near the fish rafts at any sites. Environmental impacts vary considerably between sites, and were significantly reduced at sites with good water circulation and low stocking density. Despite the high organic and nutrient loadings generated by marine fish farming activities, the impacts on water quality and sediments at all sites were localised and did not appear to extend beyond a distance of 1–1.5 km from the fish rafts. Results of the present study also do not support the suggestion that marine fish farming activities have caused eutrophication on a large scale.[5]


Reference

[1] Manohar, M., 1955. Mechanics of bottom sediment movement due to wave action.

[2] Thibodeaux, L.J. and Boyle, J.D., 1987. Bedform-generated convective transport in bottom sediment. Nature, 325(6102), pp.341-343.

[3] LaLiberte, P. and Grimes, D.J., 1982. Survival of Escherichia coli in lake bottom sediment. Applied and Environmental Microbiology, 43(3), pp.623-628.

[4] Sly, P.G., 1969. Bottom sediment sampling (No. CONF-690585-).

[5] Wu, R.S.S., Lam, K.S., MacKay, D.W., Lau, T.C. and Yam, V., 1994. Impact of marine fish farming on water quality and bottom sediment: a case study in the sub-tropical environment. Marine Environmental Research, 38(2), pp.115-145.

Editor

Leave a Reply

Your email address will not be published.