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1st International Conference on Rain Water Cistern Systems
Honolulu, Hawaii, USA - June 1982

Section 3: Design, Cost, And Policy

Page 102

Hydrographs for Roof-Top Runoff Under Varying Rainfall Conditions

Bhaskar Datta
Flood Control Dept. Gauhati, INDIA

G.C. Mishra
University of Roorkee, INDIA


With increasing population, water supply systems may fail to satisfy the demand for water. In arid and semiarid regions, especially, the source of water is often depleted. Rain water collected from roof surfaces and utilized through cistern systems can augment the usual domestic supply without large expenditures or energy and can thus meet water shortages.

To design gutters and a cistern system, knowledge of the water surface profile and flow hydrograph is necessary. The kinematics of flow over roof surfaces can be determined from available studies of overland flow. Many investigators have dealt with the problem of overland flow under uniform, excess rainfall either by using the method of characteristics (Behlke 1957; Henderson 1964; Wooding 1965; Morgali and Linsley 1965; Abbott 1966; Brakensiek 1966) or by solving the mass balance equation by assuming a linear relationship between outflow and storage (Horton 1938; Izzard 1944). Wooding (1965) has dealt with the problem of overland flow under a constant, uniformly distributed rainfall of finite duration with an analytical solution for a hydraulic model based on the method of characteristics for flow over a plane, which is part of a V-shaped catchment. In reality, the rainfall intensity is not uniform; instead, the distribution is skewed. The typical time variant design of rainfall distribution patterns described by Rodda, Downing and Law (1976) for a 10-year, 5-year and annual return frequencies are shown in Figure 1. Water surface profiles and flow hydrographs for roof surfaces for such design storms can be determined for different intervals of time by using the characteristics equations.

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