Regional flood and storm hazard assessment

The report is the outcome of a 3-year investigation of "regional flood and storm hazard over reservoired catchments" funded by the Department of the Environment's reservoir safety committee. It describes an analysis of spatial dependence in annual maximum 1, 2, 4 and 8-day rainfalls in the UK A procedure is presented for assessing the annual collective risk of an extreme rainfall being experienced at one (or more) of a network of critical sites. The concept of an equivalent number of independent sites is introduced: the number is shown to be strongly influenced by the number of sites and their spatial density, with lesser effects attributable to region, rainfall duration and return period. A corollary of spatial dependence is that, when an extreme rainfall event occurs, it may lead to design exceedances occurring simultaneously at a number of sites. The report draws conclusions for the perception of reservoir flood risk in the UK. Attention is also drawn to marked regional variations in point rainfall growth curves that are not fully represented in existing design procedures.