Moulton
Melissa
Moulton
Melissa
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ArticleOn the dynamics of the Zanzibar Channel(John Wiley & Sons, 2015-09-12) Zavala-Garay, Javier ; Theiss, Jurgen ; Moulton, Melissa ; Walsh, Connor ; van Woesik, Robert ; Mayorga-Adame, Claudia G. ; García-Reyes, Marisol ; Mukaka, D. S. ; Whilden, Kerri ; Shaghude, Y. W.The Zanzibar Channel lies between the mainland of Tanzania and Zanzibar Island in the tropical western Indian Ocean, is about 100 km long, 40 km wide, and 40 m deep, and is essential to local socioeconomic activities. This paper presents a model of the seasonal and tidal dynamics of the Zanzibar Channel based on the Regional Ocean Modeling System (ROMS) and a comparison of the model and observations. The seasonal dynamics of the channel is forced by remote processes and the local wind. Remote forcing creates the East African Coastal Current, a portion of which flows through the channel northward with a seasonally varying magnitude. The local wind enhances this seasonality in the surface Ekman layer, resulting in a stronger northward flow during the southwest monsoon season and a weak northward or occasionally southward flow during the northeast monsoon season. The tidal flows converge and diverge in the center of the channel and reduce the transport in the channel. The remotely forced, wind-forced, and tidal dynamics contain 5%, 3%, and 92% of the total kinetic energy, respectively. Despite their low kinetic energy, the remotely forced and wind-forced flows are most relevant in advecting channel water to the open ocean, which occurs in 19 days at the peak of the southwest monsoon season. The channel is well mixed, except during brief periods in the two rainy seasons, and temporarily cools between December and February. The dispersion of passive tracers is presented as an example of potential model applications.
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ArticleModeled three-dimensional currents and eddies on an alongshore-variable barred beach(American Geophysical Union, 2021-06-26) Baker, Christine M. ; Moulton, Melissa ; Raubenheimer, Britt ; Elgar, Steve ; Kumar, NirnimeshCirculation in the nearshore region, which is critical for material transport along the coast and between the surf zone and the inner shelf, includes strong vortical motions. The horizontal length scales and vertical structure associated with vortical motions are not well documented on alongshore-variable beaches. Here, a three-dimensional phase-resolving numerical model, Simulating WAves till SHore (SWASH), is compared with surfzone waves and flows on a barred beach, and is used to investigate surfzone eddies. Model simulations with measured bathymetry reproduce trends in the mean surfzone circulation patterns, including alongshore currents and rip current circulation cells observed for offshore wave heights from 0.5 to 2.0 m and incident wave directions from 0 to 15° relative to shore normal. The length scales of simulated eddies, quantified using the alongshore wavenumber spectra of vertical vorticity, suggest that increasing wave directional spread intensifies small-scale eddies ( (10) m). Simulations with bathymetric variability ranging from alongshore uniform to highly alongshore variable indicate that large-scale eddies ( (100) m) may be enhanced by surfzone bathymetric variability, whereas small-scale eddies ( (10) m) are less dependent on bathymetric variability. The simulated vertical dependence of the magnitude and mean length scale (centroid) of the alongshore wavenumber spectra of vertical vorticity and very low-frequency (f ≈ 0.005 Hz) currents is weak in the outer surf zone, and decreases toward the shoreline. The vertical dependence in the simulations may be affected by the vertical structure of turbulence, mean shear, and bottom boundary layer dynamics.
