THE INFLUENCE OF LARGE WOODY DEBRIS ON THE GEOMETRY OF THE SUBAERIAL BRAZOS RIVER DELTA, AS DEMONSTRATED THROUGH FIELD OBSERVATIONS AND A FIRST ORDER COASTAL MODEL

The Brazos River delta (Texas Gulf Coast) is a classic wave-dominated delta, and consists of a series of welded, subaerial ridges that developed following major river floods and the associated delivery of large volumes of sand. This modern delta formed within the last century after the Brazos River was artificially diverted 5 kilometers to the south in 1929. Less than fifteen years after the diversion, the subaerial and subaqueous expressions of the old Brazos delta were eroded by the coastal wave climate. Today, even with a drastically reduced water and sediment supply over the past two decades due to significant drought conditions, the modern Brazos delta is stabilized despite the prevalence of an energetic wave climate. Here we use field data to inform a first-order coastal model that examines the planform evolution of the Brazos delta since 1992—the year of the major flood that produced the most recent ridge. We explore how it is possible to maintain delta size, regardless of the reduced sediment supply to the coastline.

Our data reveal the prevalence of densely packed, large woody debris (LWD) at the ocean-delta interface, and we hypothesize that the LWD supplied by the river collects along the delta shoreface. The largest wood (up to 5-m length, 0.5 m radius) traps smaller wood (e.g., decimeter to meter length, centimeter-scale radius), and this framework attenuates wave energy, allowing for the accumulation of fine sand at the shoreface. Our modeling efforts reveal that the LWD stabilizes the delta from wave attack, promotes a growing shoreline, and aids in the creation of a relatively steep shoreline profile. Over the time period of delta development, each major flood event brings sand that accretes along the delta shoreline and wood then collects, thus attenuating wave energy.

While the woody shoreface creates an important local effect to aid delta preservation, the long term role of wood in terms of influencing the delta planform morphology (e.g., the welded delta ridges) is associated with the construction of topographic relief that aids in additional trapping of sand via aeolian processes. We use ground penetrating radar data to demonstrate that each of the growing ridges of the Brazos delta are rooted by a woody matrix. Our study reveals the critical role that LWD plays in terms of the development of a wave dominated delta.