APPLICATION OF KINEMATIC WAVE THEORY TO SMALL WATERSHEDS

Abstract

tn the recent past, in many developing tropical countries a good deal of research has been carried out to solve the problems of 'large basins' whereas not much has been done with regard to the hydrologic problems of small watersheds. Small watersheds do play important roles e.g. a village pond is catered by its own small watershed; in hilly watersheds, the generated runoff causes flash flood, resulting into disruptions of communication lines etc. Therefore, it is necessary to look into these aspects of the hydrologic problems with greater attention. The hydrologic responses of a small watershed basi cally depends upon the mechanics of surface runoff which is primarily a nonlinear process. In this thesis, the surface hydrologic behaviour of three small natural hilly watersheds [from 82.0 to 1*100.Q hectares) and of one agri cultural watershed (1073.0 hectares) have been studied in details by the application of 'Kinematic Wave (KW) Theory' During the last two or three decades considerable research work has been carried out by various researchers into different aspects of KW theory application to water sheds of varying physiography. A brief summary of these efforts is presented in Chapter II of this thesis. The details of the KW theory i.e. formulation of mathematical equations and few of their possible solutions which happened to be explicit in nature have been discussed in Chapter vi III. Out of many possible solutions, the three fully offcentred, first order, numerical finite difference schemes namely, forward-in-time and backward-in-space (termed as Scheme I), backward-in-time and forward-in-space (Scheme II) and backward-in-time and backward-in-space (Scheme lit) have been preferred and used in this work. Computations of the models are performed with the help of a digital computer. The computer code has been written in FORTRAN-IV. Suitable configuration of distributed parameter models have been suggested for applying the KW theory to compute surface runoff (Chapter - III). The data availabi lity on these watersheds is described in Chapter-IV. The procedural details for the application of KW theory to the four small watersheds have been described in Chapter-V. The physiographic parameters that have been used are namely, the overland slope and roughness; channel slope and its roughness, side slope, bed width and water depth. A systematic sensitivity analysis of these parame ters has also been carried out. The results indicated that the effective overland roughness and the overland slope happened to be the most sensitive parameters. During the identification analysis, the values of effective over land roughness parameter have been worked out for diffe rent parts of the wet monsoon period (i.e. July to October). The effective overland roughnesses showed an interesting variation with respect to time in case of all the three natural hilly watersheds. Also, suitable conclusions have been drawn with regard to the applicability of the different Vll types of finite difference computational schemes for the two distinct categories of watersheds. As an extension, application of these principles to ungauged watersheds have also been tried (Chapter-VI). Possible solutions in the form of nomograms and regression models have been suggested. It was interesting to note that the time of concentration (T ) for the same watershed did not turn out to be a fixed characteristic. Contrary to belief, it was found that T varied with rainfall excess intensity and effective overland roughness. Variations in T were studied in depths. Regression equations were found for T as a function of area, effective overland roughness, overland slope, channel slope and rainfall excess intensity. These concepts were applied to develop a suit able methodology for using these principles for time-areaconcentration (TAG) based models. The response of TAC model was found to be encouraging (Chapter-VI). Appro priate discussion of results and conclusions as arrived at in different parts of the thesis have been summarised in Chapter - VII. . i . Concludingly, it is remarked that the KW theory is a powerful tool in computing the surface hydrologic responses of small watersheds of tropical regions.