The effects of snow avalanches on the hydrologic regime of the Kunhar river, Western Himalayan, Pakistan : analysis and application to river flow forecasting

This study sets out to investigate the significance of snow avalanches on the hydrology and runoff generation in the Kunhar basin in Northern Pakistan. The objectives of this research are, to analyze the snowmelt and snow avalanche effects using the U.B.C. Watershed Model, and to produce a flow forecasting system which takes account of the snow avalanche effects. The Kunhar River is a major tributary of the Jhelum River in the western Himalayas of Pakistan. The basin area is about 2,340 km2 with an elevation range from 800 to 5,300 m above sea level. The watershed has a seasonal snow cover which develops from early November onwards, reaching a maximum depth in March or April. Also, the snowpack increases greatly at upper elevations. In the Kunhar basin the avalanching is a major source of snow redistribution from higher to lower elevations. It is estimated that on average over 200 x 106m3 water equivalent of snow is avalanched annually. The percentage of the total affected area (runout and starting zones) by avalanches in Kunhar basin is estimated to range from 12% to 21%. The starting zone lies at a mean elevation of about 4,000 m and runout zones are at mean elevations of 2,450 and 2,800 m above sea level. This means that the avalanche activities in the lower elevations are dependent on the snow precipitation at elevation 4,000 m. This study shows that about 20% of the snowpack at 4,000 m is, on average, subject to avalanching. Avalanche contribution is found to be very significant in calibrating the watershed model. On average the overall Nash-Sutcliffe coefficient of efficiency of the model was improved from 77 to 84% after introducing avalanches in the calibration which shows improved time distribution of runoff. Snowmelt pattern in the avalanche areas is significantly modified by avalanche activity. Firstly, the snowmelt in the runout zones starts about seven days later and lasts about 30 days more than in areas not affected by avalanches. The snowmelt volume in runout areas is increased by about 200 to 300% in affected areas. The maximum snowmelt from the avalanche runout areas is about 100% higher than the maximum snowmelt in the un-affected areas. The timing of the maximum snowmelt is delayed by about 15 days in the runout zones of avalanche affected area, due to high accumulation of snow. These results show that the snow avalanches increase both the volume and the period of the snowmelt in the runout zones and also change the time distribution of the snowmelt. Since the snowmelt increase in the runout zones is compensated by the decrease in snow in affected areas of the starting zones, the total snow melt from the basin is unchanged. The above results of flow simulation by using redistribution of snow were used to produce a forecasting system of avalanche activity. Linear regression analyses were performed and the linear relationships for each band were estimated. Regression analyses show very strong correlation between avalanche volume and snowpack accumulation at the upper elevations, i.e., the coefficient of determination (R2) is found to be in a range of 0.9 - 0.95. The extra snow depth acquired at elevations 2,450 to 2,800 m in the form of avalanche is also strongly correlated with the existing snow depth at 4,000 m, R2 ranged between 0.93 and 0.99. If the snowpack at 4,000 ra elevation is measured then the maximum snow accumulation, which occurs in late March or early April, can be estimated. From the developed equations the total avalanche volume, the snow avalanche depth, and the affected areas for runout and starting zones can be estimated. These estimates can then be used in the U.B.C. Watershed Model to forecast the flow for the coming season. Application of this procedure showed that the proposed forecasting system gives an improved and reliable estimation of the seasonal flow volume and the time distribution of runoff for the Kunhar river.Applied Science, Faculty ofCivil Engineering, Department ofGraduat