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Sherman, D.J., 2018. Megaripple/flat bed transition in rip current channels. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 246–250. Coconut Creek (Florida), ISSN 0749-0208.Megaripple and flat bed conditions are common bed configurations in energetic nearshore environments. Their existence is controlled by shear stresses associated with wave orbital motion, superimposed net flows, and water depth, representing flow regimes. This is based on the premise of a characteristic set of flow conditions associated with particular bed configurations. A simple flow regime sequence for surf zones is ripples, megaripples (or dunes), and flat bed. Each of these bed configurations may include several secondary characteristics. Despite the importance of understanding flow regime ranges and transitions, there have been few detailed studies for characteristic megaripple and flat bed conditions. This paper presents the results of a field experiment designed to acquire flow, sedimentological, and bedform data from rip current feeder channels in an inner surf zone of a barred nearshore. Waves and water depths were measured with pressure transducers, and currents were measured with bidirectional current meters. Sand samples were gathered and bed configurations were measured and photographed by divers. Analysis indicates that the mobility number, as calculated with a number of characteristic velocities, provides first-order bedform discrimination. Observations indicate that bedform transitions can occur over sub-hour time spans, although the response times are slower as combined wave and current velocities are reduced. Under the latter conditions, especially, non-equilibrium configurations may persist.

Shuster, R.; Sherman, D.J.; Lorang, M.S.; Ellis, J.T., and Hopf, F., 2020. Erosive potential of recreational boat wakes. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1279–1283. Coconut Creek (Florida), ISSN 0749-0208.Recreational boat wakes are important sources of erosive energy in many sheltered coastal and estuarine environments. The energy content of a wake depends on boat speed, length, hull-shape and displacement and the energy delivered to an adjacent shoreline is also a function of the distance traveled by the wake. In this study that was conducted in the estuarine waters of Georgiana Slough, which is a distributory channel of the Sacramento River in California, basic geometries and speeds of common recreational boats were recorded along with the height and period of the wakes they generated. A site was instrumented with a motion-sensing video camera and pressure transducer to record boat passages and resulting wakes. Regression analysis using a curated data set produced a coefficient of determination of 0.87 linking index wave energy and boat length cubed. These results suggest that the length of a boat traveling at typical cruising speeds is a useful predictor of wake energy when a passage is within about 30 m of the shoreline. Further research is required to quantify the effects of greater distances on wake attenuation. This method could be used to monitor boat traffic in sensitive coastal environments, assess possible erosion risk from boat wakes, and inform management responses.

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 Bagnold's (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...

Aeolian streamers are common in prototype saltation systems. Streamers are elongate, flow-aligned features within which the concentrations of saltating grains are large relative to a spanwise average concentration. The occurrence of streamers introduces substantial spatial and temporal variability in local sand transport rates. There have been few studies to attempt to characterize the scales of streamers, and the results of those studies have been constrained because they use Eulerian approaches to measure an inherently Lagrangian process. We describe the results from a field experiment designed to address this methodological problem. Field experiments were conducted at Jericoacoara, Ceará, Brazil, in October, 2011. The wind field was measured with ultrasonic anemometers and ruggedized thermal probes. Transport rates were measured using Miniphones, Wenglor Particle Counters, and hose traps. A set of three video cameras, deployed in a triangular array, was used to capture images of streamers. The field of vision for the central, upwind-facing camera was a minimum of about 15 m, expanding to more than 100 m in the middle distance. Video images were time-averaged over a number of intervals, ranging from 1-64 seconds, to establish characteristic path lengths and spatial and temporal scales. The results of these analyses show that this methodological approach is technically sound. Streamer characteristics are center-to-center spacings of about 1 m, length-scales exceeding 50 m and time scales of individual streamers exceeding 64 s. © Coastal Education & Research Foundation 2013.

Farrell, E.J. and Sherman, D.J., 2013. Estimates of the Schmidt Number for vertical flux distributions of wind-blown sand. In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.) From studies of suspended sediments in water or dust in air it is recognized that the Rouse profile represents a theoretically sound, first approximation of characteristic sediment concentration gradients. Rouse (1938) combined the influence of grain size and shear velocity changes into a universal equation. The Rouse number relates sediment size (in the form of settling velocity, w0) to shear velocity, the von Karman constant (0.4) and the Schmidt Number, typically assumed to be equal to 1.0 but with much larger values reported. The shape of the Rouse concentration profile is controlled by the Rouse number exponent. We applied the Rouse profile model to 14 vertical flux profiles of wind-blown sand measured during a field experiment in Jericoacoara, Brazil in 2008. These data were supplemented with 96 vert...

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.

Photo-Electronic Erosion Pins (PEEPs) can be used to measure short-term fluctuations in swash zone (foreshore) profiles. Because the sensors depend upon ambient light for the detection of bed elevation change, their signal indicates false changes during swash inundation. A method is presented to mitigate this source of noise and to allow a reconstruction of bed changes at swash frequencies. The protocol is illustrated with a sample data set.

