Is the construction of Groynes accelerating the degradation of channel morphology and paved the way for human encroachment in The Bengal Basin

Abstract

Abstract The present study has been exploring the influences of groynes on channel morphology in a selected reach (Approximately 2100\u202fm) of Dwarkeswar River near Rautara Village of Burdwan District, West Bengal. The selected area has been classified into three reaches and was surveyed extensively by fifteen cross-sections and fifty-nine sediment samples to understand the hydraulic geometry and sedimentological behaviour of these reaches. As well as, braided index, bar occupied area, aggradation and degradation ratio and the channel instability indices were calculated from 2003 to 2018. Simulated flow using HEC-RAS indicated that flow alteration occurred from the groyne area. Apart from this, multivariate analysis (Principal component analysis (PCA)), non-parametric tests and Canonical Discriminant Function models has successfully highlighted the impact of the groynes on the degradation of channel morphology. PCA and correlation matrix showed that the groyne is one of the dominant factor (PC 1\u202f=\u202f0.91) in this system of channel morphology and significantly altered the sedimentology, hydrology and morphology of the channel. Bankfull channel width, mean depth, cross-sectional area, width/depth ratio and length of sand-bedded river decreased along the groyne, whereas braided index, depositional rate, maximum depth (scour near groynes), the difference between maximum depth and mean depth increased significantly. Groyne area is characterized by poorly sorted sediments with multimodal character. Altogether, the channel is continuously degrading through the formation of stable bars along the banks of the river in this reach.