Modeling the influence of large woody debris on delta morphology and longshore sediment transport using the Brazos delta
Huff, Sarah Anne
Master of Science
Large woody debris' (LWD) role in coastal sediment dynamics is understudied, but yet is an extremely important consideration in the face of modern climate change and coastal erosion. In addition to amplified aeolian sediment capture due to increased roughness of the shoreface, LWD has implications for reduction of sediment transport capacity along the shoreface. Here, we examine the impact of increased woody debris concentration within the littoral zone and relate this to differential longshore sediment transport rates. We hypothesize that woody debris concentration and longshore sediment transport can be linked through a dimensionless index for use in most longshore sediment transport equations. Using a numerical model constrained by data collected from field studies, our results show that increased woody debris at the shoreface is inversely related to longshore sediment transport rates. Increased roughness due to woody debris results in a loss of stress available to mobilize sediment. Our model allows for first-order predictions of longshore sediment transport based on woody debris concentrations at the shoreline. Field surveys of the modern Brazos river delta in the Gulf of Mexico reveal an abundance of large woody debris along the coastline and within the delta matrix. There is a multitude of aerial photographs and bathymetric maps documenting the relative stability of the modern Brazos River deltas despite variable sediment discharge and the same wave climate that resulted in rapid erosion of the pre-engineered-diversion old Brazos delta. Despite their similar sediment volume and wave climate characteristics, the two deltas show very different morphologies in times of relaxed sediment discharge. With our model, we propose that the large quantities of woody debris sequestered on the modern delta may result in minimal longshore sediment transport along the delta, thereby preserving delta morphology amongst an energetic wave climate.