Publisher’s version / Version de l'éditeur:
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site
LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Proceedings of the 7th International Conference on the Application of Physical
Modelling in Coastal and Port Engineering and Science (Coastlab18), 2018-05
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
NRC Publications Archive Record / Notice des Archives des publications du CNRC :
https://nrc-publications.canada.ca/eng/view/object/?id=511b0d14-fd2a-4e41-961f-98c005f43406
https://publications-cnrc.canada.ca/fra/voir/objet/?id=511b0d14-fd2a-4e41-961f-98c005f43406
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Physical modelling of shoreline improvements in the Caribbean
Proceedings of the 7
thInternational Conference on the
Application of Physical Modelling in Coastal and Port
Engineering and Science (Coastlab18)
Santander, Spain, May 22-26, 2018
PHYSICAL MODELLING OF SHORELINE IMPROVEMENTS IN THE CARIBBEAN
SCOTT BAKER1, SETH LOGAN2, TERENCE DES VIGNES3, KEVIN MACINTOSH2
1 National Research Council, Canada, scott.baker@nrc.ca
2 W.F. Baird & Associates Coastal Engineers Ltd., Canada, slogan@baird.com & kmacintosh@baird.com 3 Unnamed Resort Developer, tdvhot@gmail.com
ABSTRACT
This paper discusses a 3D physical modelling study that was commissioned to support shoreline improvements for a new resort development in the Caribbean. The model included a recreation of the complex foreshore and shoreline at the project site at a geometric scale of 1:25. Beach sediment was modelled using fine sand that deformed naturally in response to wave forcing. The model was outfitted with a directional wave machine to generate a wide range of realistic seastates, and was equipped with instrumentation to measure wave conditions, wave-induced nearshore currents, and changes in the plan and profile shape of the model beach.
Nearly 150 individual tests were conducted to assess the impact of proposed coastal structures on the wave conditions and nearshore currents, as well as their effect on sediment transport processes and the response of the beach, for a range of prescribed wave conditions and water levels. These investigations generated a large quantity of valuable information which will be used to optimize and support the detailed design of the waterfront improvements and obtain the necessary permits required for construction.
KEYWORDS: Shoreline improvements, sediment transport, coastal processes, coastal structures, breakwaters.
1 INTRODUCTION
A resort developer plans to renovate an existing resort in the Caribbean which includes approximately 500 m of waterfront. As part of efforts to upgrade the existing site, the resort developer commissioned W.F. Baird & Associates (Baird) to design an expanded beach, sheltered swimming areas for children, improved access to the offshore coral gardens, and an overall improvement in site functionality and aesthetics. Plans for the final concept include a watersports pier, improved property and watershed drainage, and substantial measures to mitigate environmental and biological impacts. Ultimately, the project will be deemed successful if there is a net gain of marine habitat through the creation of artificial reefs and the use of sustainable construction materials and methods.
1.1 Site Conditions
The project site is located on the west coast of a small, south-eastern Caribbean island. The existing shoreline is characterized by a steep and narrow beach fronted by coral rubble, coral pavement, and areas of sandy bottom, and is classified as a typical spur and groove reef. A number of small man-made coastal structures exist along the property, and are in various states of disrepair.
The wave climate on the west coast of the island is dominated by northerly swell events. Swell waves approach the project site from a relatively narrow band of directions in the northwest quadrant. Swell events typically occur in the late fall and winter months and can have wave periods up to 16 s featuring offshore wave heights up to 1.75 m. Occasionally, storm waves generated by tropical lows will approach the project site from the southwest quadrant, with the 10-year and 50-year events featuring an offshore significant wave height on the order of 2.0 m and 4.8 m respectively. These observations are based on a 36-year wave hindcast at a location approximately 1 km north of the project site in 20 m water depth, combined with Monte Carlo-type hurricane modelling of 1,200 synthetic storm tracks affecting the island.
