Observations of bed level change in a saturated surf zone

Abstract

[1] Sonar altimeter measurements of bed level change were made in conjunction with instantaneous measurements of sea surface elevation and currents at a fixed position in a saturated surf zone (water depths less than 1 m). Two types of bed behavior were observed, classified as ''progressive,'' where mean bed level showed a steady increase or decrease with time, and ''oscillatory,'' where the bed level took a periodic motion (amplitudes 0.5-1.5 cm, periods 10-30 min). ''Mixed'' combinations of the two behaviors were also observed. Comparisons between bed behavior and a wave-based mobility number parameter (Ψ) observed oscillatory beds for Ψ < 40 and mixed behavior for 40 < Ψ < 80, suggesting that bed level oscillations were due to the presence of smallscale migratory bedforms (ripples). Bed oscillations were suppressed with increased wave energy, such that progressive bed behavior was observed when 80 < Ψ. The increase in mobility numbers associated with progressive beds was due to the presence of an energetic infragravity component (Eu ig /Eu i > 1 and Eu ig > 0.05 m 2 s À2 ) and strong mean current (>0.1 ms À1 ). A fractal analysis of the bed level time series was used to test the hypothesis that the process of ripple migration is nonlinear (''self-organised'') rather than a direct response to variations in the forcing component. For low wave energy conditions, when ripples were observable (oscillatory and mixed time series), values of the Local Hurst Exponent (0 < H L < 1) suggested fractal behavior at ripple migration timescales, indicating that the process by which ripples migrate is nonlinear and is allowed to flourish when the hydrodynamic forcing is weak.INDEX TERMS: 3022 Marine Geology and Geophysics: Marine sediments-processes and transport; 4546 Oceanography: Physical: Nearshore processes; 4558 Oceanography: Physical: Sediment transport; KEYWORDS: marine, sediments, processes, hydrodynamics, bedforms, surf zone Citation: Saulter, A. N., P. E. Russell, E. L. Gallagher, and J. R. Miles, Observations of bed level change in a saturated surf zone,

Geometry and dynamics of wave ripples in the nearshore zone of a coarse sandy beach

“…At low to moderate values of ψ , ripple dimensions gradually increase, but as ψ rises above ψ = 45 there is a notable increase in η , λ and overall scatter. This may be a reflection of the early development of plane bed or LWR conditions in the surf zone, similar to the findings of Saulter et al [2003] who find the transition from SWR to LWR occurs at ψ = 80 and the onset of plane bed conditions at ψ = 100.…”

“…Despite these larger stresses, the scale and shape of the ripples do not change significantly. The presence of vortex ripples in the outer surf zone of sandy beaches is unusual; one would expect postvortex ripples, plane bed conditions or large three‐dimensional bed forms to prevail [e.g., Hay and Bowen , 1993; Osborne and Vincent , 1993; Crawford and Hay , 2001; Hanes et al , 2001; Saulter et al , 2003]. The transformation from vortex to postvortex ripples occurs in the “break‐off region” of Grant and Madsen [1982] and commences at approximately ψ > 50 and θ > 0.2 [ Nielsen , 1981].…”

Geometry and dynamics of wave ripples in the nearshore zone of a coarse sandy beach

“…At low to moderate values of ψ , ripple dimensions gradually increase, but as ψ rises above ψ = 45 there is a notable increase in η , λ and overall scatter. This may be a reflection of the early development of plane bed or LWR conditions in the surf zone, similar to the findings of Saulter et al [2003] who find the transition from SWR to LWR occurs at ψ = 80 and the onset of plane bed conditions at ψ = 100.…”

