ABSTRACT
In the framework of inland navigation, the loss of water in a reach due to lockages has to be counterbalanced. The water may be pumped from the lower reach but this supply is energy consuming. In order to reduce the amount of water to be pumped, water may come from tributary streams by gravity. In the case of the Canal du Centre, a stream called “La Haine” may be partially diverted into the Canal du Centre, through a by-channel arriving downstream of the lock in Obourg, a city in the area of Mons in Belgium. However, this stream carries a large amount of suspended materials. It does exist a settling basin between the by-channel and the Canal du Centre to diminish the sediment supply in the canal. But at present, the by-channel is blocked. The Walloon administration would like to reuse it because the fluvial transport is expected to increase in the near future and because a new center for gathering the mud has been set up next to the channel. In this paper, the efficiency of the settling basin is analysed to help deciding whether the settling basin should be reused or not. The impact of the discharge flowing in the basin on the efficiency is accounted for in the analysis.
The study is made of a measurement campaign and numerical modeling. The results of both methods are compared.
The by-channel has been opened so that field measurements could be undertaken. A suspended-load sampler and a pressure gauge were set in two locations: (1) upstream of the by-channel and (2) downstream of the settling basin. They allow the sediment concentration and the water level to be measured. The water discharge is deduced from the known head on the downstream weir. The amount of deposited sediments is then deducted. This measurement campaigns shows that, even during peak flow, the concentration in sediments is highly decreased due to the settling basin.
A numerical model has been designed by the Hydraulics Research Laboratory in Châtelet, Belgium. The model computes the distance crossed by the particles during a steady flow, knowing the time evolution of the water discharge and the sediment concentration at the entry of the basin. It accounts for the fall velocity of the sediments sampled in situ. For the median diameter of 16.10 μm, the settling velocity is 0.205 mm/s according to the Stokes’ law. The model shows that the water discharge that ensures an efficient settling is 1m3/s. It also computes the volume of the deposited sediments.
