Rip current review MacMahan, Jamie H.; Thornton, Ed B.; Reniers, Ad J.H.M.
Coastal engineering,
02/2006, Letnik:
53, Številka:
2
Journal Article, Conference Proceeding
Recenzirano
Rip currents are shore-normal, narrow, seaward-flowing currents that originate within surf zone, extend seaward of the breaking region (rip head), and can obtain relatively high velocities. Within ...the last decade, there have been a significant number of laboratory and field observations within rip current systems. An overview of rip current kinematics based on these observations and the scientific advances obtained from these efforts are synthesized. Rip current flows are partitioned into mean, infragravity, very low frequency (vorticity), and tidal contributions, and it is found that each contributes significantly to the total. Data from the laboratory and the field suggest that the rip current strength increases with increasing wave energy and decreasing water depths. The maximum mean current occurs inside the surf zone, where the maximum forcing is present owing to the dissipation of waves.
Plankton are transported onshore, providing subsidies of food and new recruits to surf-zone and intertidal communities. The transport of plankton to the surf zone is influenced by wind, wave, and ...tidal forcing, and whether they enter the surf zone depends on alongshore variation in surf-zone hydrodynamics caused by the interaction of breaking waves with coastal morphology. Areas with gently sloping shores and wide surf zones typically have orders-of-magnitude-higher concentrations of plankton in the surf zone and dense larval settlement in intertidal communities because of the presence of bathymetric rip currents, which are absent in areas with steep shores and narrow surf zones. These striking differences in subsidies have profound consequences; areas with greater subsidies support more productive surf-zone communities and possibly more productive rocky intertidal communities. Recognition of the importance of spatial subsidies for rocky community dynamics has recently advanced ecological theory, and incorporating surf-zone hydrodynamics would be an especially fruitful line of investigation.
Cross-shore exchange between the surf zone and the inner shelf is investigated using Lagrangian and Eulerian field measurements of rip current flows on a rip-channeled beach in Sand City, California. ...Surface drifters released on the inner shelf during weak wind conditions moved seaward due to rip current pulses and then returned shoreward in an arcing pattern, reentering the surf zone over shoals. The cross-shore velocities of the seaward- and shoreward-moving drifters were approximately equal in magnitude and decreased as a function of distance offshore. The drifters carried seaward by the rip current had maximum cross-shore velocities as they exited the surf zone and then decelerated as they moved offshore. The drifters moving shoreward accelerated as they approached the surfzone boundary with maximum cross-shore velocities as they reentered the surf zone over shoals. It was found that Stokes drift was not solely responsible for the onshore transport across the surfzone boundary. The cross-shore diffusivity on the inner shelf was greatest during observations of locally contained cross-shore exchange. These field observations provide evidence that the cross-shore exchange between the surf zone and inner shelf on a rip-channeled beach is due to wave-driven rip current circulations and results in surface material being contained within the nearshore region.
Abstract
We present observations of shoaling nonlinear internal bores off the coast of central California. The dataset includes 15 moorings deployed during September–October 2017 and cross-shore ...shipboard surveys. We describe the cross-shore structure and evolution of large-amplitude internal bores as they transit from 9 km (100-m depth) to 1 km offshore (10 m). We observe that two bores arrive each semidiurnal period, both propagating from the southwest; of the total, 72% are tracked to the 10-m isobath. The bore speeds are subtidally modulated, but there is additional bore-to-bore speed variability that is unexplained by the upstream stratification. We quantify temporal and cross-shore variability of the waveguide (the background conditions through which bores propagate) by calculating the linear longwave nonrotating phase speed
c
o
and using the nonlinearity coefficient of the Korteweg–de Vries equation
α
as a metric for stratification. Bore fronts are generally steeper when
α
is positive and are more rarefied when
α
is negative, and we observe the bore’s leading edge to rarefy from a steep front when
α
is positive offshore and negative inshore. High-frequency
α
fluctuations, such as those nearshore driven by wind relaxations, contribute to bore-to-bore variability of the cross-shore evolution during similar subtidal waveguide conditions. We compare observed bore speeds with
c
o
and the rotating group velocities
c
g
, concluding that observed speeds are always faster than
c
g
and are slower than
c
o
at depths greater than 32 m and faster than
c
o
at depths of less than 32 m. The bores maintain a steady speed while transiting into shallower water, contrary to linear estimates that predict bores to slow.
