SMALL CATCHMENT-AREA FLOOD ESTIMATION

Although the UK Flood Studies Report (FSR) has been available for almo st 20 years, there remain a number of cases that arise quite frequentl y in practice which lie on, if not outside, the margins of application of the recommended methodology, One such case is that of catchment ar eas too small to be within the scope of routine gauging programmes, Un certainty as to the most appropriate method for design flood estimatio n for such basins often results in designers resorting to simplified p rocedures, such as the Rational method, A version of this method which does not suffer from the difficulty of specifying directly a time of concentration is that of B. D. Richards, Reappraisal of this approach shows it to have been an early application of Kinematic Wave principle s, A comparison between the Richards method and the FSR approach incor porating the short-cut convolution procedure on some 30 catchments les s than 55 km(2) in area from the UK demonstrated a reasonable consiste ncy in estimates by the former, although large discrepancies can be pr oduced for individual basins with particular combinations of catchment characteristics. In practice, the iterative nature of the Richards me thod calculation makes the methodology less convenient than the simpli fied FSR approach, Moreover, the critical fitting parameter in the met hod, which is broadly related to catchment roughness, is only poorly p redicted from standard catchment and rainfall characteristics. Despite the attraction of not having to specify the time of concentration dir ectly, the Richards method is seen to offer no particular advantage ov er the simplified FSR method, and its general use for small catchment areas cannot therefore be recommended.