Sarah Trimble

Accuracy Evaluation of an ASTER-Derived Global Digital Elevation Model (GDEM) Version 1 and Version 2 for Two Sites in Western Africa

This study compares the ASTER Global DEM version 1 (GDEMv1) and version 2 (GDEMv2) for two study sites with distinct terrain and land cover characteristics in western Africa. The effects of land cover, slope, relief, and stack number are evaluated through both absolute and relative DEM statistical comparisons. While GDEMv2 at times performed better than GDEMv1, this improvement was not consistent, revealing the complex nature and interaction of terrain and land cover characteristics, which influences the accuracy of GDEM tiles on local and regional scales.

Influence of Blade Flexibility on the Drag Coefficient of Aquatic Vegetation

Estimates of energy dissipation due to vegetation depend on the selection of an appropriate drag coefficient. The drag afforded by submerged and emergent vegetation depends, to varying degrees, on the incident forcing, the characteristics of the vegetation field, and the morphology of the individual blades. Recent field and laboratory evidence suggests that blade rigidity is an important, but poorly understood, control on wave attenuation, in which greater flexibility reduces drag and limits wave attenuation. The purpose of this study is to quantify the influence of blade rigidity on the drag coefficient through a laboratory experiment in which wave attenuation is measured through artificial vegetation of varying rigidity over a range of water depths and wave forcing. In general, greater flexibility caused a reduction in the drag coefficient (CD), independent of changes in water depth, although the drag would be expected to decrease as the canopy was submerged further. However, the drag coefficient of semi-rigid vegetation that pivots from the base exhibits a more complex relationship with the Reynolds number, with a significant decrease in drag as the vegetation transitions from submergent to emergent in response to the increase in oscillatory velocities at a lower point on the stem. The variation in the drag coefficient among the artificial vegetation data and previously published data from a seagrass meadow exhibits a statistically significant variation with rigidity, calculated as the ratio of Young’s modulus and wave forcing. The dependency of the drag coefficient on rigidity provides a means to estimate energy dissipation by vegetation of a given blade morphology using simple design variables.

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.

Phases of blowout initiation and stabilization on Padre Island revealed through ground-penetrating radar and remotely sensed imagery

Abstract Blowouts cover a large section of North Padre Island, Texas, and have previously been associated with overgrazing and vehicles driving between the low water line and the toe of the foredune. In the current study, the evolution and migration of 18 blowouts within Padre Island National Seashore are tracked using aerial photographs and satellite images since 1969. Representative active and stabilized trough blowouts are also examined using ground-penetrating radar surveys and remotely sensed imagery to identify the phases of blowout initiation and stabilization between 1969 and 2010. At least five distinct phases can be identified in the radar surfaces and facies of the active blowout, including both the stabilization and reactivation of the blowout throat, whereas only four phases can be identified in the surfaces and facies of a blowout that completely stabilized in 2010. Results suggest that blowout evolution on Padre Island is episodic and is initiated by the opening and closing of the blowout throat during periods with elevated storm surge. The lack of reflectors at depth is evidence that the development of blowouts on Padre Island is a relatively new phenomenon initiated by overgrazing and maintained by compaction of the beach and backshore by vehicles.

