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 x) 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 Chezy 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. Further investigations of the relation between predicted swash amplitude and frequency implies that present MOC (Method of Characteristics) frictional model is inconsistent with the physics of the problem and experimental data, but that the numerical treatment of friction is sensible. By applying a shear plate cell, direct bed shear measurements have been conducted on loose sediment beds in dam-break driven swash flows. Bed shear stress measurements for rough fixed bed and loose mobile bed configurations are presented and contrasted. Performance of the shear plate varies with the sediment grain size. For the coarse grain (d50=2.85mm) loose beds, only peak shear stress at the leading edge of the swash flow was consistently obtained, while nearly whole time series of bed shear stress measurements could be obtained over fine grain (d50=0.22mm) beds. The laboratory data indicate that bed shear stress comprises of a fluid shear stress component and a grain shear stress component, the latter which increases linearly with sediment load (i.e. dispersive shear stress). Measured bed shear stresses over mobile beds are significantly greater than those over fixed beds of the same grains. The measured shear stress increases approximately linearly with the reservoir water depth. The calculated dynamic angle of internal friction is similar to the values of Hanes and Inman (1985) under idealised steady conditions. A Lagrangian boundary layer model for swash flow is presented to account for the flow history effects. The model is based on the momentum integral method for turbulent rough flat-plate boundary layers and is forced by a 2D hydrodynamic finite volume model. Fluid particle trajectories and the evolution of the boundary layer thickness are computed in the Lagrangian framework. Good agreement has been obtained between the bed shear stress predictions and log-law estimations in the uprush phase for an existing large scale data set. Comparison with an alternative coupled boundary layer model (Briganti et al., 2015) is favourable for both bed shear stress and boundary layer development. The time series of bed shear stress for loose mobile bed conditions have been reconstructed by using the Lagrangian rough flat-plate boundary layer model. Total load transport rates were thus computed on basis of the Meyer-Peter a Muller (1948) formula and compared with measured transport rates obtained from the same small scale dam-break experiments. The varying performance of both Eulerian and Lagrangian methods in sediment transport modelling is discussed.