Tides at the project site are semi-diurnal with a tidal range of approximately 0.6 m. Mean Lower Low Water (MLLW) is approximately equal to +0.0 m, while Mean Higher High Water (MHHW) is approximately +0.6 m. An extreme water level of +1.3 m was simulated to account for a high spring tide in conjunction with a severe storm surge, sea level anomaly, and an allowance for sea level rise. This corresponds to the estimated 100-year water level.
Analysis of sediment samples taken at the project site indicate that the average grain size (beach crest and submerged samples) varies between 0.30-0.35 mm, with the north end of the site having slightly smaller sand than the south end of the site.
1.2 Proposed Shoreline Improvements
Baird developed a number of alternatives schemes featuring different arrangements of groynes and breakwaters intended to improve swimming conditions and to help stabilize the beach during storms. One such scheme (refer to Figure 1) involved constructing three emergent nearshore breakwaters and two submerged offshore breakwaters, as well as four groynes, a watersports pier, and several other minor features.
Figure 1. Conceptual shoreline improvement scheme.
2 PHYSICAL MODEL DESIGN & CONSTRUCTION
The National Research Council of Canada (NRC) was commissioned to conduct physical modelling studies to support coastal engineering services for the new development. The study objectives were to: (1) assess and measure the nearshore wave climate and wave-induced currents, and optimize the design elements to provide satisfactory conditions for swimming and water recreation; (2) assess the behaviour of the beach fill under operational and storm wave conditions, including sediment transport and reshaping of the beach; (3) assess the risk for flooding of upland areas due to high water levels combined with high waves; (4) assess various environmental considerations; and (5) assess, verify, and optimize the stability and performance of the proposed coastal structures under realistic site-specific operational and storm conditions.
2.1 Model Layout & Construction
The model study was conducted in NRC’s 50.4 m by 29.4 m Large Area Basin (LAB), a state-of-the-art facility for engineering research and performance verification of maritime structures in the ocean environment. The LAB can accommodate water depths up to 1.5m and is equipped with a powerful directional wave generator featuring 72 independent wave boards that are capable of generating a broad range of realistic wave conditions with significant wave heights up to 0.4 m.
A three-dimensional physical model of the proposed shoreline development and the surrounding bathymetry was constructed at a geometric scale of 1:25. This scale represented a reasonable compromise between minimizing scaling and boundary effects, while maximizing the extent of the model domain and the accuracy of the sedimentary processes being modelled. The lower portion of the model bathymetry was rigid, while the upper portion near the waterline was erodible. The model bathymetry covered a prototype region of approximately 735 m by 365 m. The model was oriented within the basin such that the proposed shoreline development could be tested in waves approaching from a ~80° range of directions between 240° and 320°. A side-wall reflection technique, which involved purposely reflecting waves off the basin walls on either side of the wave machine, was a crucial factor in being able to generate seastates with mean wave directions of 40° relative to the wave machine that were reasonably homogeneous across the test site. Porous gravel beaches with a mild slope were installed along portions of the side walls to control unwanted wave reflections.
All aspects of the model, except for the mobile-bed sediment and the armour stone, were designed using Froude scaling criterion. The scaling of beach sand was such that the fall velocity of the sediment was scaled according to the square root of the length scale. Therefore, the selected model sediment grain size (D50 ~0.12 mm) was considerably larger than suggested by direct Froude scaling. The influence of grain size on sedimentary processes and beach morphology is well known and has been described by many authors. Coarser sediments tend to be less mobile, travel shorter distances while in suspension, and generally form steeper beaches. In spite of these differences, the overall patterns of sediment transport and beach response observed in the model should be quite similar to the transport patterns and beach evolution that will take place in prototype under similar wave conditions and water levels. The scaling of armour stone materials ensured that the submerged stability of the stone was reproduced accurately, even though freshwater was used in the model to represent seawater.
A faithful representation of the existing seabed down to the 10 m depth contour was constructed to ensure that local nearshore wave transformations would be properly simulated in the model. The concrete bathymetry was constructed using a network of templates placed at 1 m (model scale) intervals in the alongshore direction. The model bathymetry was designed based on highly detailed seabed surveys. The templates in areas with coral reef were manually adjusted so that the model bathymetry followed the approximate base elevation of the reef. The templates were also adjusted to create a flat section at the 1 m depth contour so that the actual foreshore above this level could be replicated with a mobile-bed of fine silica sand. The model bathymetry was formed by placing a thin skin of concrete grout over a moulded bed of compacted gravel, with the templates controlling the final elevation of the bathymetry surface.
