FLOOD FREQUENCY ESTIMATION OF TODDBROOK FOLLOWING THE WHALEY BRIDGE INCIDENT


is much too low. In fact the PMF at the dam site is 292 cumecs – over 100 cumecs higher than that reported by Hughes[28]. These observations are typical of other catchments (at least in southern England) such as the upper Brue dam site where the PMF was increased from 240 cumecs to 500 cumecs with subsequent improvement works in the interests of safety. At the other end of the scale the excessively high 1.1 year flood for the Upper Stour at Bourton in Dorset was proven by current meter measurements[19].


Figure 8. Flood frequency analysis Reservoir Flood Routeing


A useful test of the estimated hydrograph just upstream of the reservoir is to route it through Toddbrook reservoir. The peak outflow discharge can then be compared with the calculated outflow, based on the capacity of the bywash channel and the head above the auxiliary spillway. These structures have less uncertainty when it comes to calculating discharges than a natural channel whose friction factor cannot be accurately determined although lies within a broad range of values. For a given time period Δt[24,29] the inflow, outflow and storage are related:


Vol XXXII Issue 1 DAM ENGINEERING 39


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