Improving the FEH statistical method

The FEH procedures have been adopted as standard practice, where applicable, by the Environment Agency and other bodies engaged in flood frequency estimation in the UK, and are used primarily for flood mapping studies, flood risk assessments, and the design of flood alleviation schemes. The results from the recently completed R & D project SC050050 (EA, 2008) recommend changes to the procedures contained in the flood Estimation Handbook (FEH) published by the Institute of Hydrology (1999) for use of statistical methods for flood frequency analysis in the UK. The changes recommended arise, in part, because the HiFlows-UK project has led to the creation of a much improved database of systematically recorded flood data. Another influence on the changed procedures has been feedback from users of the FEH, both informal and formal. The changes do not deviate from the overall framework of the FEH methodology. However, most technical details of the method have been updated to improve the performance of the procedure. These updates include: i) A new equation for estimating the median annual maximum flood (QMED) at ungauged catchments ii) An improved procedure for the use of donor catchments for estimation of QMED at ungauged catchments iii) An improved procedure for the formation of pooled growth curves. There are some cases where the analysis carried out in the science report have recommended no change to the FEH methodology these include the retention of the Generalised Logistic (GLO) distribution as default.