As seen in Figure 1, the shoreline features a notable coral reef formation interspersed with sandy channels. Most of this reef exists between the 1 m and 3 m depth contours, and it is therefore expected to exert a significant influence over the nearshore wave conditions. Hence, it was important to reproduce the coral reef as closely as possible in order that realistic wave transformations were included in the model. During initial construction of the concrete bathymetry, four large sections were left unfinished. Based on high-resolution Code Echoscope sounding data, the nearshore section of the reef was carefully reproduced by hand-placing individual stones (approximately 1 m in diameter, prototype) into the wet concrete, in an attempt to simulate the existing coral density/grouping. The offshore portions of the reef were created by random placement of individual stones. Smaller stony coral were also simulated by randomly scattering pea stone gravel throughout these areas. The reef construction process is shown in Figure 2.
Figure 2. Constructing the simulated coral reefs.
Above the 1 m depth contour, the mobile-bed material was placed to approximate existing conditions at the site, up to the +2 m elevation. Approximately 1.90 m3 of silica sand was placed to recreate existing conditions in the model. The initial sand placement was guided by a series of ‘negative’ templates that were cut to match the cross-shore profile information derived from a field survey of the prototype beach. After the sand was placed in the model, the basin was filled with water up to MSL. With the templates still in place, the beach was carefully adjusted to closely match the existing conditions and smooth out any construction irregularities. Following this, the templates were removed in preparation for subsequent testing.
On several occasions later in the test program, the beach was reshaped to match the shoreline development scheme provided by Baird. These beach ‘resets’ were achieved by precisely surveying the location of the design beach crest alignment and design beach toe alignment at 1 m (model scale) intervals using a total station. Cut and fill was performed by skimming off layers of beach material in regions of accretion and filling in of areas that had experienced erosion. This process was repeated, where necessary, until the waterline in the model was in close agreement with the proposed waterline.
Careful attention was given to the location, dimensions, composition, and methods of construction of the model shore protection structures to ensure that they replicated existing conditions or proposed designs accurately and faithfully. The alignment of each structure was precisely laid out using a total station. Each point was positioned within ±1 mm (model scale) accuracy, ensuring the overall positioning and curvature of the structures was precisely replicated.
Rock materials and gradations were prepared, as much as possible, to replicate the characteristics of the prototype materials under consideration for use in the prototype construction. Six different classes of rock material were defined, including five different sizes of armour stone and one size of core material. Equivalent armour and core materials at model scale were obtained by sieving locally available crushed limestone aggregate, effectively separating it into different sizes. Measured gradations for the core and armour rock classes were obtained by weighing a representative sample. In some cases one of the sieved materials matched the design gradation, while in other cases two or more different stone sizes were blended together in order to produce a gradation in better agreement with specifications. In this paper, the various rock classes are referred to by their M50 values.
A large number of fibreboard templates were prepared and used to guide the placement of the core and armour materials for each coastal structure. Elevations were carefully controlled by surveying the templates using a high-precision optical level, and were verified after construction for quality assurance. This technique allows the design profile (shape and elevation) to be constructed to a high degree of accuracy. Once construction was nearly complete, the templates were removed and the locally disturbed areas were patched prior to testing. Core material was bulk-placed by shovel and then gently shaped to match the desired profile using wooden trowels. The methods used to place the model armour stone attempted to mimic those to be used in the prototype, so that rock behaved similarly and displayed the same kind of interlocking characteristics. Armour stones were placed by hand, unit-by-unit, to simulate placement by crane/excavator. The outer surface of the armour layers were painted with different colours to assist in visualizing stone motion and surface damage. Figure 3 shows an overview of the physical model in NRC’s Large Area Basin.
Figure 3. Overview of the physical model.
