EXPERIMENTAL ESTIMATION OF DETENTION IN STORM SEWER SYSTEM

Abstract

In this paper, the results of experimental study on the storm runoff in urban area were described. The former investigations have been performed mainly based on theories or practical observations. The practical survey, however, is extremely difficult because of the problems of provision for sudden storm, scarcety of design storm occurrence and also the selection of proper gauge station in sewers. These difficulties may be the principal reasons why the progress of investigations of urban storm runoff is so slow. The model configuration of sewer system is not unable because the drainage area and sewer include comparatively many artificial elements. The present laboratory model herein used is only the system model of storm-plain basin-single sewer and will not be sufficient for the complete study, but would make a step further toward the better model to study the storm detention and runoff. The results of the present investigation will be summarized as follows.i) It was pointed out that the phenomena called as the flow detention, retardation, storage and flooding were the hydraulically identical phenomena and might be called “detention” in a wide sense. The detention formula may be available for the estimation of depression rate X for drainage pump capacity, provided that limits for b in Talbot's formula and the length of sewer are carefully selected, but the theoretical depression rate would become X'( = √ 2 X). The detention coefficient α is of different significance hydraulically from X and several questions are remained concerning with the theoretical procedure.ii) Experiments were made for the sewer slopes of 1/200 and 1/1 000. Applying the rational method, the rate of runoff becomes the maximum more rapidly than the actual. Sewer section design, therefore, is in safety side. The hydrograph agrees comparatively well with the actual one in case of the steep slope sewer if the roughness is selected properly. But the hydrograph is much different in the gentle slope, and the correction is difficult in the practical range of the roughness. This fact is ascribed to the so called natural detention and suggests that the Manning's formula which is the equation of motion of the rational method is no longer established in the gentle slope. If this detention is evaluated experimentally by the term of detention coefficient α, α becomes 1.30 in the slope of 1/1 000 but is unity in 1/200, so that α should be understood as the correction factor representing the difference between the hydrographs by the rational method and the actual one.iii) Any of control structure is understood to have the negative effects upon the capacity of dynamic storage except when the flooding in lower area is concerned or when the drainage pump capacity is designed to be depressed.iv) When the storage function method is used, the calculated hydrograph for steep slope agrees with the actual hydrograph. In the gentle slope area, the hydrograph is also not so good to be fitted. This method would be limited theoretically and the determination of the storage coefficients should be noted to be quite difficult.v) The flooding does not always occur by the increasing of return period because the maximum rate of runoff becomes steady state when calculated by the rational method. On the contrary, when calculated by any design method considering detention, the maximum rate of runoff is not necessarily in steady state and the flooding certainly occurs by the increase of return period. The return period should be determined in order to minimize the sum of cost and damage based on the possibility of flooding due to heavier rainfall.