DDRI: Spatial and temporal variation of sediment transport capacity and channel reach type in two mixed alluvial-bedrock rivers in central Texas

Texas State University - San Marcos, San Marcos TX

Investigators

Abstract

As end members on a continuum of a varying relationship between sediment supply and transport capacity, the general behavior of both alluvial and bedrock rivers is well known. Where rivers are a combination of bedrock and alluvium, there is a gap in knowledge and understanding of river processes and forms. The objective of this research is to explore the spatial and temporal variation in alluvial tending and bedrock tending reaches in the Guadalupe and Medina Rivers in central Texas, with focus on the sediment transport capacity of varied grain sizes. Both rivers have channel beds that alternate between predominately gravel to clean bedrock. There is a much greater urban influence, particularly in the creation of small dams, in the Guadalupe than in the Medina watershed. The extent to which a series of small, run of the river, weir type dams affect sediment transport and the resultant changes channel morphology will be investigated. A related objective of the project is to compare the Guadalupe and Medina Rivers to determine if there is a significant shift in the morphology of the Guadalupe River in the form of the evacuation of riffles. To reach these goals, the project will examine the history of sediment deposition and transport capacity, and compare the natural river processes to human altered ones through historical analysis, field mapping, and statistical techniques. The research will investigate the sediment transport processes operating in mixed alluvial-bedrock channels in an attempt to explain observed changes in the distribution of bed forms, depositional landforms, channel geometry, and channel patterns and characteristics. In comparing the Guadalupe and Medina rivers it is expected that the Guadalupe River will have more episodic, punctuated sediment transport events over space and time, a less even longitudinal distribution of sediment in the downstream direction, more differences between upstream and downstream reaches, and less regular riffle spacing than in the Medina River. With respect to temporal effects it is inferred that in the Guadalupe River pronounced changes in channel type and morphology occur immediately following dam installation with slower adjustments of channel type and morphology expected over time. It is anticipated that in segments where dams are closely spaced, morphology will be less dynamic than where dams are farther apart. It is also expected that transport capacity for large clasts in the half meter to meter size range, or the potential for macroturbulent events, occurs at several locations in both channels at flows with return intervals between 50 and 100 years. This research offers a geographic and temporal perspective on sediment transport capacity and the distribution of channel type reaches in two mixed alluvial-bedrock rivers in central Texas. Little is known about processes operating on mixed alluvial-bedrock rivers. This research project will develop a better understanding of the spatial variation of processes operating on mixed alluvial-bedrock channels and the geomorphic changes resulting from dam construction and subsequent changes to sediment transport rates and morphology. While the final results will be a comparison of the Guadalupe and Medina Rivers in central Texas, the insight gained from this study will be applicable to the region. Based on a multi-scaled approach, this research will aid the prediction of change in channel morphology subsequent to dam installation in mixed alluvial-bedrock channels and reaches. Results of this study can be used to increase understanding for watershed management, in-stream flow analyses, and regulation on the spacing of dams. Research results illustrating the effects of dam installation or removal for specific reaches can be related to resource managers to lend to help predict impacts of such alterations. Geographic perspectives on the spatial distribution of the potential for macroturbulence, sediment transport capacity, and depositional landforms will add to the scientific knowledge on mixed alluvial-bedrock channels. The project will help establish a strong research agenda and launch a promising career for a female doctoral student in the discipline of geography.