System Controls on the South Texas Sand Sheet

Semi-stabilized dune systems are important indicators of Quaternary drought variability across central North America. The South Texas sand sheet (STSS) is the southernmost relict dune system in central North America and is exposed to higher evapotranspiration and moisture variability than similar landscapes farther north. This study uses multi-scale analysis of LiDAR data, geophysical surveys, optically stimulated luminescence dates of core samples, and X-ray fluorescence analysis to identify historical periods of desertification across the STSS. These data suggest long-term relationships between climate, ecological disturbances, geological framework, and desertification. Aeolian activations dated at ca. 75, 230, 2000, 4100, and 6600 yr bp correspond to periods of persistent regional drought, changes in sediment supply, and anthropogenic disturbances of native ecology. From these results it appears that regionalized activation in semi-stabilized dune systems is controlled primarily by climatic variations that reduce the overall moisture available for maintaining vigorous vegetation growth, while localized activation patterns depend more on stresses related to site-specific morphodynamics as well as human activity. With enhanced aridity forecast for much of central North America through the 21st century, understanding the specific thresholds of desertification is an important step towards building a conceptual model of desertification in semi-stabilized dune landscapes

Sediment Transport and Wind Flow Around Hummocks

This study presents field-based observations demonstrating the relationships between vegetation density, shear stress and sediment transport surrounding hummocks, a type of small dune. While qualitative observations and holistic, large-scale experiments regarding vegetation density and sediment transport have confirmed a negative correlation, fewer papers have quantitatively described the nature of that relationship. Moreover, virtually no studies have specifically focused on hummocks. Field-based data collection recorded wind velocities using a sonic anemometer upwind of the hummock, grain impacts from four miniphones deployed on and to the side of the hummock (MICs), trap-derived sand transport, and hummock vegetation density. These data sets provide the parameters for model-estimated transport rates from Bagnold, Zingg, Kawamura, and Lettau and Lettau, as well as a slope correction coefficient from Bagnold. Observations were made in an uncontrolled natural setting so that heterogeneities in vegetation density and slope were included in the data; this study highlights the variability inherent from these conditions. Average wind velocity at z = 1.0 m was 7.8 m/s with an average shear velocity of 0.39 m/s. The average trap-based transport rate was 25.0 g/m2/s, while the unobstructed MIC transport was 89.8 g/m2/s. Trap and MIC-derived sand transport rates had an R2 of 0.39 (p0.05). These findings suggest that steering and projection of grains around and over the hummock play an important role in hummock morphology and processes

The Critical Zone of Coastal Barrier Systems

Barrier Islands represent some of the most dynamic and complex systems within the Critical Zone worldwide. Although coastal systems tend not to be recognized as Critical Zone environments, the evolution of Barrier Islands and the ecological functions they provide can be characterized in terms of a complex feedback among sediment supply (lithosphere), hydrology, the atmosphere, and ecology (biosphere). This represents an interesting departure from the traditional view of Barrier Island evolution (either regression or transgression) as a result of variations in sea level, sediment supply, and accommodation space. This chapter takes a Critical Zone approach to the response of Barrier Island evolution to sea-level rise and storm activity, explicitly recognizing the feedback among sediment supply, aeolian transport, disturbance regimes, vegetation development, and hydrology. © 2015 Elsevier B.V

Role of the Foredune in Controlling Barrier Island Response to Sea Level Rise

The height, volume, and alongshore extent of the foredune are primary controls on the response of barrier islands to the elevated storm surge that accompanies hurricanes and extra-tropical storms. In this respect, the ability of the foredune to recover following a storm determines whether a barrier island can maintain elevation as sea level rises and the island migrates landward through the redistribution of sediment to the back of the island through washover and breaching. This chapter provides a review of a body of recent fieldwork on the role of the foredune in controlling island transgression. It is argued that the role of the foredune to control washover and island transgression is analogous to that of a variable resistor in an electrical circuit, with the strength of the resistor dependent on the ability of the dune to recover in height and extent following each storm. Recovery of the foredune requires that sediment removed to the nearshore during a storm be returned to the beachface through the landward migration and welding of the innermost bars where it is eventually transported to the backshore and trapped by vegetation. Field observations from Padre Island in Texas, Santa Rosa Island in Florida, and Assateague Island in Virginia suggest that the recovery of dune height can be modeled using a sigmoidal growth curve, and that recovery can take up to a decade. The slow rate of dune recovery suggests that the resiliency of barrier islands to sea level rise is dependent on whether there is a change in the frequency and magnitude of storm events or an interruption to the exchange of sediment among the nearshore, beach, and dune. Ultimately, the height and volume of the foredune can be controlled by the framework geology (to varying degrees), which determines beach and nearshore state through the availability and texture of sediment and structural controls. In this respect, the response of barrier islands to sea level rise can be expected to vary regionally and alongshore as a reflection of diverse framework geology. The local response to sea level rise depends on the ability of the dune to recover following storms. Assuming no new sediment from alongshore or offshore sources, an increase in the frequency of washover will limit the ability of the dune to recover, and recent field evidence suggests that a change in dune height and volume is self-reinforcing, which suggests a lack of island resiliency. Further testing is required to determine how the field observations and modeling described in this chapter from a select group of barrier islands around the United States are applicable to other islands and consistent throughout the evolution of a barrier island