A Comparison of Beach Nourishment Methodology and Performance at Two Fringing Reef Beaches in Waikiki (Hawaii, USA) and Cadiz (SW Spain)

Abstract

Fringing reefs have significant impacts on beach dynamics, yet there is little research on how they should be considered in beach nourishment design, monitoring, and conservation works. Thus, the behavior and characteristics of nourishment projects at two reef protected beaches, Royal Hawaiian Beach (RHB) in Hawaii, USA, and Victoria Beach (VB) in Cadiz, Spain, are compared to provide transferable information for future nourishment projects and monitoring in fringing reef environments. The nourishment cost at RHB was nine times higher than VB. This is partly due to lower total volume and a more complex placement and spreading method at RHB, despite the much closer borrow site at RHB. There was a significant difference in post-nourishment monitoring frequency and assessment of accuracy. RHB elevation was monitored quarterly for 2.7 years at 30 m-spaced profiles, compared to 5 years of biannual surveys of 50 m-spacing at VB. An additional problem related to the presence of reefs at both RHB and VB was estimating the beach volume increase after nourishment, due to variable definitions of the 'beach' area and high alongshore variability in reef topography. At sites where non-native sediment is used, it is imperative to understand how wave and current energy changes due to reefs will influence nourishment longevity. Thus, differences in erosion and accretion mechanisms at both beaches have been detected, though are still little understood. Moreover, discrepancies in sediment porosity between the two sites (which should be surveyed in future nourishments) have been found, probably due to differences in the nourishment sand transportation and distribution methods. In summary, more dialogue is needed to explicitly consider the influence of fringing reefs on coastal processes and beach nourishment projects. time [2,3]. However, there is increasing research showing that the geologic setting plays a dominant role in contemporary beach morphodynamics [4][5][6][7]. In fact, some studies have taken into account the former geologic setting to design multifunctional artificial reefs that serve several purposes [8].One common type of geologically-controlled beach is situated landward of areas of hard bottom, such as rock or coral reefs. Reefs have large impacts on coastal hydrodynamics, sediment transport, and morphology, such as by influencing water level fluctuations [9], wave set-up and cross-reef currents [10], wave breaking, wave-induced flows, and wave attenuation over reef platforms [11][12][13][14][15], which affect the beach morphodynamics tidal range at multiple scales [16][17][18][19], even under hurricane conditions [20,21].Despite clear evidence that beaches with reefs behave differently to their non-reef counterparts, there has been scant attention given to how reefs should be considered in beach nourishment projects. A recent study by Habel et al. [22] focused on the methodology of a beach nourishment at Royal Hawaiian Beach (RHB), Waikiki, which is fronted by a fringing coral reef. They also analyzed the subseque...

A Design Parameter for Reef Beach Profiles—A Methodology Applied to Cadiz, Spain

“…The maritime climate was obtained using the real (REMRO) and virtual (SIMAR) buoy data sets available at (http://www.puertos.es). Moreover, cartographic, bathymetric, morphologic, and sediment data were obtained from the "Ecocartographic Study of the Province of Cádiz" [25] and from other studies carried out previously on the SW coast of Spain [26][27][28][29].…”

Geologically controlled sandy beaches: Their geomorphology, morphodynamics and classification

“…leaky systems), which is likely to increase in nourished geologically controlled beaches. The adjustment of a nourished beach profile when there are cross-shore geological constraints is also likely to depart from theoretical models of cross-shore sediment redistribution used in coastal engineering (Muñoz-Perez et al, 2020). Scaling up to a local sediment cell, there is scant understanding of and limited modelling tools to predict the rates of sediment transport from geologically controlled beaches to other coastal systems, or between these beaches (Naylor et al, 2016).…”

“…For example, a geologically controlled beach might be an important supply of sediment to a nearby spit (as in Westward Ho!, North Devon, UK), but we have limited understanding of the process and of how much geologically controlled beach material serves as a key source of sediment to an economically and socially valuable beach spit. Moreover, it is largely unclear how to account for spatial variations in geological controls to quantify beach nourishment volumes and costs, and how to include the effect of this variation on the beach morphodynamics and hence nourishment performance and longevity (Muñoz-Perez et al, 2020).…”

The Influence of Reef Topography on Storm-Driven Sand Flux

“…Thus, these studies suggest that the effect of reefs on beach dynamics is highly complex, and variable alongshore, even at a single beach. Moreover, while there has been extensive research on the cross-shore response of hydrodynamics over reefs, such as wave transformation [17][18][19], less attention has been paid to the overall alongshore variations in both cross-shore and longshore sediment transport [20] and the resulting beach dynamics. This is a complex task because, in addition to the cross-shore process of wave attenuation, the alongshore variability of reefs is a key factor in controlling sediment transport and beach morphodynamics drivers.…”