Ephemeral sand waves in the hurricane surf zone

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Marine Geology 250 (2008) 276 – 280 www.elsevier.com/locate/margeo

Letter section

Ephemeral sand waves in the hurricane surf zone Andrew B. Kennedy a,⁎, K. Clint Slatton a,b , Tian-Jian Hsu a , Michael J. Starek a , Kittipat Kampa b a

b

Department of Civil and Coastal Engineering, University of Florida, Gainesville, FL, 32611, USA Department of Electrical and Computer Engineering, University of Florida, Gainesville, FL, 32611, USA Received 19 October 2007; received in revised form 9 January 2008; accepted 18 January 2008

Abstract Airborne bathymetric LIDAR observations along the Florida panhandle after Hurricane Dennis (2005) show the first unequivocal observations of surf-zone sand wave trains. These are found in depths of ∼ 5m along the trough of the hurricane bar, where hindcasts show strong longshore currents only during severe storms. The waves extend over tens of kilometers of coast after Dennis but are absent from the same area in four other datasets. Observed wavelength to water depth ratios are comparable to river dunes and tidal sand waves but height to depth ratios are smaller, with the largest wave heights around 0.1 times the water depth. The sand wave generation mechanism is hypothesized to be from wind-and-wave-induced longshore currents, which were hindcast to be large during Dennis, with destruction from water wave orbital velocities. © 2008 Elsevier B.V. All rights reserved. Keywords: sand waves; dunes; hurricanes; currents; water waves; LIDAR; Hurricane Dennis

1. Introduction Investigations of small-scale sandy bedforms in surf zone longshore currents have repeatedly observed linear, irregular and cross-ripples with cm to 10s of cm horizontal scales, and megaripples with length scales 1–5m. These features are believed to follow a characteristic progression with increasing wave orbital velocities and longshore current strengths through several types of (usually) asymmetric ripples, and often megaripples before transitioning to a planar bed (Clifton, 1976; Thornton et al., 1998; Hay and Mudge, 2005). Long trains of sandy bed features analogous to river dunes and sand waves are conspicuously missing from this description, even though depths and mean velocities for typical and storm conditions in surf zone longshore currents (Ruessink et al., 2001) are squarely in the range where such features would be prominent in riverine environments (van Rijn, 1993; Best, ⁎ Corresponding author. Currently at University of Notre Dame, Department of Civil, Engineering and Geological Sciences, Notre Dame, IN, 46556, USA. Tel.: +1 574 631 6686; fax: +1 574 631 9236. E-mail address: [email protected] (A.B. Kennedy). 0025-3227/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2008.01.015

2005; Parson et al., 2005). Possibly related features such as nearshore megaripples (Gallagher et al., 1998) do not appear to scale with water depth, unlike sand waves and dunes which have wavelengths many times the local water depth (van Rijn, 1993; Hulscher and van den Brink, 2001; Best, 2005; Barnard et al., 2006). Konicki and Holman (2000) have presented indirect video evidence of alongshore topographic features being generated during longshore currents, but the detailed forms, forcing and extent are not conclusive. From visual observations using SCUBA, Hunter et al. (1979) reported apparent nearshore sand waves in the trough of a barred system in Oregon, but provided little detail. In this paper we are able to present more conclusive observations of longshore current-generated surf-zone sand waves found in the Florida Panhandle after Hurricane Dennis (2005). 2. Surf zone sand waves after Hurricane Dennis (2005) Fig. 1 shows bathymetric measurements from a US Army Corps of Engineers (USACE) airborne LIDAR survey near Panama City Beach, Florida (see, e.g., Wozencraft and Lillycrop, 2003), which spanned 3–15days after the 105knot

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Fig. 1. (a) Topographic and bathymetric LIDAR after Hurricane Dennis showing section of sand waves; (b) Elevation of transect in (a) from left to right.

(54m/s) Hurricane Dennis landfall on July 10, 2005. A long train of shore-perpendicular sand waves is clearly visible in the trough of the hurricane bar, but absent in other areas of the cross-shore profile. Fig. 2(a) shows the area considered was ∼ 130km from landfall at Santa Rosa Island, Florida; however environmental forcing was still strong as shown in Table 1, and will be demonstrated shortly to have produced a vigourous Eastto-West longshore current. Sand waves were near-continuous along the 24km distance shown, are mildly three dimensional in their limited cross-shore extent of approximately 100–150m,

and do not correspond to any known LIDAR artefacts. The region of prominent sand waves was in an area of fine sand with median grain sizes of around d50 = 0.24mm and numerous offshore borrow pits (Robertson et al., 2007). Some sand waves were also apparent outside this region but measurements were hindered by poor water clarity. These sand waves are not permanent features as they do not appear in four other datasets in this area taken from 2004–2006. To quantify sand wave properties, an elevation transect was taken through the approximate midpoint of the hurricane trough

Fig. 2. (a) Sand wave locations after Hurricane Dennis ( ), showing tracks of hurricanes Ivan and Dennis and locations of measured and hindcast winds, waves, and water levels. Bathymetry contours are at 50 m intervals. ( ) Ivan and Dennis wave hindcast station, ( ) WIS wave hindcast station for average conditions, ( ) NOAA/ NOS station PCBF1/8729210. NOAA buoy 42039 is south of the figure at 28.79N, 86.02W; (b) Sand wave heights traveling from west to east along the 23.7 km transect of (a): ( ), RMS height over 200 m sections, ( ), maximum height over 200 m sections.

