Carbonate Beaches 2000

This is a review of the geological aspects of carbonate beaches that a geologist thinks may be useful for an engineer. Classical geologic problems of carbonate beaches, for example how ancient examples are recognized in rock sequences, are of little interest to engineers. Geologists not involved in engineering problems may find it difficult to know what an engineer should understand about carbonate beaches. Nevertheless, there are at least eleven topics that are potentially very useful for engineers to keep in mind. These eleven are chosen with as much thought going into what has been omitted as has been given to the eleven included topics. Some qualifications are in order: First, this paper does not discuss certain kinds of carbonate shorelines that are beyond the scope of engineering issues. For example, this review does not discuss very high-energy carbonate boulder beaches. These beaches are comprised of pieces of carbonate material ganging in size from ten centimeters to meters. Typically, these are high-energy storm deposits formed from pieces of either eroded carbonate rock or other large carbonate pieces such as pieces of large corals. This paper focuses on sand-sized (0.0625–2.0 mm) coastal carbonate deposits. Second, offshore beaches will not be discussed. There are many carbonate beaches that form on banks or shoals exposed at low tide, but our discussion is confined to what most people think of when they go to some tropical island and/or resort and walk out to lay on the beach. Third, this paper does not consider mixed carbonate/quartz sand beaches. While mixed beaches are common, only the end member of purely carbonate sand beaches is considered. Fourth, there will be no order of preference of the eleven topics. And lastly, these eleven topics are not consensus items. These are simply one geologist s thoughts about the aspects of carbonate beaches that would be useful for engineering colleagues to keep in mind. Where possible, general reference is made to textbooks that will lead the reader to extensive literature on carbonate sediments. Several of the topics are not so broad as to have had a large general treatment in texts, and in those cases some original literature is cited.

Exchange of ideas, concepts and scientific knowledge is important to the development of understanding between engineers and geologist. Engineers must understand and incorporate into their studies and designs the fundamentals that the geologist derives from their investigations into beaches and beach materials. As solutions are continually sought to combat the erosion problems around the nation, the design process becomes more complete and inclusive of the natural phenomenon causing the problems and the nature of the materials. Description of the coastal processes and material properties that geologists provide, such as in Komar are fundamental to successful coastal projects. Understanding these concepts is even more important when dealing with carbonate beaches. Carbonate beaches are seldom discussed as part of formal coastal engineering training since most schools are not located near carbonate beaches. To help the geologist determine the most appropriate way that engineers can be aided in understanding the material and process properties, a brief discussion of the engineering process is presented below. Armed with what engineers consider in designing a beach nourishment project, geologist will be better prepared to assist the engineers understanding of the role of the particular characteristics of carbonate material on beaches.

The US Army Corps of Engineers (USACE) SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) system is an ideal tool for surveying carbonate beaches. Mounted in an airborne platform, the SHOALS system is unconstrained by environmental conditions that may hamper traditional survey operations, such as the very shallow water over coral reef flats. SHOALS data provide an accurate source of elevation data to assist in coral reef mapping and from which to compute short-term changes in beach morphology. This paper describes the SHOALS system and typical survey operations. Several surveys of carbonate beaches and coral reefs are presented. An analysis of consecutive SHOALS data sets reveals changes in beach morphology.

In this study, sea-level predictions based on harmonic tidal models with their parameters estimated by the Kalman filter algorithm are tested for Macau. It is shown that Kalman filter is able to solve harmonic sea-level models from short observation record once they are put in the state-space form. Current case study indicates that harmonic models with four to five dominant tidal constituents may give the best prediction performance in Macau giving that their harmonic parameters are estimated by Kalman filter with as little as four to twenty days record. These encouraging results indicate that Kalman filter is a strong contender for solving harmonic tidal forecaster in Macau as well as other places in the world with limited monitoring data.

