| dc.description.abstract | In countries. such as Kenya.where industrialization nnd urbanization trends are emerging in the rural selting.lhe hydrological water balance is subjected to rapid changes. These changes pose problems to design engineers who rely on parameters that may no longer be applicable. In order to understand the overall performance of such rapidly changing watersheds in terms of water inflow and outflow. the catchment characteristics and any changes to them need to be understood. This analysis can be conducted on real catchments or by simulating the catchment processes on suitably prepared models. In this study physical models were set up to study a wide range of catchment processes. The results have shown that it is possible to study real catchmen~hydrology on suitably prepared physical models. Rainfall was simulated by using a hypodermic type of rainfall simulator and a comparison between the simulated and natural rainfall showed similarity in perfomance. Two types of physical model catchments were pr~ared and used in the study. The indoor model catchment was fabricated in the form of a rectangular box in which both runoff and seepage were investigated. The outdoor model catchments were in forms of runoff plots which were composed of pre-designed soil-cover complexes. The results from both types of catchments were similar and in agreement with the results encountered in nature. It was possible to vary and investigate some catchment characteristics, such as slope and shape. that are normally considered fixed in practice. The results of th-ese findings can therefore be used alongside those in real catchment studies for better catchment understanding and planning. The impact of man's activities on catchment Ilerformance were investigated on the physical models and produced resulls in short. durations of time such as minutes. Similar activities in natllre would take years or decades to monitor. Such results can tllerefore help in better catchment management without having to fully monitor each aspect in nature. From the wide range of catchment types simulated,it was possible to confirm the high variability of results observed in nature. The explanations derived from this study for the differences in the results can be applied in real catchment investigations. Rainstorms are often a result of a moving storm. This is confirmed by the observation of the strong winds that normallY accompany most rainstorms. The effect of storm movement is however rarely taken into consideration ·n the design of most hydraulic structures. Storm movement was simulated and investigated in this study. The results have &hown that moving storms have significant effects on runoff and that critical conditions for peak rates of flow occur when storms are moving over the catchment. Creation of more surface depressions increased the groundwater recharge potential and decreased both runoff and erosion. Increase in urbanization generally increased the runoff volume, the peak runoff rate and decreased the time to peak and the infiltration capacity of the catchment. Decrease in vegetation cover generally increased runoff. The runoff did not show uniform response to vegetation cover variation. The differences which are gimilar to those observed in nature were related to the catchment characteristics. The rate of erosion wh ich was measured in terms of sediment concentration was found to vary rapidly at vegetation cover between 40 to 60 %. At less than 40 % vegetation cover the erosion rates were high but there was little alteration in erosion rates even though the vegetation cover was varied. Beyond 60 % vegetation cover. the decrease in erosion rate with further increase of cover was small and there was some erosion even at 100 % cover. Minimum rates of erosion were not neces§arily at 100 % vegetation cover. Some ~atchments showed minimum rates of sediment yield at about 75 % vegetation cover. Soil and Water conservation measures sucbas terraces and cutoff drains were introduced and their effects were to decrease the runoff volume. decrease the peak runoff rate and drastically reduce sediment yield. | |
