SPACING OF SPURS AS BANK PROTECTION MEASURE-A CASE STUDY

Abstract

Training the riversin alluvium is rather difficult because of enverchanging nature of these rivers and river Ghagra over which a present case study has been taken up, is one of them. Rivers carry on their natural activities in the form of changing the riverine geomorpho logical fea- tures by way of erosional as well as aggradational activities. Here, river bank erosion in curved reach is a major problem. It generally occurs in curves of river channels, where the interaction between the vertical gradient of the velocity and the curvature of flow 'generates spiralling flow. It moves the high velocity near surface current outward and, low velocity near bed current inward. This mechanics of flow produces larger depths and velocities near the con-cave bank. The deepening of the channel around the concave bank diminishes the toe support of the bank, and the larger velocities attack., it, setting the stage for bank erosion. The migrating trend and the morphological changes of river Ghacra has been studied due to construction of spurs over its affected reach near chuinti Dand between Gola Bazar and Dohrighat. The earliest form of river control consisted simply of a retired embankment or bund to prevent the spill of floods. At a later stage, control was effected indirectly by means of spurs constructed transverse to the flow extending from the bank towards the main channel by repelling off the flow from the Bonk. Dimensionsof spacing and length of spur as an. anti bank erosion structure posses a very interesting but challenging subject before the river engineers as well as research workers. Considerable amount of Laboratory analysis in flumes and scale models were conducted by many investigators to understand the intricacies involved in flow behaviour with spurs and the length of bank protected by them. A number of studies entirely based on field ex-perience and prototype behaviour of spurs have also been conducted to evolve a semi theoretical basis for finding the spacing of spurs. Notwithstanding, these vigorous efforts at the laboratory as. well as at actual field as cited above, evolution of a sound theoritical approach has continued to remain illusive till today, because behaviour of the same river changes differently in different reaches. Even some of the recent findings of flume studies on spa-cing of spurs really are not fit in with actual prototype behaviour in an eroded .bank line due to many factors such as adoption of a straight channel reach in a flume, where it as4s curved in plan. As such, the practicing river engineers often are required to rely on emperical approach based on their judgement, experience and sometimes even thumb rules. The study has yieled information about - (i) Changes in bank line migration in U/S, along the spur system and D/S of it. (_) (ii) Changes in river bed profile in termso f percentage aggradation-degradation. (iii) Efficacy_,- of the existing spur system with reference to their spacing and length. So, in the context of P.F. 1985 river configuration, three number of spurs has been suggested to protect greater length of the affected bund. In the second suggestion, to check the erosional activity of the river on the U/S of the existing spur No. 1 on the basis of F.F. 1990 river configuration, three number of small spurs along with one fork spur on spur No. 1 has been proposed. Finally, it has emerged from the study that satellite imageries can be advantageously used to monitor the mor-phological changes of a river, to evaluate the limiting value of arc-chord ratio ( rmax) of a river in the different reaches and also for an appraisal of the efficacy of costly river training works. Further, the use of a mathe-matical model simulating bed level changes in combination with the satellite data appears to be promising for better understanding of the river behaviour.