2.2 Instrumentation
Eighteen capacitance wave gauges were deployed throughout the domain, including one gauge for performing directional measurements. Orbital velocities and wave-induced currents within the model were measured using three 2-axis electromagnetic current meters. The velocity measurements were analyzed to resolve the speed and direction of the low- and high-frequency components of the flow at each sensor. Circulation patterns along the shoreline and in the vicinity of the various structures were also qualitatively assessed by observing plumes of coloured dye injected into the model.
A photographic damage analysis system comprising six remotely-operated digital cameras was used to monitor the movement of armour stone on the surface of the various coastal structures. Since each camera remained fixed throughout a test series, the movement of individual stones could be detected by comparing photographs taken at different times. Four digital video cameras were also used to record all tests: three cameras provided oblique overhead views while the fourth provided close-up views of wave-structure interactions at ground level.
Two different techniques were used to monitor and document the dynamic response of the model beach (see Figure 4). The first technique involved markers with coloured flags that were placed to denote the position of the still waterline at the beginning and end of selected tests. The evolution of the beach morphology was recorded by measuring the horizontal distance from each marker to a reference baseline located a short distance inland from the model beach. The second technique involved cross-shore beach profiles surveys that were measured at four different locations to investigate how various wave conditions and structure layouts affected deposition and erosion patterns. These four locations were chosen to coincide with the locations of beach profiles previously surveyed by Baird at the existing site.
Figure 4. Two different techniques were used to monitor and document the dynamic response of the model beach.
2.3 Wave Calibrations
Following construction of the bathymetry and existing conditions, a series of undisturbed wave calibrations were conducted to tune the wave generator command signals and verify the incident wave conditions to be used in the model study (the existing site features only a few small structures that were unlikely to cause any significant reflections that might contaminate the nearshore wave measurements). The target wave conditions were developed based on analysis of available wave climate data by Baird. Operational waves were conducted with long-crested waves, since the day-to-day conditions measured at the site tend to come from a very narrow range of directions. Significant wave heights at the nearshore calibration point ranged from 0.65 to 0.80 m, with wave periods ranging from 8 to 14 s. Extreme waves were conducted with multi-directional (short-crested) waves prescribed with a mild degree of spreading, since they provide a more realistic simulation of storm conditions. Significant wave heights at the calibration point ranged from 1.2 to 5.5 m (annual maximum wave height up to the 50-yr hurricane) with wave periods ranging from 6 to 10 s. Wave calibration was performed at the 6 m depth contour. The scaled wave conditions generated and measured in the model agreed very well with specifications.
3 RESULTS & DISCUSSION 3.1 Existing Conditions (Test Series A)
Prior to testing the proposed shoreline improvements in the physical model, the existing site conditions were constructed and simulated. The objectives of the existing conditions tests were to validate the physical model through comparison of modelled and prototype data, and to provide baseline data for comparison with the proposed shoreline improvements. Existing conditions tests included a variety of operational and extreme wave and water level combinations to investigate the performance of the existing beach under both typical and extreme conditions.
The beach planform was shown to be generally stable under operational conditions, with some erosion occurring during the annual event. Longer return period events resulted in more substantial beach erosion and flooding of the site, particularly when extreme waves were combined with extreme water levels. When operational conditions were simulated after an extreme event, the beach demonstrated the ability to recover to a certain degree, with some sand that had been previously transported offshore during the extreme event being returned to the shoreline. The location and elevation of the modelled beach crest was shown to agree well with the prototype beach profile surveys, though modelled beach slopes were generally found to be slightly steeper (6:1 to 7:1) than the corresponding prototype slopes (8:1 to 10:1). This was expected as a result of scaling effects due to the exaggeration of the prototype grain size in the physical model, as described above. The planform position of the beach waterline showed excellent agreement to prototype conditions after operational tests. A comparison of the modelled and prototype beach after operational and annual conditions is shown in Figure 5.
