A geomorphic approach for floodplain zonation in an anthropogenically impacted reach of the Narmada River, Central India

Modern floodplain ecosystems, despite being valuable resources, are increasingly threatened by urban and infrastructural development, particularly in their riparian areas. Therefore, monitoring floodplain ecosystems and processes is crucial for their protection and sustainable development. This study uses a topographic and geomorphic approach to carry out floodplain zonation along a 22 km urban stretch of the Narmada River in Central India. This reach is located about 20 km downstream of the Bergi dam- a large major anthropogenic intervention on the Narmada River. The Narmada River valley in the study reach is partly confined, and is marked by a litho-geomorphic complex region that includes ravine topography and dynamic fluvial systems. The river flows through a transition zone, the channel changes from a fractured bedrock riverbed (FBR) to a mixed or sediment-covered bedrock riverbed (SBR). We use satellite images (1965-2024; e.g., Corona, Landsat, ASTER, Sentinel, Planetscope), drone images, high resolution topographic data, field measurements for flood zonation. We employed a drone to capture inaccessible parts of the channel and floodplain, and Differential GPS (DGPS) for topographic measurements of the channel, banks, and floodplain. Acoustic Doppler Current Profilers (ADCP) was used to measure the bathymetry and flow velocity of the river. Anecdotal information about the past flood and associated inundation in the Narmada River was collected from the local population. The stretch experienced the largest flood event in the year 1991 as all spillway gates were opened to enable downstream movement of floodwater. Based on the geomorphic and topographic analysis, we demarcate and validate flood zones (No Go Zone, Regulatory Zone, Warning Zone). These zones exhibit a good match with flood extent attained historically by Narmada River. However, minor mismatches are noted, in places. We observed, through mapping and surveys, that the anthropogenic modifications have altered the geometry, slopes, sediment supply, riverbank heights, riparian vegetation, floodplain land use, bank stability, and natural flow patterns. Ex post facto analysis highlights those alterations due to the construction of the Bergi dam, rapid urbanization, infrastructure developments, and activities like agriculture, riverbed mining, and quarrying leading to anthropogenic pressure on the Narmada riverscape.  These activities cause several disturbances in the channel and floodplain domains, for example, the agriculture activities disrupt the banks and gullies due to removal of riparian vegetation- a key factor for bank stability. We propose strategies for restoring natural flood pulses, stabilizing riverbanks, and improving flood risk management through measures that protect the riparian vegetation, bank gullies; as well as the implementation of an effective flood zonation policy. This study emphasizes the importance of geomorphic and topographic mapping and analysis for adapting effective restoration strategies in river management in order to manage and counteract anthropogenic pressures in the context of a part of the Narmada River in central India.

Keywords: Floodplain zonation, Geomorphic analysis, Anthropogenic interventions, Urban reach, Narmada River