The relative importance of nutrient loadings and hydrographical and meteorological conditions for oxygen conditions in the Thisted Bredning, Denmark were evaluated by intensive survey and model calculations. The influence of wind speed on the vertical mixing in the unstratified shallow waters was quantified and tested in an oxygen model. The significance of oxygen consuming biological processes on the oxygen conditions were tested. By the use of a eutrophication model, the influence of local discharges of sewage on the size of the oxygen producing and oxygen consuming processes in the Thisted Bredning was calculated. Calculations showed that only a minor increase in rates of the oxygen consuming processes result in a drastic rise in the probability of oxygen depletion.
An area of the Niger River Delta was studied from October 1974 to October 1975 in connection with feasibility studies and preliminary design for the development of a deep draught port in the Western portion of the Delta. The provision of a"100-kilometer, 8 or 10 m navigation channel through one of the entrances from the sea, up to new port facilities at Warri required comprehensive hydraulic, hydrographic and sedimentation surveys over such period.
Sea pipelines are for various reasons frequently laid in trenches in the seabed rather than on the seabed itself. In certain areas it is requested, for security reasons, that the pipeline should be covered to a certain extent, either through natural backfilling or through artificial backfilling. For the planning of the pipeline protection, it is therefore important to be able to calculate the rate of natural backfilling. The present paper describes how the natural backfilling may be calculated on the basis of the following parameters: currents, waves, bed material, shape of trench and water depth. The backfilling (sedimentation) in the trench is partly caused by a gravity effect on the bed load transport and partly by settling of the suspended load due to decreasing transport capacity over the trench. The physics behind these two sedimentation mechanisms are described. A theoretical calculation of the sedimentation, due to gravity effects on the bed load transport is developed in two steps. The first step contains only current, crossing the trench at an arbitrary angle. In the second step, it is shown how the same solution may be applied to bed load transport caused by wave action. The sedimentation due to settling of suspended load is also calculated for currents alone and for combined currents and waves. From the developed mathematical model, it is possible to calculate which portion of the suspended load will be able to settle as the current passes the trench. Four examples of how to calculate natural backfilling are presented. Two examples in current only and two examples in combined currents and waves. INTRODUCTION The aim of the studies presented in the present paper has been to provide mathematical models for calculation of natural backfilling in pipeline trenches under the assumption that the sea bed consists of non-cohesive sediments and that the depth of the trench is small compared to the water depth, see Fig. 1. The local scour around the pipeline is not considered in the present paper. The transportation of non-cohesive sediment is normally separated into the transportation of bed load and suspended load. In many contexts, such a distinction is of minor importance, but for the present purpose it will be demonstrated that such a distinction is necessary. As an illustrative example, the especially simple case where the alignment of the dredged trench is parallel to the direction of the current is considered. In this case, no suspended sediment is carried from the undredged regions to the trench by the current, if secondary currents are neglected. As long as the variations in water depth are small, the magnitude of secondary currents is negligible. However, while the suspended load is carried in the current direction, the direction of the transport of bed load deviates from the current direction due to gravity effect on the bed load moving on the slopes of the dredged trench.
The feasibility of a harbour project, which involves dredging of an access channel, may to a large extent depend on the future maintenance dredging in the channel. It is therefore important to be able to calculate sedimentation in dredged channels with sufficient accuracy. In 1974 and 1975 the Danish Hydraulic Institute (DHI) carried out a study of the most feasible access channel to Warri Port situated in the Western Niger Delta, Nigeria. Two alternative entrances were studied and in conclusion it was recommended to improve the existing access channel through Escravos Entrance.
In order to describe and evaluate the effect on the oxygen concentrations of a planned sewage outlet in the Port Shelter Bay in Hong Kong, a relatively simple BOD-DO-box model has been established. The model describes the oxygen concentration in 6 horizontal boxes in the central part of Port Shelter. Each box is vertically divided into 4 layers (sub-boxes). The oxygen concentration in each layer is described as a function of the vertical and the horizontal mixing, the oxygen demand of the sediment and the concentration of organic matter in the water. A finite difference transport dispersion model provided input data for the BOD-DO model. Two main calculations have been made: one simulating the cold season (15°C) and one simulating the warm season (30°C). Furthermore sensitivity analyses have been carried out. The calculations show an oxygen depletion of approximately 1 and 2 mg O2/l at 15° and 30°C respectively in the bottom layer over an area of approximately two sq. km. The highest oxygen depletion is calculated in the bottom layer. From the calculation it can further be concluded that the decay rate of organic matter in the water and the vertical mixing will influence the oxygen depletion considerably. In periods where either the decay rate is higher than 3 d−1, or the vertical mixing is considerably low an oxygen depletion higher than the calculated can be expected. The lowest measured oxygen concentration in Port Shelter during the period July-August 1982 was 1.7 mg O2/l. Therefore it can be expected that the planned outlet will cause oxygen-free or nearly oxygen-free conditions in the bottom layer in warm periods with low vertical mixing.
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