Hydrology

The most popular methods used for computation of peak runoff rate in Nepal are:

(a) Rational Method The most widely used method for small catchments (generally less than 50 km², common in Nepali hill watersheds and urban drainage design). Peak discharge is computed as:

$Q_p = C \cdot i \cdot A$

where $C$ = runoff coefficient, $i$ = intensity of rainfall (mm/hr) for a duration equal to the time of concentration, and $A$ = catchment area. It is popular because of its simplicity and minimal data requirement.

(b) Empirical Flood Formula Methods Used where rainfall and streamflow records are scarce. The common ones in Nepali practice are:

• Dickens Formula: $Q = C \cdot A^{3/4}$ (A in km², C varies with region)
• Ryves Formula: $Q = C \cdot A^{2/3}$
• Inglis Formula: developed for the Bombay region, widely applied to Indian and Nepali catchments
• Fuller's Formula: $Q_{max} = C_f \cdot A^{0.8}(1 + 0.8 \log_{10} T)$

(c) Unit Hydrograph Method Used for medium-sized catchments where rainfall and runoff records are available. A unit hydrograph is derived from observed storm data and then used to compute peak runoff for design storms of various return periods. The Snyder's Synthetic Unit Hydrograph and SCS (Soil Conservation Service) Unit Hydrograph are commonly adopted for ungauged Nepali catchments.

(d) Frequency Analysis Method Used for gauged catchments. Observed annual peak flows are fitted to probability distributions such as Gumbel's Extreme Value Distribution, Log-Pearson Type III, or Log-Normal Distribution, and the peak for a given return period (e.g., 25, 50, 100 years) is estimated. This is the standard approach for major river basins in Nepal.

(e) SCS Curve Number Method Used to estimate runoff volume from rainfall using the curve number (based on soil group and land use), which is then routed to obtain the peak rate. Popular for watershed management and small dam design in Nepal.

(f) Regional Flood Frequency Methods Developed for ungauged catchments by regionalization of flood data — e.g., the WECS/DHM method and the HEC-HMS based regional approach used in Nepal for design of hydropower and irrigation structures.