3.2 Proposed Shoreline Improvements (Test Series B through F)
The focus of test series B through F (denoted TSB – TSF) was to document the effect of the proposed shoreline improvements on waves, currents and beach morphology, and to achieve iterative optimizations to the proposed shoreline improvements. Minor adjustments were made to the proposed concept between each test series, in response to observations and data collected throughout the model study. The principal adjustments are summarized in Table 1. The overall objective of the proposed shoreline improvements was to improve the nearshore conditions with respect to swimmer safety (waves and currents) and to create a wider stable beach. Furthermore, optimizations in the structural details of each coastal structure were sought to reduce the overall material and construction cost of the works.
Figure 5. Existing condition tests showed excellent agreement in modelled and prototype beach planform under typical and annual conditions.
Effect on Currents
The primary objective with respect to currents was to understand those that develop around and behind the offshore structures, and document any changes in current speeds and circulation patterns compared to existing conditions. Dye tests were used to identify general circulation patterns and localized areas of higher current speeds. Current meters were subsequently placed to capture the highest currents throughout the model domain for each test series. The following key observations were made:
Ambient longshore currents are north to south for typical swell wave conditions. A very small longshore current is observed along the beach face and a larger longshore current is present along the edge of the nearshore reef flat in depths of -1.5 m to -6.0 m. This corresponds to the region in which waves are breaking for the majority of tests (operational and extreme).
Currents in the sandy channel between Breakwaters 2 and 3 were shown to increase by no more 0.3 m/s during extreme storm conditions.
Currents in the gaps between the central offshore reef (Breakwater 4) and the nearshore breakwaters (1 and 2) were shown to decrease under all wave and water level combinations when compared to currents measured at the same locations during existing conditions tests.
Rip currents in the southernmost beach cell between the watersports pier and Breakwater 1 remained generally consistent when compared to those observed under existing conditions.
Currents at all other locations of interest were generally within 0.1 m/s of those measured during existing conditions tests.
In general, the highest nearshore currents observed throughout the model domain were found to be 0.6 m/s or less for all conditions up to and including the annual storm event. This is a slight improvement over existing conditions, and adheres to the widely-adopted criteria for safe swimming of 0.6 m/s (2 ft/s), as published by the United States Fish & Wildlife Service. Although this current speed is exceeded for larger return period events, it is anticipated that lifeguards will be on duty and swimming will be prohibited during storms of these magnitudes. It is also noted that swimmer safety concerns due to currents are subjective, and the above criteria are not necessarily all-encompassing (i.e. weak swimmers may be unsafe in currents less than 0.6 m/s, while strong swimmers may be comfortable in much higher currents). Ultimately, the decision as to whether conditions are safe for swimming will be made by the lifeguards, and standard procedures for determining swimmer safety on public beaches will apply.
Table 1. Physical Model Test Series and Description of Each
Test Series Description
TSA Existing conditions tests (with existing beach and coastal structures). TSB Initial proposed concept layout.
TSC Lowered Breakwater 3 crest elevation (from +2.5 m to +2.0 m);
Shortened north and south groynes by 10 m;
Lowered Breakwater 2 crest elevation (from +2.5 m to +1.75 m) and removed 10 m from north end; Lowered Breakwater 1 crest elevation (from +2.5 m to +2.0 m) and increased armour size on
roundhead to 3-5 tonne armour stone; and,
Lowered watersports pier deck (from +2.5 m to +2.0 m).
TSD Reoriented south end of Breakwater 1 seaward;
Modified Breakwater 1 crest elevation to +2.25 m for central 35 m, tapering down to +1.5 m at both roundheads;
Returned Breakwater 1 to 2-4 tonne armour stone, except for northernmost 20 m and submerged sill which were changed to 1-3 tonne armour stone;
Lowered Breakwater 2 crest elevation (from +1.75 m to +1.25 m); Reoriented north end of Breakwater 3 seaward;
Rebuilt south half of Breakwater 3 with 1-3 tonne armour stone, while north half built with 2-4 tonne armour stone;
Altered north and south groynes to have 10 m flat trunks (at +1.8 m crest elevation), sloping down 15 m to the head at crest elevation +0.5 m;
Shortened central offshore reef (Breakwater 4) by 5 m at both ends and split into three armour stone sizes, from south to north: 3-5 tonne, 4-6 tonne, 5-8 tonne; and,
Shortened north offshore reef (Breakwater 5) by 10 m at south end and split into two stone sizes, from south to north: 3-5 tonne, 4-6 tonne.
