DR COLIN CLARK


Figure 3. Catchment rainfall based on TBR data at the Cat and Fiddle


Although the temporal distribution of rainfall at Lamaload is different the orientation of both high ground and total storm depth strongly suggest that the concentration of rainfall during the main storm was prevalent over the upper part of Toddbrook Valley. Since this is where the flood hydrograph is generated the Cat and Fiddle data has been adjusted downwards by the ratio of the catchment average rainfall and that at the Cat and Fiddle: 84/105 to give the design storm for the flood of 31 July of 63.5mm in 16.5 hours.


Field Investigations: Estimation of Peak Discharge


Just under 1km upstream from the Toddbrook reservoir the river overflowed its channel to a depth of up to 1m. This was enough to leave a trash mark showing that the river had cut off its meander and flowed across the floodplain. Figure 4 shows the local site conditions and Figure 5 part of the trash mark the week following the flood. Two cross-sections were surveyed, as shown in Figure 6. The first showed a typical pan-handle shape: calculation of the discharge was carried out in two parts – main channel and floodplain – with a threshold flood depth of 0.4m on the floodplain.


28 DAM ENGINEERING


Vol XXXII Issue 1


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