Abstract:

The Texas coast is a system of barrier islands, lagoons, estuaries, plains, and rivers on a low-lying coastal plain with gently sloping topography. Embedded in this natural and dynamic system are a variety of human developments and activities including oil and gas production, heavy industry, shipping, commercial fishing, recreational fishing, agriculture, tourism, and small and large communities dotted throughout the landscape. The natural systems of the coastal plain, however, are dynamic and subject to sudden hazards such as floods, storm winds, storm surge, and erosion superimposed on longer-term processes of ongoing erosion caused by sediment supply changes, shifting habitats, sea level rise (SLR), and climate change. Given the vulnerability of the Texas coast, this study assesses the impacts of relative sea level rise (RSLR) and associated enhanced storm surge to better understand the relative susceptibility to negative impacts on the natural and built environments. This study is part of the third publication of the Texas Coastal Resiliency Master Plan (TCRMP) published in April 2023. TCRMP is an ambitious coastal planning effort by the Texas General Land Office (TGLO) to make the Texas coast more resilient to hazards. For this study, the Harte Research Institute (HRI) developed a dynamic modeling framework to assess quantitative information regarding the impacts of SLR and associated enhanced future storm surge caused by higher sea levels and changes in land cover. For the 2023 TCRMP, two SLR scenarios are modeled - Intermediate-Low scenario (0.5 m of SLR by 2100) and Intermediate-High scenario (1.5 meters of SLR by 2100). The relative component of sea level rise is determined on a regional basis by deriving an average trend from long-term records of coastal tide gauges. This study employs the Sea Level Affecting Marshes Model (SLAMM) to model the possible effects of sea level rise on coastal habitats under two SLR scenarios. This land cover change model provides baseline conditions and predicts future landscape trends based on rising sea levels. The model takes into account key processes that affect the distribution of environments on the coast, such as erosion, vertical accretion, inundation, and saltwater intrusion. This dataset contains the present land cover based on National Wetland Inventory (NWI) and predicted 2100 land cover with two SLR scenarios from the SLAMM modeling for 4 coastal planning regions covering the Texas coast. These maps illustrate the areas where current critical environments are, areas where those environments will be lost or gained in 2100 under different SLR scenarios, as well as areas that will become new environments but are currently undeveloped dry land. The results of the SLAMM model show that in this century, with no mitigating action taken, significant portions of the Texas coast will be at risk of land loss, as current habitats and low-lying areas around communities are converted into open water.

Suggested Citation:

Subedee, Mukesh, Marissa Swift, Lihong Su, Pu Huang, Jessica Magolan, and James Gibeaut. 2023. Geospatial model output of the coastal wetland landscape change modeling under two projected sea level rise scenarios along the Texas Coast. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/4vn3xzdq

Purpose:

The Texas coast is a low-lying coastal plain with gently sloping topography and eroding coastal environments. Embedded in this natural and dynamic system are large- and small-scale human developments. The coastal plain is vulnerable to sudden hazards such as floods and storm surge superimposed on other processes that affect long-term coastal change, including the cumulative impact of storms and sea level rise (SLR). Given the vulnerability of the Texas coast, this study assesses the impacts of SLR and associated enhanced storm surge on natural and built environments. This study is part of the third publication of the Texas Coastal Resiliency Master Plan (TCRMP), a coastal planning effort by the Texas General Land Office (GLO) to make the Texas coast more resilient to hazards. For this study, the Harte Research Institute (HRI) developed a dynamic modeling framework to assess quantitative information regarding the impacts of SLR and associated enhanced future storm surge caused by higher sea level and changes in land cover. The framework is comprised of state-of-the-art models including SLAMM, ADCIRC, SWAN, and HAZUS-MH. This study employs the Sea Level Affecting Marshes Model (SLAMM) to project the possible effects of sea level rise on coastal habitats under two SLR scenarios by year 2100. For the 2023 TCRMP, two SLR scenarios are modeled - 0.5 meter (Intermediate-Low scenario) and 1.5 meters (Intermediate-high scenario) of global mean SLR by 2100. The relative component of sea level rise is determined on a regional basis by deriving an average trend from long-term records of coastal tide gauges. The coupled hydrodynamic storm surge model, Advanced CIRCulation (ADCIRC) and Simulating Waves in the Nearshore (SWAN), is used to identify the threats posed by storm surge and nearshore waves to people and the ecosystem in the current and 2100 environments. Nineteen synthetic storms, ranging in severity from Category 1 to 3, making landfall near major bay systems or city centers across the Texas coast, are modeled for both the current and two 2100 landscapes. The 2100 surface and land cover predicted by SLAMM is used as a representative of future elevations and land cover type for input to the ADCIRC+SWAN model. The result of this modeling serves as input for geohazards and vulnerability maps that show current and future exposure to changing environmental dynamics in an area. This study provides a range of coastal vulnerability, and therefore, better defines the requirements for projects or programs to address resiliency now, and in the future. Results from this study will help decision makers prioritize projects, and understand the value provided by the coast, the inherent risks the coastal communities face, and the opportunities available to manage a dynamic coastal environment in a more resilient manner.

