4-Variations in Daily Sediment Loads of Narmada river

Abstract

River Narmada is the seventh largest river in India and flows mostly through the Deccan volcanics before debouching into the Arabian Sea. Absence of any significant deltaic feature allows all sediments to reach into the ocean. Observations of daily sediment loads are important to understand earth surface processes, especially erosion rates in continents and flux to the oceans. A comprehensive study has been undertaken to focus on the daily variation of sediment loads of river Narmada, factors controlling sediment loads and to understand the impacts of manmade structures on sediment loads on the river course. In this study data for a period of three water years (1996-1999) obtained from Central Water Commission, India, based upon the daily sediment load observed at 12 sites on Narmada and 4 sites on different tributaries have been used. The present paper discusses the observations on three sites on Narmada River at Manot (Upper Narmada Basin), Hosangabad (Middle Narmada Basin), Garudeshwar (Lower Narmada Basin) and one site on one of major tributaries of Narmada, Orsang at Chandwara (from Lower Narmada Basin). Narmada River is characterized by high- suspended sediment loads in wet (monsoon) periods and low loads during the rest of the year. Monsoon loads vary between 85 to more than 98% generally, or even become 100% of total annual loads in case of some tributaries. Most of the sediment load is fine (diameter less than 0.075 mm) in nature and makes 90 to more than 98% of total suspended sediments carried annually. Coarse and medium size particles are transported mainly during the monsoon seasons only. Interpretation of data shows intimate relationship of sediment loads with water discharge and indicates that sediment load in river basin is directly controlled by average annual rainfall and its intensity, and geology of the basin. The water discharge and sediment load at Garudeshwar (extreme downstream CWC site over river Narmada) is 33550 m.cum. and 6.70 mmt respectively. The presence of a number of manmade structure such as dams and reservoirs in the basin, show a significant impact over the sediment load carried to the further down stream or to oceans. Sardar Sarovar Dam, largest dam in Narmada River Basin traps a large proportion of sediments being carried by river from upstream. This study indicates that the dam traps nearly 60 to 80% of suspended sediments. The presence of this dam allows to settle 70 to 90% of coarse sediments and nearly 50% of medium sized particles.