Optimizing Regional Flood Frequency Analysis for Floodplain Mapping and Risk Reduction

ABSTRACT

The estimation of flood hazards in river basins under monsoon conditions and data‐scarce situations is a challenging task due to the scarcity of the streamflow measurements and the high variability of the distribution of the rainfall. This paper provides a detailed framework for flood modeling and inundation mapping for a specific sub‐catchment of the Cauvery River in Kushalnagara taluk, Kodagu District, Karnataka, India. The catchment area of 1144.56 km ² and the outlet is situated at Mysore–Kodagu bridge (12°26′54.69″ N, 75°58′13.32″ E). Various datasets were used such as IMD–CWC rainfall records (2000–2019), a 30 m SRTM DEM from USGS, streamflow data and land‐use information to analyze the physiography and hydrological behavior of the basin. Design floods were estimated with regional flood frequency analysis techniques (Gumbel and Log‐Pearson Type III) and the Rational method and HEC‐HMS rainfall‐runoff modeling. A 1‐h Synthetic Unit Hydrograph was established for five sub‐basins obtained from the DEM and the resultant hydrographs were routed using Muskingum routing and HEC‐RAS to generate hydraulic inundation maps within a GIS framework. The model had virtuous precision with NSE of 0.88 and bias of ∼5%. HEC‐RAS–GIS outcomes showed that the simulated peak flood discharge of 5064.7 m ³ /s caused the inundation of approximately 51.2 km ² along a 42.7 km river reach, with flood depths more than 7 m near Keralapura Bridge. Most of the areas affected were low‐lying farms and homes. The proposed RFFA‐SUH‐HEC‐HMS‐Muskingum‐HEC‐RAS framework provides useful flood risk information for floodplain zoning, vulnerable zone identification and planning for future flood risks in monsoon affected basins with limited data.