This research presents a segmentation-based, image processing method to automate the extraction of tidal datum referenced shorelines from airborne light detection and ranging (LIDAR) data. The method first segments a LIDAR digital elevation model (DEM) into a binary image, consisting of land and water pixels, by intersecting the LIDAR DEM and the tidal datum surface. A chain of image processing algorithms, including region grouping and labeling, two passes of image region scanning, a mathematical morphology operation, line tracing, and vectorization, is sequentially applied to the segmented binary image. Our applications to the upper Texas gulf coast show that the method is efficient, accurate, objective, and replicable. Spatially detailed shorelines can be derived from the LIDAR data with minimal human intervention. The method is a substantial technical improvement over standard cross-shore profile and contouring methods. We also examine and quantify the effects of vertical measurement error of ...

Aeolian sand transport on beaches and in dune environments shows a great spatial and temporal variability that has important implications for modeling and monitoring of coastal systems. Yet there have been few quantifications or statistical characteristics of transport variability in natural environments. Transport variability can result from bed surface control in the form of differentiation in grain size, surface moisture, and microtopography, or can be induced by fluid forcing in the form of gusts, burst-sweep events, and streamwise vortices. A field experiment was conducted on a coastal dune near Guadalupe, California, to quantify transport variability over spatial scales of 0.1–4.0 m and temporal scales of 1–120 seconds. Results show that spanwise (lateral) variability increases with spatial scale and decreases with temporal scale. Minimum transport variability over the smallest distances and longest time scales is on the order of 30%, providing error margins for transportrate measurements a...

Sediment transport across a foredune and beach at Ocean City, New Jersey, was examined to identify the effect of houses, dune topography, sand fences, vegetation, and wrack lines during an offshore wind. Houses are as close as 18 m from the crest of the 2- to 3-m-high foredune and are up to 9.0 m high and 12.8 m wide and spaced 4.0 to 5.0 m apart. Data were gathered during a 1-day study on 3 March 2003 with the use of 15 vertical sand traps, 13 erosion pins, and eight sets of anemometers placed 0.3, 0.6, and 0.9 m above the ground surface. Wind speeds 0.9 m above the ground ranged from 1.61 to 3.75 m/s. Trapping rates were negligible on the vegetated dune crest and up to 2.03 kg m−1 h−1 near mid foreshore. The highest rate of trapping was 2.4 kg m−1 h−1 at a

Longshore sediment transport rates were estimated on a microtidal estuarine beach in Great South Bay, N.Y., during two dyed sand tracer experiments using a temporal sampling method. Mean onshore wind speeds of 5.8 and 9.9 m/s resulted in root-mean-square wave heights of 0.07 and 0.08 m and wave angles of 3.0 and 10.1, causing transport rates of 0.468 and 0.972 m3/h. Rates were 3.1 to 6.5 times greater than predicted by existing equations using standard coefficients. Greater rates are attributed to the concentration of sediment transport in the energetic swash zone under plunging breakers.

The subaqueous portion of a levee bank in the Sacramento-San Joaquin River Delta of central California was instrumented to quantify the impact of boat-generated waves. Typical erosion rates associated with recreational craft are too small for direct measure- ment of bank retreat on a per-boat-passage basis; therefore, two independent analytical methods of estimating linear erosion were developed based on colocated suspended sediment concentration and velocity time series. The algorithms were tested using data measured during a field experiment in which a 7.5 m boat was driven past the site over a range of speeds to generate waves of varying size. A cross-shore array of electromagnetic current meters and optical back-scatterance sensors measured the character of boat-generated waves and the resultant sediment suspension. In near-bank, shallow-water ( d,0.5 m) locations, sediment suspension was closely correlated with the primary boat-wake waves (Hmax,0.21 m), indicating that maximum near-bottom orbital velocities were sufficient to erode the fine-grained~mud-silt! bottom materials. Suspension events were short lived ~order of 1-5 min!, despite very long particle settling times ~order of hours!, because river currents swept the suspension plumes downstream. This implies negligible sedimentation and resuspension locally. Both algorithms produced strikingly similar erosion estimates, and these values ~0.01-0.22 mm/boat passage! compare favorably with direct measurements of cumulative bank erosion in response to multiple, sequential boat passages. Field conditions for which the algorithms are appropriately applied are discussed. DOI: 10.1061/~ASCE!0733-950X~2002!128:4~152! CE Database keywords: Sediment suspension; Levees; Erosion; Ship motion; California; Measurement.