TSE Altered central offshore reef (Breakwater 4) including splitting crest into three “terraces”, at roughly
-2 m, -1 m and +0 m (lowest terrace on seaward side, highest terrace on shoreward side);
Installed two “coral trays” in the central offshore reef (Breakwater 4), one at ~ -1.5 m, and the second installed at ~ -1.0 m. Trays were 4 m x 4 m with 1 m wide flange and 2 m x 2 m central tray with 0.5 m high walls; and,
Added 20 m long stone sill extending from the south roundhead of Breakwater 1 (comprised of 1-3 tonne stone sitting on concrete).
TSF Removed both offshore reefs (Breakwaters 4 and 5). Effect on Waves
A principal objective of the proposed shoreline improvements was to reduce the amount of wave energy reaching the shoreline to promote both safer/calmer swimming, and to reduce the erosive forces impacting the beach during storm events. This was accomplished through the careful placement and design of nearshore emergent breakwaters and offshore submerged artificial reefs.
Both artificial offshore reefs (Breakwaters 4 and 5) provide a significant reduction in the transmitted wave height. The reduction in wave height is directly related to the water depth over the structures, which is a function of both water level and crest elevation. The crest width is also an important factor, particularly for the long period swell waves that are prevalent at the site through the winter months. With an established crest elevation of +0.0 m (MLLW), and a crest width of 13 m, the offshore reefs produced an average reduction in wave height of 20% under operational (swell) conditions at mean sea level. For annual storms and hurricane waves however, the average reduction in wave heights over both reefs was much higher, at 35% to 50%. As a result, a moderate improvement in swimming conditions behind the structures is observed under operational swell conditions, while during storm conditions a much more significant reduction in the erosive force reaching the shoreline is realized, as is the principal objective of the offshore submerged reefs. The physical model was critical in establishing both the crest width and the crest elevation of the structures just below the waterline, thereby improving the overall aesthetics of the proposed design. The physical model also allowed for a reduction in the combined length of the two offshore structures of approximately 40 m while maintaining the wave sheltering objectives of the design.
As was the case for the analysis of currents, published swimmer safety criteria for waves was interpreted from a literature review of the subject. From this review, a reasonable swimming limit of 0.8 m for swell waves (Tp > 10 s) and 1.2 m for shorter period waves was established. Based on this criteria, swimming conditions within the central
beach cell would be considered potentially hazardous approximately 25% of the time (~90 days out of the year) under existing conditions based on existing conditions tests (TSA). With the proposed offshore reefs and nearshore breakwaters in place, the occurrence of potentially hazardous swimming conditions due to waves was reduced by nearly half, to less than 13% of the time. It is noted that the safer swimming criteria adopted for this assessment is subjective, and likely conservative, in that many swimmers would be comfortable swimming in more severe wave conditions. Swimmer safety is dependent on the individual’s age and swimming ability, as well as the physical characteristics of the immediate environment, including water depth, visibility, and distance from shore. Finally, the determination of whether or not conditions are safe for swimming will ultimately be the responsibility of the lifeguards, and may be influenced by several other factors.
An important component of swimmer safety is water access, as some swimmers have difficulty accessing the water when large waves are breaking directly on the foreshore. The proposed design resulted in a significant reduction of waves impacting the beach directly in lee of the three nearshore emergent structures (Breakwaters 1, 2, and 3), thereby creating safer water access points for beach users.
Effect on Sediment Transport and Beach Morphology
The effect of the proposed shoreline improvements on beach morphology was documented in the physical model by tracking the position of the beach waterline from test to test, and by taking regular beach profiles at four locations along the shoreline. Figure 6 and Figure 7 illustrate the stable beach position after operational (swell) and annual storm conditions for one iteration of the proposed design, in relation to the stable beach position under existing conditions.
