Tools · Hydrology

River design discharge

Guided step by step: the 50-year flood by RDSO’s RBF-16 for small catchments, the CWC Flood Estimation Report unit hydrograph method for each subzone (25 to 2500 km²), the empirical formulas of Dicken, Ryve and Inglis, the area–velocity method from the HFL at site, then compare them and adopt a design discharge.

Guided · step 1 of 7

Collect the catchment data

From the topo sheet: catchment area A up to the bridge site, length L of the longest stream, and the fall H from the farthest point of the catchment to the bridge bed. From the rainfall atlas of your subzone: the 50-year 24-hour point rainfall R. RBF-16 applies to catchments smaller than 25 km²; larger ones (25 to 2500 km²) use the CWC Flood Estimation Report method for the subzone.

A = 2.54 km², L = 2.5 km, H = 67.25 m, R = 16 cm

Soil: red soil, clayey loam, cultivated plains, tall crops, wooded areas

Catchment

Area A (km²)Longest stream L (km)Fall H (m)Farthest point to bridge bedR₅₀ 24-hour (cm)From the subzone rainfall mapSoil and cover (Table 2)Ratio tc-rain / 24-h rainFig. 4; approx. 0.282 at tcR₅₀ 1-hour (cm), if knownLeave blank to use the 24-h value

Catchment and stream

Q₅₀

36.43m³/s

0.604h

36.3 min

0.691

I₅₀

74.7mm/h

RBF-16 tables

Table 1: F	tc < 30 min	30 to 60 min	60 to 100 min
Below 2.5 km²	0.72	0.81	0.88
2.5 to 5.0 km²	0.71	0.8	0.87
5.0 to 13.0 km²	0.7	0.79	0.86
13.0 to 25.0 km²	0.68	0.78	0.85

Table 2: soil and cover	X
Sandy soil, sandy loam, arid areas	0.249
Alluvium, silt loam, coastal areas	0.332
Red soil, clayey loam, cultivated plains, tall crops, wooded areas	0.415
Black cotton, clayey soil, lightly covered, plain and barren	0.456
Hilly soils, plateau and barren	0.498

Short-duration rainfall ratio (after Fig. 4)

Approximate curve fitted to RBF-16’s worked examples. Read the exact ratio from Fig. 4 for your subzone and type it in on the left if it differs.

Full solution, all steps at once

Collect the catchment data
A = 2.54 km², L = 2.5 km, H = 67.25 m, R = 16 cm
Soil: red soil, clayey loam, cultivated plains, tall crops, wooded areas
Time of concentration tctc = (L³H)^0.345
tc = (2.5³67.25)^0.345 = (15.62567.25)^0.345 = 0.2323^0.345
tc = 0.604 h = 36.3 min
Areal reduction factor F (Table 1)
A = 2.54 km² → row “2.5 to 5.0 km²”; tc = 36.3 min → column “30 to 60 min”
F = 0.8
Runoff coefficient CC = X·(R·F)^0.2
X = 0.415 (Table 2)
C = 0.415 × (16 × 0.8)^0.2 = 0.415 × 12.8^0.2 = 0.415 × 1.6651 = 0.691
Rainfall during tcR₅₀(tc) = R₅₀(24 h) × ratio(tc)
ratio at tc = 0.6 h: 0.282
R₅₀(tc) = 16 × 0.282 = 4.512 cm
Rainfall intensity I₅₀I₅₀ = R₅₀(tc)tc
I₅₀ = 4.5120.604 = 7.465 cm/h = 74.65 mm/h
Design discharge Q₅₀Q₅₀ = 0.278·C·I₅₀·A
Q₅₀ = 0.278 × 0.691 × 74.65 × 2.54 = 36.43 m³/s
This is the 50-year flood used for railway bridge waterway, scour depth and HFL.