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Table 1: Assumptions employed in the guides on optimum diameter of diversion tunnel

Type Top width, B (m) Average length, W(m) Average side slope Freeboard, F (m)
Embankment 5 100 to 1200 1H:2.25V 1.5
Length, L (m) Diameter, D (m) Longitudinal slope, S (-) Lining thickness, T(cm)
100 to 1400 3 to 15 0.005 (0.001 to 0.015) 30 to 75, varying linearly with D
Discharge, Q (m3/s) Manning Roughness Coefficient, n Flow regime Exit
50 to 1900 0.014 for RC lining 0.022 for shotcrete 0.030 for natural rock Pressurised (full flow) Unsubmerged
Type I (n=0.014 Type II (n=0.022) Type III (n=0.030) Rock bolts Shotcrete
Rock bolts + Shotcrete + Reinforced concrete (RC) Rock bolts + shotcrete Little rock bolts + little shotcrete ~3m long 10cm thick with steel wires
Tunnel excavation, C1 (US$/m3) Concrete lining C2 (US$/m3) Support (rock bolts + shotcrete), C3 (US$/m2) Fill for the cofferdam, C4 (US$/m3)
8 to 48 40 to 140 24 to 70 2 to 8