The following observations were made with respect to sediment transport and beach response:
The shoreline was found to be relatively stable under operational conditions, with only minor deviations from the initial target beach fill line. The target beach fill line represents a beach nourishment volume of approximately 15,000 m3 and an overall average widening of the beach by 5 to 10 m across the site. The widening of the beach in lee of the proposed north and south nearshore breakwaters (Breakwaters 1 and 3) was much larger, in the range of 15 to 25 m. Beach fill material was generally found to remain within each beach cell under operational and annual storm conditions, with some offshore transport observed during larger hurricane events.
In the south beach cell (adjacent to the watersports pier), the beach conditions were reasonably stable with little deviation from those observed in the existing conditions tests, aside from the additional beach width provided by beach fill. A large tombolo formed on the north end of this beach cell, in lee of Breakwater 1, and was generally stable up to the 50-year event. To the south of the beach cell, a flow-through groyne beneath the watersports pier showed an ability to retain beach width while allowing sediment to bypass both around the head and over the crest during more energetic conditions.
In the central beach cell, the overall alignment of the beach responds to the wave direction with sediment being pushed to the north under southwesterly storm conditions, and to the south under typical northwesterly swell conditions. The beach cell is anchored to the south by a large, stable tombolo which forms behind Breakwater 1, which was designed to a higher elevation than the other structures in order to support the tombolo. To the north, large salients formed behind Breakwaters 2 and 3, with additional beach stability provided by a flow-through groyne attached to Breakwater 3. The central salient in lee of Breakwater 2 is highly dynamic in part due to the structure having a lower crest elevation to promote circulation in the beach cell, and will take on different planforms depending on wave direction, period, and height. Under operational conditions, beach fill material was not shown to leave the central beach cell, though some offshore transport was noted through the sandy channel at the center of the site during the annual and more extreme storm events.
The north beach cell alignment shifts depending on the incoming wave direction, with sediment accreting at the south end of the cell behind Breakwater 3 under operational conditions. Southerly storm waves push this sediment to the north of the cell, where it is anchored by the northernmost flow-through groyne. Like the southernmost groyne, this structure showed good ability to anchor the beach cell, while allowing some sediment to bypass (from either direction) during large storm events. In general, the northern beach cell is relatively stable and performs similarly to the existing conditions at the site, although with an additional stable beach width of 5 to 10 m. This additional beach width is provided by beach fill in combination with the shelter provided by the proposed north offshore reef (Breakwater 5).
Longshore transport along the beach face was shown to generally remain within each of the beach cells under operational conditions, which is consistent with the results of the existing conditions tests. Sediment bypassing at the site was not shown to be significantly affected, as the primary longshore current which passes from north to south along the outer edge of the nearshore reef flat (depths of 1.5 m to 6.0 m) is similar both with and without the proposed shoreline improvements in place.
Beach crest elevations along the property were shown to stabilize between +2.0 m and +2.5 m for operational and annual storm conditions. This is consistent with prototype beach surveys and existing conditions tests. During extreme hurricane events, the beach slope will become over-steepened at and above the waterline, with beach crests increasing by as much as 1 m.
Figure 6. Approximate position of stable beach during existing conditions tests (left), and with the proposed shoreline improvements in place after both operational (center) and annual storm (right) waves, looking South.
Figure 7. Approximate position of stable beach during existing conditions tests (left), and with the proposed shoreline improvements in place after both operational (center) and annual storm (right) waves, looking North.
Structure Stability
Structure stability was assessed for several structural geometries (position, length, crest width and height, stone materials) throughout each of the test series, and for a variety of extreme wave and water level conditions. The stability assessment focused on 20-year and 50-year hurricane waves, coupled with water levels ranging from mean sea level to extreme high water (100-year). Given the presence of extensive nearshore reef flats (to a depth of 1.5 to 2.0 m), the nearshore design wave conditions and subsequent structural requirements for the nearshore structures are depth-limited, and are controlled largely by the water level.
