Spatial Variation in Bed-material Load as Captured by Dune-form Analysis and its Connection to Geomorphology of the Backwater Zone on the Trinity River, East TX, USA

Abstract:

Recent observations made in the Trinity River of East Texas reveal that systematic spatial changes in bedform geometry, coverage, and inferred activity correlate with documented shifts in the larger-scale geomorphology of the river. Acoustic imaging data was collected through the transition into the backwater zone, or the reach of river where flow is affected by hydraulic readjustment between quasi-uniform flow further upstream and gradually varying flow towards the river mouth. Measurements collected immediately following a minor flood record spatial changes in bedforms with dune height systematically decreasing from roughly 0.4 m to 0.2 m and dune length decreasing from 13.4 m to 7.3 m, maintaining a constant value of 29 for the ripple index over a 6 km reach that covers 7 river bends. It appears that bedform height is depth-limited within the quasi-uniform flow, and gradually shifts to occupy a smaller fraction of the increasing flow depth within the backwater zone. Over the same reach after a period of extended low river discharge, dune height decreases from 0.3 m to 0, while dune length decreases from 9.0 m to 4.4 m before dunes are completely absent. Ripple index stays relatively constant until the last two bends, a streamwise distance of 2 km, where it rapidly increases from a value of 30 to 44 in the 6^th bend and then to infinity in the 7^th most downstream bend. Accompanying the disappearance of the dune forms is a systematic reduction in the slopes of their lee faces until the bed is completely flat. The location of these shifts in bed-material load coincides nicely with and likely accounts for documented geomorphic changes to the river, including a reduction in point bar surface area and volume and a decrease in channel-bend migration rates (Smith, 2012). Results have obvious implications for understanding coastal fluvial geomorphology and can help elucidate relationships between bedforms, bed-material load, point bars, and river bend kinematics.