267-6 Unexpected Sediment Loading of an Urban Creek by Anthropogenic Sources: Waller Creek, Austin, Texas

Session: Advances in Fluvial Processes and Sediment Transport (Posters)

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Abstract:

Our study focuses on sediment in Waller Creek, an urban stream entirely within the city borders of Austin, Texas. Its bedrock channel is 11-km long with a steep bed slope that ranges from 0.008 to 0.01. Waller Creek is an ideal system for studying anthropogenically derived sediment because its entire catchment is underlain by a single rock type and formation, the Austin Chalk. It is a soft fine-grained carbonate with a distinctive texture and color that makes it easy to identify and distinguish from other rock types. 

Waller Creek base flow is insufficient to transport any of the sediment that partially covers its creek bed. However, the creek is prone to flash flooding during intense rainfalls and stage can increase by as much as 3 m. In flood stage, all sediment sizes are transported, including boulders. This transport has received increased attention since construction of the 1.6-km long Waller Creek Tunnel that diverts flood water beneath downtown Austin. Sediment accumulating at the tunnel’s inlet must be occasionally removed, prompting interest in its origin and motivating our study. 

In order to identify possible sources, we have collected observations along the entire creek length and sampled bed sediment at 7 roughly evenly spaced locations upstream from the tunnel inlet. These samples consist of grains ranging from sand to cobbles in size. Surprisingly, Austin Chalk is not the major constituent in many of these samples. Our preliminary analysis has it only composing between 20% and 60% by weight. The fraction of chalk grains is correlated with particle size; coarsest clasts are primarily chalk, while chalk sand is rare. Non-chalk grains mostly are chert, quartz, and feldspar, with lesser amounts of other limestones, glass, concrete, ceramic, brick, and plastic. Observations from thin soils resting directly on top of the Austin Chalk suggest they are not the source of the exotic sediment. Instead, the chert, quartz, feldspar and other limestones are similar to commonly used landscaping materials that we have observed being delivered to the creek by overland flow during intense rains. Future work will focus on further understanding the connections between grain size and composition, as well as sources for the exotic sediment.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8188

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