Swash
Swash denotes the turbulent uprush and subsequent backwash of water across the beach face following the breaking of incident waves at the shoreline.[1] This oscillatory motion defines the swash zone, the intermittently inundated region between maximum runup and rundown limits, where supercritical flows prevail with velocities typically ranging from 2 to 5 m/s.[2] The swash zone serves as a critical interface between surf zone hydrodynamics and subaerial beach morphology, governing cross-shore sediment fluxes through bedload sheet flow and suspended load advection.[2] On dissipative beaches, infragravity waves dominate swash energetics, promoting erosion, whereas reflective steep slopes enhance backwash dominance and potential accretion via berm formation.[1] Key parameters, such as the surf similarity parameter \epsilon_b = \frac{4\pi^2 H_b}{2g T^2 \tan^2 \beta}, delineate dissipative (\epsilon_b < 2.5) from reflective (\epsilon_b > 20) regimes, influencing runup extents and sediment transport efficiency.[2] Swash processes drive significant alongshore transport, contributing up to 50% of littoral drift on steep beaches, and underpin features like beach cusps and rhythmic patterns via instabilities in flow-sediment interactions.[2] Empirical models, including runup formulations like R_{2\%} = 1.1 (0.5 \sqrt{H_s L_p (0.563 \beta^2 + 0.004)} + 0.35 \beta \sqrt{H_s L_p}), enable prediction of morphological responses to wave forcing and sea-level variations.[2] These dynamics are pivotal for coastal management, as swash-mediated erosion controls shoreline retreat rates during storms.[3]Hydrodynamics
Uprush and Backwash Mechanics
The swash cycle comprises the uprush, the landward surge of water following wave bore collapse on the beachface, and the backwash, the gravity-driven seaward retreat.[2] Uprush initiates as a thin, turbulent sheet flow with rapid propagation of the wave tip, decelerating under gravity and bottom friction, while backwash forms shallower flows that accelerate uniformly downslope until friction dominates in thin depths.[2] Field observations indicate free-stream velocities of 2-5 m/s during swash events, with peaks at the onset of uprush and linear decreases through backwash.[2] Uprush depths exceed backwash depths, and uprush durations are typically shorter than backwash durations, contributing to hydrodynamic asymmetry.[2] Laboratory flume experiments on impermeable beaches reveal uprush velocities reaching 1.5-1.7 m/s post-bore arrival, transitioning to depth-uniform profiles, whereas backwash velocities peak at 0.9-1.6 m/s with pronounced near-bed shear on steeper slopes.[4] Boundary layer development differs markedly: during uprush, the layer is thinnest at the seaward edge and thickens landward before vanishing at flow reversal, then regrows in backwash.[2] Friction coefficients range from 0.02 to 0.1, exerting significant drag in shallow flows.[2] Beach slope modulates these mechanics; milder slopes (e.g., 1:35) yield longer swash cycles (∼17.5 s) and greater uprush velocity asymmetry, while steeper slopes (e.g., 1:10) produce shorter cycles (∼7.6 s), hydraulic jumps in backwash, and enhanced turbulence.[4] Swash-swash interactions, including overtaking of uprush by subsequent bores or collisions of backwash with incoming waves, induce flow reversals and skew velocity moments offshore, influencing overall dynamics.[2] Gravity remains the primary force, but pressure gradients, infiltration, and wave setup contribute to net cross-shore flows.[5]Infragravity and Sea-Swell Interactions
Infragravity waves, defined by frequencies lower than 0.05 Hz (periods exceeding 20 seconds), emerge primarily from nonlinear interactions among higher-frequency sea-swell waves during wave shoaling and breaking in the nearshore.[6] These long-period oscillations contrast with sea-swell waves (frequencies above 0.05 Hz), which drive primary surf zone processes but saturate through breaking, whereas infragravity energy often amplifies shoreward due to reduced dissipation.[7] In the swash zone, infragravity waves contribute significantly to total velocity variance, often exceeding 50% of the signal on dissipative beaches, modulating uprush duration and backwash strength.[8] Sea-swell wave groups generate infragravity waves via two main mechanisms: bound waves, which are phase-locked to the group envelope and release as free waves post-breaking, and breakpoint forcing from turbulent release during sea-swell collapse.[9] Observations indicate weak seaward coupling between infragravity waves and sea-swell groups offshore, but stronger shoreward propagation as free modes that reflect from the shoreline, enhancing standing wave patterns in the inner surf and swash.[10] This reflection sustains infragravity energy, with heights reaching meters nearshore even when open-ocean amplitudes are small (centimeters).