Preprints
https://doi.org/10.5194/nhess-2022-64
https://doi.org/10.5194/nhess-2022-64
 
03 Mar 2022
03 Mar 2022
Status: this preprint is currently under review for the journal NHESS.

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

Melanie Fischer1, Jana Brettin1, Sigrid Roessner2, Ariane Walz1, Monique Fort3, and Oliver Korup1,4 Melanie Fischer et al.
  • 1Institute of Environmental Science and Geography, University of Potsdam, Potsdam, Germany
  • 2Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany
  • 3Département de Géographie, Université Paris-Cité, Paris, France
  • 4Institute of Geosciences, University of Potsdam, Potsdam, Germany

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.

Melanie Fischer et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on nhess-2022-64', Adam Emmer, 14 Apr 2022
  • RC2: 'Comment on nhess-2022-64', Anonymous Referee #2, 15 Apr 2022

Melanie Fischer et al.

Melanie Fischer et al.

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Short summary
Nepal’s second largest city is rapidly growing since the 1970s although its valley has been affected by rare, catastrophic outburst floods in recent and historic times. We analyse potential impacts of such floods on urban areas and infrastructure by modelling ten physically plausible flood scenarios along Pokhara’s main river. We find that hydraulic effects would largely affect a number of squatter settlements, which have expanded rapidly towards the river by a factor of up to 20 since 2008.
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