Wave-Driven Circulation of a Coastal Reef–Lagoon System

Abstract The response of the circulation of a coral reef system in Kaneohe Bay, Hawaii, to incident wave forcing was investigated using field data collected during a 10-month experiment. Results from the study revealed that wave forcing was the dominant mechanism driving the circulation over much of Kaneohe Bay. As predicted theoretically, wave setup generated near the reef crest resulting from wave breaking established a pressure gradient that drove flow over the reef and out of the two reef channels. Maximum reef setup was found to be roughly proportional to the offshore wave energy flux above a threshold root-mean-square wave height of 0.7 m (at which height setup was negligible). On the reef flat, the wave-driven currents increased approximately linearly with incident wave height; however, the magnitude of these currents was relatively weak (typically <20 cm s−1) because of (i) the mild fore-reef slope of Kaneohe Bay that reduced setup resulting from a combination of frictional wave damping and its relatively wide surf zone compared to steep-faced reefs, and (ii) the presence of significant wave setup inside its coastally bounded lagoon, resulting from frictional resistance on the lagoon–channel return flows, which reduced cross-reef setup gradients by 60%–80%. In general, the dynamics of these wave-driven currents roughly matched predictions derived from quasi-one-dimensional mass and momentum balances that incorporated radiation stresses, setup gradients, bottom friction, and the morphological properties of the reef–lagoon system.

Download Full-text

Unforced Oscillation of Rip-Current Vortex Cells

Journal of Physical Oceanography ◽

10.1175/jpo-d-11-0164.1 ◽

2013 ◽

Vol 43 (3) ◽

pp. 477-497 ◽

Cited By ~ 7

Author(s):

Joseph D. Geiman ◽

James T. Kirby

Keyword(s):

Incident Wave ◽

Surf Zone ◽

Low Frequency ◽

Surface Gravity Wave ◽

Wave Forcing ◽

Forcing Function ◽

Single Dipole ◽

Wide Range ◽

Simple Scaling ◽

Good Agreement

Abstract A numerical simulation of a monochromatic surface gravity wave–driven flow over an alongshore quasi-periodic rip-channeled beach using the wave-resolving model Funwave is used to investigate coherent, very low-frequency (VLF) motions with characteristic frequencies f < 4.0 mHz inside of the surf zone generated by wave breaking. These oscillations of the nearshore cellular vorticity pattern occur for shore-normal waves over a wide range of amplitudes of the incident wave field and occur despite the wave forcing being essentially constant. The oscillations occur at the lower end of the VLF spectrum or around fp = 0.55 mHz. For small incident wave amplitudes, an equilibrium state consisting of a staggered counterrotating vortex array generates a net weak alongshore current that is also seen in drifter trajectories observed in the field. Using a simpler pseudospectral vorticity model of a single dipole generated by a smooth, stationary in time forcing function sb, this study shows show that the Strouhal number of the vortex shedding process responsible for the oscillation is dependent on the circulation strength of the vortices in the dipole, as well as the bottom friction parameter. This process includes the pinching off, advection, and eventual regeneration of the vortex in the dipole. A simple scaling argument shows good agreement with the frequencies observed in the simulations.

Download Full-text

WAVE BREAKING AND WAVE SETUP OF ARTIFICIAL REEF WITH INCLINED CROWN

Coastal Engineering Proceedings ◽

10.9753/icce.v32.waves.18 ◽

2011 ◽

Vol 1 (32) ◽

pp. 18

Author(s):

Keisuke Murakami ◽

Daisuke Maki

Keyword(s):

