FLOOD FREQUENCY ESTIMATION OF TODDBROOK FOLLOWING THE WHALEY BRIDGE INCIDENT
The percentage run-off for a given rainfall intensity using the following formula:
where R = rainfall intensity; Mid value R-Ksat = mid value of each Ksat class (0-5, 5-10...) of conductivity; A = proportion of catchment covered by each soil association. Values of % run-off are summed for each conductivity class until the mid value equals or exceeds the value of R.
The UH ordinates of overland or quickflow are non-linear using an alternative growth curve of the form for the rising limb:
where Tp = time to peak; Qp= peak run-off rate = 330/Tp) 0.001 CA; where CA= catchment area. For the falling limb:
where TB = time base = 2.52Tp, t1 = TB - t; where t = time since start of storm. These equations have been found to mimic the hydraulics of overland flow, which are themselves
non-linear processes, and agree with direct observations made during the historic floods of 1979 and 1982 on the upper Brue: slow rise to begin with, then rapidly increasing and finally reaching peak discharge at a slower rate of change. The falling limb repeats these changes but at a slower rate. The temporal distribution of rainfall will determine the shape of the unit hydrograph. The depth of quickflow and delayed flow run-off is calculated using the sine of catchment slope, the Ksat relationship, leaving the remaining rainfall as delayed flow. This part of the flow model has remained unchanged since it was first put into operational use in 2004.
Results
Table 6 is an extract from the calculation of quickflow and delayed flow. The data are 0.5 hour interval and the rainfall in mm. Slope run-off = rainfall x sin α, where α is the average catchment slope.
Vol XXXII Issue 1
DAM ENGINEERING
33
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