Morphological Response of Rivers to Withdrawal of Water

Morphological Response of Rivers to Withdrawal of Water

Crosato, A.

Civil Engineering and Geosciences

Hydraulic Engineering

1989-12-01

It is very convenient to make use of rivers as water supply for various purposes, i.e. irrigation, industrial and domestic uses. Even if it is a very common practice, the utilization of river waters must be carefully planned and its consequences analysed. For example, before starting on a new withdrawal of water the need of water supply more downstream must be considered. Furthermore, whenever those needs are satisfied, the morphological changes of the river must be taken into account. Characteristic consequences of withdrawal of water are: aggradation of the river bed; eventual rise of the water level and decrease of the water depth downstream of the extraction point. All these morphological changes can affect the existing structures along the river and the navigability of the channel. For this reason the morphological response of the river must be predicted. In this way it is possible to take measures in advance, when necessary. Here the river response to water withdrawal is analyzed by means of mathematical models. After a short description of the underlying theory, the morphological changes are investigated computationally by using the computer program "ODIRMO" (Vermeer [1985]) available at the Delft University of Technology. In order to give a more general insight into the involved processes the computations are carried out for several conditions. Furthermore, the input data and the output layout are comparable with those adopted in the study made by Hendrickx [1988]. In this way the morphological changes due to water withdrawal can be compared with the response generated by sediment mining. The results of the morphological changes due to water withdrawal show the importance of investigating the transitional period and not only the final situation. During the transitional period the water level initially drops and then rises. The decrease of the waterlevel can temporarily affect, for example, the intake of irrigation canals further downstream. Consequently its intensity and duration must be taken into account when planning a new water withdrawal. The performed computations show how effecting the schematization of the computational model can be on the results. The decoupling of flow and bed computations might yield to undesired oscillations of the bed when the celerity of the bed rise is two high. Here a criterion for a restriction of validity of that schematization via a dimensionless parameter is suggested. The lack of computational examples, together with the uncertainty added by (i) the problems related to the accuracy of the computations, (ii) the non-optimization of the flow computation routine, lead to the impossibility of estimating a critical value of the dimensionless parameter by means of the computational results of this study only.

morphology
water withdrawal
computational model
morphological change

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