Jean T. Ellis

Measuring Aeolian Saltation: A Comparison of Sensors

Abstract We report the results of field experiments designed to compare four types of aeolian saltation sensors: the Safire; the Wenglor® Particle Counter; the Miniphone; and the Buzzer Disc. Sets of sensors were deployed in tight spatial arrays and sampled at rates as fast as 20 kHz. In two of the three trials, the data from the sensors are compared to data obtained from sand traps. The Miniphone and the Buzzer Disc, based on microphone and piezoelectric technologies, respectively, produced grain impact counts comparable to those derived from the trap data. The Safire and the Wenglor® Particle Counter produce count rates that were an order of magnitude too slow. Safires undercount because of their large momentum threshold and because its signal is saturated at relatively slow transport rates. We conclude that the Miniphone and the Buzzer Disc are appropriate for deployment as grain counters because their small size allows them to be installed in closely-spaced sets.

10.10 Beach and Dune Interaction

This chapter examines beach–dune interactions, the sediment transport, and morphological connection between beach and dune systems, from a process perspective. We introduce the components of beach-dune interactions from small to large temporal scales: process, tidal and storm, and Holocene. The chapter concludes by integrating the previously discussed concepts and introducing dominant beach–dune interaction models. Linking these processes over a range of spatial and temporal scales has proven quite difficult, remains an important focus of the science, and provides opportunity for future investigations.

Estimating the impact threshold for wind-blown sand

ABSTRACT Li, B., Ellis, J. T., Sherman, D. J., 2014. Estimating the Impact Threshold for Wind-Blown Sand. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 627–632, ISSN 0749-0208. In many aeolian studies, it is commonplace to use Bagnolds (1936) equation to calculate threshold shear velocity (u*t), which includes an empirical constant, A, typically set at about 0.082 for maintaining saltation (the dynamic, or impact, threshold). Here, we present data from a pilot study to assess the variability of A to improve estimations of u*t, which in turn, should improve transport rate predictions. Using field data from three coastal environments, we measured or calculated all parameters within the Lettau and Lettau (1978) model and u*t equation. In Jericoacoara, Brazil (BRA), Inch, Ireland (IRE), and Esposende, Portugal (POR) wind velocities were measured with cup anemometer towers and transport rates were measured using traps for 31 data runs lasting 120 to 1020 seconds each. Mean grain sizes were 0.17 mm (IRE), 0.31 mm (POR), and 0.30 mm (BRA), and mean shear velocities were 0.38 m s−1 (IRE), 0.40 m s−1 (POR), and 0.49 m s−1 (BRA). Empirically determined, adjusted A values ranged from 0.02 to 0.21 with a mean and standard deviation of 0.11 and 0.04. No relationship exists between estimates of A and grain Reynolds number. A statistically significant (p < 0.001), negative relationship was found between A and mass transport rate, leading to substantial over-prediction of transport rates near the threshold and under-prediction during fast winds if a constant of 0.082 is used.

Dynamics of sediment storage and release on aeolian dune slip faces: A field study in Jericoacoara, Brazil

Sediment transport on the lee sides of aeolian dunes involves a combination of grain-fall deposition on the upper portion of the slip face until a critical angle is exceeded, transport of a portion of those sediments down the slip face by grain flows and, finally, deposition at an angle of repose. We measured the mean critical and repose angles and the rate of slip-face avalanching using terrestrial laser scanning (TLS) on two barchans of different size in Jericoacoara, Brazil. Wind speeds and sand fluxes were measured simultaneously at the dune crests. We found that the mean critical and repose angles decrease with increasing wind speed. We attribute this to turbulent shear stresses, the magnitude of which are predicted using 3D Large-Eddy Simulation (LES) modeling, that episodically act down the slip face (i.e. in the direction of gravity) to trigger grain flows at lower angles than with gravity stresses alone. The rate of avalanching is a maximum in the morning at our study site and coincides with the maximum rate of increase in wind speed and not with the maximum rate of sediment supply to the slip face. We developed and tested a new predictive model for the rate of avalanching that includes both sediment flux delivered to the slip face and the derivative of the critical angle with time. Our data also suggest that the mean critical angle varies inversely with slip-face height. These results have important implications for aeolian dune evolution, interpretations of aeolian stratigraphy, and granular mechanics.

Storm-Driven Hydrodynamic and Sedimentological Impacts to an Engineered Coast

ABSTRACT Ellis, J.T. and Cappietti, L., 2013. Storm-driven hydrodynamic and sedimentological impacts to an engineered coast. We replicated a breakwater, seawall, and artificial gravel beach from Marina di Pisa, Italy in a 1:40 scale-model. The model was tested in a wave-current flume that is 50 m long, 0.8 meters wide and high, and located at the University of Florence. The gravel beach, berm, and breakwater models were constructed based upon in situ beach surveys. The laboratory quartz boulder beach and breakwater D50 approximated 3.0 mm and 3.6 cm, respectively. The laboratory hydrodynamics aimed to simulate a portion of storm measured at a local wave buoy during an event in October 2003 that lasted 54 hours. Ten wave runs were conducted with durations ranging from 9.5 - 43 min with programmed peak wave periods of 1.4 - 2.1 s and significant wave heights of 9.6 - 17.5 cm. Seven wave gauges were distributed along the wave flume, two adjacent to wave maker, three just offshore the breakwater, and two just onshore of the breakwater. The clear glass side of the flume was delineated to capture millimeter scale changes of beach and berm morphology. Wave transmission (Kt) values calculated using field-based measurements ranged from 0.33–0.52, suggesting that between 11–27% of the offshore energy is transmitted through the breakwater. In the laboratory simulation of the storm, the modeled Kt values under-predicted measured Kt values by 1–16% and the berm migrated onshore to the upper shoreface and flattened the overall beach profile.

