On the Amount of Bottom Sediments in Reservoirs

Abstract

This paper presents the results of the study on the amount of bottom sediments in reservoirs.The study concerns the geology, topography, annual precipitation in the watershed, discharge of rivers flowing into the reservoir, and amount of bottom sediments in the reservoir. The characteristic features of erosion and transportation in the watershed could be found from this study.1. As regards the 52 reservoirs studied, the amount of the sediments (the mean annual amount per unit area of the watershed) has little or no relation directly with any one factor of geology, relief and annual precipitation during heavy rains in the watershed. Each of the reservoirs based upon the geological characteristics (rock distribution) of their watersheds, can be classified into the following three groups, namely, A: Reservoirs which have the watershed consisting mainly of Paleozoic and Mesozoic rocks.B: Reservoirs which have the watershed consisting mainly of acidic plutonic, hypabyssal and their metamorphic rocks (Granitic rocks).C: Reservoirs which have the watershed consisting mainlyf Cenozoic sedimentary rocks, effusive rocks or crystalline schists, or sedimentary, plutonic and effusive rocks of nearly equal distribution.2. For the reservoirs of each group included in the three groups by the characteristics of rock distribution in the watershed, the larger amount of bottom sediments is found for the reservoir with the watershed which has larger numerical value of product of those representing the relief and the annual precipitation during heavy rains.3. Moreover, from comparison of the amount of bottom sediments in the reservoirs of these three groups with each other, it is clear that the finer the watershed products, the more the sediments are deposited. The same relation is also found for the reservoirs.of the three subgroups (B1, B2 and B3) of B group which were classified based upon the climatic condition of the watershed.4. Based on the aforementioned relations between the amount of bottom sediments in the reservoirs and the several factors of the watersheds, it is concluded that for each reservoirs of A, B and C groups, the larger the numerical value of the product of those representing the relief and the annual precipitation during heavy rains, the more the clastics are eroded in the watershed and transported into the reservoirs. Furthermore, for the reservoirs with the watershed of which combined factors of relief and annual precipitation during heavy rains is nearly equal, the finer the sizes of the clastics, the more clastics are eroded in the watershed and transported into the reservoir. 5. For each of the several main reservoirs selected from the aforementioned 52, the annual variation of the amount of bottom sediments is little relatad with that of the annual discharge of inflow, whereas it is controlled by that of the yearly maximum flood discharge. Therefore, it is considered that for all the reservoirs which have their own rock distribution and relief, the amount of the clastic sediments transported by the rivers has a close relation with the flood discharge.