Contents preface (VII) introduction 1—37

Table 2.5 Guidelines for selecting design floods (14)

S. No.	Structure	Recommended design flood
1.	Spillways for major and medium	(a) PMF determined by unit hydrograph and
	projects with storage more than 60 Mm^3	probable maximum precipitation (PMP)
		(b) If (a) is not applicable or possible, flood-
		frequency method with T = 1000 years to 5000
		years
2.	Permanent barrage and minor dams	(a) SPF determined by unit hydrograph and
	with capacity less than 60 Mm^3	standard project storm (SPS) which is usually
		the largest recorded storm in the region
		(b) Flood with a return period of 100 years.
		(a) or (b) whichever gives higher value.
3.	Pickup weirs	Flood with a return period of 100 or 50 years
		depending on the importance of the project.
4.	Aqueducts
	(a) Waterway	Flood with T = 50 years
	(b) Foundations and free board	Flood with T = 100 years
5.	Project with very scanty or inadequate	Empirical formulae
	data

The PMF is used for structures such as dams, spillways etc. whose failure would result in huge loss of life and property. Table 2.6 provides guidelines for this purpose (15).

Table 2.6 Design flood for dams

Size/class of dam	Gross storage	Hydraulic Head	Design Flood
	(Mm^3)	(m)
Small	0.5 to 10.00	7.5 to 12.0	100 – year flood
Medium	10.0 to 60.00	12.0 to 30.0	SPF
Large	more than 60.0	more than 30.0	PMF

90 IRRIGATION AND WATER RESOURCES ENGINEERING
EXERCISES

1. 
   There are five rain gauges uniformly spread in a small watershed. The depths of rainfall observed at these rain gauges and the area of Theissen polygons for the corresponding rain gauges are as follows :
   

Rain gauge no.	1	2	3	4	5
Rainfall depth (cm)	47.3	46.4	43.8	52.3	48.5
Area of Theissen polygon (1000 m^2)	95	102	98.67	80.52	85.38

Determine the average depth of precipitation.

2. 
   The amounts of rainfall for 6 successive days on a catchment were 2, 5, 7, 8, 5, and 1 cm. If the φ-index for the storm is 4 cm/day, find the total surface runoff.
   

2. 
   The normal annual precipitation at four rain gauge stations is 120, 100, 84, and 120 cm, respec-tively. During a particular storm the rain gauge at station A became non-functional. The rainfall depths recorded at B, C, and D are respectively, 10, 7 and 11 cm. Estimate the rainfall depth at station A during the storm.
   

2. 
   In a drainage basin, there exist 6 rain gauges, and average rainfall at these stations is 106, 91, 65, 55, 62, and 48 cm. Determine if these rain gauges are adequate to give reliable measure-ments of rainfall with an error of less than 10% ? If not, how many additional rain gauges are needed ?
   

2. 
   The ordinates of three unit hydrographs derived from separate storms of 4-hour rains each are as follows :
   

Hours	Storm 1	Storm 2	Storm 3	Hours	Storm 1	Storm 2	Storm 3
0	0	0	0	8	135	165	214
1	115	20	16	9	100	125	164
2	370	120	57	10	70	85	121
3	505	353	173	11	45	55	90
4	395	460	335	12	27	30	60
5	315	400	442	13	15	15	35
6	240	300	400	14	10	3	16
7	175	215	283	15	0	0	0

Find the average unit hydrograph.

6. 
   The observed stream discharges resulting from a storm of 3-hr duration are as follows :
   

Hour	Day 1	Day 2	Day 3	Hour	Day 1	Day 2	Day 3
3 a.m	550	4500	1800	3 p.m.	8000	2600	1000
6 a.m	500	3900	1600	6 p.m.	7000	2300	900
9 a.m	6000	3400	1400	9 p.m.	6000	2000	800
Noon	9500	3000	1200	Mid-night	5200	1800	700

The drainage area of the stream is 2000 km². Derive the unit hydrograph assuming constant base flow of 500 m³/s.

7. 
   For storm 1 of Exercise 2.5, obtain the S-hydrograph and derive the 2-hr and 6-hr unit hydrographs.
   

HYDROLOGY

91

REFERENCES

1. 
   Mays, L.W., Water Resources Engineering, John Wiley and Sons Inc., Singapore, 2004.
   

2. 
   Kurino T. (1997), A Satellite Infrared Technique for Estimating ‘‘deep/shallow’’ precipitation, Adv. Space Res., Vol. 19, No. 3, pp. 40-63.
   

3. ...... Guide to Hydrometeorological Practices, 3rd edition, WMO no. 168, Geneva, 1974.

4. ......Bureau of Indian Standards (IS: 8389-1983); IS Code of Practice for Installation and Use of Rain gauges and Recording.

5. 
   Kohler, M.A., Double—Mass Analysis for Testing Consistency of Records and for Making Required Adjustments, Bulletin of American Meteorological Society, Vol. 30, no. 5, May 1949.
   

5. 
   Dhar, O.N. and A.K. Kulkarni, Estimation of Probable Maximum Precipitation for Some Selected Stations in and Near Himalayas, Proc. of National Symposium on Hydrology, Roorkee, India 1975.
   

5. 
   Weisner, C.J., Hydrometeorology, Chapman and Hall, London, 1970.
   

5. 
   Subramanya, K., Engineering Hydrology, Tata McGraw-Hill Publishing Company Ltd., New Delhi, 1994.
   

5. 
   Linsley R.K. (Jr.), Kohler, M.A., and Paulhus, J.L.H., Hydrology for Engineers, McGraw-Hill Book Co. 1988.
   

5. 
   Sherman, L.K., Streamflow from Rainfall by the Unit-Graph Method, Engg. news Record, Vol. 108, pp. 501-505, 1932.
   

11. ...... Flood Estimation Report for Lower Godavari (sub-zone 3F), Design Office report no. 3/1980, Directorate of Hydrology (Small Catchments), Central Water Commission, New Delhi, 1980.

12. ...... Flood Estimation Report for lower Narmada and Tapi (sub-zone 3B), Design Office report no. M/5//1981, Directorate of Hydrology (Small Catchments), Central Water Commission, New Delhi, 1980.
13. ...... Flood Estimation Report for Mahanadi (sub-zone 3D), Design Office report no. LNT/4/1981, Directorate of Hydrology (Small Catchments), Central Water Commission, New Delhi, 1982.
14. ...... Estimation of Design Flood Peak, Report no. 1/73, Central Water Commission, New Delhi 1973.
15. ...... IS: 11223-1985—Guidelines for fixing Spillway Capacity, Bureau of Indian Standards.