Mixing and Transport in the Surfzone

Transcription

1 Mixing and Transport in the Surfzone Robert T. Guza Scripps Institution of Oceanography La Jolla, CA phone: (858) fax: (858) Grant Number: N LONG-TERM GOALS The long-term goal is to improve our understanding of wave-driven processes on ocean beaches. OBJECTIVES Specific objectives include analyzing existing NCEX (Nearshore Canyon Experiment) field observations, and developing new capability to quantify breaking wave-driven mixing in the surf zone. APPROACH Field observations are used to develop and calibrate numerical models. WORK COMPLETED NCEX ANALYSIS: Observations of waves and currents were obtained near the La Jolla and Scripps submarine canyons on the southern California coast (Figure 1) as part of the Nearshore Canyon Experiment (NCEX, Fall 2003). Studies of the effects of complex shelf bathymetry on shoaling sea and swell waves were led by Dr Herbers (NPGS). Refraction and scattering by the canyons creates strong alongshore variations in wave height in shallow water onshore of the canyons, with the location of wave energy maxima and minima dependent on the period and direction of the incident waves (see Magne et al, 2007 and references therein). Wave amplification can be locally large, consistent with the surfing renown of this site, Black s Beach. Observations in the surfzone have been used to study breaking-wave-driven processes onshore of the irregular bathymetry. Infragravity waves (motions with periods of a few minutes) are strongly affected by both changing tide levels (Thomson et al., 2006) and refraction and reflections over the canyons (Thomson et al.,2007). In the surfzone, wave setup varies alongshore as the wave energy varies (Apotsos et al, in press), and alongshore setup gradients sometimes driving strong currents from the regions of high waves and setup towards regions of low waves and setup (Apotsos et al, submitted). On 27 October (Figure 2), both near and far from the canyon, waves approached the beach from the south (the radiation stress gradient S xy / x is negative), and the setup gradients provide an opposing force [(ρgh) η y is positive]. Far from the canyon (green), pressure gradients are weak, and alongshore flows are northward (positive). Near the canyon, pressure gradients dominate, and alongshore flows (with one exception) are southward (negative), and buck into the approaching waves. 1

2 Figure 1: NCEX bathymetry (after Apotsos et al, submitted). Isobaths are in 1.0 m intervals (black curves) from 2 m above (darkest yellow) to 10 m below (darkest blue). MSL is the dashed curve. Currents meters were colocated with buried (circles) and unburied (diamonds) pressure sensors. North is to the figure top. On 27 Oct 2003, waves on the 10m-depth contour approached from the South (black arrows show direction and energy), with maximum (minimum) energy near alongshore distance 1600 (1000 m). Surf zone currents (red arrows) reversed direction. Figure 2: From Apotsos et al, submitted. The alongshore velocity observed with the 1.0-m isobath sensors versus forcing [circles and pluses are S xy / x; squares and crosses are (ρgh) η y ] at y = 2321 m (green symbols, far from canyon) and y = 1450 m (orange symbols, near canyon) on Oct 27, The 1.0-m isobath sensor was well within the surfzone for these 8.5-min data records (50 far from, and 45 near, the canyon). 2

3 MIXING: Pollution, fine sediments, and some chemicals act as passive tracers in the surfzone and nearshore. That is, over a few hours they are approximately conserved and water following. Passive tracer evolution and transport in the nearshore are important to Navy/Marine objectives including chemically detecting mines, and predicting where optical clarity will be effected by fine sediments and silt. Improved understanding of tracer transport would also allow improved prediction of the fate of terrestrial runoff pollution that drains directly onto the beach, degrading water quality. Although important to a range of applications, our ability to predict the transport and dispersal of passive tracers in the surfzone and nearshore is severely limited by the lack of concurrent detailed observations of waves, currents, and tracer concentration needed to develop and validate models. Dye is a proxy for pollutants, chemicals from ordnance, or other tracers of interest. Newly adapted fixed (frame mounted) and mobile (jet ski mounted, Figure 3) methods for in situ surfzone fluorescent Rhodamine WT dye measurement were tested in the field Fand laboratory (Clark et al, submitted). Bubbles and sand, suspended by breaking waves in the surfzone, interfere with optical fluorometer dye measurements, increasing the lower bound for dye detection and reducing (quenching) measured dye concentrations. Simultaneous turbidity measurements are used to estimate the level of bubble/sand interference, and empirically correct for quenching. Errors in raw and corrected dye measurements are estimated from lab experiments with known dye concentrations (Figure 4). Smoothing of sharp dye gradients and delay time (between drawing in a water sample and measuring the sample) in the mobile flow-through fluorometer system were also examined, giving estimates of effective spatial resolution and error. The system can also sample chlrophyl A (Omand et al, in prep). The dye sampling jetski and fixed fluorometers were deployed at Huntington Beach, as part of the HB06 field experiment (Figure 5). SIO graduate PhD student David Clark, co-advised with Dr Feddersen, is using these observations to begin to characterize mixing in terms of environmental conditions (Figure 6). Figure 3: GPS-tracked jet ski on a transect across the surfzone. On-board, flow-through dye sampling apparatus (inset) includes an electricpump, a debubbler, a turbiditysensor (Turner SCUFA), and a Rhodamine WT dye fluorometer (WETLabs Wetstar), sampling ast 5Hz. To reduce effects of bubbles and sand, the jetski is driven in the relatively low turbulence region just in front of a bore. Data from the seaward bound transects are usually not useful because the bore-avoiding ski path is highly irregular, and the intake pumps of the flow-through sampling system suck air when the ski is airborne. A handlebar-mounted screen displays realtime data and a local position map, allowing repeated sampling of pre-determined transects. Position and sensor data are transmitted ashore, allowing real-time analysis and adaptive sampling. 3