“…Despite these larger stresses, the scale and shape of the ripples do not change significantly. The presence of vortex ripples in the outer surf zone of sandy beaches is unusual; one would expect postvortex ripples, plane bed conditions or large three‐dimensional bed forms to prevail [e.g., Hay and Bowen , 1993; Osborne and Vincent , 1993; Crawford and Hay , 2001; Hanes et al , 2001; Saulter et al , 2003]. The transformation from vortex to postvortex ripples occurs in the “break‐off region” of Grant and Madsen [1982] and commences at approximately ψ > 50 and θ > 0.2 [ Nielsen , 1981].…”

The Contribution of Sand Ripple and Slope Driven Sediment Flux to Morphologic Change of an Idealized Mound Under Waves

“…During bedload transport dominated conditions, sediment flux driven by bedform translation and deformation can contribute to the depositional and erosion patterns in the nearshore (Aagaard et al., 2001; Coco & Murray, 2007; Hay & Mudge, 2005; Saulter et al., 2003; van Rijn, 2007; van Scheltinga et al., 2020; Wengrove et al., 2018). The sediment flux mechanisms important for the diffusion of a three‐dimensional sediment mound include ripple migration and propagation (Figures 1a and 1c), ripple growth and decay (Figure 1e), ripple splitting leading to higher rates of lateral migration (Figure 1d), and gravitational influence leading to sediment flux driven by mobile ripple layer avalanching (Figure 1b) (Aagaard et al., 2004; Allen & Collinson, 1974; Dalrymple & Rhodes, 1995; Dietrich & Smith, 1984; Lefebvre et al., 2013; Passchier & Kleinhans, 2005; Ruggiero et al., 2016; van Scheltinga et al., 2020; Winter et al., 2008).…”

Bed level changes in the surf zone during post-storm beach recovery

“…The change in the beach bed level is a hot topic in the research on the range of the surf zone. Most of the research on bed level changes is conducted under normal wave conditions (Kroon and Masselink, 2002;Masselink et al, 2008;Biausque and Senechal, 2019), and only a few researchers have performed high-frequency measurements of bed-level elevation (BLE) within tidal cycles (Saulter et al, 2003;Puleo et al, 2014;Pang et al, 2021). Puleo et al (2014) used a new conductivity concentration profiler on a dissipative beach to quantify the bed level changes throughout the tidal cycle.…”

Geometry and dynamics of wave ripples in the nearshore zone of a coarse sandy beach

“…At low to moderate values of ψ , ripple dimensions gradually increase, but as ψ rises above ψ = 45 there is a notable increase in η , λ and overall scatter. This may be a reflection of the early development of plane bed or LWR conditions in the surf zone, similar to the findings of Saulter et al [2003] who find the transition from SWR to LWR occurs at ψ = 80 and the onset of plane bed conditions at ψ = 100.…”

“…Despite these larger stresses, the scale and shape of the ripples do not change significantly. The presence of vortex ripples in the outer surf zone of sandy beaches is unusual; one would expect postvortex ripples, plane bed conditions or large three‐dimensional bed forms to prevail [e.g., Hay and Bowen , 1993; Osborne and Vincent , 1993; Crawford and Hay , 2001; Hanes et al , 2001; Saulter et al , 2003]. The transformation from vortex to postvortex ripples occurs in the “break‐off region” of Grant and Madsen [1982] and commences at approximately ψ > 50 and θ > 0.2 [ Nielsen , 1981].…”

The Contribution of Sand Ripple and Slope Driven Sediment Flux to Morphologic Change of an Idealized Mound Under Waves

“…During bedload transport dominated conditions, sediment flux driven by bedform translation and deformation can contribute to the depositional and erosion patterns in the nearshore (Aagaard et al., 2001; Coco & Murray, 2007; Hay & Mudge, 2005; Saulter et al., 2003; van Rijn, 2007; van Scheltinga et al., 2020; Wengrove et al., 2018). The sediment flux mechanisms important for the diffusion of a three‐dimensional sediment mound include ripple migration and propagation (Figures 1a and 1c), ripple growth and decay (Figure 1e), ripple splitting leading to higher rates of lateral migration (Figure 1d), and gravitational influence leading to sediment flux driven by mobile ripple layer avalanching (Figure 1b) (Aagaard et al., 2004; Allen & Collinson, 1974; Dalrymple & Rhodes, 1995; Dietrich & Smith, 1984; Lefebvre et al., 2013; Passchier & Kleinhans, 2005; Ruggiero et al., 2016; van Scheltinga et al., 2020; Winter et al., 2008).…”