Erosion of unprotected levee banks decreases their structural integrity and increases the likelihood of failure. Several types of restoration structures for levee protection and stabilization have been used in the Sacramento–San Joaquin River Delta, California, to reduce erosion. The purpose of this paper is to describe the results of a field experiment designed to measure the effectiveness of organic restoration structures (brush bundles) in altering the hydrodynamic regime in the vicinity of levees, with specific focus on changes in boat wake energy. Two simple hypotheses were tested: 1) restoration structures reduce boat wake energy, and 2) energy reduction is dependent on water depth. Field work was conducted August 29–31, 2000 on Georgiana Slough, which is a tidally influenced (spring tidal range of 2 meters) distributary of the Sacramento River. Pressure sensors were deployed offshore and landward of the restoration structures. Data collection occurred with the bundles in place and with the...

The sediment budgets of most beaches in southern California are dominated by sand contributions from coastal streams. Extensive alteration of fluvial systems by the construction of dams and debris basins has reduced substantially the volume of sand reaching the shore. This is one factor in the chronic erosion of these beaches. The purpose of this research is to assess the magnitude of the impacts of coastal dams and debris basins on sand delivery using direct measurements of sediment impoundment rates within eight watersheds. Sediment impoundment data were obtained for 28 dams and more than 150 debris basins in the watersheds of eight rivers: the Santa Ynez, Ventura, Santa Clara, Los Angeles, San Gabriel, Santa Ana, San Dieguito, and San Diego. The cumulative effect of these structures is the impoundment of more than 4,000,000 m3 yr−1 of sediment. This is equivalent to a potential deprivation of beach sand of roughly 3 m3 yr−1 per m of shoreline in the five southern California coastal counties. T...

[1] We evaluate the increase in apparent roughness length during aeolian saltation. Using a data set comprising 291 wind profiles measured in wind tunnels or in the field, we tested the Charnock (1955), modified Charnock (Sherman, 1992), and Raupach (1991) relationships for the entire data set and for segregated wind tunnel and field data sets. We assessed the abilities of the models to predict enhanced roughness and investigated scaling differences between wind tunnels and field environments. For the entire data set, the Charnock relationship worked best, with an r2 of 0.25. For the segregated data sets, the Charnock relationship also worked best, with an average r2 of 0.675, as opposed to 0.66 for the modified Charnock and 0.61 for the Raupach model. For conditions with shear velocity ≤1.0 ms−1, the comparable, average values of r2 are 0.54, 0.625, and 0.645, respectively. There was an order of magnitude difference in the roughness length responses for wind tunnel and field data for each model evaluated, convincing evidence of a fundamental scaling constraint in the wind tunnel data. The scaling differences are attributed to conceptual and bias errors that include the use of the law of the wall versus the wake-defect law or differences in turbulence characteristics, especially regarding the role of large coherent structures. Raupach's (1991) A was found to be an extremely volatile constant, making the modified Charnock relationship most suitable for predicting aerodynamic roughness lengths over movable sand beds.

Received 5 December 2004; revised 31 March 2005; accepted 9 June 2005; published 7 September 2005. (1) Field experiments were conducted to determine the characteristic spatial and temporal dimensions and behavior of aeolian streamers and to identify the processes responsible for their formation. An instrument array that included anemometer towers, piezo-electric transport sensors (Safires), and hot-film probes was deployed to measure streamers and airflow dynamics on both a coastal dune and a desert sand mound in California. Aeolian transport occurs predominantly in the form of families of intertwining and bifurcating streamers, while under intense wind forcing, more complex patterns of nested streamers and clouds with embedded streamers develop. The streamers display a characteristic width of approximately 0.2 m and an average lateral spacing of about 1 m. These dimensions appear to be independent of mean airflow characteristics. The observations and measurements of streamers under different environmental conditions suggest that bed surface control in the form of differentiation in moisture content, grain size, or microtopography is not a necessary condition for the formation of streamers. The results show little correlation between possible streamwise vortices or burst sweep events and streamers. It is proposed that streamers are generated by near-surface gusts that originate from large eddies propagating to the ground from higher regions of the boundary layer. These elongated and stretched eddies scrape across the surface and initiate saltation along their path. This concept accounts for the characteristic size and spacing of streamers, their rapid propagation, and the fast response of saltation to wind speed fluctuations.

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 se...

Information from a bidirectional electromagnetic current meter and surface‐piercing, continuous resistance wave staff is shown to provide good estimates of the angle of wave approach within the surf zone. Spectra of surface elevation fluctuations (η) define limits to the primary frequencies in the incident wave field, which are then used to define the wave‐driven components in the onshore (x) and alongshore (y) velocity spectra. Cross spectra between η and x and y give squared coherence values used to weight the wave contribution to the motion in any frequency band. A weighted orbital vector for each frequency band is calculated: the angles corresponding to each band are then further weighted according to the proportional contribution of the variance in the velocity in that frequency band to the total variance in the onshore and offshore velocities. Summation of the individual angle contributions gives a wave angle representative of the energy distribution in the peak of the incident wave spectrum. A test...