Surface Hydrology | Tutorial – Unit Hydrographs | Date: 09/11/2020
1. The IUH of a basin can be approximated to a triangle of base 36 hr and peak of 30 cumec at 9
hr from the start. Derive a 3-hr UH for this basin.
2. The 30-min unit hydrograph for a 2.25 km2 catchment is given. Calculate the 40-min unit
hydrograph for the catchment using S-curve method, and verify that your calculated unit
hydrograph correspond to a 1-cm rainfall excess.
3. The following information for watershed A and its two-hour unit hydrograph has been
determined: area = 259 sq. km, Lc = 16.1 km, L =38.6 km, tR = 2 h, tpR = 6 h, Qp = 107.5
cumecs/cm, W50 = 4.1 h, and W75 = 2 h. Watershed B, which is assumed to be hydrologically
similar to watershed A, has the following characteristics: area = 181.3 sq. km, L = 25.1 km, and
Lc = 15.1 km. Determine the one-hour synthetic unit hydrograph for watershed B.
4. The watershed with area of 101.8 sq. km; the length of the main channel is 29.1 km, and the
main channel length from the watershed outlet to the point opposite the centroid of the
watershed is 9.7 km. Using Ct = 2.0 and Cp = 0.625, determine the standard synthetic unit
hydrograph and the two-hour unit hydrograph for this watershed.
5. Construct a 10-minute SCS unit hydrograph for a basin of area 3.0 km2 and time of
concentration 1.25 h.
6. A triangular synthetic unit hydrograph developed by the Soil Conservation Service method
has Qp = 32 cumecs/cm, Tp = 50 min, and tr = 10 min. Compute the direct runoff hydrograph
for a 20-minute storm, having 1.7 cm rainfall in the first 10 minutes and 4.3 cm in in the second
10 minutes. The rainfall loss rate is 1.52 cm/h throughout the storm.