Dynamic flood topographies in the Terai region of Nepal

Abstract

Flood hazard maps used to inform and build resilience in remote communities in the Terai region of southern Nepal are based on outdated and static digital elevation models (DEMs), which do not reflect dynamic river configuration or hydrology. Episodic changes in river course, sediment dynamics, and the distribution of flow down large bifurcation nodes can modify the extent of flooding in this region, but these processes are rarely considered in flood hazard assessment. Here, we develop a 2D hydrodynamic flood model of the Karnali River in the Terai region of west Nepal. A number of scenarios are tested examining different DEMs, variable bed elevations to simulate bed aggradation and incision, and updating bed elevations at a large bifurcation node to reflect field observations. By changing the age of the DEM used in the model, a 9.5% increase in inundation extent was observed for a 20-year flood discharge. Reducing horizontal DEM resolution alone resulted in a <1% change. Uniformly varying the bed elevation led to a 36% change in inundation extent. Finally, changes in bed elevation at the main bifurcation to reflect observed conditions resulted in the diversion of the majority of flow into the west branch, consistent with measured discharge ratios between the two branches, and a 32% change in inundation extent. Although the total flood inundation area was reduced (À4%), there was increased inundation along the west bank. Our results suggest that regular field measurements of bed elevation and updated DEMs following large sediment-generating events, and at topographically sensitive areas such as large river bifurcations, could help improve model inputs in future flood prediction models. This is particularly important following flood events carrying large sediment loads out of mountainous regions that could promote bed aggradation and channel switching across densely populated alluvial river systems and floodplains further downstream.

Geomorphic diversity of the Indian sub‐continent: Progress and updates

“…This is particularly important in highly aggrading and dynamic systems in the Himalayan foreland where large sediment flux derived from their active hinterland makes the river channel superelevated in several reaches (Sinha et al, 2014). Dingle et al (2020) have examined the role of dynamic topographies on flooding in the 3) elevation adjustment at bifurcations in addition to scenario 2 in accordance with field observations. Modelling results show a 9.5% increase in flood inundation extent by changing the DEM resolution from SRTM to TanDEM-X for depths greater than 0.5 m. However, resampling TanDEM-X (10 m) data to SRTM scale (30 m) resulted in a 1% decrease in inundation area.…”

“…This is particularly important in highly aggrading and dynamic systems in the Himalayan foreland where large sediment flux derived from their active hinterland makes the river channel superelevated in several reaches (Sinha et al, 2014). Dingle et al (2020) have examined the role of dynamic topographies on flooding in the Terai region of Nepal using a two‐dimensional (2D) hydrodynamic flood model. The authors use the term ‘dynamic flood topographies’ to emphasize such mobile alluvial landscapes.…”

Toward Improved Comparisons Between Land‐Surface‐Water‐Area Estimates From a Global River Model and Satellite Observations

“…Moreover, models are the only way to hindcast the water surface in the past before satellites were launched (Lewin & Hughes, 1980) and forecast the future changes when no observational results exist (Hirabayashi et al., 2013). Over the past two decades, several hydrodynamic models have been developed (e.g., LISFLOOD‐FP, HEC‐RAS, MIKE‐Flood, DELFT3D, and CaMa‐Flood) and tested under various conditions (Bates & De Roo, 2000; Dingle et al., 2020; Pappenberger et al., 2005; Patro et al., 2009; Yamazaki et al., 2011). For a detailed review of flood inundation models, refer to Teng et al.…”

“…DELFT3D, and CaMa-Flood) and tested under various conditions (Bates & De Roo, 2000;Dingle et al, 2020;Pappenberger et al, 2005;Patro et al, 2009;Yamazaki et al, 2011). For a detailed review of flood inundation models, refer to Teng et al (2017).…”

Environmental drivers of space-time dynamics in floodplain vegetation: grasslands as habitat for megafauna in Bardia National Park (Nepal)

“…From a hydrological perspective, there are indications that a major change occurred during monsoonal floods in 2009 (Sinclair et al, 2017): near the apex of the highly dynamic braided river system of the Karnali River, the dominant discharge channel relocated from its eastern branch, the Geruwa River that borders BNP, to the western branch (Kauriala River). The current distribution of discharge is considered to be about 80% for the Kauriala river and 20% for the Geruwa River during low discharges (Sinclair et al, 2017;Dingle et al, 2020a) and the distribution is thought to be higher in 85 the western branch (55-65%) during peak flow for monsoonal discharges (Dingle et al, 2017(Dingle et al, , 2020a. Reduced fluvial dynamics in the Geruwa River could possibly favour higher successional stages of vegetation in and near the floodplains at the western boundary of BNP.…”