Rural earthen roads impact assessment in Phewa watershed, Western region, Nepal

Abstract

Background: This work describes current research being conducted in the Phewa watershed, near Pokhara in Nepal's Middle hills, a moist sub-tropical zone with the highest amount of annual rainfall in Nepal (4,500-5,000 mm). The main purpose of this study is to focus on the increase and impact of rural earthen road construction in the Phewa watershed as part of land use changes over 30 years in one of Nepal's most touristic regions. Research methods were interdisciplinary and based on a combination of remote sensing, field observations and discussions with community members. The study compared 30 year-old aerial photos with current high resolution satellite images to detect changes in the watershed road network. Secondly, 42 % of the watershed's roads (138 km) were surveyed in order to inventory and quantify shallow landslide events. Using an erosion-characteristics grid, their main features were measured (location, size, type and dimensions of damaged areas, etc.) and a Geographic Information System data base was created. We then estimated economic impacts of these events in terms of direct agriculture lands losses and road maintenance. Results: Results of the remote sensing analysis demonstrate that the road network increase is following an exponential trend, which enables us to predict future watershed road network scenarios. Field work investigations have demonstrated that unplanned road excavations are producing mainly embankment shallow planar soil slides and/or gullying which primarily affect roads themselves, wiping them out and blocking vehicle circulation, and secondly, destroying or burying agriculture lands. Along the 138 km surveyed, we calculated an amount of soil material released of about 70,000 cubic meters, which amounted to 99 cubic meters per kilometer annually. Of 179 cases of roadside erosion processes sampled, about 85 % directly impact roads or agricultural lands. Conclusion: The current mode of road construction which is currently occurring in Nepal is largely related with erosion and shallow landslide processes. Considering the exponential growth of rural earthen road networks, we would expect an increase of sediments released by roads and serious consideration must be taken if roads continue to be made without more careful methods. Through simple technologies using low cost and local resources along the lines of 'green road' or 'eco-safe road' approaches, it may be possible to reduce the impacts of rural road construction.

Shrinkage of Nepal’s Second Largest Lake (Phewa Tal) Due to Watershed Degradation and Increased Sediment Influx

“…The construction of the Pokhara-Baglung Highway in the upper part of the catchment (1988)(1989) preceded this period of lake-area decrease. A rapid and accelerating increase in rural road construction in the high-relief upper catchment is amplifying landslide activity [4], which is projected to increase the rate of sediment delivery into Phewa Lake. Vuillez et al (2018) found that, following extreme rainfall in July 2015, 40% of newly triggered landslides intersected a road and 84% were within 40 m of a road.…”

“…Landslides triggered by heavy rain or earthquakes often lead to fatalities in the monsoon-dominated high-relief Himalayan landscape [1][2][3], and urban expansion is increasing socio-economic exposure to landslides. Additionally, rural road-building without proper geotechnical considerations is associated with an upward trend in landslide occurrences and sediment mobilisation in Nepal [2,[4][5][6]. The highest rates of sediment transport occur during the monsoon, where pore water pressures are high and landslide activity peaks [7,8].…”

Development of Monsoonal Rainfall Intensity-Duration-Frequency (IDF) Relationship and Empirical Model for Data-Scarce Situations: The Case of the Central-Western Hills (Panchase Region) of Nepal

“…There are four distinct hydrological seasons: pre-monsoon (April and May: AP), monsoon (June to September: JJAS), post-monsoon (October and November: ON) and winter or dry period (December to March: DJFM) in the country [46]. The monsoonal rainfall is highly variable over the country (~1000 mm-~4500 mm) and is intense in nature where more than 80% of the annual rainfall occurs in the 4 months of the monsoon [18,47], resulting in landslides, debris flows, flooding and sedimentation, threatening livelihoods and properties in numerous ways [48].…”

“…In order to evaluate the rainfall variability and to establish the IDF relationship of the region, the historical rainfall data of 11 weather stations were collected from the DHM (Table 1). The region is one of the most studied areas in the country [48][49][50][51][52][53] due to the importance of Phewa Lake that promotes economic activities and biodiversity. However, the study of rainfall patterns, their intensity and impact have not been properly addressed yet.…”

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“…In rural Nepal, there has been a prolific growth of earthen road expansion in recent years. Due to the mountainous topography, steep slopes, and weak soils, these poorly constructed rural roads have increased the probability of landslides especially during the heavy monsoonal rainfall (Leibundgut et al, 2016). Therefore,…”