FLOOD FREQUENCY ESTIMATION OF TODDBROOK FOLLOWING THE WHALEY BRIDGE INCIDENT


Calculation of quickflow is in two stages. The first is the calculations of Tp, TB and


Qp, as shown in Table 8. The second is the convolution of the ordinates at each stage of the storm.


Table 8. Parameters of the non-linear flow model CA = 15.41km2, MSL = 7.952, TB = 2.52 Tp


Values of the ordinates for the rising and falling limb are then calculated and to ensure there is a perfect match between the time to peak and data interval for the time to peak of the highest rainfall intensity. In this case it is 2.8 hours. Hence, with a time interval of 0.5 hour the times become 0.3, 0.8, 1.3, 1.8, 2.3, 2.8, 3.3.....


The convolution of the unit hydrograph takes place in the normal way except that the values in the body of Table 9 are ordinates not discharges. The total discharge for each time step is the sumof the products of quickflow, which is the sum of slope run-off and Ksat run-off and the ordinates. Table 9 gives an extract from this process. Time (hour) abscissa, ordinates total quickflow (mm).


Vol XXXII Issue 1 DAM ENGINEERING 35


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