290-2 Morphological Evolution of a Flood-Tidal Delta in a Tidal Lake (Bahia Grande) of the Coastal Rio Grande, TX, USA

Session: Delta Evolution from Rivers to the Shelf: Past, Present and Future Perspectives for Society

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Abstract:

Flood-tidal deltas are unique systems where flux both accumulates and distributes as the flow direction and magnitude change through a tide cycle. Deltas frequently develop in shallow coastal environments, including tidal lakes and back-barrier lagoons, which are particularly susceptible to the effects of sea-level changes. However, the early stages of delta formation under natural conditions have not been extensively researched, largely due to limited data availability, which presents challenges in understanding the mechanisms behind their development. The Isabel Delta, a recently formed flood-tidal delta in Bahia Grande, a tidal lake within the broad Rio Grande Delta Plain in Texas, offers an opportunity to explore deltaic evolution shaped by both natural and human-induced factors. This study investigates the growth, sediment supply, and morphodynamic changes of the Isabel Delta following the construction of the Pilot Channel in 2005 and its enlargement in 2021 (i.e., change in boundary conditions of water and sediment influx), which restored tidal connectivity to the Bahia Grande as part of an ecological restoration project. We utilized imagery and topobathymetric data, including Planet Labs, Texas LiDAR, and echo sounder surveys, to map shoreline and channel network changes in a GIS. These measurements are combined with a field data-informed sediment transport model to understand shoreline mobility, sediment deposition, and the hydrodynamic processes that influenced the delta's development. Preliminary results indicate that the shoreline progradation rate has been 11.2 m/yr since 2005, with an autogenic timescale of ~1.5 years. This rapid progradation reflects significant sediment accumulation and an active process of delta formation, primarily driven by the interactions of tidal forces and wind in the area. Furthermore, our observations have highlighted changes in island and channel areas due to the enlargement of the Pilot Channel. This research enhances our understanding of small-scale deltaic systems in coastal restoration and highlights the effects of engineered hydrological changes on sediment distribution and landscape evolution in a changing climate.

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

doi: 10.1130/abs/2025AM-7471

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