Estimates of animal abundance are widely used to support conservation and resource management. For populations in open systems, abundance estimates from tagging data can be highly uncertain or ...biased. Here, we develop a novel approach to estimate abundance of an open population by pairing two models, each utilizing distinct tagging data. Using data from telemetry tags, we infer movement rates to and from the study site with a Markovian model allowing for an environmental effect. Then, using data from conventional passive tags, we apply a Lincoln–Petersen abundance estimator modified to account for mortality and movement. After developing the model within a Bayesian framework, we demonstrate its application to data on gray triggerfish (Balistes capriscus) tagged in the Atlantic Ocean off North Carolina, USA. For this open population, we estimate site abundance to be ∼1000 fish (∼2000 fish·km
–2
) and additionally find evidence for an effect of hurricanes on movement. The general approach may be useful for fisheries, wildlife, and other ecological studies utilizing multiple tag types, particularly for estimating abundance of an open population.
Abstract
Temperature and velocity measurements from 42 moorings were used to investigate the alongshore variability of nonlinear internal bores as they propagated across the central California inner ...shelf. Moorings were deployed September–October 2017 offshore of the Point Sal headland. Regional coverage was ~30 km alongshore and ~15 km across shore, spanning 9–100-m water depths. In addition to subtidal processes modulating regional stratification, internal bores generated complex spatiotemporal patterns of stratification variability. Internal bores were alongshore continuous on the order of tens of kilometers at the 50-m isobath, but the length scales of frontal continuity decreased to
O
(1 km) at the 25-m isobath. The depth-averaged, bandpass-filtered (from 3 min to 16 h) internal bore kinetic energy
was found to be nonuniform along a bore front, even in the case of an alongshore-continuous bore. The pattern of along-bore
variability varied for each bore, but a 2-week average indicated that
was generally strongest around Point Sal. The stratification ahead of a bore influenced both the bore’s amplitude and cross-shore evolution. The data suggest that alongshore stratification gradients can cause a bore to evolve differently at various alongshore locations. Three potential bore fates were observed: 1) bores transiting intact to the 9-m isobath, 2) bores being overrun by faster, subsequent bores, leading to bore-merging events, and 3) bores disappearing when the upstream pycnocline was near or below middepth. Maps of hourly stratification at each mooring and the estimated position of sequential bores demonstrated that an individual internal bore can significantly impact the waveguide of the subsequent bore.
Abstract
Cross-shore heat flux (CHF) spatiotemporal variability in the subtidal (ST), diurnal (DU), and semidiurnal (SD) bands is described for 35 days (summer 2015) from collocated vertical measures ...of temperature and currents obtained by moorings deployed from 50- to 7-m water depths near Pt. Sal, California. The CHF is largest in the ST and SD bands, with nearly zero contribution in the DU band. The sum of CHF and surface heat flux (SHF) account for 31% and 17% of the total change in heat storage on the midshelf and inner shelf, respectively. The ST CHF for the midshelf and inner shelf is mostly negative and is correlated with upwelling-favorable winds. A mostly positive SD CHF on the midshelf and inner shelf decreases linearly in the shoreward direction, is correlated with wind relaxations, and is attributed to warm-water internal tidal bores (WITBs) that are observed to propagate to the edge of the surf zone. A negative SD CHF is correlated with upwelling-favorable winds on the midshelf at 15–25-h time lags, and is believed to be associated with cold-water internal tidal bores. The WITBs have characteristics of progressive waves on the midshelf and transition to partially standing waves on the inner shelf potentially reducing the SD CHF contribution on the inner shelf. Heat accumulation over the midshelf and inner shelf is primarily driven by WITBs and SHF, which is largely balanced by cumulative cooling by ST processes over the midshelf and cumulative cooling by alongshore heat flux (AHF) over the inner shelf.
This study investigates the potential of rotary wing unmanned aerial vehicles (UAVs) to monitor the surfzone. This paper shows that these UAVs are extremely flexible surveying platforms that can ...gather near-continuous moderate spatial resolution and high temporal resolution imagery from a fixed position high above a study site. The rotary wing UAVs used in this study can fly for ~12 min with a mean loiter radius of 1-3.5 m and a mean loiter error of 0.75-4.5 m. These numbers depend on the environmental conditions, flying style, battery type, and vehicle type. The images obtained from the UAVs, and in combination with surveyed ground control points (GCPs), can be georectified to a pixel resolution between 0.01 and 1 m, and a reprojection error-that is, the difference between the surveyed GPS location of a GCP and the location of the GCP obtained from the georectified image-of O(1 m). The flexibility of rotary wing UAVs provides moderate spatial resolution and high temporal resolution imagery, which are highly suitable to quickly obtain surfzone and beach characteristics in response to storms or for day-to-day beach safety information, as well as scientific pursuits of surfzone kinematics on different spatial and temporal scales, and dispersion and advection estimates of pollutants.