Cross Section ◽

Wave Height ◽

Energy Flux ◽

Incident Wave ◽

Wave Energy ◽

Wave Breaking ◽

Artificial Reef ◽

The Other ◽

Wave Setup ◽

Other Hand

INTRODUCION The beach protection facilities are required in some situations to harmonize with coastal environments and utilizations(National Association of Sea Coast,2004). This study investigates some hydraulic functions of proposed multipurpose artificial reef which has an inclined reef crown. The reef is expected to protect beaches against storm waves, and also facilitates the surfing activities under mild wave conditions. The forced wave breaking on the reef sometimes causes a mass transport and wave setup. This study focuses on the following hydraulic characters to clear the efficiencies of proposed artificial reef with inclined crown. 1) Wave energy dissipation by the reef, 2) Generation of suitable wave breakers for surfing, 3) Wave setup behind the reef. HYDRAULIC EXPERIMENT SETUP A series of hydraulic experiments were carried out with using a two-dimensional wave flume. The model scale was assumed 1/30. Three kinds of cross sections were employed as the model of artificial reef (Fig.-1). Both Case-A and Case-B have the inclined crown. The cross section of Case-C, which has a flat crown, is set as a typical cross section of conventional artificial reef. The reef length of Case-A is the same as that of Case-C, and Case-B is double the length of Case-A. Two different submerged depths of crown, hc=2cm and 5cm, were set in the experiments. In Case-A and Case-B, their submerged depth of crown, hc, were defined as the minimum depth at the onshore edge of the crown. The incident wave heights were changed at 1cm intervals from 4cm to 9cm, and the periods were also chanced at 1 sec. intervals from 1 sec. to 2.4 sec. for each wave height. SUMMARY OF RESULTS The energy flux ratio of transmitted waves in Case-B shows similar values in Case-C under the stormy wave conditions in the case of hc=2cm. The longer reef shows favorable characters in dissipating wave energy as well as in maintaining a wider breaker zone on its crown. On the other hand, the transmitted energy flux in Case-A become slightly higher than that in Case-C. The inclined shape of the reef crown closely relates to both the type of wave breakers and the generation of higher order waves. Wave breakers observed in Case-A and Case-B are almost Plunging breaker or Collapsing breaker, and these breakers are suitable for surfing(Walker, et.al.,1972). In Case-C, on the other hand, most incident waves break at the offshore edge of the crown with backwash(Fig.-2). This means that the slope on the reef crown play an important roll in generating suitable breakers for surfing. Type of wave breakers on the inclined reef were summarized by surf similarity parameter(Battjes,1974). Fig.-3 shows the normalized wave setup behind each reef. The wave setup differs depending the reef sections. Case-A and Case-B check the wave setup effectively in comparison with Case-C. This excellent checking effect can be observed in the wide range of incident wave height and wave period. Through a series of hydraulic experiments, it is cleared that the difference of wave setup observed behind the reefs relates to the wave breaker type and wave breaker point on the reefs.

Download Full-text

Environmental controls on surf zone injuries on high-energy beaches

Natural Hazards and Earth System Science ◽

10.5194/nhess-19-2183-2019 ◽

2019 ◽

Vol 19 (10) ◽

pp. 2183-2205 ◽

Cited By ~ 2

Author(s):

Bruno Castelle ◽

Tim Scott ◽

Rob Brander ◽

Jak McCarroll ◽

Arthur Robinet ◽

...

Keyword(s):