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Table 1 Peak wind and water level (NOAA/NOS station PCBF1/8729210) and wave (hindcast) conditions near Panama City Beach during Hurricanes Dennis and Ivan Property

Dennis (peak)

Ivan (peak)

Dennis (representative model inputs)

Wind (m/s) Direction (degrees relative to shore-normal) Storm Surge (m above MSL) Wave height Hs(m) Direction (degrees relative to shore-normal)

25 88

27 77

20 60

1.9 6.4 50

1.8 8.9 25

1.5 5 45

over the entire 24km region. This was filtered to remove short wavelength noise and long wavelength formations unrelated to the sand waves using a Daubechies filter (Strang and Nguyen, 1996) with band-pass wavelengths of approximately 8–250m. Both zero-upcrossing and zero-downcrossing wave heights and lengths were computed along the filtered transect and the resulting series of individual wave heights was used to compile sand wave statistics. One small area showing a scour depression seaward of a pier was manually removed from the results. The mean wavelength over the entire transect was L = 33m, with a standard deviation of 12m. This gives average relative length to depth (h) scales of L/h = 5–7 based on the storm surge depth and mean water depths of h ≈ (6.5m, 5m), which scale well

within the range of both river dunes and sand waves (van Rijn, 1993; Hulscher and van den Brink, 2001; Best, 2005; Parson et al., 2005; Barnard et al., 2006). Fig. 2(b) shows that sand wave heights varied considerably over the region, with broad areas of several km showing larger and smaller waves. Using a representative depth of h = 5m, all RMS heights, HRMS, averaged over 200m sections were found to have HRMS/h b 0.1, and few individual wave heights exceeded this. Although a 10% change in water depth is significant in the surf zone and would certainly affect wave breaking, these relative heights are smaller than typical for many river dunes and tidal sand waves (van Rijn, 1993; Hulscher and van den Brink, 2001; Best, 2005). We thus hypothesize that persistent water wave orbital velocities act as an opposing process to limit sand wave heights during strong longshore current events. Surf zone wave orbital velocities have magnitudes comparable to or greater than longshore current velocities even in the strongest flows (LonguetHiggins, 1970), and Langhorne (1982) has demonstrated attenuation of tidally-driven sand waves in deeper water by strong wave action, so this hypothesis seems aligned with previous results. By extending the hypothesis to include total attenuation in the days and weeks after longshore current events, we may also explain why sand waves are prominent and often permanent in many riverine and tidal flows but smaller and episodic in the present dataset. Strong longshore currents are themselves highly episodic (Ruessink et al., 2001), particularly near a hurricane

Fig. 3. Modeled representative waves and currents for Hurricane Dennis (all blue lines) and typical (all black lines) conditions. (a) Significant wave height Hs (solid), and wave height required to mobilize bottom sediment (dashed); (b) Mean longshore current velocity (solid), and RMS wave orbital bottom velocity (dashed); (c) typical bottom elevation from local MSL, showing location of sand waves. Representative Dennis conditions are adapted from hindcast ( Jensen et al., 2006) and measured data in Table 1: Equivalent deep water wave height Hs0 = 5 m, and relative angle θ0 = 45°, Period T = 13 s, Surge elevation from MSL η = 1.5 m, Wind velocity U10 = 20 m/s, and relative angle θw = 60°. Typical conditions adapted from WIS hindcasts: Hs0 = 0.8 m, θ0 = 30°, T = 4s, η = 0 m, U10 = 5 m /s, θw = 60°.