Wave Transmission and Water Setup Behind an Emergent Rubble-Mound Breakwater

ABSTRACT Cappietti, L.; Sherman, D.J., and Ellis, J.T., 2013. Wave transmission and water setup behind an emergent rubblemound breakwater. Wave transmission and water setup landward of emergent detached breakwaters play a major role in altering nearshore hydrodynamics, circulation, and morphodynamics and influencing the safety of recreational bathers. These phenomena have been extensively studied by means of small-scale laboratory experiments and numerical simulation, but field measurements have been extremely limited. In this work, wave transmission and setup were measured landward of a detached, emergent rubble-mound breakwater at Marina di Pisa, Italy. Water-surface elevations were measured with pressure transducers onshore and offshore of the breakwater. Measurements were made for 14 hours during a storm with significant wave heights up to 3.5 m and mean periods of 8.5 s. In the present work, the data were used to quantify and develop statistical relationships describing wave transmission and water setup and to evaluate the performance of empirical models of transmission and setup for emergent breakwaters. We found that the incident wave height explained (statistically) 94% of the variability in wave transmission and 96% of the variability in setup for the Marina di Pisa breakwater. There was a 99% coefficient of determination when explaining variation in setup using incident wave height and transmissivity. Of the empirical models we tested, those that best predicted observed wave transmission landward of the breakwater had root mean square (RMS) errors of 33%. The best available models to predict observed setup had RMS errors of 66%.

Intermittent Aeolian Saltation: A Protocol For Quantification

Abstract The quantification of saltation intermittency—the proportion of time when sand transport occurs—provides valuable information about the nature of aeolian systems, including insights concerning threshold conditions for sand transport. Intermittency has been measured in numerous studies, but a lack of measurement standardization often makes the comparison of results difficult. Four methodological factors influence estimates of intermittency: sample frequency, sample duration, sample area, and sensor elevation. For a given transport rate, the value of intermittency may decrease with sensor elevation, increased sample frequency, or decreased sensor area; it may increase or decrease as a consequence of sample duration. We therefore simulate the influences of these factors and suggest a common protocol for measuring and reporting intermittency based on measurements at 1 Hz with 100 mm2 sensor area at 50 mm elevation above the surface, or equivalent values.

Gulf of Mexico processes

Abstract This contribution is part of the Gulf Regional Sediment Management Master Plan and serves as a broad overview of the general setting, geologic history, coastal processes, hydrodynamics, and sediment sources for the Gulf of Mexico (GOM). The GOM is approximately 1,500,000 km2. The five U.S. states bordering the Gulf comprise over 75,000 km of coastline. Gulf circulation is dominated by the Loop Current and gyres. The general trend of longshore sediment transport is in the westward direction west of the Mississippi River and in the eastward direction east of the Mississippi River, and is driven primarily by the wave and tidal energy. The GOM is a shallow basin and its climate is strongly dependent on precipitation and temperature averages. The general coastal climate is subtropical with warm to hot summers and cool winters with precipitation and high relative humidity throughout the year. The GOM coast is predominantly microtidal (<2-m range) with coastal processes and associated morphodynamics strongly influenced by storms. Tropical cyclones have affected every GOM coastal county or parish since 1900. Over 150 rivers flow into the Gulf; 85% of the fluvial water contribution is from U.S. rivers and of that, 64% is from the Mississippi River, which discharges approximately 2.4 billion kg of sediment annually. Humans are altering the natural coastal sediment budget through beach nourishment and dredging that affects the beach–dune sediment exchange. Six of the 10 most socioeconomically vulnerable coastal counties in the United States are in the Gulf region, which emphasizes the need for a sediment management master plan.

Geospatial method for computing supplemental multi-decadal US coastal land use and land cover classification products, using Landsat data and C-CAP products

This paper discusses the development and implementation of a method that can be used with multi-decadal Landsat data for computing general coastal US land use and land cover (LULC) maps consisting of seven classes. With Mobile Bay, Alabama as the study region, the method that was applied to derive LULC products for nine dates across a 34-year time span. Classifications were computed and refined using decision rules in conjunction with unsupervised classification of Landsat data and Coastal Change and Analysis Program value-added products. Each classification’s overall accuracy was assessed by comparing stratified random locations to available high spatial resolution satellite and aerial imagery, field survey data and raw Landsat RGBs. Overall classification accuracies ranged from 83 to 91% with overall κ statistics ranging from 0.78 to 0.89. Accurate classifications were computed for all nine dates, yielding effective results regardless of season and Landsat sensor. This classification method provided useful map inputs for computing LULC change products.

Behavior and identification of ephemeral sand dunes at the backshore zone using video images

The backshore zone is transitional environment strongly affected by ocean, air and sand movements. On dissipative beaches, the formation of ephemeral dunes over the backshore zone plays significant contribution in the beach morphodynamics and sediment budget. The aim of this work is to describe a novel method to identify ephemeral dunes in the backshore region and to discuss their morphodynamic behavior. The beach morphology is identified using Argus video imagery, which reveals the behavior of morphologies at Cassino Beach, Rio Grande do Sul, Brasil. Daily images from 2005 to 2007, topographic profiles, meteorological data, and sedimentological parameters were used to determine the frequency and pervasiveness of these features on the backshore. Results indicated that coastline orientation relative to the dominant NE and E winds and the dissipative morphological beach state favored aeolian sand transport towards the backshore. Prevailing NE winds increase sand transportation to the backshore, resulting in the formation of barchans, transverse, and barchanoid-linguiod dunes. Precipitation inhibits aeolian transport and ephemeral dune formation and maintains the existing morphologies during strong SE and SW winds, provided the storm surge is not too high.