Evolution of Return-Flow Channels Cut Into San Jose Island, Texas, Caused by Hurricane Harvey
Abstract
Coastline resilience relies on predicting storm impacts on beach morphologies. However, return-flow channels, a morphodynamic consequence of storm surge ebb, have not been well-studied. Our study investigates the erosional evolution of return-flow channels on San Jose Island in Texas as a result from Hurricane Harvey in 2017. These channels are up to 300 m in length and have multiple upstream heads with single, elongated downstream necks. Depth measurements along and across four channels show the deepest parts of most channels being near their upstream heads and associated knickpoints. Channel-bottom elevations here ranged from 0.78 - 1.95 m below mean sea level. These deep spots resemble plunge pools of horseshoe waterfalls, implying a similar set of developmental mechanisms associated with flow focusing. Subaqueous channel margins can be stepped, indicating a lithologic control on the pattern and amount of erosion. A rack line of debris was observed 5 m above mean sea-level on the landward side of the island. Four large barges drifted across the island during return-flow, coming to rest at the landward edge of the coastal dune complex without producing drag marks on the surface. We interpreted the rack line, the barges, and the 'bathtub rings' within some of the channels as indicators of a high and slowly draining water level during channel cutting. Scours preserved on the landward side of dunes document a seaward flow direction; there was no evidence of washover deposits. Vegetation density appears to have controlled erosion rates as sediment samples collected at seaward and landward points along the channels show a similar grain-size distribution. A pre-hurricane airborne lidar survey collected by the U.S. Army Corps of Engineers in 2016 and a post-hurricane survey flown by the Bureau of Economic Geology at UT-Austin in 2017 are used to quantify beach morphological change associated with channel formation, including the volume of eroded material, and channel dimensions and density. Channels are shown to take advantage of pre-existing low topography between dune ridge lines. The field data and lidar analyses will guide the construction of a return-flow channel-evolution model using ANUGA, a morphodynamic modeling suite.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP23C2347R
- Keywords:
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- 0442 Estuarine and nearshore processes;
- BIOGEOSCIENCESDE: 1625 Geomorphology and weathering;
- GLOBAL CHANGEDE: 1641 Sea level change;
- GLOBAL CHANGEDE: 3020 Littoral processes;
- MARINE GEOLOGY AND GEOPHYSICS