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bar, but water wave orbital velocities are more persistent. The historical lack of sand wave observations in the surf zone may then be attributed to the difficulty in measuring nearshore bathymetry during strong longshore currents when the waves would be generated. Additionally, the widely-spaced, crossshore survey profiles which have been standard for nearshore researchers (Birkemeier and Mason, 1984; Gallagher et al., 2003) are poorly suited for resolving intermediate alongshore bathymetric scales of 5–15 times the water depth. For these reasons, longshore current sand wave features may be generated and destroyed regularly but not have been measured until the advent of airborne bathymetric LIDAR. Some evidence supporting this hypothesis may be found by considering the difference between storm and typical conditions using the one dimensional hydrodynamic model of Ruessink et al. (2001) over a typical cross-shore profile, with their suggested coefficients. Although no wave or current measurements are available at this location, representative storm wave conditions can be taken from a Hurricane Dennis hindcast shown in Table 1 (Jensen et al., 2006), while typical conditions use average heights and periods from USACE Wave Information Study (WIS) hindcasts (Tracy and Cialone, 2004). Fig. 3 shows wave and current predictions: during storms, water wave orbital velocity amplitudes and longshore current velocities are very strong and roughly comparable but orbital velocities dominate in the area of sand waves during more typical conditions. Predicted sand mobility numbers in the area of sand waves during storm conditions, Ψ ≡ U2 + V2 / ((s − 1)gd), are all Ψ N 250, where (U,V) are total cross-shore and longshore velocities, respectively, s is the specific gravity of sediment taken as 2.65, g is gravitational acceleration, d = 0.25mm is the sediment diameter, and angle brackets denote the averaging operator. This is predicted to be in the region of sheet flow with no small scale bedforms expected according to standard nearshore relations (Nielsen, 1992). For smaller waves, conditions for initiation of sediment motion are estimated using the critical Shields parameter θc defined as θc = 0.5 fwUc2 / (s − 1)gd, where Uc is the critical wave velocity amplitude. The wave friction factor fw is determined following Swart (1974) as fw = exp[5.213(Ks/(Uc /ω))0.194− 5.997] with the wave frequency ω = 2π / T, and the roughness height Ks = 2.5d. The critical Shields parameter under waves is around θc = 0.06 for d = 0.25mm (Nielsen, 1992; Madsen and Grant, 1976). The critical wave velocity amplitude for the initiation of sediment motion then becomes Uc = 0.21m/s for T = 13s, and Uc = 0.17m/s for T = 4s, using linear wave theory to compute orbital amplitudes. These are quite moderate velocities and because of this, Fig. 3(a) shows that typical conditions will still mobilize sediment easily in the area of sand waves. These conclusions are insensitive to moderate changes in forcing and sediment properties, and to different sediment transport parameterisations. Regular sediment mobilization even in the absence of a longshore current explains, at least qualitatively, why sand waves are not seen in typical datasets since orbital velocities may act to smooth out these features. As noted previously, degradation by orbital velocities is also a leading hypothesis for why the observed sand waves have relatively small amplitudes

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compared to river dunes and tidally driven sand waves, with short storm durations potentially acting as a further limiter. 3. Discussion and conclusions Of the four additional datasets without surf zone sand waves, the most intriguing was taken 10 weeks after Hurricane Ivan (2004). As seen in Fig. 2 and Table 1, Ivan's landfall was more distant to the site but environmental conditions were quite similar to Dennis. Unfortunately, with the evidence available we cannot distinguish whether sand waves were not generated during Ivan, or whether sand waves were generated during Ivan but dissipated before the survey. Between Ivan and the survey completion, there were eleven occasions when significant wave heights at NOAA buoy 42039 (150km S of Panama City Beach) exceeded 2m, while there were none after Dennis. Thus, generation-but-dissipation is plausible, but so little is known about the detailed conditions required to generate sand waves that no definitive conclusions can be made. However, the sand waves observed following Hurricane Dennis appear clearly to have been generated by strong waveand-wind-induced longshore currents. Sand wave non-appearance in other datasets demonstrates that they are not permanent features and implies they are destroyed during more moderate conditions when longshore currents are small but wave orbital velocities are sufficient to mobilize sediment. These observations are suggestive that sand waves may also appear in even shallower depths for strong longshore flows. If they scale with water depth, typical alongshore wavelengths in 1.5m depth might be 8–15m, with heights of 0.05–0.15m. These scales should be measurable with a targeted plan although generation and destruction time scales would undoubtedly also decrease in shallower depths, further complicating observations. We are hopeful that the present results will stimulate a reexamination of existing data for further examples of surf zone sand waves. Finally, details of how waves and currents act together to generate and destroy these features, and how they in turn will affect sediment transport and bottom friction are almost nonexistent and would be a fruitful topic for further study. Acknowledgements This work was supported by the Joint Airborne LIDAR Bathymetry Technical Center of Expertise (JALBTCX), the USACE System-Wide Water Resources Program, the USGS Center for Coastal and Watershed Studies, and the NASA EAARL program. ABK was supported by the National Science Foundation under grant CBET 0423877 and the Office of Naval Research under grant N0001406IP20020. References Barnard, P.L., Hanes, D.M., Rubin, D.M., Kvitek, R.G., 2006. Giant sand waves at the mouth of San Francisco Bay. EOS Trans. AGU 87 (29), 285–289. Best, J.L., 2005. Fluid dynamics of river dunes: a review and some future research directions. J. Geophys. Res. 110, F04S02. Birkemeier, W.A., Mason, C., 1984. The CRAB: a unique nearshore surveying vehicle. J. surv. eng. 110, 1–7.

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