Abstract
In Mauritius, during heavy rains, there is quite a lot of sediment load which is transported by the rivers. There have been many estimates of the actual load being carried away. In some cases, these sediments have formed deltas at the estuaries, such as at Grand River North West or river St. Louis. A basic understanding of the process of sediment transport is essential to the scientist, engineers, and technologist who may be engaged in geology, hydrology, irrigation and other aspects of civil, marine or environmental engineering. This study was proposed so that an estimate might be made on the values that actually occur in Mauritius, as well as understanding the parameters that are involved in the process. Sediment load in rivers is caused by river flow arising from rainfall. This is why the study started with an investigation of rainfall patterns to channel towards erosion and river flow. Instead of measuring the sizes of sediments carried by the rivers, it was considered more convenient to carry out experimental work in the laboratory, where many factors could be controlled. This can then be compared with actual sediment data collected on various rivers. Data has been collected by the Central Water Authority during floods following heavy rainfalls or just after cyclones. It has been found more sensible to relate sediment load to river flow. River flow suffers a time lag from the rainfall causing it. In this sense, when there is a heavy rainfall, it causes some soil erosion which is carried to the river. Initially even before the flow has reached its time of concentration, the river may be carrying a substantial sediment load. Later on, when the flow reaches its peak value, the sediment load being carried has subsided. This is, of course, one scenario among many. It is possible to describe the relationship between flow and sediment load through a series of minimum and maximum lines. The minimum line shows the minimum sediment load that may be expected in the river as a function of the flow. The maximum line shows a higher bound value for the sediment load. It must be emphasised that these sediment loads occur mainly after heavy rains or cyclones. Once the occurrence of heavy rain is over, after some time (a few hours to a few days), though the river flow is still high, there might be no appreciable sediment load in the river. After rain has stopped, river flow will take place through contribution from groundwater flow via the recession curve. This explains why the flow may still be high without any sediment being carried. During an actual flood, these lines will give an indication of what may be carried in the river. Integrating the values over time, will give an estimate of what load is being carried by the river. Estimates made by the Central Water Authority give values of 20 tons/yr/km' for Grand River North West and 9-23 tons/yr/km' for River La Chaux. Recently, the confluence of the St. Louis river was dredged as a huge soil/mud deposit had been formed and was alleged to be blocking the flow in the river during heavy rains. Some 8000 tons of material have been dredged. This works out to a figure of21.6 tons/yr/km'. The experimental channel work carried out gives results which are comparable to those obtained by other people in other countries. It has been possible to make estimates on Eau Bleue reservoir. These work out to a sediment load of around 60-120 tons/yr/km'. Both the estimates of20 and 60-120 tons/yr/km' from rivers and from Eau Bleue reservoir are lower than figures recorded elsewhere or in the literature. It must be agreed, however, that these are estimates made from actual measurements in Mauritius. These low values of sedimentation, however, are borne out from deposits observed in other reservoirs, namely La Ferme, Mare Longue, Piton du Milieu and Mare aux Vacoas, which are in fact much lower than would have been expected from world figures. There may be very many varied explanations for this low sedimentation rate. Suffice it to retain that the sedimentation rate in Mauritius is of the order of 20-120 tons/yr/km'. This may seem a wide range, but clearly indicates lower bound and higher bound values.
Keywords
Water Quality,Rivers,Monitoring,Sediment Load