TEMPORAL AND SPATIAL VARIATIONS IN BEDFORM ACTIVITY AND GEOMETRY WITH VARYING STREAM DISCHARGE IN THE BACKWATER ZONE OF THE TRINITY RIVER, TEXAS

Acoustic imaging of the bed of the coastal Trinity River during varying stream discharges reveals spatial changes in bedform geometry, coverage, and inferred activity. We see rapid changes in dune properties beginning 60 km upstream of the coast where the river progresses into the backwater zone, specifically the reach of river where flow is affected by hydraulic readjustment between quasi-uniform flow in the upland system and the gradually varying flow approaching the river mouth. Measurements collected immediately following a minor flood document 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. These observations imply a pulse-like process of delivery of bed-material load to the coastal zone of the Trinity River. The goal of the project is to connect bedform behavior and its characterization of bed-material load to the spatially changing geometry of point bars and ultimately to the kinematics of the river bends themselves.