Wave Height ◽

Surf Zone ◽

High Tide ◽

High Energy ◽

Water Levels ◽

Small Scale ◽

Beach Morphology ◽

Environmental Controls ◽

Flow Activity ◽

Incident Waves

Abstract. The two primary causes of surf zone injuries (SZIs) worldwide, including fatal drowning and severe spinal injuries, are rip currents (rips) and shore-break waves. SZIs also result from surfing and bodyboarding activity. In this paper we address the primary environmental controls on SZIs along the high-energy meso–macro-tidal surf beach coast of southwestern France. A total of 2523 SZIs recorded by lifeguards over 186 sample days during the summers of 2007, 2009 and 2015 were combined with measured and/or hindcast weather, wave, tide, and beach morphology data. All SZIs occurred disproportionately on warm sunny days with low wind, likely because of increased beachgoer numbers and hazard exposure. Relationships were strongest for shore-break- and rip-related SZIs and weakest for surfing-related SZIs, the latter being also unaffected by tidal stage or range. Therefore, the analysis focused on bathers. More shore-break-related SZIs occur during shore-normal incident waves with average to below-average wave height (significant wave height, Hs = 0.75–1.5 m) and around higher water levels and large tide ranges when waves break on the steepest section of the beach. In contrast, more rip-related drownings occur near neap low tide, coinciding with maximised channel rip flow activity, under shore-normal incident waves with Hs >1.25 m and mean wave periods longer than 5 s. Additional drowning incidents occurred at spring high tide, presumably due to small-scale swash rips. The composite wave and tide parameters proposed by Scott et al. (2014) are key controlling factors determining SZI occurrence, although the risk ranges are not necessarily transferable to all sites. Summer beach and surf zone morphology is interannually highly variable, which is critical to SZI patterns. The upper beach slope can vary from 0.06 to 0.18 between summers, resulting in low and high shore-break-related SZIs, respectively. Summers with coast-wide highly (weakly) developed rip channels also result in widespread (scarce) rip-related drowning incidents. With life risk defined in terms of the number of people exposed to life threatening hazards at a beach, the ability of morphodynamic models to simulate primary beach morphology characteristics a few weeks or months in advance is therefore of paramount importance for predicting the primary surf zone life risks along this coast.

Download Full-text

Surf zone transformation of wave height to water depth ratios

Coastal Engineering ◽

10.1016/0378-3839(92)90013-k ◽

1992 ◽

Vol 17 (1-2) ◽

pp. 49-70 ◽

Cited By ~ 19

Author(s):

R.C. Nelson ◽

J. Gonsalves

Keyword(s):

Wave Height ◽

Water Depth ◽

Surf Zone

Download Full-text

Wave height, setup and currents around a detached breakwater submitted to regular or random wave forcing

Coastal Engineering ◽

10.1016/s0378-3839(96)00053-1 ◽

1997 ◽

Vol 31 (1-4) ◽

pp. 77-96 ◽

Cited By ~ 19

Author(s):

Mathieu Mory ◽

Luc Hamm

Keyword(s):

Wave Height ◽

Random Wave ◽

Wave Forcing

Download Full-text

FIELD MEASUREMENTS AND MODELLING OF NEARSHORE CURRENTS AT A HIGH-ENERGY GEOLOGICALLY-CONSTRAINED BEACH

Coastal Engineering Proceedings ◽

10.9753/icce.v36v.currents.18 ◽

2020 ◽

pp. 18

Author(s):

Arthur Mouragues ◽

Philippe Bonneton ◽

Bruno Castelle ◽

Vincent Marieu

Keyword(s):

Surf Zone ◽

Low Frequency ◽

Field Measurements ◽

High Energy ◽

Oblique Wave ◽

Wave Forcing ◽

Circulation Patterns ◽

Wave Conditions ◽

Nearshore Currents ◽

Set Up

We present field measurements of nearshore currents at a high-energy mesotidal beach with the presence of a 500-m headland and a submerged reef. Small changes in wave forcing and tide elevation were found to largely impact circulation patterns. In particular, under 4-m oblique wave conditions, our measurements indicate the presence of an intense low-frequency fluctuating deflection rip flowing against the headland and extending well beyond the surf zone. An XBeach model is further set up to hindcast such flow patterns.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/EiqnjBIkWJE

Download Full-text

MACH-REFLECTION AS A DIFFRACTION PROBLEM

Coastal Engineering Proceedings ◽

10.9753/icce.v15.45 ◽

1976 ◽

Vol 1 (15) ◽

pp. 45 ◽

Cited By ~ 2

Author(s):

Udo Berger ◽

Soren Kohlhase

Keyword(s):

Wave Height ◽

Incident Wave ◽

Water Waves ◽

Gas Dynamics ◽

Mach Reflection ◽

Diffraction Problem ◽

Vertical Wall ◽

Oblique Wave ◽

Wave Heights ◽

The Waves

As under oblique wave approach water waves are reflected by a vertical wall, a wave branching effect (stem) develops normal to the reflecting wall. The waves progressing along the wall will steep up. The wave heights increase up to more than twice the incident wave height. The £jtudy has pointed out that this effect, which is usually called MACH-REFLECTION, is not to be taken as an analogy to gas dynamics, but should be interpreted as a diffraction problem.