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PreprintResonances in an evolving hole in the swash zone( 2011-08-06) Elgar, Steve ; Raubenheimer, Britt ; Thomson, James M. ; Moulton, MelissaWater oscillations observed in a 10-m diameter, 2-m deep hole excavated on the foreshore just above the low-tide line on an ocean beach are consistent with theory. When swashes first filled the initially circular hole on the rising tide, the dominant mode observed in the cross-shore velocity was consistent with a zero-order Bessel function solution (sloshing back and forth). As the tide rose and swash transported sediment, the hole diameter decreased, the water depth inside the hole remained approximately constant, and the frequency of the sloshing mode increased according to theory. About an hour after the swashes first reached the hole, it had evolved from a closed circle to a semi-circle, open to the ocean. When the hole was nearly semi-circular, the observed cross-shore velocity had two spectral peaks, one associated with the sloshing of a closed circle, the other associated with a quarter-wavelength mode in an open semi-circle, both consistent with theory. As the hole evolved further toward a fully semi-circular shape, the circular sloshing mode decreased, while the quarter-wavelength mode became dominant.
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ArticleComparison of rip current hazard likelihood forecasts with observed rip current speeds(American Meteorological Society, 2017-08-28) Moulton, Melissa ; Dusek, Gregory ; Elgar, Steve ; Raubenheimer, BrittAlthough rip currents are a major hazard for beachgoers, the relationship between the danger to swimmers and the physical properties of rip current circulation is not well understood. Here, the relationship between statistical model estimates of hazardous rip current likelihood and in situ velocity observations is assessed. The statistical model is part of a forecasting system that is being made operational by the National Weather Service to predict rip current hazard likelihood as a function of wave conditions and water level. The temporal variability of rip current speeds (offshore-directed currents) observed on an energetic sandy beach is correlated with the hindcasted hazard likelihood for a wide range of conditions. High likelihoods and rip current speeds occurred for low water levels, nearly shore-normal wave angles, and moderate or larger wave heights. The relationship between modeled hazard likelihood and the frequency with which rip current speeds exceeded a threshold was assessed for a range of threshold speeds. The frequency of occurrence of high (threshold exceeding) rip current speeds is consistent with the modeled probability of hazard, with a maximum Brier skill score of 0.65 for a threshold speed of 0.23 m s−1, and skill scores greater than 0.60 for threshold speeds between 0.15 and 0.30 m s−1. The results suggest that rip current speed may be an effective proxy for hazard level and that speeds greater than ~0.2 m s−1 may be hazardous to swimmers.
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ArticleA surfzone morphological diffusivity estimated from the evolution of excavated holes(John Wiley & Sons, 2014-07-14) Moulton, Melissa ; Elgar, Steve ; Raubenheimer, BrittDownslope gravity-driven sediment transport smooths steep nearshore bathymetric features, such as channels, bars, troughs, cusps, mounds, pits, scarps, and bedforms. Downslope transport appears approximately as a diffusive term in the sediment continuity equation predicting changes in bed level, with a morphological diffusivity controlling the rate of seafloor smoothing. Despite the importance of surfzone sediment transport and morphological evolution, the size of the downslope transport term in nearshore models varies widely, and theories have not been tested with field measurements. Here observations of the infill of large excavated holes in an energetic inner surf zone provide the first opportunity to infer the morphological diffusivity in the field. The estimated diffusion coefficient is consistent with a theoretical bedload morphological diffusivity that scales with the three-halves power of the representative bed shear stress.
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ArticleExtremely low frequency (0.1 to 1.0 mHz) surf zone currents.(American Geophysical Union, 2019-01-02) Elgar, Steve ; Raubenheimer, Britt ; Clark, David B. ; Moulton, MelissaLow‐frequency surf zone eddies disperse material between the shoreline and the continental shelf, and velocity fluctuations with frequencies as low as a few mHz have been observed previously on several beaches. Here spectral estimates of surf zone currents are extended to an order of magnitude lower frequency, resolving an extremely low frequency peak of approximately 0.5 mHz that is observed for a range of beaches and wave conditions. The magnitude of the 0.5‐mHz peak increases with increasing wave energy and with spatial inhomogeneity of bathymetry or currents. The 0.5‐mHz peak may indicate the frequency for which nonlinear energy transfers from higher‐frequency, smaller‐scale motions are balanced by dissipative processes and thus may be the low‐frequency limit of the hypothesized 2‐D cascade of energy from breaking waves to lower frequency motions.
