LONG TERM SEDIMENT YIELD ESTIMATION IN A SUB-CATCHMENT OF RIVER YAMUNA USING AERIAL PHOTOGRAPHS

Abstract

Sediment, the end product of erosion, has a two fold effect : Ci.> it depletes the Land front which i t is derived and Cii) it impairs the quality of the water-resources in which it is entrained and deposited. Sediment becomes a problem when it occupies water-stor-a e reservoirs, settles on productive Lnerd-. degrades water- qua i ty, c logs streams and eventual ly resultin& in floods. Hence it is easy to understand the importance of the sediment-yield-surveys, as preventive and corrective measures can then be adopted to control the dama6es caused by erosional-processes. Traditional methods to compute the sediment-yield include: Ci.) Measuring the sediment-deposited in a reservoir/lake in a certain period of time and ,getting the sediment-yield as, the sediment-deposited per unit time per unit area of the drairia -e-basiro, and Cii) Samptine the sediment-toad at various points of the channel. But these field-methods are quite Laborious- and in India, it is. difficult to monitor all the basins through out the year as many Large-catchments are Located in remotely-inaccessible areas. In such a situation, Rerrcote-Seresirt-techniques and data may prove to be are inexpensive toot for sediment-yield-studies, as continuous monitoring of the remotely Located watersheds through out the year is possible with a considerable reduction in field-work. In the present study, this new-approach has been used as a pretimznar-y work, takin€ the catchment of Artdawa-river a tributary of the river-Yczrrnuna in the Lesser-Himalayas. The Amlawa-Catchment with a thainage area of about 127 kmC is• Located in a part of DehradcrrL and Tehri-Gar-hwctl-districts of Ut tar-Pradesh. Though sediment-product ion due to frequent mas•s - x.zs t ins, raid remouaL of sL ided -debris and erosion of weathered-materials on steep slopes by running water is also present, but its contribution is -rLe,gLieible in comparison to the amount of sediment due to the vat Ley-zone- incision. An assumption has been made that the sediment - pr-oduc t i o-n from the incised-tone is a measure for the effects of all the sources for sedimentation. -A hypothesis has also been postulated that the incision has- started after the Last eLaciat-r-etreat. Hence, volume/•mass eroded from t he catchment divided by the time-period of incision divided by the area of the drainage-basin may give the Average-L One- term- sediment-yield of the catchment. B & W aerial-photographs of year 1975 at a scale of 1:40,000 were used in the study. The whole catchment was divided into 33 srrrctLL sub catchments and cross-sections across the stream-tines were plotted at about 200 points alone the strecura-Lines. Har•hi-rte the former-fill-level, the ercctasirc,q areas-, in between the farmer-fill-level and the present-Landscape were then measured. Finally, the vo L umes•/masses eroded were computed sub-catchment wise or for whole-catchment. ALthoueh a very simplified hypothesis was used and effects of many parameters accounted far, the results obtained were very near to the actual averaee field-values.