[6] Interactions intensify in the swash zone, where infragravity motions modulate sea-swell runup by altering instantaneous water depths and breaking thresholds; for instance, infragravity crests can promote sea-swell breaking, while troughs delay it, leading to asymmetric swash envelopes.[11] Numerical models like SWASH reveal beach steepness controls these transfers: on steeper profiles, nonlinear infragravity-sea-swell energy exchanges peak in intermediate depths, diminishing in shallow swash due to saturation, whereas gentler slopes sustain bound-mode dominance into the shoreline.[12] Field data from high-energy events confirm infragravity swash exceeds sea-swell contributions during swell-dominated conditions, correlating with offshore groupiness (correlation coefficients >0.8 with swell energy).[13] Such dynamics drive net offshore sediment flux under infragravity asymmetry, distinct from onshore-biased sea-swell transport.[8]Flow Reversals and Turbulence
In the swash zone, flow reversal denotes the abrupt transition from onshore-directed uprush to offshore-directed backwash, driven by the imbalance between wave-induced momentum and gravitational drainage on the beachface.[2] This reversal typically occurs within seconds, with near-bed flows reversing prior to those higher in the water column due to enhanced frictional drag at the boundary layer.[2] Laboratory experiments indicate that reversal initiates near the bed during the thinning of the swash film, leading to flow attachment and internal circulation cells that persist briefly into the backwash phase. Turbulence during flow reversal intensifies as velocity gradients sharpen, particularly in the boundary layer, where shear from accelerating backwash generates high turbulent kinetic energy (TKE) levels. Field observations reveal peak near-bed TKE immediately following wave crest passage and during reversal, often exceeding Froude-scaled values by factors linked to bore collapse and sediment interaction.[14] In the uprush phase, turbulence profiles are relatively uniform vertically, reflecting bore-propagated eddies, whereas backwash turbulence becomes bottom-dominated, decaying offshore due to reduced water depth and increased bed roughness.[14] Bore-driven mechanisms contribute significantly to swash turbulence, with breaking waves injecting kinetic energy that persists through reversal and influences subsequent flow instability.[15] Numerical simulations confirm that air entrainment at the swash front enhances turbulence production during run-up reversal, releasing bubbles that amplify dissipation rates at the rear.[16] Experimental data from steep beaches show turbulence scales following a -5/3 Kolmogorov spectrum during bore development at reversal, transitioning to steeper cascades under strong shear, underscoring the role of nonlinear wave-bore interactions.[17] These dynamics vary with beach slope and wave period, with steeper profiles (tan β > 0.1) exhibiting more pronounced reversal-induced bursts.[4]Morphology
Beachface Profiles
The beachface constitutes the steep foreshore segment of the intertidal profile, spanning from the berm crest seaward to the low-tide shoreline, where recurrent swash uprush and backwash govern erosion, accretion, and sediment sorting.[18] Its morphology reflects a dynamic equilibrium between incident wave forcing, sediment grain size, and tidal modulation of the active swash zone width.[2] Typical slopes range from tan β ≈ 0.01–0.05 on dissipative beaches to tan β > 0.1 on reflective ones, with gravel-dominated faces often exceeding tan β = 0.18.[19][20] Equilibrium beachface profiles approximate Dean's parabolic form, expressed as depth h(y) = A y^{2/3}, where y is the offshore distance from shore and A ≈ 0.1–0.3 m^{1/3} scales with median grain diameter D_{50} via A ∝ D_{50}^{1/3}, yielding steeper nearshore gradients for coarser sediments under constant wave energy dissipation.[21] This model assumes uniform energy flux decay, though field observations reveal deviations, such as planar steep faces on reflective beaches under short-period, high waves.[22] Morphodynamic state hinges on the surf scaling parameter ε_b = \frac{4\pi^2 H_b}{g T^2 \tan^2 \beta}, where H_b denotes breaker height, T peak period, g gravitational acceleration, and β the face slope; ε_b < 2.5 characterizes dissipative profiles with gentle slopes, wide surf zones, and spilling breakers, while ε_b > 20 defines reflective profiles featuring steep, linear gradients, surging waves, and prominent berms.[23] Intermediate states (2.5 < ε_b < 20) exhibit transitional barred or rhythmic features.[24]| Profile Type | Slope (tan β) | Key Features | Wave Conditions | ε_b Range |
|---|---|---|---|---|
| Reflective | > 0.05–0.1 | Steep, planar; berms, cusps; minimal surf zone | Short T, high H_b; surging breakers | > 20 |
| Dissipative | < 0.02–0.03 | Gentle, concave; multiple bars; broad intertidal | Long T, low steepness; spilling breakers | < 2.5 |