THE STUDY OF RUNOFF MECHANICS OF A NATURAL - WATERSHED

Abstract

I'hc present work deals with the study of runoff mechanics of the natural watershed of Bridge No 719, For this purpose the mechanics of runoff generation has been studied through the unit hydrograph suggested by Sherman (1932) and Nash (1957). Also the Kinematic wave model which is based on non-linear theory has been applied to have a comparison of linear approaches with non-linear response of the watershed for uniform rainfall excess function. Roughness Coefficients play a significant role on the response parameters of the hydro logic system. As an academic exercise some hypothetical conditions of varied channel roughnesses have been considered to see the over all effects on the shapes of the hydrographes of surface runoff obtained at the out let and the other response parameters of the hydrologic system, viz., time of concentration, velocity of flow, depth of flow etc. by deriving S-hydrographes. For this purpose separate computer runs were obtained by using four channel roughnesses namely 0.04, 0.1,0.2 and 0.3. For each of these roughnesscs, three sets of overland roughness i.e. 0.1,0.2 and 0.3 were adopted. From this study it was observed that with the increase in roughness value, time of concentration, area and depth of flow increased while velocity of flow decreased. As stated above, a comparison was also made between the outflow hydrograph obtained from Sherman's linear unit graph approach and non-linear analysis based on Kinematic wave theory for the watershed of Bridge No. 719-using the concepts of S-hydrograph. This analysis indicated that none of the hydrographes obtained through non-linear Kinematic Wave approach for different combinations of channel and overland roughnesses mentioned above, matched with the S-hydrograph derived from Sherman's unit graph. Finally, it was concluded that both the linear and non-linear hydrologic response of the watershed may match when channel and overland roughness values are allowed to vary with time or depth of flow.