KINEMATIC WAVE MODELLING OF A SEMI-URBAN WATERSHED

Abstract

Kinematic wave routing method, adopted in this dissertation relates basin and flow characteristics directly to two routing parameters a and m. These parameters are directly related to measurable geographic features of the basin such as slopes, drainage length, boundary roughness for both channel and overland flow surface. In the case of channel a also relates to the channel geometry. In this exercise, the primary objective is to produce a model that would emulate the watershed behaviour as closely as possible. As such the model has to be essen- tially a distributed one rather than a lumped model. Distri-buted model is more versatile than a lumped one because minor modifications, such as changing a channel geometry or size in a subbasin can be done without reevaluation of the overall watershed characteristics. However, a fair amount of simplifi-cation has to be done to make the model efficient and econo-mical to execute. As such the distributed model obtained is actually a composite of lumped model of subbasins of the watershed. Roorkee watershed, that is the area east of Ganga Canal has been chosen to be studied. The area is some 2.93 square kilometers and allows easy access to confirm drainage pattern, length and other physical features. In the study for the effect of urbanization, four different years has been considered viz. 1949, 1960, 1970 and 1990. The pervious and impervious areas for the Roorkee watershed corresponding to the above years are computed using available data. These are then used to compute the outflow S-hydrographs for both the lumped and distributed basis. Three different return period storms have been used viz. 2 years, 10 years and 100 years to study their effect on the watershed. These have been done for the dura-tions of 15 minutes, 30 minutes and 60 minutes on lumped model only. The study for the effect of roughness has been done using 'N' values (modified Manning's roughness values for overland surface) of 0.35, 0.30 and 0.20 for the pervious cover only. Impervious cover used 'N' value of 0.15. S-hydrographs are obtained for both the lumped and distributed model. After analysing the hydrographs, it is found that between the periods of. 1949 to 1990, time of concentration for the watershed has been reduced by about 30%. Maximum discharge increased by upto 10%. Discharge per hectare is found highest in subwatershed 'C' which is the area located within the vicinity of the road extending from CBRI junction to Azad Wing of Roorkee University. It is found that time of concentration for a given rainfall intensity of infinite duration differs between the lumped and distributed model by upto 40%. This difference is less marked for lower intensity rainfall.. It can be seen that kinematic wave method proves to be a very useful tool in relating the physiographic features of a watershed directly to its outflow characteristics.