Estimation And Analysis of Low, High and Mean Monthly Flow For Ungauged Manohara River

Abstract

Manohara River, an important tributary, of Bagmati River has a catchment area of66.34sq.km. with length andperimeter as 24 km and 47.25 km respectively. The formfactor is 0.115, elongation ratio as 0.38, the circulatory ratio as 0.37 and compactnesscoefficient is 1.5. These statistics relating to the shape of the catchment indicate that thecatchment is not symmetrical in shape. It has narrow width at the lower reach and has afan shape at the upper reach with relatively short river length and high relief of 1038meters. There is a possibility of flash flood with high intensity of monsoon rain which isfrequently experienced. The Manohara River is ungauged and this catchment has fertileagriculture fields supplying vegetables to Kathmandu city in all seasons. The settlement in this catchment is rapidly growing due to overcrowding of the nearbyKathmandu city. In the near future the agriculture-fields will require irrigation, thegrowing settlement will require more water supply and more link-roads have to beconstructed with many culverts and bridges. Since, no discharge data for Manohara riverwere available, low flows and high flows have been estimated using transposition datawith the help of Cudworth equation, as referred by WECS/DHM method. The dischargedata of Bagmati River at Khokana, Chovar and Sundarijal have been transposed for theManohara River forthe estimation of high flows and mean monthly flows. The low flowshave been estimated with WECS/DHM method using the catchment area with associatedparameters. The monthly minimum low flow for the return period of 2 year has beenfound to be 1m 3 /s. Similarly the monthly minimum flows have been calculated as 72 m /sand .56m 3 /s for the return periods of 10 and 20 years respectively. The one day minimumlow flow value for the return period of 2 years, 10 years and 20 years have beencalculated as 1m 3 /s, 0.65m 3 /s and 0.5m 3 /s. These values are very important for irrigationand domestic water supply projects that are likely to be implemented in the near future forthis ungauged river. The flow duration curve has also been constructed for this river withthe WECS/DHM method. The flow duration curve indicates that exceedence probabilityof 100% flow is only 0.14m 3 /s. Similarly exceedence probability of 20% and 60% are10m 3 /s and 2m 3 /s respectively. These values are also very important for irrigation projectsand drinking water projects in near future. Regarding high flood analysis for ManoharaRiver, discharge data were transposed to the Manohara River from Chovar and Khokanaand Sundarijal sites. Estimated floods for 5 years, 20 years, 50 years and 100 years have i 3 been calculated as about 201m 3 /s, 303 m 3 /s, 372 m ii 3 /s and 422 m 3 /s respectively. Thedischarge data of Bagmati River at Chovar and Khokana with transposed datamethodology for Manohara River gives reasonable estimated floods agreeing with thevalues estimated by WECS/DHM method. This is because the bridge site near Pepsi ColaFactory, Chovar site, and Khokana site all are situated in the flood plains of Kathmanduvalley, butSundarijal gauging site is located on the higher elevation near the origin ofBagmati River. The bridge site of Manohara near the Pepsi cola factory, Chovar site andKhokana site have same type of physiographic characteristics but Sundarijal site hascompletely different physiographic characteristics. Therefore, data from Sundarijal site isnot suitable for estimation of high flood for Manohara by data transposition method. The estimated high floods by transposition method will be useful for designing the bridgenear the Pepsi Cola Factory and many link roads with culverts in near future.The resultsfrom the discharge data transposition method for ungauged Manohara River which will beuseful for water resources project in the near future. Such techniques will also be usefuland can be applied for many ungauged catchments in various partsof the country.