Hydro-torrential hazard vs. anthropogenic activities along the Seti valley, Kaski, Nepal: Assessment and recommendations from a risk perspective

Abstract

The Seti River originates from the Annapurna Massif in the Higher Himalaya of Nepal and flows through the Pokhara valley in the Lesser Himalaya. The Seti River witnessed a disastrous flash flood on May 5th, 2012 causing the death of 72 people, obliterating dozens of homes and damaging infrastructures worth millions of dollars. Despite the 2012 flood event and several warnings by scientists for more yet bigger scale future floods in the Seti valley, fluvial risk is being aggravated by anthropogenic activities such as unplanned human settlement, encroachment of riverbanks, haphazard construction of road, drinking water, and hydropower projects in potential flood hazard areas in addition to the increased impacts of climate change on geological and hydro-metrological hazards as in other parts of Hindu Kush Himalayan Range. Covering some 40-km distance from the Seti headwater (Sabche Cirque) down to Pokhara city, the study is carried out based on hydro-geomorphological mapping, analysis of land-use and land-cover change, hydrological analysis including HEC-RAS modelling, historical archives, and interviews with local people. The study shows a significant change on the land use and land cover of the Seti catchment, mainly the urban/built-up area, which is increased by 405% in 24 years period (1996 to 2020) and by 47% in 7 years period (2013 to 2020). Further the study reveals that anthropogenic activities along the Seti valley have increased fluvial risk and are likely to invite more disasters. From the HEC-RAS analysis, two motor bridges built over Seti River were found to have insufficient freeboard to safely pass the highest flood discharge for 100 years return-period. Instead of relocating people to safer places, the government and local authorities rather seem to have encouraged people to live in the floodplain by providing basic amenities such as drinking water, electricity and access road. Given the context of climate change and Pokhara valley and the Seti catchment being in a high-seismic gap zone, there is a strong possibility of similar flood to the scale of 2012 or even greater in Seti River. Though the fluvial risk can be managed in a sustainable way through the application of functional space concept, i.e., by allowing more space (freedom) for rivers, this economic and environment friendly approach of the fluvial risk management has not been implemented yet in the Seti valley nor in Nepal. Rather the encroachment of floodplains by anthropogenic activities along the Seti valley is on an increasing trend. Many settlements and infrastructures along the valley have been identified vulnerable to hydro-torrential hazards, therefore it is utmost necessity to implement functional space river concept, land use and land plan policy, early warning system and public awareness education in order to mitigate and manage the future impact of fluvial hazards along the Seti valley.

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

“…Our results offer a first comprehensive set of outburst flood scenarios along the Seti Khola, and thus expand on previous studies of a more qualitative (Rimal et al, 2015(Rimal et al, , 2018 or local focus (Gurung et al, 2021;Thapa et al, 2022). By intersecting modelled inundation extents with spatial data on individual buildings and the route network, we are able to outline relative hazard zones on the assumption that smaller outburst flood magnitudes are more frequent than larger ones.…”

“…Further downstream, our mapping of the recent spread of informal settlements along the Seti Khola at Kaseri and Yamdi between 2008 and 2021 substantiates residence interviews about this rapid growth (Gurung et al, 2021). We found that unregulated sand and gravel extraction at Yamdi has, like in many other Nepalese rivers, increased in past decades, but has especially accelerated after large amount of sediments were deposited by the May 2012 flood (Dahal et al, 2012;Fort et al, 2018).…”

“…Even larger outburst floods may have occurred in the Seti Khola in Medieval times (Fort, 1987;Schwanghart et al, 2016). Yet appraisals of flood hazard have so far largely focused on the 100-year meteorological flood as estimated from rainfall data (Basnet and Acharya, 2019;Gurung et al, 2021). The associated risk estimates rely on land use and land cover (LULC) mapping and projections from Landsat data (Gurung et al, 2021;Rimal et al, 2015Rimal et al, , 2018, floodplain mapping (Rimal et al, 2015(Rimal et al, , 2018, and hydrodynamic modelling for selected reaches (Basnet and Acharya, 2019;Gurung et al, 2021).…”

“…Previous studies of HEC-RAS for outburst floods have used mostly coarser digital elevation data than the 5-m ALOS DEM we used here (Mergili et al, 2011;Somos-Valenzuela et al, 2014;Wang et al, 2018;Zhang and Liu, 2015). The stereo satellite imagery forming the basis for this DEM was acquired between 2006 and 2011 and excludes channel changes by the May 2012 flood (Gurung et al, 2021). We minimised potential resulting effects on our models by manually adjusting cross-sections with our additional field-surveyed elevation data.…”

“…Apart from annual monsoonal floods, this river has a history of rare outburst floods. On May 5, 2012, a hyperconcentrated flood killed 72 persons and destroyed roads, bridges, and drinking water pipelines in the northern Pokhara valley (Gurung et al, 2015(Gurung et al, , 2021. The exact sequence of events remains debated, but is likely to have been initiated by rock-slope failures on the western flank of the Annapurna IV massif at 7,525m a. s. l., observed by chance by a pilot (Hanisch et al, 2013;Kargel et al, 2013).…”