Relative Significance of Background Erosion and Spreading Losses: An Aid for Beach Nourishment Design

Abstract

The evolution of beach nourishment projects includes the background (pre-nourishment) erosion effects, herein referred to as ‘background erosion’ and the project evolution in the absence of background erosion, namely the ‘spreading out’ effects and profile adjustment. This paper examines the relative significance of background erosion and volumetric spreading for idealized initial nourishment planforms. According to approximate beach nourishment theory, it is possible to simply superimpose these two effects. It is useful in the design process to develop estimates of the approximate relative contributions of these two components. It will be shown that in addition to the magnitude of background erosion, the relative effects of background erosion are more significant for the longer projects and increase with time. The interpretation is that the spreading out losses are less for the longer projects and, after evolving for some time, the project behaves as a longer project and thus the relative importance of the background erosion is greater under these two scenarios.Design aids are presented in the form of graphs to assist in evaluating the relative importance of these two components of beach nourishment project evolution and for preliminary design purposes. These aids provide a basis for determining the significance of background erosion and thus the level of effort warranted in establishing the background erosion rate as compared to the wave forcing which drives the longshore spreading processes. For cases in which the background erosion is secondary, the design engineer can concentrate efforts on the factors (waves) which govern the spreading out losses. Two beach nourishment settings are considered for nourishment with initially rectangular planforms: (1) on a long straight beach, and (2) on a long barrier island with one end of the nourishment adjacent to an inlet. These design aids are illustrated with idealized situations and additional examples are presented for five projects constructed in Florida.KeywordsBarrier IslandShoreline PositionBeach NourishmentTotal Volume ChangeSpreading LossThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.