An idealised study for the evolution of a shoreface nourishment

Abstract

We develop an idealised one dimensional (cross-shore) morphodynamic model that 6 couples wave, tide and sediment dynamics to study the effect and evolution of a 7 shoreface nourishment. Sediment fluxes driven by wave skewness, wave asymmetry 8 (both onshore) and return flow (offshore) are considered. With the aid of new an-9 alytical expressions for the skewness and standard deviation of wave velocity and acceleration, sediment fluxes are calculated. Nourishment is viewed as a perturbation to the system in equilibrium that is subject to the divergence of the perturbed sediment flux and a gravity driven diffusion term. Depending on the location, a nourishment may provide a feeder or lee effect. In moderate and mild wave conditions, the evolution of a nourishment primarily depends on the relative location of nourishment and break point. Placed well offshore of the break point, the nourishment induces an overall positive perturbation in sediment flux, resulting in onshore migration (feeder effect). Located closer to the break point, the nourishment induces an earlier wave breaking, which dissipates part of the wave energy (lee effect), leading to a negative sediment flux perturbation around this break point and a positive sediment flux perturbation around the break point of the un-nourished beach. Depending on the intensity of the earlier breaking, the nourishment either migrates onshore (weak break) or splits into onshore and offshore moving parts (strong break). The relative importance of the diffusion term and the divergence of perturbed sediment flux may lead to a primarily migrating or decaying evolution of nourishment. In storm wave conditions, the nourishment tends to move offshore due to the predominance of return flow driven sediment flux. The sensitivity to wave period and tide are also studied. Model results are consistent with observations, as well as prevailing theory on cross-shore sediment transport.

A nonlinear perturbation study of a shoreface nourishment on a multiply barred beach

“…Roos and Hulscher (2003) also provided a general linear framework of looking at bed evolution as a perturbation problem. Model results of Chen and Dodd (2019) are consistent with observations, as well as prevailing theory on cross-shore sediment transport.…”

“…To circumvent these problems Chen and Dodd (2019) developed an approach that can be described as a "nourishment-as-a-perturbation" (hereafter NaaP) concept. In this approach they focus on the dynamics of the nourishment alone (i.e.…”

“…However, the study of Chen and Dodd (2019) was limited to an idealised, plane beach. For the concept to be useful, it must be valid for non-planar beaches, on which there may be multiple regions of breaking and shoaling, aside from the nourishment itself.…”

“…We therefore present here a study, in which the nonlinear NaaP model of Chen and Dodd (2019) is applied to a real beach that possesses a multiple barred background profile on which a nourishment is placed. To do this we must also develop further the NaaP approach so as to include wave streaming, which is thought to play an important role in cross-shore sediment flux (Fuhrman et al, 2009).…”

Morphodynamic Evolution of a Nourished Beach with Artificial Sandbars: Field Observations and Numerical Modeling

“…A lot of previous morphodynamic model studies of beach nourishment assumed a single-fraction sediment with uniform diameter [36][37][38][39][40]. During beach nourishments, however, it is a common practice to apply filling sand slightly larger than the original beach sand in order to slow down the erosion the introduced sand from the beach system [41].…”