Coastal dune vulnerability, resilience and ecological modeling and restoration at South Padre Island, Texas

About 40% of the world’s population currently live within 200 km of coastal zone and are exposed to erosion and flooding risks. Dunes are a critical component of the coastal landscape that can absorb the destructive hydrodynamic wave energy, thus reducing storm surge and hurricane impacts on coastal communities and wildlife habitat. Vegetation is essential both for building dunes through trapping of windblown sand and for structural stability with their root system. However, dunes and dune vegetation are exposed to both human and climate related erosion. This study developed a new method for dune vulnerability assessment through integration of exposure criteria (marine, aeolian and human vulnerability parameters), sensitive criteria (dune geomorphology and vegetation vulnerability parameters), and adaptation criteria at South Padre Island (SPI), Texas, U.S.A. Relevant vulnerability parameters for the SPI area were preselected using the Analytical Hierarchy Process (AHP). Remote sensing and landscape metrics techniques were applied to study the dune ecosystem’s resilience through a temporal and spatial dynamic evaluation of the vegetation in response to Hurricane Dolly in 2008. Finally, an innovative methodology was developed to help quantify the contribution of dune vegetation to dune structural stability and erosion reduction. A new resistance coefficient determined from direct field measurements of dune shear strength and root density was introduced into an analytical model to estimate relative dune erosion volume reduction due to the stability of the vegetation. The integrated vulnerability assessment showed that geomorphological and vegetation characteristics were the major drivers of dune vulnerability at the South Padre Island sites. The geomorphology was characterized by high fragmentation, inadequate dune width and height, while vegetation on the primary frontal dunes were mainly annual species with no diverse root architecture for dune structural stability. Where adaptation practices such as dune walkover boardwalks, designated parking and signage were implemented, dune vulnerability indices significantly decreased as compared to locations without community adaptation programs. Remote sensing application showed a dynamic resilience of dune vegetation at South Padre Island. Landscape metrics from classified images before and after Hurricane Dolly showed a 5% decrease in dune vegetation cover, a reduction in patch density (from 6,890 patches/100 ha to 3,589 patches/100 ha), and edge density (from 2,035 m/ha to 1,521 m/ha). However, despite hurricane and routine human disturbances, vegetation cover increased by 38% from 1996 to 2016. Aggregation and the largest patch indices also increased during this period. Overall, classification and characterization of coastal dune vegetation using publicly available images gave adequate accuracy for spatial and temporal quantification using landscape metrics Based on a new vegetation induced erosion resistance coefficient range of 0.2 – 0.8, results of the analytical model showed that reduction of dune erosion volume appears to be linearly related to in-situ shear strength and root density within the collision regime. Dune resistance to shear also varied with the type of vegetation; the dunes vegetated with monocot species (sea oats) were twice as resistant to shear as those vegetated with dicot species (railroad vines and gulf croton). Therefore, sea oats and other native monocot species are more suitable for structural reinforcement and stability of dunes than dicot vegetation species for restoration activities. This study quantifies the contributions of coastal community’s adaptation and stimulates further engagements by all stakeholders. A decision support system for dune vulnerability mitigation was proposed to assign appropriate adaptation strategies based on associated vulnerability characteristics. The methodology and findings from this study have led to an improved understanding of dune ecosystem vulnerability, resilience and execution of adaptation programs that can be applied to other global coastal dune systems of similar makeup