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Catchment scale flood management through spatial targeting: a case study of the East Rapti catchment, Nepal

Callum Pearson, Sim M Reaney, Nick J Rosser and Andy R Large

AGU Fall Meeting, 2019

Practical approaches for managing flood hazards are moving away from mitigation solely at the point of the impact, and towards an integrated catchment management approach which considers flood source areas, flow pathways of flood waters and impacted communities. Our three stage approach acknowledges this and, through developing understanding of the spatial and temporal complexity of flood events at the catchment scale, tests sustainable flood management solutions. Our approach adopts SCIMAP-Flood, a catchment scale decision support framework that has the advantage in these remote areas of only requiring limited data input. SCIMAP-Flood identifies critical source areas for flood waters using a combination of mapping flood water generation areas based on spatial rainfall patterns and land cover, the incorporation of travel times across a catchment, and modelling of hydrological connectivity. Outputs are used to create catchment scale flood management scenarios which target flood source areas.

These scenarios are then tested using CRUM3, a physically-based, spatially distributed, catchment scale hydrological model. Sustainable flood management measures including targeted afforestation and abandoned terrace restoration are implemented in the areas identified as floodwater generating. The impact of the flood management measures can be evaluated throughout the catchment using the modelled change in discharge.

The final stage uses LISFLOOD-FP, a 2D flood inundation model, to establish the flood extent at key areas within the catchment from the created flood management scenarios. At this stage cost-benefit analysis of a given scenario can be undertaken with the potential impact on the flooded community fully understood.

The approach has been applied to East Rapti catchment in Nepal. The 3,084 km2 catchment is representative of a typical medium-sized Nepalese catchment with monsoonal flood waters originating in the mountainous regions impacting on the more densely populated Terai plains to the south. The research identifies locations where flood management measures would be optimum to get positive effects in multiple flood impacted areas across the catchment. Based on the combined modelling approach, we propose strategies to reduce dependency on hard engineering solutions for flood protection.