Pulsations in Surf Zone Currents on a High Energy Mesotidal Beach in New Zealand
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Gallop, S.L.; Bryan, K.R.; Pitman, S.J.; Ranasinghe, R., and Sandwell, D., 2016. Pulsations in surf zone currents on a high energy mesotidal beach in New Zealand. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 378–382. Coconut Creek (Florida), ISSN 0749-0208.The exchange of material between the surf zone and continental shelf can be driven by pulsations in rip current velocities. However, there is a poor understanding of the relationship of these pulsations to surf zone morphology and material exchange. Moreover, understanding of rip current dynamics has focused mainly on single-barred beaches in an intermediate state, and there have been few studies on high energy beaches. Therefore, this paper undertakes preliminary research on surf zone current velocity pulsations, on a high energy beach in New Zealand. This initial analysis presents results from two days of measurements using Acoustic Doppler Velocimeters and Lagrangian GPS drifters. Drifters revealed pulsations in current velocities on the order of ∼0.5–2 m s−1 throughout the surf zone, whether inside a rip current circulation cell or not. More infragravity wave energy was associated with constant pulsations in current velocity, and lower infragravity energy with pulsation bursts, lasting 5–10 minutes, interspersed with periods of relatively constant velocity lasting 15–25 minutes. However, higher wave conditions also reduced the exit rate from the surf zone.Keywords:
Surf zone
Rip current
Energy exchange
Energy current
Surf zone
Rip current
Current meter
Drifter
Neutral buoyancy
Stokes drift
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Coastal processes are natural processes that operate along coastal zones, resulting in morphological changes in erosion and deposition. The western coast of India is affected by extreme monsoonal wave activity, which can lead to the loss of beaches and the vulnerability of the dunes. As a result, understanding actual near-shore physics and long-shore sediment transport becomes a prerequisite for the development of an effective coastal zone management strategy. The aim of this study is to quantify and investigate longshore sediment flux as a result of wave action based on sediment trap experiments (Kraus 1987). The Kraus (1987) method, along with wave hydrodynamics and current measurements, is performed using acoustic instruments across the surf zone.
Surf zone
Rip current
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Coastal engineering
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Surf zone
Rip current
Drifter
Wave setup
Shoal
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Rip currents are the main cause of beach rescues and fatalities. Key drivers of rip current hazard are: (1) fast current speeds; and (2) the exit rate of floating material from inside to outside of the surf zone. Exit rates may vary temporally, such as due to Very Low Frequency (VLF) motions, which have a period on the order of 10 minutes. However, there is little field data to determine the driver(s) of exit rate. Therefore, the aim of this research was to determine rip current circulation patterns, and specifically, determine their relationship to surf zone exits, on a high-energy dissipative beach. Three days of field measurements were undertaken at Ngarunui Beach, New Zealand. Three daily surf zone flow patterns were found: (1) alongshore; (2) surf zone eddy with high exit rate; and (3) surf zone eddy with no exits. There were strong infragravity peaks in energy within the surf zone, at 30-45s, although none at VLF (around 10 minute) frequencies. Further research is underway to determine what drove the high surf zone exit rate observed at Ngarunui Beach.
