Morphological evolution and dynamics of a large, sand braid-bar, Jamuna River, Bangladesh

The initiation and evolution of a kilometre-scale, sand braid-bar was monit ored during a 28-month survey period from 1993 to 1996 in one of the world' s largest braided rivers, the Jamuna River, Bangladesh. Repeated bathymetri c surveys through two monsoon flood seasons, combined with bar-top surveys during exposure of the bar at low flow, provide the most detailed chronolog y of braid-bar growth yet compiled for a large sand-bed river. During risin g and peak flow of the 1994 monsoon flood, a 1.5-km-long, 0.5-km-wide, 12-m -high, symmetrical mid-channel bar was deposited in the centre of a major c hannel downstream of a zone of flow convergence and significant bank erosio n. Initial deposition and growth of the bar core were probably caused by am algamation of dunes that are present in the Jamuna channels at all flow sta ges. Bar-top aggradation continued through downstream migration of an 'accr etionary dune front', a 3-m-high, angle-of-repose slipface that was compose d of amalgamated, 0.5- to 1-m-high dunes. At waning and low flow, the mid-c hannel bar widened by up to 1 km through the lateral accretion of dunes ont o the margins of the initial bar core. A low-velocity zone in the sheltered wake region of the bar-tail led to the accumulation of substantial volumes of silts and clays. During the rising and peak flows of the next monsoon f lood, the mid-channel bar extended its bar-tail by up to 1.5 km, as one of the anabranches became dominant, and flow was deflected across the bar-tail . Accretion at the bar-tail generated a lobate, transverse bar-front with a 10-m-high, angle-of-repose avalanche face. Emergence of several smaller ba rs along this depositional front produced an overall reach morphology that more closely resembled an alternate bar rather than several mid-channel bar s. The conversion of a mid-channel bar to an alternate bar is contrary to m any previous descriptions of the braiding process.