Evaluating Transport Formulations for Application to Nearshore Berms

Rachel Bain,Austin Hudson,Douglas Krafft,Brian Mcfall

doi:10.1061/(asce)ww.1943-5460.0000668

Abstract

Dredged sediment is commonly placed as a submerged nearshore berm to nourish the beach or to dissipate high-energy waves, but the lifespan of such features is not easily predicted by existing methods. This paper presents a simple technique for generating order-of-magnitude estimates of the sediment transport rate of nearshore berms using offshore hindcast wave characteristics transformed to the nearshore. Total longshore transport for the entire nearshore region is calculated using eight published longshore transport equations (e.g., CERC equation and Kamphuis equation), which were evaluated for their relative performance. Because nearshore placements occupy only a portion of the cross-shore profile, the total longshore transport rate is scaled by an empirically-based fraction between 0 and 1, which is determined by the nearshore berm’s position in nondimensional space. The cross-shore transport rate is calculated independently using the near-bed orbital velocity from stream-function wave theory. The longshore and cross-shore transport rates are then superimposed to generate a total transport rate for the nearshore berm’s constructed footprint. The total transport rates were calculated at 11 historical nearshore berms and evaluated based on accuracy, inclusion of relevant coastal processes, and sensitivity to input parameters. The recommended total transport rate technique resulted in an average percent error magnitude of 72% and a maximum percent error magnitude of 167% at the historical placement locations. This technique is recommended for generating rapid, order-of-magnitude estimates of nearshore berm deflation rates for project design, particularly in scenarios when application of a full numerical model is prohibitive.

Highlights

Placing dredged sediment in the nearshore as a submerged mound or elongated bar is a common practice for the beneficial use of dredged sediment
These comparisons consider the average rate of transport over the monitoring duration; i.e., all values of Qberm referenced were calculated as Vloss/(tend − tstart), with Vloss as defined in Eq (12)
Several simple sediment transport models based on a superposition of longshore and cross-shore transport have been evaluated for estimating the deflation rate of nearshore berms, which can be used to predict the lifespan of proposed placements

Summary

Introduction

Placing dredged sediment in the nearshore as a submerged mound or elongated bar is a common practice for the beneficial use of dredged sediment. This type of nearshore nourishment is called a nearshore berm and can be considered a soft submerged breakwater. These features are categorized as either active or stable (McLellan et al 1990; Hands and Allison 1991). New results from Hudson et al (“Sediment mobility, closure depth, and the littoral system–Oregon and Washington coast.” in prep., ERDC/CHL CHETN, Vicksburg, MS: US Army Corps of Engineers) indicate that the rate of volume removal from a nearshore placement at the mouth of the Columbia River could be accurately calculated by balancing near-bed, wave-driven transport with downslope diffusion

Methods

Results

Discussion

Conclusion