Seawalls and Signage: How Beach Access Management Affects Rip Current Safety

Rip currents are concentrated flows of water flowing out to sea faster than the surrounding waves. These currents form as a result of alongshore variations in wave set-up driven by variable nearshore morphology or hard structures that interrupt the longshore current. Recent research from the United States, Costa Rica, Australia, and the United Kingdom suggests that the beach going public is mostly unaware of how to identify, avoid, and escape rip currents. As a result, hundreds of rip current related deaths occur worldwide each year, making rip currents a global health hazard. While an increasing number of programs are created in coastal countries, many aimed at increasing public awareness and education, signage, or improving lifeguard programs, there is increasing evidence that existing warning systems and signage are ineffective because beach users are unable to translate the warning into a real-world feature. Further evidence suggests that beach access management can inadvertently steer unsuspecting beach users towards rip-prone areas, increasing the chances of a drowning occurring on that beach. For example, alongshore variations in the offshore bathymetry at Pensacola Beach, Florida responsible for semi-permanent rip-prone sections of the beach are also responsible for the development of relatively small dunes in the backshore. Beach access points were preferentially built in the areas with smaller dunes, thereby focusing beach-users towards the most rip-prone sections of the beach. In another example from Jaco Beach, Costa Rica, public beach access points are adjacent to stream outlets that are responsible for creating a nearshore terrace and rip morphology, and are focusing beach users’ access and activity towards rip-prone sections of the beach. In contrast, the evenly spaced beach access ramps from the seawall down to the sand of Australia’s famous Bondi Beach in Sydney do not focus beach-users and activity towards rip-prone sections of the beach. However, the placement of a popular bus stop and hostels invite the most vulnerable and unaware beach users swimmers towards the southern end of the beach with a large semi-permanent rip current called the “Backpacker’s Express.” Through these examples, we conclude that when developers do not consider beach and nearshore geomorphology in their designs for beach access management, they may lead unsuspecting and unaware beach users towards the rip hazard and increase the potential for drownings.

Determining Beach User Knowledge of Rip Currents in Costa Rica

ABSTRACT Llopis, I.A.; Echeverria, A.G.; Trimble, S.; Brannstrom, C., and Houser, C., 2018. Determining beach user knowledge of rip currents in Costa Rica. Rip currents account for over 50 drownings a year in Costa Rica, with most drownings involving young single male students at beaches in close proximity to San Jose. The hazard posed by a rip current in Costa Rica and elsewhere depends in part on beach user knowledge of the rip current hazard and their ability to identify the situations in which there is the potential for drowning or need for rescue. This study describes the results of beach user surveys (n = 171) completed in English and Spanish at Jacó Beach on the central Pacific coast and Cocles Beach on the Caribbean coast of Costa Rica in 2013 and 2014. Rip current knowledge amongst national (domestic) and foreign tourists was estimated on both beaches. Results suggest that the amount of time spent in the water, activities on the beach, and self-assessment of swimming abilities help to explain why more males drown despite similar populations of males and females at both beaches. The personal and group behaviors that increase the potential for drowning are exacerbated by problems with the rip current warning system used at each beach. Approximately 58% of respondents did not observe the warnings, and 41% self-reported not changing their behavior after observing the sign, with 40% of respondents noting that the messages on the signs were confusing. Results of the study highlight the need to design and employ more effective warning signs, to set up a national certified lifeguard corps, and to plan educational activities aimed at those who are at greatest risk to drowning. This assessment and the identified need to develop a national policy can serve as a model for other countries in Central America and elsewhere where rip current–related drownings are a public health concern.

The psychology of beach users: importance of confirmation bias, action, and intention to improving rip current safety

The rip current hazard on beaches is a global public health issue. While physical controls on rip current formation and flow behavior are relatively well understood, there has been a recent increase in studies examining the less-understood social dimensions of the hazard. This paper reviews how these studies provide insight into beach safety practices and rip current knowledge among beach users, their ability to spot a rip, and their ability to understand and heed posted warnings. However, we identify how these studies are hindered by methodological limitations and problematic sample choices. It is argued that beach user behavior is affected by confirmation bias, a cognitive shortcut by which a person selectively attends to evidence confirming their preexisting beliefs and ignores disconfirming evidence, and to make decisions in accordance with this bias. Evidence is presented to suggest that there is a potential for beach-going behaviors to be influenced by other beach users and past observations, whether the behavior of others conforms to warnings about the dangers posed by rips in general or at a specific time and place. We also suggest that beach users’ self-reported intentions and beliefs about beach safety may not correspond to their actions at the beach. This suggests a need for active intervention and creation of social norms that address cognitive errors associated with unsafe beach behavior, which in turn requires that coastal scientists and beach safety practitioners should collaborate with social scientists and psychologists for more effective safety outcomes.

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.