MORPHOLOGY AND GROWTH PATTERNS OF THE USRI RIVER POINT BARS, BIHAR, INDIA

Morphology and growth patterns of four point bars of the river Usri, Bihar, India were studied by plane table mapping for two to three successive years before and after the monsoon. In plan, all these bars are crescentic in shape with downstream tail. The central parts of the bars are wider than their two ends. The bar represents aggregate of strips of sediments added laterally. The elevation contour lines demarcate the individual strips by abruptly cutting at the junction of two adjacent strips. In transverse profiles, the strips show up as series of steps. General slope of the bar is towards the adjacent channel though this slope is interrupted by these steps. In 3-dimensions, all these bars exhibit avalanche faces near the downstream end. The longitudinal profile of the bar over this avalanche face resembles the profile of an asymmetrical ripple, the lee face being this avalanche face. Trenches across the avalanche face reveal cross-stratification with gradation of grain sizes within individual cross-strata, similar to the features generated in front of migrating ripples. All these features suggest that the point bar is a mega ripple (macro bedform) which migrates downstream during the high flood when the entire bar is inundated. SEPM bedform classification also records that dunes are bedforms with wave length greater than 100m and height greater than sin. The equation of relative discharge as worked out by Sundborg (personal communication) from the Chezy and Manning equations, has been applied to establish a relationship between relative discharge and relative volume of sedimentation in point bars.