Data Parameters and Units:

Elevations are in meters relative to the NAVD88 datum. Each land cover maps show 6 land cover classes - 1: Developed Dry Land; 2: Undeveloped Dry Land; 3: Freshwater, non-Tidal Wetlands; 4: Saltwater and Brackish Tidal Wetlands; 5: Flats and Beaches; 6: Open Water

Methods:

To facilitate the presentation of vulnerabilities and potential solutions, the Texas coast was divided into four regions. The four regions are based on major bay systems and habitats: Region 1 extends from Sabine Pass to Galveston Bay covering Brazoria, Chambers, Galveston, Harris, Jefferson, and Orange counties; Region 2 surrounds the entire Matagorda Bay system covering Calhoun, Jackson, Matagorda and Victoria counties; Region 3 extends from San Antonio Bay to the Baffin Bay covering Aransas, Kleberg, Nueces, Refugio, and San Patricio counties; and Region 4 extends from southern edge of Baffin Bay to the Texas-Mexico border covering Cameron, Kenedy, and Willacy Counties. Due to high population density, Region 1 was subdivided into Regions 1A (from the Sabine River to the west side of Galveston Bay) and 1B (from the west side of Galveston Bay to the Brazos River). Several map-based inputs and numeric parameters along with global mean sea level rise scenario (GMSLR) by the year 2100 were required to run the SLAMM model that simulates the dominant processes involved in wetland conversion and shoreline change for the SLR scenario. Two SLR scenarios were modeled - Intermediate-Low scenario (0.5 m of SLR by 2100) and Intermediate-High scenario (1.5 meters of SLR by 2100). SLAMM provides maps of updated elevations and land cover classes in the year 2100 along with other numerical outputs. The future topographic surface predicted by the SLAMM model was used to update the ADCIRC+SWAN model computational mesh. The future land cover conditions predicted by the SLAMM model, combined with the 2100 land cover datasets developed by the US Geological Survey (Sohl et al., 2014), was used to generate ADCIRC+SWAN model friction parameters (Manning’s n) representative of future conditions. Both GMSLR scenarios used in the SLAMM modeling were incorporated in the ADCIRC+SWAN model setup by increasing the initial water surface elevation from the current sea level condition. The ADCIRC+SWAN model was forced using meteorological wind and pressure fields of nineteen selected hypothetical hurricane events making landfall in different parts of the Texas coast. A total of 57 ADCIRC+SWAN simulations were performed. The same nineteen hurricane events were forced to both the present-day condition to assess the storm surge impact in present land cover and topography, and the two future conditions (2100) to assess the combined impact of RSLR and future land cover and topography. For more information on the models, input data, modeling scenarios, and methodology, please refer to the 2019 TCRMP technical report and 2023 TCRMP technical report. Texas General Land Office (GLO), 2019. Texas Coastal Resiliency Master Plan Technical Report – May 2019 https://library.ctr.utexas.edu/Presto/content/Detail.aspx?ctID=UHVibGljYXRpb25fMTE2MTA%3D&rID=MzQwNjE%3D&ssid=c2NyZWVuSURfMjEzMjI%3D&bmdc=MQ==

Provenance and Historical References:

Sohl, T.L., Sayler, K.L., Bouchard, M.A., Reker, R.R., Friesz, A.M., Bennett, S.L., Sleeter, B.M., Sleeter, R.R., Wilson, T., Soulard, C. and Knuppe, M., 2014. Spatially explicit modeling of 1992–2100 land cover and forest stand age for the conterminous United States. Ecological Applications, 24(5), pp.1015-1036. Subedee, M., M. Swift, L. Su, P. Huang, J. Magolan and J. Gibeaut. 2023. Integrated Modeling and Mapping for Coastal Resiliency: Informing the 2023 Texas Coastal Resiliency Master Plan (TCRMP). Final technical report to the Texas General Land Office, Contract Number 21-060-022-C817. Harte Research Institute, Corpus Christi, Texas, USA, 169 pp. Subedee, M., C. R. Pollard, M. Dotson, B. Lupher, L. Su and J. Gibeaut. 2019. Sea Level Rise and Storm Surge Modeling in support of the 2019 Texas Coastal Resiliency Master Plan (TCRMP). Final technical report to the Texas General Land Office, Contract Number 17-244-000-A189. Harte Research Institute, Corpus Christi, Texas, USA, 162 pp.

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