The Economical Evaluation in the Design of Main Drainage Systems
In order to have an instrument with which a system of watercourses can easily be evaluated on its merits the model EWAS, a model for the design and Evaluation of WAtermanagement Systems, has been developed (Volp 1986).
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The Economical Evaluation in the Design of Main Drainage Systems C. Volp and C.F.J.M. Veraa Department oj Civil Engineering, Delft University oj Technology, The Netherlands
INTRODUCTION In order to have an instrument with which a system of watercourses can easily be evaluated on its merits the model EWAS, a model for the design and Evaluation of WAtermanagement Systems, has been developed (Volp 1986). The main objective of the model can be formulated as follows: It should be possible with the model to evaluate the hydraulic properties of a watermanagement system and at the same time give the economical consequences. In this paper the model EWAS is introduced and an example is treated to demonstrate the application of the model. As subject for the example the problem is chosen of the balance between investment costs and maintenance costs in the design of watercourses. The design of drainage canals is usually determined by the choice of the design discharge, the freeboard that is to be maintained and the assurance that a certain flow velocity is not exceeded. A relatively simple calculation routine gives the dimensions of the various watercourses in the drainage system. In this calculation routine the designer has to make a choice for the roughness coefficient. This roughness coefficient strongly depends on the type of bed material and the bed vegetation and so implicitly on the rate of maintenance that is foreseen. On the other hand the choice directly affects the investment costs mainly determined by the excavation costs. The question arises whether it is possible to reduce the net present value of the costs for a given time period and interest rate by designing for a lower level of maintenance and larger dimensions. The importance of this question is clearly shown by the fact that according to the annual reports of the Dutch Waterboards the ratio of costs for depreciation and interest, maintenance and administration and management are 30, 55 and 15 % respectively, with a total budget of about Dfl. 400. million.
K. V. H. Smith et al. (eds.), Hydraulic Design in Water Resources Engineering: Land Drainage © Springer-Verlag Berlin Heidelberg 1986
558 MODEL DESCRIPTION Basically the purpose of the development of a tool for the designer of watermanagement systems is to simplify the determination of the optimal system in terms of lay - out, dimensions and water management. Optimal has in this case an economical meaning. In that case the costs for investment, operation and maintenance of the future system must be compared with the benefits, or the reduction of damage, that can be obtained with the system. The benefits that can be obtained are related to the area usage, the reference level, the frequency and the extent of exceedances of that reference level and the duration of these exceedances. For some user groups in a given area it is practically impossible to determine the waterlevel - benefits relation. Extreme examples are recreational and nature reserve areas. It is obvious that in these cases the waterlevels ar
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