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Rare flood scenarios for a rapidly growing high-mountain city: Pokhara, Nepal

Abstract. Pokhara (ca. 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
3 decades. Construction materials are in high demand in rapidly expanding built-up areas, 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 (7555 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 a sudden 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 flood scenarios using the hydrodynamic model HEC-RAS (Hydrologic Engineering Center’s River Analysis System).
We simulate the potential impacts of peak discharges from 1000 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 2 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 3- and 20-fold, thus likely raising local
flood exposure well beyond changes in flood hazard. Besides these drastic
local changes, about 1 % of Pokhara's built-up urban area and essential
rural road network is in the highest-hazard zones highlighted by our flood
simulations. Our results stress the need to adapt early-warning strategies
for locally differing hydrological and geomorphic conditions in this rapidly
growing urban watershed.
