Community Assessment of Flood Risks and Early Warning System in Ratu Watershed, Koshi Basin, Nepal

Nepal is highly vulnerable to flood-related disasters which cause considerable loss of lives and property. The vulnerability of communities to flood-related hazards can be reduced by proper planning, preparedness, and responses using various structural and nonstructural measures. The community-based flood early warning system is one such tool that enables local communities to enhance their resilience to flooding risks. This paper highlights the efficacy of the community assessment of flood risks and early warning systems. Using qualitative and quantitative methods, this paper evaluates the progress of a community-based flood early warning system implemented in the Ratu River—a small tributary of the Koshi River. The establishment of a community network in 2015 was instrumental in the dissemination of flood early warning information and in building local capacities to understand the risks and take timely action. The flood early warning resulted in awareness-raising, strengthened upstream–downstream linkages, and resulted in a greater willingness among communities to help each other prepare for flood disasters in the Ratu watershed.

The Saptakoshi high dam project and its bio-physical consequences in the Arun river basin

The big project called Saptakoshi high dam is a bilateral project of Indian and Nepalese government under the Koshi agreement. At present, high dam issue is being the great issue especially raising by the inhabitants of upstream in Koshi basin. Therefore, this research paper attempts to examine the bio-physical consequences due to high dam in the upstream of Arun river basin. Geographic Information System (GIS) and Remote Sensing (RS) technology have been used for the spatial analysis to prepare this paper. Spatial data have been taken from topographical map and Digital Elevation Model (DEM) and the attribute data have taken from Central Bureau of Statistics (CBS), Kathmandu. It revealed that approximately 11777 hectors arable plain land will be inundated in Arun river basin only by water with river deposits due to high dam if the high dam will be built at the proposed place and proposed dam height. The proposed place lies 1.6 km upstream from Barahakshetra temple of Sunsari district and the dam height will 269 meters. Similarly, more than 10000 population might be displaced in future. There is a strong sense of fear and sadness among the people of affected communities. Till now, they have unknown about the clear information about high dam. As a result, they are sometimes demonstrating against Saptakoshi high dam. So, government should inform and ensure the alternative management of the concerned people in the initial stage to manage the future disputes.

Glacial Lake Inventory and Lake Outburst Flood/Debris Flow Hazard Assessment after the Gorkha Earthquake in the Bhote Koshi Basin

Glacial lake outburst floods (GLOF) evolve into debris flows by erosion and sediment entrainment while propagating down a valley, which highly increases peak discharge and volume and causes destructive damage downstream. This study focuses on GLOF hazard assessment in the Bhote Koshi Basin (BKB), where was highly developed glacial lakes and was intensely affected by the Gorkha earthquake. A new 2016 glacial lake inventory was established, and six unreported GLOF events were identified with geomorphic outburst evidence from GaoFen-1 satellite images and Google Earth. A new method was proposed to assess GLOF hazard, in which large numbers of landslides triggered by earthquake were considered to enter into outburst floods enlarge the discharge and volume of debris flow in the downstream. Four GLOF hazard classes were derived according to glacial lake outburst potential and a flow magnitude assessment matrix, in which 11 glacial lakes were identified to have very high hazard and 24 to have high hazard. The GLOF hazard in BKB increased after the earthquake due to landslide deposits, which increased by 216.03 × 106 m3, and provides abundant deposits for outburst floods to evolve into debris flows. We suggest that in regional GLOF hazard assessment, small glacial lakes should not be overlooked for landslide deposit entrainment along a flood route that would increase the peak discharge, especially in earthquake-affected areas where large numbers of landslides were triggered.

Role of supraglacial lakes in recession of Himalayan glaciers: A case study of Dudh Koshi basin, Nepal

Climate change has been adversely affecting glaciers causing them to advance and recession worldwide. Existing studies have primarily attributed temperature as the leading factor causing glacier recession. However, detailed studies that investigate effect of other factors like presence of debris cover, slope, and contact with water bodies are still scarce. This research, thus investigated the role of supraglacial lakes in recession of debris-covered glaciers (DCG). Such glaciers were studied since these lakes are found in debris-covered glaciers only. For this purpose the interannual variation in area of supraglacial lakes of Dudh Koshi basin was computed to test the hypothesis that these lakes play a significant role in glacier recession. Supraglacial lakes were delineated using Google Earth Pro at five year intervals to assess interannual variation in lake area. Slope, elevation and change in supraglacial lake area were the predictors influencing average decadal change in area of glaciers. Two models prepared using multiple linear regression in Excel were compared. The first model used elevation and slope as predictors while the second model used change in supraglacial lake area as the additional predictor. The second model had a higher coefficient of determination (R square) and Adjusted R-square values of 99 % and 96 % compared to the first model. Further test statistics from Analysis of Variance (ANOVA) results were compared to test the hypothesis. Moreover the Root mean square error (RMSE) of second model was also less than the first one. Hence both the regression statistics and RMSE confirmed that change in area of supraglacial lakes was an important factor that influences overall recession of debris-covered glaciers. Nevertheless, use of high spatial and temporal resolution imageries along-with increase in number of glaciers sampled should be incorporated in future studies to ensure robust outcomes. Thus this research can bolster the overall understanding between glacier and glacial lake dynamics which will improve the resilience of downstream inhabitants from climate induced hazards, such as glacial lake outburst floods (GLOFs).

Spatial Distribution and Temporal Change of Extreme Precipitation Events on the Koshi Basin of Nepal

Climate change, particularly at South Asia region is having a huge impact on precipitation patterns, its intensity and extremeness. Mountainous area is much sensitive to these extreme events, hence having adverse effect on environment as well as people in term of fluctuation in water supply as well as frequent extreme weather events such as flood, landslide etc. So, prediction of extreme precipitation is imperative for proper management. The objective of this study was to assess the spatial distribution and temporal change of extreme precipitation events on Koshi basin of Nepal during 1980-2010. Five indicators (R1day, R5 day, R > 25.4 mm, SDII and CDD) were chosen for 41 meteorological stations to test the extreme events. Inverse distance weighting and kriging interpolation technique was used to interpolate the spatial patterns. Result showed that most extreme precipitation events increased up to mountain regions from low river valley; and then it decreased subsequently up to Himalayan regions (south to north direction). However, there is high value of indices for lowland Terai valley also. Most of the indices have hotspot with higher value at north western and southern part of the study area. For temporal change, most of the extreme precipitation indices showed increasing trend within 30 years’ period. The spatial distribution of temporal change in indices suggests that there is increasing trend in lowland area and decreasing trend in mountainous and Himalayan area. So, adaptive measure should be adopted through proper land use planning, especially at those hotspot areas and their tributaries; to reduce adverse effect of extreme precipitation events.