Swash zone boundary conditions and direct bed shear stress measurements over loose sediment beds

Abstract

The hydrodynamic and bed boundary conditions in the swash zone are investigated through laboratory, field and numerical experiments. The swash hydrodynamics induced by monochromatic waves were numerically investigated by using the CoulWave model. The correlation between the swash flow asymmetry parameter k and offshore wave parameters (e.g. wave height H, wave period T and Iribarren number ξ) was examined. The computed magnitude of k agrees with the results (-1, 1.5) from remote sensing field data but falls in a relatively narrow range of [0.68, 0.89]. A low correlation was observed between k and wave parameters examined in the present study.The role of the hydraulic resistance in the swash zone has been investigated by comparisons with an existing analytical solution (Shen and Meyer, 1963) and a semi-analytical swash hydrodynamics model (Guard and Baldock, 2007). The characteristic form equations incorporated with a Chézy type friction term were solved by the finite difference mid-point method. The swash hydrodynamics appear to be damped in the presence of the friction term and the resolved shoreline motions were retarded as well. However, the swash asymmetry (k) is not significantly affected by the friction term.

A Swash-Zone Seaward Boundary Condition for Impermeable Beaches

“…Where: α(t) is the value of the characteristic variable (at time t) and k is an introduced constant introduced. Despite noteworthy improvements, the selection of k is subjective and without any significant physical meaning/relation to offshore wave conditions [2].…”

“…In general, the hydrodynamics between each are significantly different. Comparatively, higher maximum velocities are typical of the uprush phase, whilst longer flow periods are typical of the backwash phase [2]. Despite the generic differences between uprush and backwash, the overall hydrodynamics of the swash event is strongly influenced by whether breaking or non-breaking waves occur at the shore [1].…”

“…These models predominantly employ the depth-averaged nonlinear shallow-water theory or Boussinesq simplifications and have significantly advanced understanding of the swash-zone. Nonetheless, the complex processes of the zone's hydrodynamics are not completely understood, warranting the need for further in-depth investigations [2]. Alternatively, depth-resolving models, which implement some form of the full Navier-Stokes Equations, presents a viable option for an improved flow description in the zone.…”