4 Figure 4: Rhodamine WT concentration versus time in bubbly, sandy (laboratory) water with a known dye concentration (60ppb). Errors in the raw concentrations are reduced using corrections estimated with the accompanying (not shown) turbidity time series. Figure 5: HB06. Cross-shore transect of frame-mounted current meters, dye-concentration sampling jetsk), and dye-sampling grad student (using bottles to sample about once every 4 min). 4

5 Figure 6: (a) Schematic of HB06 circulation and plume mixing experiment. Fixed instruments (current meters and pressure sensors) measured waves and currents on cross- and alongshoreoriented transects. Dye was measured at (usually) 4 locations with fixed fluorometers, and wth the jetski. Dye was released as a single patch, and continuously forming a plume. (b) ski-sampled dye concentrations across the plume are indicated by colors (scale to the right). During a 4-hr plume release on 11 Oct 2006, the mean alongshore current near the shoreline was 20 cm/s, and the breaker height was 50 cm. About 10 passes were made along each transect. (c) Dye concentration versus cross-shore distance with colors corresponding to alongshore distance from the source (see legend). (d) plume cross-shore half- width versus alongshore distance from the source. IMPACT/APPLICATIONS Tracer evolution and transport in the nearshore is important to Navy/Marine objectives including chemically detecting mines, avoiding contact with dangerous substances, and predicting where optical clarity will be effected by fine sediments and silt. A long-term goal is a portable (between sites) model suite that, given bathymetry and incident wave conditions, predicts tracer transport and dilution. RELATED PROJECTS NCEX observations of surfzone waves and circulation were obtained in collaboration with Drs Elgar and Raubenheimer (WHOI). NCEX data are available to the public on Steve Elgar s website ( HB06 observations and analysis are collaborative with 5

6 Feddersen and O Reilly (SIO). With support from the State of California, nowcasts of surfzone waves and alongshore currents at Huntington Beach are available REFERENCES Apotsos, A., B. Raubenheimer, S. Elgar, & R.T. Guza, Testing and Calibrating Parametric Wave Transformation Models On Natural Beaches, Coastal Eng., in press. Apotsos, A., B. Raubenheimer, S. Elgar & R. T. Guza, Wave-Driven Setup and Alongshore Flows Observed Onshore of a Submarine Canyon, J. Geophy. Res., submitted. Clark, D.B., F. Feddersen, M. Omand, & R. T. Guza,Measuring Fluorescent Dye Concentration in the Surfzone, J. Atm. & Oc Tech., submitted. Magne, R., K. A. Belibassakis, T. H. C. Herbers, F. Ardhuin, W. C. O Reilly, and V. Rey, Evolution of surface gravity waves over a submarine canyon, J. Geophys. Res., 112, C01002, doi: /2005jc003035, Omand, M., F. Feddersen, P. Franks, J. Leichter, & R. T. Guza, Observing chlorophyll in the surfzone, in preparation. Thomson, Jim, S. Elgar, T.H.C. Herbers, B. Raubenheimer, & R.T. Guza, Tidal modulation of infragravity waves via nonlinear energy losses in the surfzone, Geophys. Res. Letters 33, L05601, doi: /2005gl025514, Thomson, J., S. Elgar, T. H. C. Herbers, B. Raubenheimer, and R. T. Guza, Refraction and reflection of infragravity waves near submarine canyons, J. Geophys. Res., 112, C10009, doi: /2007jc004227, PUBLICATIONS Apotsos, A., B. Raubenheimer, S. Elgar, R.T. Guza, & J. Smith, The effects of wave rollers and bottom stress on setup, J. Geophys. Res., 112, C02003, doi: /2006jc003549, 2007 [published, refereed]. Apotsos, A., B. Raubenheimer, S. Elgar, & R.T. Guza, Testing and Calibrating Parametric Wave Transformation Models On Natural Beaches, Coastal Eng. [in press, refereed]. Apotsos, A., B. Raubenheimer, S. Elgar & R. T. Guza, Wave-Driven Setup and Alongshore Flows Observed Onshore of a Submarine Canyon, J. Geophy. Res. [submitted, refereed]. Clark, D.B., F. Feddersen, M. Omand, & R. T. Guza, Measuring Fluorescent Dye Concentration in the Surfzone, J. Atm. & Oc. Tech. [submitted, refereed]. Henderson, S., R.T. Guza, S. Elgar, & T.H.C. Herbers, Refraction of surface gravity waves by shear waves, J. Phys. Oceanog., 36, , 2006 [published, refereed]. 6

7 Henderson, S, R.T. Guza, S Elgar, T.H.C. Herbers, & A.J. Bowen, Nonlinear generation and loss of infragravity wave energy, J. Geophy. Res, 111, C12007, doi: /2006jc003539, 2006 [published, refereed]. Spydell, M., F. Feddersen, R.T. Guza, & W. Schmidt, Observing surfzone dispersion with drifters, J. Phys. Oc. [in press, refereed]. Thomson, Jim, S. Elgar, T.H.C. Herbers, B. Raubenheimer, & R.T. Guza, Tidal modulation of infragravity waves via nonlinear energy losses in the surfzone,, Geophys. Res. Letters, 33, L05601, doi: /2005gl025514, 2006 [published, refereed]. Thomson, J., S. Elgar, T. H. C. Herbers, B. Raubenheimer, and R. T. Guza, Refraction and reflection of infragravity waves near submarine canyons,, J. Geophys. Res., 112, C10009, doi: /2007jc004227, 2007 [published, refereed]. 7