Institute of Hydrology, 1988. Collective risk assessment for sites sensitive to heavy rainfall. Flood Studies Supplementary Report No. 18, Institute of Hydrology, Wallingford, 8 pp.
Collective risk assessment for networks of impounding reservoirs in common ownership is one application of a procedure developed at the Institute of Hydrology. If the N sites under scrutiny are few and widely scattered, the effect of spatial dependence in heavy rainfall will be small and the collective risk of an exceedance of the T-year event can be calculated from:
r = 1 - (1 - 1/T)N
However, if the sites of interest are many and closely grouped it is essential to take spatial dependence into account when assessing the collective risk of a design exceedance. Such a method has been developed in research for the Department of the Environment's reservoir safety commission and is presented here for general use.