DR COLIN CLARK


Table 9. Extract of the UH convolution for quickflow


Total flow is quickflow x Scf + delayed flow, where Scf is the slope correction factor. This factor is calculated by the equation [Σx/A] /100, where x = sum of changes in elevation of 10m on 1:50,000 OS maps. This is the tangent of the land slope. The inverse is the slope. The formula for the Scf[16] is:


In this case Scf = 1.699. As the rarity of the flood increases the effect of the land slope


(Scf) tends toward unity. This is because the effect of slope gradually becomes overtaken by the depth of overland flow. The adjustment of Scf for flood rarity is given by Clark[16]:


where Scfrare = Scf for the given rarity, Scf = baseline Scf, S = [(eLog Rp)-1 + 1] [0.02729 Log Rp + 0.93264], where Rp = return period of flood, as calculated below.


In the present case Rp = 30 years and Scfrare = 1.216.


Thus the peak flow = 18.91 (quickflow) x 1.216 + 3.218 (delayed flow) = 26.2 cumecs This result is slightly lower than the field-based estimate. Figure 7 shows the complete modelled hydrograph.


The same flow model was applied in order to estimate: 1. The probable maximum flood 2. The 1 in 10,000 year flood 3. The 2-year flood - winter rainfall 4. The 2-year flood - summer rainfall


36 DAM ENGINEERING


Vol XXXII Issue 1


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