Outburst flood scenarios and risks for a rapidly growing high-mountain city: Pokhara, Nepal

Abstract

Abstract. Pokhara (c. 850 m a.s.l.), Nepal’s second largest city, lies at the foot of the Higher Himalayas and has more than tripled its population in the past three decades. Rapidly expanding built-up areas are high in demand for construction materials and several informal settlements cater to unregulated sand and gravel mining in the Pokhara valley’s main river, the Seti Khola. This river is fed by the Sabche glacier below Annapurna III (7,555 m a.s.l.), some 35 km upstream of the city, and traverses one of the steepest topographic gradients in the Himalayas. In May 2012 an outburst flood caused > 70 fatalities and intense damage along this river and rekindled concerns about flood-risk management. We estimate the flow dynamics and inundation depths of outburst flood scenarios using the hydrodynamic model HEC-RAS. We simulate the potential impacts of peak discharges from 1,000 to 10,000 m3 s-1 on land cover based on high-resolution Maxar satellite imagery and OpenStreetMap data (buildings and road network). We also trace the dynamics of two informal settlements near Kaseri and Yamdi with high potential flood impact from RapidEye, PlanetScope, and Google Earth imagery of the past two decades. Our hydrodynamic simulations highlight several sites of potential hydraulic ponding that would largely affect these informal settlements and sites of sand and gravel mining. These built-up areas grew between three and twentyfold, thus likely raising local flood risk well beyond changes in outburst hazard. Besides these drastic local changes, about 1 % of Pokhara’s urban built-up area and essential rural road network is in the highest hazard zones highlighted by our outburst simulations. Our results stress the need to adapt early-warning strategies for locally differing hydrological and geomorphic conditions in this rapidly growing urban watershed.