Dry season discharge and sediment yield of the northern tributaries of the Kathmandu Valley, Central Nepal

Abstract

upon hydraulic regime, geological setting, land use and human influences. When sediment load increases due to high erosion rate than sediment yield rate, the normal range exceeds resulting problems to mankind and manmade structures, and reduce environmental quality and elements of rivers. Therefore, to overcome such situations it is important to study estimation of sediment yield and sediment transport capacity of the rivers. According to Tamrakar and Shrestha (2008) rivers in the Lesser Himalaya are capable to flush out sediment compared to such in the Siwaliks because of unstable slope. Various studies have shown that the rivers in the INTRODUCTIONRiver system is a natural and dynamic process which carries sediments generated due process of weathering, mass wasting and erosion of rocks and soils present in the nature. In this point, on the basis of the characteristics of the river system, particular system of the river has its own capacity of transporting sediment and yielding sediment from the river basin. The sediment yield and the amount of sediment load depend Population growth, urbanization and improper land use practice cause change in natural behavior of the river system. In this point soil erosion and sediment deposition in the river channel adversely bring environmental problems like variation in discharge, surface water physical quality, sediment transportation capacity, sediment yield and stream power. In lack of proper research and planning, disasters like flooding, failure of engineering structure, bank erosion and imbalance in aquatic ecosystem can be seen. To better understand the stream, present work was carried out to determine the discharge, sediment transportation capacity, sediments yield and stream power of northern major tributaries of the Bagmati River. Subsequent to this, relations between the two or more than two morpho-hydraulic parameters with the discharge and sediments load were evaluated.The northern Bagmati River basin occupies an area of 3750 sq. km and lies within 26 0 42' and 27 0 50' N latitude and 85 0 02' and 85 0 58' E longitude. The Bagmati River, non-glacial perennial river, consists of four main tributaries contributing from the northern part of the Bagmari River basin. They are the Manahara River, Bagmati River, Dhobi Khola and the Bishnumati River. The four major tributaries including the Bagmati River were upto sixth stream order. The study revealed that water discharge varied from 0.033 to 1.983 m 3 /s and the grain size distribution d 50 ranged from 0.0002 to 0.0250 m. The study found that the suspended load for dry season varied from 5.93 to 916 mg/L and the bed load from 6 to 393711 tonnes/day. The total sediment yield ranged from 0.58 to 22029.64 tonnes/km 2 /day. The boundary shear stress and critical shear stress ranged from 0.03 to 30.87 N/m 2 and 0.05 to 0.06 N/m 2 , respectively. The stream power per unit channel width was found to vary from 0.001 to 0.06 KNm/s/m 2 for tributaries. Except at few sites, the rest of the sites of the studied river exhibit comp...