Precise determination of Turkish spillway sizes from synthetic unit hydrographs to prevent flood damage

Abstract

In Turkey, calculations of runoff hydrographs are made in practice by synthetic methods, among which Snyder and Mockus are the most widely used synthetic unit hydrographs. Using these methods without modification of the original coefficients, C t and C p for the Snyder and K and H for the Mockus, characterizing climatic, geological, topographic, soil texture, land use and cover features of a watershed, introduces problems in designing the capacity of spillways, in turn bringing risks for safety. The coefficients were optimized by the analysis of 127 unit hydrographs, using directly measured runoff values in 17 different experimental watersheds located in different part of Turkey between the years 1975 and 1999. After runoff analysis and statistical evaluation, C t and C p coefficients for the Snyder method were found between 0.10 and 1.22, and 0.52 and 0.92 while the original values ranged from 1.8 to 2.2 and from 0.5 to 0.7, respectively. K and H coefficients for the Mockus method were obtained between 0.192 and 0.330, and 0.45 and 2.23, respectively for Turkey in general, whereas the original values were 0.208 and 1.67, respectively. The size of spillways built in Turkey to date was found to be too small, and should be increased using these new coefficients to prevent flood damage. © 2004 Taylor & Francis.