The Manning equation though developed for uniform flow conditions has been used extensively for the estimation of average velocities or discharges in mountain rivers. Despite the availability of a few excellent guidelines in the form of photographs particle size (d) data etc. the selection of an appropriate value of Manning’s roughness Coefficient (n) without considerable experience becomes quite difficult. This difficulty to a larger extent has been overcome in the Jarret’s model (1984) for n which is based on easily onsite measurable parameters such as water slope(s) and hydraulic depth (D) or radius (R) neglect of bed-material size data and providing variation in n with change in depth.
In this paper the prediction ability of Jarret’s model has been tested with the aid of data from 71 field measurements on 32 mountain rivers with 0.0020 <S <0.0373, 0.10 m< D <2.09 m, and 0.43 <D/d84<.24.6. The average standard error of estimate obtained as 29%- quite comparable to spme of the highly technical predictors - suggests a wider use of Jarret’s model.
Further using the slope-area method and n values from Jarret’s model the predicted discharges have been compared with those measured at sites. Since for the 71 data points errors in predicted discharge are -3 °A on average a suggestion has been made to give a fair trial to his methodology in the estimation of water resource from ungauged mountain rivers.
Flow Resistance, Slope-Area Methods, Mountain Stream Gauging, Relative Roughness/Manning’s Roughness Coefficient.
