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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 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. The discharge was estimated using the Manning equation:
Q = VA V = R0.666
Figure 2: Rainfall 30-31 July 2019
j (Clark, 2013). The results were applied on a daily basis and the equations relating rainfall and pan evaporation (Clark, 2013) were updated to 2020 and given in Table 3.
The storm The rainfall of 30-31 July was centred over the
Figure 3: Catchment rainfall based on TBR data at the Cat and Fiddle
High Peak area to the south of Whaley Bridge as shown in Figure 2. Over 100mm were recorded at the Cat and Fiddle and at Lamaload Reservoir, both sites just outside of the catchment area. The 15 minute tipping bucket rainfall (TBR) data for the Cat and Fiddle is given in Figure 3, where two main pulses of rainfall are visible, with the latter of much greater depth and therefore importance. Although the temporal distribution of rainfall at Lamaload is different the orientation of both high
S 0.5 n-1 no
where Q = discharge (cumecs) R = hydraulic radius (A/P) where A = channel area, P = wetted perimeter, S = water surface slope m/m, n = Mannings friction factor. Assignment of the n value followed the component method (Chow, 1959) and for the floodplain the method of Jarrett (1987). Chow: n = (no – n4
+ n1 + n2 + n3 + n4 ) m5 , where are factors for channel material, degree of
irregularity, cross section changes, obstructions and vegetation, m5 a factor for meandering. Jarrett (1987) n = 0.39 R-0.16
S0.38 , where R =
hydraulic radius, S = water surface slope. The latter method was chosen because the friction factor n, increases with a decrease in hydraulic radius: the relatively low depth of flow across the floodplain gives a low value of R. The results of the survey and application of the methods are given in Table 4. Thus the average discharge is 28.4m3
/sec.
The uncertainty of these estimates depends mainly on the friction factor, and without direct measurements made during the flood this remains a problem. However, by using a hydrological model to estimate the discharge and then routeing this through the reservoir, the field based outflow can then be compared with the estimated outflow from the bywash channel and the auxiliary spillway. Photographs taken during the flood allow a good estimate of the discharge over the auxiliary spillway.
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
14 | August 2022 |
www.waterpowermagazine.com
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