MAUSAM,49, 2(Apil I998l, 211·216

55I.m.36 : 556.51

Probable maximum precipitation for the catchment of Koyna dam

P.R. RAKHECHA, A.K.KULKARNI, B.N. MAND AL,R.B. SANGAM and N.R. DESHPANDE

Indian Institute ofTropical Meteorology, Pune-s l 1008, India

(Received J April 1996, Modified 19 March 1997)

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ABSI1tAcr. Eitimares of Probable MuinNm Precipitation (PMP) for different durati ons were: made for lhc:e:ateIunent above Koyna dam 00 the Kayna river. The cnlchment spans an area of 892 km2and the PMP esnmetes were nlo'ldefor . range of durations of I to 3 dayl .ThePMPestimates for Kayna dam were found to be48,87 and 117em by l.he physicalmethod and 54.89 and 124em by statistical method for 1. 2 and 3 day respect ively. These estimates can be used to check (heexisti"l spillway desian flood of Koynadam.

ICeJ _ - Probable maximum procipillllioo (PMP), Spillway desian flood.

I. Introdudlon

The Probable Maximum Precipitation (PMP) studyover the catchment of the Koyna river above Koyna dam(sec Fig, I) with a total area of 892 km2has been carried out.The objective of this study is to provide estimates of PMPdepths for duratio ns of I to 3 days over the catchment so thatthese values can be used to check the adequacy of the currentspillway design flood (SDF) of Koyna dam.

2. Koyna dam and need for revle.. of SDF

Koyna dam on the river Koyna for hydropower genera ­tion was built near Helwak village in Salata distri ct ofMaharashtra state during 1956-64 . The project supplieselec tric energy to Bombay · Pune area and the adjoiningdistricts. The dam is ':,Pproximalel y 103 m high with acapacity of 2796 x 10 m3. The spillway is designed todischarge 7845 m3,s which was com puted using empiricalformu la. The Koyna catchment though has a comparativelysmall drainage area but large national economic imponance.

Since the Koyna dam was desig ned in 1960's, themethod used to estimate SDF has greatly changed with theadvent of hyd rometeorolog ical techniques, The ewe

(1972) has given ~uidel ines that all large darns with storagesmore than 60x 10 m3 and hydraulic heads greater than 30mshould be designed for the Probable Maximum Flood (PMF)resulti'l,8 from the PMP and dams with storages less than60x 10 m) for standard projec t flood , The PMP den otes theestimated largest possible rainfall thaI will occur for a givenduration over a given area . It is derived by a physicall y basedmethod using daily rainfall records over a long period oftime at several stations in and around the catchment, supple­mented with meteorol og ical conditions. Keeping in view thefact that the existing spillway capacity of Koyna dam is notin accordance with the hydrometeorological method. thedam design authorities as also the ewe ~ave felt the needto carry out comprehensive investigations into the adequacyof the current spillway capacity of Koyna dam . The purposeof the investigation is to incorporate a spillway capable ofpassing the PMF.

3. Topography of th e Koyna river catchment

The Koyna river catchment is situated in the WesternGha ts approximately between the latitudes 17°20' and17°55' N and longitudes 73°35' and 73°55' E (Fig. I). The