FLOOD FREQUENCY ESTIMATION OF TODDBROOK FOLLOWING THE WHALEY BRIDGE INCIDENT
Field Investigations: Estimation of Bankfull Discharge
Of all the measures of flood response, bankfull discharge is the most straightforward but not necessarily a straightforward parameter to estimate. Uncertainties remain with the exact level at which the channel is reckoned to be full and if the slope area method is used, with the choice of roughness factor. The cross-sections CS1 and CS2 differ in their shape so that there is a difference in bankfull discharge:
The component method gave n values of 0.067 and 0.035, respectively, for the two cross-sections. Taking the average of the two estimated discharges gives a result of 3.7m3/sec. It is noted that the two discharges are associated with flow velocities of about 1.3m/sec, which has been measured in similar river channels. CS2 is a shallower cross- section and may have a slightly steeper water surface slope since it is in the position of a channel riffle, but as the WSS at bankfull stage was not measured in the field this point remains uncertain.
Modelling of the Flood of 31st July
The non-linear unit hydrograph flow model was initially developed for flood warning below a flood detention reservoir[11], and has been in use for over 15 years. It was later extended for use throughout England and Wales[16], and flood warning at Boscastle and Lynmouth[20]. The main features of the model are as follows:
The depth and duration of design storm is calculated by first obtaining an estimate of the 2-year rainfall using the FEH CDROM, or by reference to original autographic rainfall records. Since the frequency is very high, the error will be low. Second, an estimate of the 24 hour probable maximum precipitation (PMP) is obtained from the map produced by Clark & Dent[23], a revised and extended version produced by Clark in 2002[10]. The estimate of the 1 hour PMP comes from the maximisation of two historic storms[15]. The T hour PMP is estimated by a log linear regression of duration and depth. The area reduction factor (ARF) for PMP comes from a nomogram[13,16] of ARF, area and duration. As the storm rarity declines the ARF moves towards unity, which is in agreement with the observed spatial concentration of rare storms and more even distribution of common ones. The ARF nomogram bears a striking resemblance to that of Rezacova et al[41].
The detailed temporal distribution of the rainfall comes from another nomogram[16] which was based on an envelope curve of the most extreme historic storms. The resulting data are for summer events. For winter events a seasonal correction factor from the ReFH software[34] is used, although direct reference to original records could also be made. Rainfall frequency
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