DR COLIN CLARK The discharge was estimated using the Manning equation:
where Q = discharge (cumecs) and R = hydraulic radius (A/P), where A = channel area, P = wetted perimeter, S = water surface slope m/m and n = Mannings friction factor. Assignment of the n value followed the component method[7], and for the floodplain the method of Jarrett[33]:
• Chow: n = (no + n1 + n2 + n3 + n4 ) m5, where no - n4 are factors for channel material, degree of irregularity, cross-section changes, obstructions and vegetation, and m5 a factor for meandering.
• Jarrett: n = 0.39, R-0.16, S0.38; where R = hydraulic radius and 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.
Table 4. Estimated discharge at CS1 and CS2
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.
30 DAM ENGINEERING
Vol XXXII Issue 1
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48
