Sediment Transport Studies in Punatsangchhu River

Abstract: Sediment transport pattern in the Himalayan River is complex and sediment sampling in these rivers are often difficult. Sediment load in the river varies largely from year to year. Major part of the sediment load is transported in the monsoon months. Reliable and consistent sediment rating equation is not found for the Himalayan Rivers. The change in the flow rate and suspended sediment concentration is very rapid and unpredictable. In Bhutan, there are no published records on sediment transport studies. There are four sediment sampling stations in the whole country. Bhutan being a mountainous country has a huge potential for hydropower development. The most common problem related to hydropower projects is the reservoir sedimentation. In this report, the sediment concentration and river flow data from one gauging station along Punatsangchu River is presented to increase the understanding of sediment transport pattern in this river. Sediment transport in the Himalayan Rivers with regard to factors affecting sediment transport is addressed and some comparison is made with the Punatsangchu River. The factors affecting sediment transport are hydrology and climate, geology, land use and topography. The correlation between sediment concentration and river discharge for the year 2007 for Punatsangchhu River was found to be very good with an R2 value of 0.8. The correlation of average discharge and concentration over the record period of 1993 and 1996 to 2008 was found to be 0.53. For some years the correlation was very poor with an R2 value of 0.13. This shows that river discharge is not a reliable variable to predict the sediment concentration in Punatsangchu River. Most of the sediments are transported during the monsoon months which account for about 90% of the total load transported in a year. The bed load and suspended load are calculated using the formulae developed for alluvial rivers and the results were similar to the estimated load from the gauging station.