Risk Assessment of Attabad lake Outburst Flooding using integrated Hydrological and Geo-spatial Approach

Abstract

Attabad Lake is a debris dammed lake in Gojal valley, Pakistan. It is likely to outburst because of the geological and hydro-meteorological settings of the region. This study is an effort to assess the Attabad Lake outburst flooding using integrated hydrological and geo-spatial approach. Both primary and secondary date have been utilized to achieve objectives of the study. The onset of such a hazard is dependent on the intensity and frequency of rainfall, the melting of glaciers, River Hunza discharge, type of soil, extent of mass movement, and the seismic activity as the region lies on an active fault line. The vulnerability of the forty-eight villages downstream can be easily assessed on the basis of various parameters of social, economic, and structural nature which include type and size of house, size of household, male to female ratio, monthly income and expenditure and the estimated damage cost. Considering the parameters of house type, occupation and access to safe place, there are around 21 villages that are highly vulnerable including Askurdas, Budalas, Chaprot, Dadimal, Garelt, Ghamadas, Guoro, Guvachi, Hakuchar, Isfahan, Jehgot, Maiun, Miachar, Minapin, Phakkar, Pissan, Rabbat, Sarat, Shayyar, Sumayyar and Thol. The exposed elements are basically divided into three categories of structural, social and economic exposure. The socially exposed elements are the household size which is an average of five persons per household. The structural elements exposed are the 48 villages and 8 bridges. The economic exposure can be gauged on the basis of monthly income which is less than PKR 30,000 for almost 80% of the population in the downstream region. In case of flooding, only the house damage cost is averaged at PKR 1,50,000 for the houses of more than 76 percent houses. The Gumbel frequency estimated the probability for 5, 10, 25, 50, 100 and 200 years. It shows that as the return period increases, the chances of flooding decreases, but there is increase in flow discharge even with the decreasing probability of occurrence of flood. The return period of 5 years has 20 percent chances of inundation. However, this decreases to only 0.5% in two centuries but with the flood discharge of 2170 m3/s. The dam breach concludes that over 35, 46, and 63 km of the surrounding area is at risk with 10,739 m3/s, 44,904 m3/s and 175,145 m3/s of flood discharge for 25, 50 and 100 percent dam breach, respectively.