Fixed acoustic Doppler current profiler (ADCP) velocity measurements are used to investigate headland vorticity generation and recirculation in ∼20 m depth around the small (∼1 km) central California ...headland Pt. Sal. To reduce vorticity estimation noise, velocities are reconstructed from the first two Empirical Orthogonal Function modes representing ≈73% of the variance. Using fixed ADCPs, depth‐averaged vorticity is estimated west and south of Pt. Sal. Only one west‐location vorticity component is estimated, leading to negative vorticity bias for northward flow. The south location vorticity is consistent with estimates from parallel vessel transects on one day. The observed depth‐averaged flow V was primarily along‐bathymetric contours and varied ±0.2 ms−1 across subtidal and tidal frequency bands. The depth‐averaged normalized vorticity ζ¯/f varied ±8 across all frequency bands. Vorticity distributions are skewed with opposite sign at west and south locations, and ζ¯/f<−1 is more likely at the west location. At both locations, depth‐averaged vorticity and velocity are inversely related, with relationship asymmetric with sign of V, indicating headland and farther upstream vorticity generation. Binned‐mean ζ¯/f depends on both V and its time‐derivative, and indicates vorticity recirculation across the headland. The ∼2 h vorticity adjustment timescale and the associated short excursion distances indicate vorticity generation between south and west locations. Potential vorticity changes across the headland are different for positive and negative V indicating headland asymmetric vorticity generation. Pt. Sal occupies a nondimensional parameter space that is unique relative to other well studied headlands.
Plain Language Summary
Coastal ocean flows past topographic features, such as headlands and islands, lead to a variety of processes which can enhance how much mixing and stirring occurs and have implications on the movement of marine biota and sediment. One measure of this mixing and stirring is ‘vorticity,’ which is a gauge of how fast the water spins. Observations from September and October 2017 are used to investigate how vorticity is created and moved around the small (∼1 km) central California headland Pt. Sal. Raw data are filtered using statistical methods to remove noise. Vorticity is estimated west and south of Pt. Sal from groups of fixed instruments measuring flow speed and direction. Separate vorticity observations from a pair of research vessels corroborate the fixed observations on a single day. Currents at the headland were a mix of oscillatory tidal (twice a day) and longer time‐scale flows. The generation of vorticity is found to be stronger for northward flow relative to southward flow. The vorticity can also recirculate back with the oscillatory tidal flow. Pt. Sal is a unique study site, especially relative to other well studied headlands which are either larger in size or in deeper water.
Key Points
At a 1 km scale headland (Pt. Sal CA), depth averaged vorticity varied {plus minus}8f and was asymmetrically related to along‐headland flow
Vorticity also depends on flow acceleration, indicating short (2 h) adjustment timescale, recirculation, and headland generation
Estimated potential vorticity across the headland indicates asymmetric vorticity generation stronger for northward flow
Approximately 32% of the measured wave energy flux by sea and swell waves was dissipated over distances less than 130 m, outside of wave breaking on the inner shelf, over a rocky shore in southern ...Monterey Bay, CA. The bottom roughness of the rocky shore is defined by the standard deviation of bottom vertical variability, σb, that is 0.9 m, which is of similar magnitude to previously measured σb for rough coral reefs. Spectral wave energy flux balanced by bottom friction is modeled and compared with observations. Measured average wave reflection was 0.08 and is neglected in the model. The average energy dissipation owing to bottom friction over the rocky shore results in energy friction factors, fe, ranging 4 to 34. The observed fe are larger than previously measured fe on coral reefs. An empirical power law relationship is developed for fe as a function of the ratio of wave orbital excursion amplitude, Ab, and σb, based on combined data from coral reefs, rocky platforms, and this rocky shore. As σb increases, fe increases. Numerical simulation by Yu et al. (2018, https://doi.org/10.9753/icce.v36.waves.57) of waves over large bottom variations, similar to observed on coral reefs, suggests that drag forces do not account for the large observed fe. Therefore, it is hypothesized that bottom friction on rocky shores is a function of multiscale physical and biological roughness.
Plain Language Summary
During the spring and the fall of 2018, two experiments were conducted to examine how waves change as they progress from offshore to onshore over a rough rocky reef on the Central California coast. Wave statistics from a buoy offshore, which marked the edge of the rocky reef, were compared to wave measurements acquired farther onshore located seaward of wave breaking. Wave heights decreased between the edge of the reef and the corresponding onshore wave measurement stations. Since the observations are outside of wave breaking, the decrease in wave height is attributed to energy dissipation by bottom friction. Bottom friction is a function of bottom roughness, which was found to be as much 7 times larger than the roughest coral reef. Wave energy dissipation is hypothesized to be a result of the large and small rocks and biological growth on the rocky reef. Understanding wave transformation due to friction is important as this region is the home to a diverse ecosystem.
Key Points
Very large bottom roughness and energy dissipation factors are observed on a rocky shore
Energy dissipation by bottom friction on rocky shore is hypothesized to be a function of multiscale physical and biological roughness
Energy transformation on a rocky shore outside of wave breaking is dominated by bottom friction
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