Thomas M. Ravens

Does vegetation prevent wave erosion of salt marsh edges

This study challenges the paradigm that salt marsh plants prevent lateral wave-induced erosion along wetland edges by binding soil with live roots and clarifies the role of vegetation in protecting the coast. In both laboratory flume studies and controlled field experiments, we show that common salt marsh plants do not significantly mitigate the total amount of erosion along a wetland edge. We found that the soil type is the primary variable that influences the lateral erosion rate and although plants do not directly reduce wetland edge erosion, they may do so indirectly via modification of soil parameters. We conclude that coastal vegetation is best-suited to modify and control sedimentary dynamics in response to gradual phenomena like sea-level rise or tidal forces, but is less well-suited to resist punctuated disturbances at the seaward margin of salt marshes, specifically breaking waves.

Causes of Salt Marsh Erosion in Galveston Bay, Texas

Abstract There is major salt marsh loss in Galveston Bay and other estuarine environments. In Galveston Bay, the causes of marsh loss include wave action, subsidence, eustatic sea-level rise, and insufficient sediment supply. To assess the relative importance of these factors in marshes of West Galveston Bay, wave action, sediment supply, and sedimentation rates were studied. Analysis of the data indicated a significant gap between the historic sediment accretion rate of 0.20 cm y−1 and the relative sea-level rise (the rate of rise of the water depth due to the combined effects of eustatic rise and subsidence) of 0.65 cm y−1. Furthermore, in 94% of the eroding marshes, where 20% exceedance wave height was less than 0.17 m, the role of wave-induced erosion was relatively small. Thus, the major cause for salt marsh loss is insufficient sediment supply. These findings indicate that in the many eroding marshes in Galveston Bay, where wave action is not the major cause of marsh loss, marsh restoration efforts need to enhance sedimentation rather than wave protection.

Process-Based Coastal Erosion Modeling for Drew Point, North Slope, Alaska

A predictive, coastal erosion/shoreline change model has been developed for a small coastal segment near Drew Point, Beaufort Sea, Alaska. This coastal setting has experienced a dramatic increase in erosion since the early 2000s. The bluffs at this site are 3-4 m tall and consist of ice-wedge bounded blocks of fine-grained sediments cemented by ice-rich permafrost and capped with a thin organic layer. The bluffs are typically fronted by a narrow (∼5 m wide) beach or none at all. During a storm surge, the sea contacts the base of the bluff and a niche is formed through thermal and mechanical erosion. The niche grows both vertically and laterally and eventually undermines the bluff, leading to block failure or collapse. The fallen block is then eroded both thermally and mechanically by waves and currents, which must occur before a new niche forming episode may begin. The erosion model explicitly accounts for and integrates a number of these processes in- cluding: (1) storm surge generation resulting from wind and atmospheric forcing, (2) erosional niche growth resulting from wave-induced turbulent heat transfer and sediment transport (using the Kobayashi niche erosion model), and (3) thermal and mechanical erosion of the fallen block. The model was calibrated with historic shoreline change data for one time period (1979-2002), and validated with a later time period (2002-2007). DOI: 10.1061/(ASCE)WW.1943-5460.0000106.

Measurement of Longshore Sediment Transport Rates in the Surf Zone on Galveston Island, Texas

Abstract Longshore sediment transport in the surf zone on Galveston Island, Texas, was studied to develop a new technique involving optical instruments rapidly calibrated in situ and to compare measured transport rates with those predicted by the well-known Coastal Engineering Research Center (CERC) formula. This method used an instrumented sled equipped with a LISST-100 for particle size distribution determination, four optical backscatter sensors (OBSs) for turbidity measurements, and two velocity sensors for longshore current measurements. The sled was pulled across the surf zone, occupying 10 to 15 stations spaced about 10 m apart, for approximately 3 minutes each. The OBS data were calibrated with the LISST-100 particle size distribution data, thereby overcoming the difficulties associated with the use of these sensors in the presence of a mix of sand and fine particles. Subsequently, these data were fit to a logarithmic profile to determine the average vertical distribution of suspended sand concentration, assuming that the fine particles were vertically well mixed at each station. A logarithmic profile of average longshore current was also computed based on the measured velocity data. The longshore sediment transport rate was calculated as the spatial integral of the product of suspended sand concentration and velocity and related to the wave conditions at the point of breaking. Measured rates ranged from 86,000 to 231,000 m3/y, and transport was found to be greatest in the vicinity of the sand bars. The popular CERC formula gave sediment transport rates significantly greater than the observed values, with the difference between the two on the order of 100%. The average CERC transport coefficient, K1, computed from our measurements was determined to be 0.19 ± 0.12.

Numerical Modeling and Analysis of Shoreline Change on Galveston Island

Abstract Galveston shoreline data from 1956, 1965, 1990, and 2001 were analyzed with a sediment budget to infer the longshore and cross-shore (out of the littoral system) transport. The analysis indicated a relatively calm period from 1990 to 2001, which was dominated by longshore transport, as well as two hurricane-prone periods (1956–65 and 1965–90), which had both longshore and cross-shore transport. The Wave Information Study wave data (1976–95) were then examined to identify those years where the waves gave the “expected” longshore transport. Five wave years were selected for further study. Numerical modeling of shoreline change on Galveston Island, based on the five selected wave years, was conducted in order to gain an understanding of, and to design remedial action for, an erosional hotspot at the end of the islands seawall. The GENESIS model was chosen after careful consideration of its assumptions and performance and after analysis of site conditions. It was judged to be a suitable shoreline change model for the relatively calm 1990–2001 period, but not for the excessively hurricane-prone 1956–90 period. Model calculations of shoreline change from 1990 to 2001 were in agreement with the measurements in the vicinity of the hotspot. The model was used to design beach nourishment at the hotspot for 2001–11. The calculations indicate that about 100,000 m3/y of sand would be needed to maintain the 2001 shoreline. Under storm conditions, a sediment budget indicated that an additional 300,000 m3/y might be necessary to maintain the 2001 shoreline.

Abrasion Testing of Critical Components of Hydrokinetic Devices

The objective of the Abrasion Testing of Critical Components of Hydrokinetic Devices (Project) was to test critical components of hydrokinetic devices in waters with high levels of suspended sediment – information that is widely applicable to the hydrokinetic industry. Tidal and river sites in Alaska typically have high suspended sediment concentrations. High suspended sediment also occurs in major rivers and estuaries throughout the world and throughout high latitude locations where glacial inputs introduce silt into water bodies. In assessing the vulnerability of technology components to sediment induced abrasion, one of the greatest concerns is the impact that the sediment may have on device components such as bearings and seals, failures of which could lead to both efficiency loss and catastrophic system failures.