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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 &lsquo, beach&rsquo, 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.

Source: https://erdc-library.erdc.dren.mil/jspui/ Hydraulic model studies were conducted at the U. S. Army Engineer Waterways Experiment Station to aid in determining the technical feasibility and optimum design factors of the perched beach concept for widening the existing beach to provide right-of-way for a freeway along a portion of the Santa Monica Bay coastline. The original study proposal consisted of six main parts. However, during the course of the model studies, the California Legislature deleted this section of the freeway from the California Freeway and Expressway System. As a result, the Division of Highways terminated their freeway location project and canceled further model testing. Consequently, only the following three parts of the study were completed: (a.) an undistorted, three-dimenaional, fixed-bed model (scale 1:100) was used to determine the effect of the perched beach on rip currents. If adverse interactions were present, the model was used to determine means of minimizing them; (b.) a distorted-scale (1:100 horizontal, 1:50 vertical), two-dimeneional, movable bed model was used to estimate the amount of sand which might be lost seaward over the toe structure due to normal and storm wave actions and to determine the optimum crown elevation of the submerged structure and the length of stone riprap apron required to reduce the seaward migration of sand to a minimum; and (c.) an undistorted, two-dimensional model (scale 1:30) was used to determine the structural design of the proposed rubble-mound toe structure for various depths. The following three parts of the study were not completed: (a.) an office data (environmental data) analysis, including a hydrographic survey with sand sampling, a re-evaluation of geographical information, and a computation of littoral transport; (b.) the construction and testing of a surf beat model to evaluate the wave runup characteristics with and without a perched beach; and (c.) an office study to determine the feasibility of constructing a three-dimensional, movable-bed model. This report describes the testing and results up to the premature termination of the model studies. It was concluded from test results that: (a.) installation of the perched beach in the model had no adverse effect on rip currents and reduced rip current velocities about 20 percent for the configuration tested, (b.) normal wave friction (waves expected to occur a high percentage of the time) on the perched beach caused no appreciable loss of beach fill; (c.) for the larger storm waves of any significant duration, a large net seaward loss or fill material can be expected; (d.) the installation of a 100-ft stone apron in conjunction with a 350-ft-wide (measured from toe structure to 0.0-ft -mllw contour) perched beach (plan 2A) will have little or no effect on the loss of fill material; (e.) the installation of a 100-ft stone apron in conjunction with a 700-ft-wide (measured from toe structure to 0.0-ft-mllw contour) perched beach (plan 1A) will significantly reduce the amount of beach fill lost seavard of the toe structure; (f.) if the beach fill is extended as far as 1100 ft seaward of the 0.0-ft-mllw contour (plans 3 and 3A), the toe structure will have little or no beneficial effect on reducing the amount of beach fill lost; (g.) of all plana tested , plan 1A appears to offer the greatest degree of protection against seaward loss or beach fill material; (h.) 5000-lb armor stone will be needed if the rubble-mound toe structure is located in water depths ranging from 20 to 35 ft; 3000-lb armor stone will be needed in depths from 35 to 45 ft; and 1000-lb armor stone will be needed in depths greater than 45 ft; (i.) quarry-run stone whose 50 percent weight is about 100 lb will be adequate bed material for the toe structure; (j.) 250-lb stone wlll be stable for the riprap apron shoreward of the toe structure; (k.) additional wave flume tests will be needed with armor units in the specific gravity range of 1.3 to 1.6 prior to performing a three-dimensional, movable-bed model investigation; and (l.) provided adequate prototype data become available for use in model verification, a three-dimensional, movable-bed model investigation appears to be feasible and should result in a valid indication of the relative merits of various project designs.