MODELING OF FLOOD FLOWS IN NATURAL WATERSHEDS

Abstract

In this research work an attempt has been made to develop a model having capabilities of accounting for the spatial and temporal variations of rainfall as well as of physiographic characteristics which do prevail in most of the tropical countries. Aliterature survey conducted during the course of this study (Chapter-II) revealed that the Kinematic Wave (KW) theory and the Dynamic Wave (DW) theory based hydrologic models currently being used for solving the St. Venants equations have the capabilities of taking into account the distributed nature of the input function as well as of the physiographic characteristics. Thus, these mathematical theories have been applied to develop the following two physiographic models (Chapter-Ill). CD Physiographic Model-I; consisting of tributary subwatersheds and a single consolidated main channel subwatershed til) Physiographic Model-II consisting of tributary subwatersheds and distributed main channel subwatersheds. The details of these models have been discussed (Section 3.5). The later model is an extension to the first model. The main tributaries are identified and the watershed under consideration is split up into its tributary subwatersheds which remain common to both the models. The remaining area is considered as a single main channel subwatershed in the first case whereas it is further split up into smaller units in the second physiographic model given above. Drainage characteristics of the areas happen to be the criteria adopted for the demarcation of the subwatersheds. In order to obtain the conceptual configuration, the surface runoffs coming from each of these subwatersheds (i.e. tributary subwatersheds and main channel subwatersheds) are folded onto the main channel (Section 3.5). The final physiographic pattern so arrived at will remain unique for the watershed under consideration. The surface runoffs from the overlapping overland planes are superimposed to compute the lateral flows coming to the main channel. Flows are routed through the main channel to compute the outflow hydrographs at the outlet. For the proposed configuration each of the subwatersheds becomes the elementary unit from which the runoff responses are to be computed. Any changes in its landuse can be appropriately taken care of by suitably modifying the values of the 'physiographic parameters' and thus affecting the runoff process. For the application of the proposed physiographic models the KW theory is applied for routing the flows on the overland planes. The Lax-Wendroff explicit scheme has been used for the mathematical formulation of the KW equations. The criteria adopted for the 2 applicability of the KW theory is Fr K > 5, where Fr is Froude number and K is KW number. The computed overland runoffs form the lateral flows to the channel. For routing the flows through the channel, the DW theory has been preferred. The mathematical formulation of St. Venant equations have been sought through the four point implicit scheme. The application of the proposed models have been discussed in depth and details for the watershed of Kolar river. However, in order to draw logical conclusions about the applicability of the proposed models, the applications have been repeated onto the watershed of the Railway Bridge No. 719 and the Kassilian watershed. The 'Open Book Type' physiographic model has also been applied for comparing its performance with proposed models. The comparison of the computed hydrographs with the observed ones as well as the model efficiencies, do suggest that the physiographic model-II consisting of VI tributary subwatershed and the distributed main channel subwatersheds gave better results. At the same time the phys.ographic „odel-I „ comparatively simpler and easy to apply. The performance of open boo type Physiographic mode. In general was net found to be satisfactory. Th. proposed mode.s are advantageous ,„ asense that the distributed response, of the surface runoff coming from different parts of the ohannel can separately be estimated. The model can be further strengthened and Improved In future by linking It by infiltration based ground water models.