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Surf zone
Wave setup
Plage
Forcing (mathematics)
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Abstract : As the Navy thrusts operations into global brown environments, a more complete understanding of the phenomena ships and swimmers will encounter in nearshore regions is necessary. Rip currents remain infamous and important characteristics of the nearshore environment. These events not only impinge upon swimmers' safety, but may play a key role transferring water, containing nutrients, biologics, and even shore-/ship-borne pollution, between the surf zone and open ocean environments. Vertical and temporal behavior of rip currents outside of the surf zone is poorly understood due to a paucity of comprehensive observations. Observations of two upward-looking Acoustic Doppler Current Profilers (ADCP) deployed in 3 m and 5 m water depth within a rip current (nominally 1.0 and 1.2 surf zone widths from the shoreline respectively) were obtained during April-May 2008 as part of the Rip Current EXperiment (RCEX) at Sand City, Monterey Bay, CA. The ADCPs sampled continuously at 1 Hz. Energetic seaward-directed episodic pulses associated with the rip current obtained velocities up to 0.5 m/s with a frequency of occurrence varying from 1-15 times a day depending upon coincident wave and tidal conditions. Vertical variations of the episodic rip current pulsations ranged depth-uniform to surfacedominated. Cross-rotary analysis and complex correlation, performed in the vertical to describe rotational behavior and temporal lags, show rip currents in the inner shelf exhibit more rotation, up to 20 degrees in both CW and CCW directions, than in the surf zone. High coherence is limited to near-surface levels in the inner shelf, versus more depth-uniform values in the surf zone. Mean vertical profiles show these phenomena exhibit significant shear and structure.
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The nearshore zone is an active zone that can be quite inhospitable to humans due to violent wave breaking and strong rip currents. Rip currents are shore normal jet-like currents that typically extend from near the shoreline out past the line of breaking waves. Observations have concluded that a rip current system generally consists of 4 parts. Part 1 is the shoreward mass transport due to the waves carrying water through the breaker zone in the direction of wave propagation. Part 2 is the movement of this water mass parallel to the coast known as a longshore current. Part 3 is the rip current itself, a seaward flow of water through a narrow rip channel. And part 4 is an alongshore movement outside the breaker zone of the expanding rip head. With the use of the numerical model XBeach, in which a non-hydrostatic model based upon the numerical scheme as developed by Stelling and Zijlema (2003)was implemented, the fluid motions in the nearshore zone are simulated. The method of Stelling and Zijlema utilizes an edge based compact difference scheme for the approximation of the vertical gradient of the non-hydrostatic pressure. This ensures accurate wave breaking and dispersion characteristics, which is important for an accurate simulation of the nearshore hydrodynamics. Two test cases are used to verify the model for replication of the hydrodynamics in the nearshore zone. The first case consists of irregular wave breaking in a laboratory barred surf zone. The second case is a wave induced and bathymetry driven rip current in a directional wave basin. The numerical model is further developed with the addition of an eddy viscosity model and a non-reflecting boundary condition. With these additions the depth averaged model gave very satisfactorily results for both cases. The XBeach model is an accurate and efficient simulation package for the dynamics in the nearshore zone. This study shows that application to real world situations should give realistic and accurate results. Therefore the model could be applied in coastal engineering applications and in the research for energy extraction methods from wave induced currents.
Surf zone
Rip current
Pressure gradient
Longshore drift
Hydrostatic equilibrium
Wave height
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Surf zone
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Bar (unit)
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Gallop, S.L.; Bryan, K.R.; Pitman, S.J.; Ranasinghe, R., and Sandwell, D., 2016. Pulsations in surf zone currents on a high energy mesotidal beach in New Zealand. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 378–382. Coconut Creek (Florida), ISSN 0749-0208.The exchange of material between the surf zone and continental shelf can be driven by pulsations in rip current velocities. However, there is a poor understanding of the relationship of these pulsations to surf zone morphology and material exchange. Moreover, understanding of rip current dynamics has focused mainly on single-barred beaches in an intermediate state, and there have been few studies on high energy beaches. Therefore, this paper undertakes preliminary research on surf zone current velocity pulsations, on a high energy beach in New Zealand. This initial analysis presents results from two days of measurements using Acoustic Doppler Velocimeters and Lagrangian GPS drifters. Drifters revealed pulsations in current velocities on the order of ∼0.5–2 m s−1 throughout the surf zone, whether inside a rip current circulation cell or not. More infragravity wave energy was associated with constant pulsations in current velocity, and lower infragravity energy with pulsation bursts, lasting 5–10 minutes, interspersed with periods of relatively constant velocity lasting 15–25 minutes. However, higher wave conditions also reduced the exit rate from the surf zone.
Surf zone
Rip current
Energy exchange
Energy current
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