FLOOD FREQUENCY ESTIMATION OF TODDBROOK FOLLOWING THE WHALEY BRIDGE INCIDENT
Figure 7. Inflow and outflow hydrographs Flood Frequency Analysis
Flood rarity is the product of the return periods of the storm and the critical or threshold soil moisture deficit. This relationship exists because there is no correlation between SMD and storm rarity[21]. In the present case the area adjusted rainfall in 16.5 hours is 63.5mm. The 2-year 16.5 hour rainfall = 33.8mm. 1 hour PMP = 160mm[15]; 24 hour PMP = 475mm[23,10]. The steps in the calculation are shown below:
1. Relate the 1 hour and 24 hour PMP with Log of duration: PMP = 228.225 Log D + 160. 2. Thus 16.5 hour PMP = 438mm. 3. Use the nomogram for ARF[13,16] for 16.5 hour PMP over 15.41km2 = 0.810. 4 Therefore a real 16.5 hour PMP = 438 x 0.810 = 354.8mm. 5. Correlate 2-year areal 16.5 hour rainfall and areal 16.5 hour PMP with the modified reduced variate (Equation 1) to give the 16.5 hour areal rainfall frequency equation:
6. Thus the rarity of the storm (R) of 63.5mm in 16.5 hour (y) = 2.655, which gives a value of T, the return period of 14 years.
7. The rarity of the SMD is assessed using the method described by Clark[21]. This is based on 20 years’ weighing lysimeter and open pan data gathered at CHRS, and 142 years regional rainfall corrected using 30 years local rainfall. The result for a threshold SMD of 16mm over a time period of 142 years’ data = 2.15 years.
8. Thus the flood rarity = Rp ER x Rp SMDcrit; where ER = effective rainfall after the SMD is reduced to the critical value as determined from weighing lysimeter data at CHRS[21]. In the present case the critical SMD had already been attained so the effective rainfall is the storm rainfall.
9. Thus, in the present case the return period of the flood = 14 x 2.15 = 30 years.
Vol XXXII Issue 1
DAM ENGINEERING
37
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