Structural stability was assessed for all rubblemound structures, including offshore reefs, nearshore breakwaters, and groynes. The structures were fitted with various sizes of armour stone throughout the model tests to optimize their design with respect to stability and cost. Stability was assessed through flicker photography, from which destabilized and/or displaced armour stones could be identified and quantified. In general, the offshore reefs were designed to be porous, with large local armour stone used as core material in an effort to provide fish nursery and coral habitat. The outer armour layers on the offshore reefs required imported armour stone on the order of 5 to 8 tonnes in order to remain stable during the annual storm, with only minor (acceptable) damage experienced during the 20-year and 50-year event.
The nearshore emergent structures were designed as conventional, non-permeable breakwaters with a core comprised of smaller material to maximize their ability to retain tombolos and salients in their lee. All three structures were optimized with a crest width of 4 armour stones, front and back slopes of 1.5:1, and an outer armour stone size of 3-5 tonnes. This gradation was observed to be stable on the structure roundheads under annual conditions and experienced only minor damage during hurricane events.
Environmental and Benthic Considerations
A critical objective of the proposed shoreline works was to mitigate impacts on the benthic environment, including coral reefs and fish habitat. Through the physical model study, offshore and nearshore coastal structures were re-aligned and re-positioned specifically to mitigate impacts to existing coral heads, where possible. Those coral heads that could not be avoided will be transplanted during construction, and returned to the site post-construction, to be affixed to the newly constructed offshore artificial reefs. This coral transplanting effort will be conducted as a component of an extensive research and monitoring program on the creation of artificial reefs and success of coral relocation efforts in the Caribbean.
As was previously discussed, the offshore artificial reefs have been designed using large, bulky, locally sourced stone as the core material, therefore resulting in an extremely porous structure. This was done in an effort to create fish nursery habitat and opportunities for the development of corals on the newly constructed artificial reef. Moreover, the porous nature of the offshore structures, combined with the low crest elevation, will promote circulation in the beach cells and improved water quality when compared to the effects of a more conventional, impermeable structure.
In addition to the introduction of two new artificial reefs to the site, coral enhancement areas are proposed in lee of the central offshore reef. In these areas, local stones and man-made coral recipient structures will be placed on sandy bottom to promote the expansion of the reef and growth of new and transplanted corals.
4 CONCLUSIONS
A three-dimensional physical model was designed and constructed at a geometric scale of 1:25 to support the detailed design of shoreline improvements at a new resort on the west coast of a small Caribbean island. The beach sediment was modelled using a fine sand that deformed naturally in response to wave forcing. The model was outfitted with a multi-directional wave machine to generate a wide range of realistic seastates and equipped with instrumentation to measure wave conditions, wave-induced nearshore currents, and changes in the shape of the model beach.
Nearly 150 tests were conducted to assess the impact of proposed coastal structures on the wave conditions and nearshore currents, as well as their effect on sediment transport processes and the response of the beach, for various wave conditions and water levels. These investigations generated a large quantity of valuable information concerning the potential impact of the proposed shoreline improvements on coastal processes at the project site. This information was used by Baird to support the detailed design of the waterfront improvements and will be critical in obtaining the necessary permits required for construction.
The proposed shoreline improvement scheme (including some modifications) performed adequately under the conditions selected for testing in the model. Minor damage to nearshore structures and substantial reshaping of the beach (plan shape and profile) were experienced in extreme hurricane events. A factor of safety could be applied to the design if statically resisting these hurricanes is desired; otherwise some contingency should be allowed for repairs.
ACKNOWLEDGEMENT
The authors would like to acknowledge Baird and an unnamed source for funding this study, and would like to thank the technical staff at NRC for their outstanding efforts and dependable support.
REFERENCES
Dean, R. (1985). Physical Modeling of Littoral Processes. In: R. Dalrymple, ed., Physical Modeling in Coastal Engineering, Boston. Hughes, S. (1993). Physical Models and Laboratory Techniques in Coastal Engineering. New Jersey: World Scientific.
United States Fish & Wildlife Service. (1978). Methods of Assessing Instream Flows for Recreation, Cooperative Instream Flow Service Group, Instream Flow Information Paper No. 6, June 1978.
