Probabilistic approach to determination of the dimensions of gravitational settling basins

Abstract

Operating experience is presented for gravitational settling basins.As component parts, diversion hydroelectric power plants and irrigation systems constructed on foothill and mountain sections of rivers, as a rule, possess gravitational settling basins, which are characterized by large dimensions and resource outlays for their construction and operation. Despite this, they have come into widespread use in world practice, since they perform an important task -protecting the structures from siltation, and the linings of conduits and equipment from abrasive wear. The high and ever-increasing cost of land, and ecological problems caused by the construction of water-storage complexes, as well as the deficiency of electric power and lack of natural fuel resources dictate the need to expand the construction of hydroprojects with gravitational settling basins on mountain and foothill sections of rivers. In the republics of Northern Kavkaz, for example, these factors will, in the near future, determine the construction of diversion HPP with settling basins on rivers of Baksan (Él'bursskaya, Upper Baksan, Tyrnyauz, Zhankotsk) with a total capacity of 250 MW, and Cherek Balkar (Vysokogornaya, Balkar, and Golubye Lakes) with a total capacity of 270 MW, etc. Mandatory inclusion of settling basins as component parts of these water-storage complexes is determined by the fact that mountain rivers bear a large amount of suspended particles of various sizes, and there are no alternatives to settling basins.Operating experience with gravitational settling basins indicates, however, that in many cases, they fail to fulfill their function satisfactorily. This is associated with the fact that the turbidity of rivers with respect to size and amount of suspended particles varies appreciably over time. The problem of cleaning water of dangerous suspended detritus to the required level during the course of the daily service of structures and equipment is one of the most complex.In designing settling basins, the following are factors that determine the quality of their performance: correct designation of the design turbidity of the flow and the sizes of dangerous particles, well-founded assignment of the degree of clarification of the flow (probability of detritus deposition), correct determination of the dimensions of the settling basins, and high in-service quality. This paper is devoted to the problem of improving computational reliability of the dimensions required for the basins.Previously proposed methods for the design of gravitational settling basins were based on available results of investigation of the deposition of uniform suspended particles passing through the intake section at the same height. To achieve a result, it was necessary in these methods to partition the computed working depth of flow into elementary layers (25 -30 layers), and assume a normal distribution for the particles that had been deposited from each elementary layer; this appreciably schematizes the character of the scatter of settled uniform and nonunifor...