Design Storm Estimation

Abstract

Construction of dams and reservoirs across rivers for collecting and storing the runoff water to serve the needs of the people has been in vogue for many centuries. It was known that these dams often breached due to overtopping or seepage instability. Later, the discovery of concrete in the late 19^th century gave a tremendous push to the dam construction work throughout the world. In all these dams, a spillway is built that not only allows water to pass over the dam structure in normal day to day operation but is also a safety device to pass the largest flood discharge arising from heavy rainfall after the reservoir is filled so that the dam would not be damaged. The magnitude of the largest flood, called the spillway design flood, was earlier based on professional judgement and historical flood marks. As a consequence, the largest flood that would be expected at the dam site was estimated from flood marks and the spillway capacity was provided accordingly. In the later part of the 19^th century, the magnitude of the spillway design flood was determined by empirical formulae relating discharge to drainage area in the form Q = K A ⁿ, where Q is the peak flood discharge, A is the drainage area, K is an empirical coefficient depending upon the rainfall-runoff characteristics of the drainage area, and n is a constant whose value usually lies between 0.5 and 1.0. The empirical formulae were based on the catchment area, with the assumption of constant coefficients. The degree of conservatism implied by these formulae was obviously unknown. The dams based on these formulae were often breached due to inadequate spillway capacities as pointed out in Chapter 8.