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PreprintImproving the time resolution of surfzone bathymetry using in situ altimeters( 2013-06) Moulton, Melissa ; Elgar, Steve ; Raubenheimer, BrittSurfzone bathymetry often is resolved poorly in time because watercraft surveys cannot be performed when waves are large, and remote sensing techniques have limited vertical accuracy. However, accurate high-frequency bathymetric information at fixed locations can be obtained from altimeters that sample nearly continuously, even during storms. A method is developed to generate temporally and spatially dense maps of evolving surfzone bathymetry by updating infrequent spatially dense watercraft surveys with the bathymetric change measured by a spatially sparse array of nearly continuously sampling altimeters. The update method is applied to observations of the evolution of shore-perpendicular rip current channels (dredged in Duck, NC, 2012) and shore-parallel sandbars (observed in Duck, NC, 1994). The updated maps are compared with maps made by temporally interpolating the watercraft surveys, and with maps made by spatially interpolating the altimeter measurements at any given time. Updated maps of the surfzone rip channels and sandbars are more accurate than maps obtained by using either only watercraft surveys or only the altimeter measurements. Hourly altimeter-updated bathymetric estimates of five rip channels show rapid migration and infill events not resolved by watercraft surveys alone. For a 2-month observational record of sandbars, altimeter-updated maps every 6 h between nearly daily surveys improve the time resolution of rapid bar-migration events.
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ArticleRip currents and alongshore flows in single channels dredged in the surf zone(John Wiley & Sons, 2017-05-08) Moulton, Melissa ; Elgar, Steve ; Raubenheimer, Britt ; Warner, John C. ; Kumar, NirnimeshTo investigate the dynamics of flows near nonuniform bathymetry, single channels (on average 30 m wide and 1.5 m deep) were dredged across the surf zone at five different times, and the subsequent evolution of currents and morphology was observed for a range of wave and tidal conditions. In addition, circulation was simulated with the numerical modeling system COAWST, initialized with the observed incident waves and channel bathymetry, and with an extended set of wave conditions and channel geometries. The simulated flows are consistent with alongshore flows and rip-current circulation patterns observed in the surf zone. Near the offshore-directed flows that develop in the channel, the dominant terms in modeled momentum balances are wave-breaking accelerations, pressure gradients, advection, and the vortex force. The balances vary spatially, and are sensitive to wave conditions and the channel geometry. The observed and modeled maximum offshore-directed flow speeds are correlated with a parameter based on the alongshore gradient in breaking-wave-driven-setup across the nonuniform bathymetry (a function of wave height and angle, water depths in the channel and on the sandbar, and a breaking threshold) and the breaking-wave-driven alongshore flow speed. The offshore-directed flow speed increases with dissipation on the bar and reaches a maximum (when the surf zone is saturated) set by the vertical scale of the bathymetric variability.
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ThesisHydrodynamic and morphodynamic responses to surfzone seafloor perturbations(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2016-06) Moulton, MelissaHoles and channels were excavated in the surf zone on an ocean beach near Duck, NC, and observations of the subsequent evolution of waves, currents, and the modified seafloor were used to investigate nearshore dynamics. In one set of seafloor perturbation experiments, deep holes with steeply sloping sides were excavated in the inner surfzone seafloor. Observations of the infilling holes were used to make the first field estimates of the surfzone morphological diffusivity, which describes the rate of seafloor smoothing by downslope sediment transport. To improve the temporal resolution of bathymetric estimates, a mapping method was developed to combine infrequent, spatially dense watercraft surveys with continuous, spatially sparse in situ altimeter estimates of the seafloor location. In another set of seafloor perturbation experiments, channels were dredged across the surf zone with the propellers of a landing craft. Alongshore variations in wave breaking caused by the perturbed bathymetry resulted in strong rip currents in the channels under some conditions, whereas alongshore currents bypassed the channels under other conditions. The dynamics of the circulation response for changing wave forcing, bathymetry, and tidal elevation are investigated using the observations, a numerical model, and a parameter based on wave properties and bathymetry.