Spatial Change in the Geometry and Kinematics of the Trinity River, TX, USA, Defining the Morphodynamics of the Fluvial Coastal Zone

Abstract:

Changes to the geomorphology of meandering rivers as they approach the coast are now recognized as being connected to a transition from quasi-normal flow upstream to a zone of gradually varying flow near the shoreline. In spite of this recognition, there remains a great deal to learn regarding the morphodynamics of coastal rivers. Here we present data from repeat bathymetric and topographic surveys, aerial photo analyses, and bed-material sampling that are aimed at defining the morphodynamics of the coastal Trinity River. Initial change in the geomorphology of the river channel is coincident with the average elevation of the river bottom dropping below mean sea level. Over the next 50 river km downstream of this point, the vegetated channel width systematically decreases by 44 percent and channel depth systematically increases 1.95 times. The net result is a 66 percent reduction in the width-to-depth ratio, which is not evenly distributed in the downstream direction. All of the change occurs in the first 28 river km, with the channel maintaining a statistically constant value of 17 for the next 22 river km. The size of point bars also begins to systemically decrease at the point where the river bottom drops below sea level. Over the next 50 river km the surface area of these bars decreases by an order of magnitude. This substantial reduction is not only a function of the decrease in channel width, but also a decrease in point-bar length. At about 23 km downstream the bars undergo a change in their cross-sectional shape. Upstream they occupy roughly 80 percent of the channel, while downstream they occupy only 70 percent of the channel width. This coincides with the point where lateral migration rates for channel bends drop by roughly 75 percent. We show how these changes in channel geometry and kinematics are connected to time and space changes in hydrology and sediment transport in the backwater zone.