Download Full-text

Spectral Analysis of Irregular Wave Impact on the Structure in Splash Zone

21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 1 ◽

10.1115/omae2002-28091 ◽

2002 ◽

Cited By ~ 1

Author(s):

Bing Ren ◽

Yongxue Wang

Keyword(s):

Spectral Analysis ◽

Wave Height ◽

Incident Wave ◽

Impact Pressure ◽

Splash Zone ◽

Spectral Moment ◽

Wave Impact ◽

Large Wave ◽

Irregular Wave ◽

The Impact

The spectral analysis from experimental data of irregular wave impact on the structures with large dimension in the splash zone is presented. The experiments were conducted in the large wave-current tank in the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology. In the experiment, the target spectrum is JONSWAP spectrum, the significant wave height H1/3 is in the range from 0.1m to 0.3m, and the peak period of spectrum Tp in the range from 1.0s to 2.0s. The ratio of s/H1/3, which refers to the clearance of the subface of the structure above still water level (s) to the incident wave height, is between −0.1 and 0.4. The spectral analysis results of the irregular wave impact pressure on the subface of the structure under various case studies are presented. The distribution of spectral moment of the impact pressure on the structure along the subface is given. And the influence of different incident wave parameters and relative clearance s/H1/3 on the average spectral moment of impact pressure are discussed.

Download Full-text

Studi Hindcasting Dalam Menentukan Karakteristik Gelombang dan Klasifikasi Zona Surf Di Pantai Uluwatu, Bali

Journal of Marine and Aquatic Sciences ◽

10.24843/jmas.2019.v05.i01.p15 ◽

2018 ◽

Vol 5 (1) ◽

pp. 119

Author(s):

Karina Santoso ◽

I Dewa Nyoman Nurweda Putra ◽

I Gusti Bagus Sila Dharma

Keyword(s):

Wave Height ◽

Calculation Result ◽

Significant Wave Height ◽

Surf Zone ◽

Wind Data ◽

Wave Transformation ◽

Breaking Wave ◽

Significant Wave ◽

The World

Bali is one of the islands where there are many surf zones with various characteristics. In addition, Bali is also a heaven with a classy wave for the surfers of the world. One of the most challenging places to surf in Bali is Uluwatu Beach. Uluwatu Beach is ranked the 3rd best surf spot in the world version of CNN Travel in 2012. Wind causes sea waves, therefore wind data can be used to estimate the height and direction of the waves. Wave Hindcasting with Sverdrup, Munk and Bretschneider (SMB) method is calculated based on wind data for 10 years (2001 - 2010) from BMKG Ngurah Rai Station - Denpasar to obtain a significant wave height and period. In this research, it is necessary to approach through Hindcasting procedure, wave transformation analysis and surfing Terminology in determining the type of breaking wave and classification of surf zone in Uluwatu Beach area. Wave calculation result in Uluwatu Beach dominated by wave that coming from west side with significant wave height (Hs) of 0.98 m and significant wave period (Ts) of 5.21 s. The wave height due to the influence of wave refraction and shoaling is 0.976 m. The breaking wave height obtained from the calculation is 1.04 m at a depth of 0.849 m. From the result in this research, it can be concluded that the breaking wave type that occurred at Uluwatu Beach is plunging type according to the calculation result from its Irribaren number (0.4 <Ni <2.3). The classification of the surf zone at Uluwatu Beach based on its breakup type of wave is thought to be a good zone for surfers on intermediate level.

Download Full-text