Bed level changes in the surf zone during post-storm beach recovery

“…The change in the beach bed level is a hot topic in the research on the range of the surf zone. Most of the research on bed level changes is conducted under normal wave conditions (Kroon and Masselink, 2002;Masselink et al, 2008;Biausque and Senechal, 2019), and only a few researchers have performed high-frequency measurements of bed-level elevation (BLE) within tidal cycles (Saulter et al, 2003;Puleo et al, 2014;Pang et al, 2021). Puleo et al (2014) used a new conductivity concentration profiler on a dissipative beach to quantify the bed level changes throughout the tidal cycle.…”

Geometry and dynamics of wave ripples in the nearshore zone of a coarse sandy beach

“…At low to moderate values of ψ , ripple dimensions gradually increase, but as ψ rises above ψ = 45 there is a notable increase in η , λ and overall scatter. This may be a reflection of the early development of plane bed or LWR conditions in the surf zone, similar to the findings of Saulter et al [2003] who find the transition from SWR to LWR occurs at ψ = 80 and the onset of plane bed conditions at ψ = 100.…”

“…Despite these larger stresses, the scale and shape of the ripples do not change significantly. The presence of vortex ripples in the outer surf zone of sandy beaches is unusual; one would expect postvortex ripples, plane bed conditions or large three‐dimensional bed forms to prevail [e.g., Hay and Bowen , 1993; Osborne and Vincent , 1993; Crawford and Hay , 2001; Hanes et al , 2001; Saulter et al , 2003]. The transformation from vortex to postvortex ripples occurs in the “break‐off region” of Grant and Madsen [1982] and commences at approximately ψ > 50 and θ > 0.2 [ Nielsen , 1981].…”

The Contribution of Sand Ripple and Slope Driven Sediment Flux to Morphologic Change of an Idealized Mound Under Waves

“…During bedload transport dominated conditions, sediment flux driven by bedform translation and deformation can contribute to the depositional and erosion patterns in the nearshore (Aagaard et al., 2001; Coco & Murray, 2007; Hay & Mudge, 2005; Saulter et al., 2003; van Rijn, 2007; van Scheltinga et al., 2020; Wengrove et al., 2018). The sediment flux mechanisms important for the diffusion of a three‐dimensional sediment mound include ripple migration and propagation (Figures 1a and 1c), ripple growth and decay (Figure 1e), ripple splitting leading to higher rates of lateral migration (Figure 1d), and gravitational influence leading to sediment flux driven by mobile ripple layer avalanching (Figure 1b) (Aagaard et al., 2004; Allen & Collinson, 1974; Dalrymple & Rhodes, 1995; Dietrich & Smith, 1984; Lefebvre et al., 2013; Passchier & Kleinhans, 2005; Ruggiero et al., 2016; van Scheltinga et al., 2020; Winter et al., 2008).…”

Bed level changes in the surf zone during post-storm beach recovery

“…The change in the beach bed level is a hot topic in the research on the range of the surf zone. Most of the research on bed level changes is conducted under normal wave conditions (Kroon and Masselink, 2002;Masselink et al, 2008;Biausque and Senechal, 2019), and only a few researchers have performed high-frequency measurements of bed-level elevation (BLE) within tidal cycles (Saulter et al, 2003;Puleo et al, 2014;Pang et al, 2021). Puleo et al (2014) used a new conductivity concentration profiler on a dissipative beach to quantify the bed level changes throughout the tidal cycle.…”