Physical linkages between an offshore canyon and surf zone morphologic change

Abstract

The causes of surf zone morphologic changes observed along a sandy beach onshore of a submarine canyon were investigated using field observations and a numerical model (Delft3D/SWAN). Numerically simulated morphologic changes using four different sediment transport formulae reproduce the temporal and spatial patterns of net cross‐shore integrated (between 0 and 6.5 m water depths) accretion and erosion observed in a ∼300 m alongshore region, a few hundred meters from the canyon head. The observations and simulations indicate that the accretion or erosion results from converging or diverging alongshore currents driven primarily by breaking waves and alongshore pressure gradients. The location of convergence or divergence depends on the direction of the offshore waves that refract over the canyon, suggesting that bathymetric features on the inner shelf can have first‐order effects on short‐term nearshore morphologic change.

Extremely Low Frequency (0.1 to 1.0 mHz) Surf Zone Currents

“…Regardless of the incident wave direction, there is significant alongshore variability in ELF power (Figures a–c). When incident waves were from the west‐northwest (10 October), integrated ELF power (black symbols in Figure c) was maximum near ~1.5 < X < 2.0 km, where surf zone flows converged (black arrows in Figure d; see also Apotsos et al, , Long & Özkan‐Haller, ; Hansen et al, ) and where time‐lapse images of the surf zone are consistent with offshore flow (white arrows in Figure e; Long & Özkan‐Haller, ). Although there is indication of offshore flow in the time‐lapse image (Figure e) near X = 0.7 km, ELF power there is low, possibly owing to smaller wave heights in the shadow of the submarine canyon (depth contours in Figure d), consistent with observations in 2.5‐m depth (Gorrell et al, ).…”

“…Observations from the surf zone on a San Diego, CA, beach onshore of a large submarine canyon during the NCEX project (2003, for instrument array details see Apotsos et al, 2008;Gorrell et al, 2011;Hansen et al, 2017) also have ELF peaks at f~0.5 mHz (Figures 4a and 4b). During the periods examined here, the bathymetry did not change significantly, and there was little variation in wave height in 5-m water depth for 1.1 < X < 2.7 km (with X the alongshore coordinate; not shown), but surf zone circulation patterns differed for different incident wave directions (Apotsos et al, 2008;Hansen et al, 2017;Long & Özkan-Haller, 2016). Regardless of the incident wave direction, there is significant alongshore variability in ELF power (Figures 4a-4c).…”

“…Regardless of the incident wave direction, there is significant alongshore variability in ELF power (Figures 4a-4c). When incident waves were from the west-northwest (10 October), integrated ELF power (black symbols in Figure 4c) was maximum near~1.5 < X < 2.0 km, where surf zone flows converged (black arrows in Figure 4d; see also Apotsos et al, 2008, Long & Özkan-Haller, 2016Hansen et al, 2017) and where time-lapse images of the surf zone are consistent with offshore flow (white arrows in Figure 4e; Long & Özkan-Haller, 2016). Although there is indication of offshore flow in the time-lapse image (Figure 4e) near X = 0.7 km, ELF power there is low, possibly owing to smaller wave heights in the shadow of the submarine The sum of the cross-shore plus alongshore velocity power spectral densities (average of spectra from eight consecutive detrended 2.8-hr records of 2-Hz samples averaged to 1 min) for the BARGAP rip channel bathymetry (shown in Figure 3b; solid black curve, H sig = 0.8 m, f cent = 0.12 ± 0.01 Hz, Θ = 5°± 2°), the BARGAP alongshore uniform bathymetry (dashed black curve, H sig = 1.2 m, f cent = 0.17 ± 0.01 Hz, Θ = −3°± 6°), the RODEX crescentic-shaped bars bathymetry (shown in Figure 3c; solid red curve, H sig = 2.3 m, f cent = 0.14 ± 0.01 Hz, Θ = 6°± 2°), and the RODEX alongshore uniform bathymetry (dashed red curve, H sig = 1.7 m, f cent = 0.17 ± 0.00 Hz, Θ = 5°± 2°), and of vorticity on the crescentic-bar bathymetry (green curve (arbitrary units), H sig = 2.3 m, f cent = 0.14 ± 0.01 Hz, Θ = 6°± 2°) versus frequency.…”

Sixteen years of bathymetry and waves at San Diego beaches

“…Sand was stored in an offshore bar and partially returned to the subaerial beach the following summer 21 . In autumn 2003, waves, currents, and morphology near Scripps Canyon and at South Torrey Pines Beach were observed during the Nearshore Canyon Experiment (NCEX) (refs 24,39,40 , and references therein). In 2004, monitoring expanded to span 8 km alongshore, including both North, Central, and South Torrey Pines (Fig.…”

“…These datasets have been used to investigate equilibrium behavior of the shoreline 12 and beach profile 13 , beach response to two energetic El Niño winters 14–19 and the evolution of four beach nourishments 17,20–23 . The performance of 1-D (imensional) storm erosion models (XBeach) 22 , and 2-D morphological evolution models (Delft3D) 24 have been assessed. These bathymetry data also have been used in tests of surfzone circulation (funwaveC) 25 , and coastal flood models (XBeach and EurOtop) 26 .…”