Geophysical data obtained during an offshore geotechnical sand-search defined the surficial extent and sub-bottom configuration of geomorphological features such as reef tracts, infills of carbonate sediments within back reef troughs, and karstified rock outcrops along the upper shoreface. Coastal process zones (CPZ) were derived from interpretation of spatial distribution patterns of morphostructural units. Recognition of discrete morphostructures and CPZs additionally provided a basis for conceptualizing a new model for exploration of offshore sand deposits that are suitable for beach renourishment. Morphostructural mapping showed that the present carbonate setting can be explained in terms of representative CPZs. The relationships between form and function, including paleoforms and processes as relics in the present environment, distinguish sediment distribution patterns and CPZs that typify this carbonate arena. Uniformly graded clastic sediments are deposited in inter-reefal flats that act as a depocenters for accumulating calcareous sands. The intersection of para- and diabathic process zones at reef gaps produces mixed sand bodies that are also useful for beach replenishment. Study of morphostructural units and coastal process zones, provides a useful exploration paradigm for seabed sand sources within the dimensional framework of the southeast Florida Sand Search Model.

The Halekulani Sand Channel and the Makua Shelf off the south shore of Oahu contain at least 1.3 million m³ of sediment that is a possible resource for nourishing degraded sections of Waikiki Beach. A sidescan sonar survey indicates continuous sediment cover within the channel and on the shelf, and samples from the top and bottom of vibracores from the channel and shelf contain from 29% to 77% of grains between 0 to 2.5 phi (1 to 0.177 mm), the size range of four samples from Waikiki Beach. Compositional analyses indicate high variability, but the vibracore samples normally have relatively high Halimeda content compared to beach sand samples. Laboratory tests show a positive correlation of abrasion with Halimeda content, suggesting that the offshore sediment would abrade more than beach sediment under nearshore wave action. The common gray color of the offshore sediment can be aesthetically undesirable for sand on popular tourist beaches such as Waikiki; however, visual observation of native beach sand indicates that a significant component of gray color is endemic to many Hawaiian beaches. The gray color was removed in the laboratory by soaking in heated hydrogen peroxide. The geological properties of the offshore sediment indicate potential as a resource for beach nourishment, but industrial treatment might be necessary to remove excess fine and coarse grains, and possibly the gray color. Further, the abrasion potential might have to be considered in calculating beach sand losses over time.

A combined geophysical and geotechnical study was conducted at a reconnaissance level in order to determine the potential for recovery of beach quality sand in the deep water area of Dade County beyond the outer reef line. The geophysical survey plan included 121 track lines covering a total of approximately 150 line-miles over a total area of more than 15 square miles. The reconnaissance study area can be broadly divided into three sub-sections according to depth and corresponding lithology. The three sub-sections lie generally in the depth ranges of 55–110 feet, 130 to 240 feet and 250 to 300 feet. The division between the two deep-water sections is probably a wave-cut scarp that is persistently present at depths of 250 to approximately 290 feet. The results ot tne deep-water reconnaissance investigation indicate that there is significant potential for beach compatible sand in water depths of 250 to 300 feet as well as dredgeable volumes within the depth of 55 to 110 and 130–240 feet within the surveyed area. The resolution of this reconnaissance survey is too coarse to specifically define borrow areas and the details of sediment textures. However, the central and southern portions of the surveyed area may include a significant volume of beach compatible sand estimated to potentially be approximately 20,000,000 cubic yards. Based on the results of the survey it is recommended that sand resources in the deep water area be evaluated in more detail using high resolution sub-bottom profiling methods in combination with coring methods applicable to deep water investigations.

Beach profiles from selected Oahu and Maui beaches quantitatively document beach volume variation and change between 1994 and 1999. Along exposed, high-energy beaches, large fluctuations in beach volume, characterized primarily by the formation and erosion of extensive berms, dominate the seasonal changes. Beaches along more protected stretches of coastline show much less variation in profile morphology. Beaches on the west (leeward) coast of Oahu experienced the most seasonal variation in profile volume, followed by the north shore, east (windward) shore, and south shore. Similar to Oahu, beaches along the west coast of Maui showed the greatest overall profile variation. However, the mean variation for profiles along a single coastal reach showed little difference compared to other coastal segments. Although some beaches showed net gain or loss during the study period, most beaches remained relatively stable with change limited to a finite envelope. No island-wide trends in beach erosion or accretion were observed during the study period. However, no extreme events, such as tropical storms or hurricanes, directly influenced the Hawaiian Islands during the study period. This data set should therefore be considered as representative of typical annual beach activity. Greater variation and possible long-term change would be expected during extreme events.

In the tropical Central and West Pacific as well as in the East Indian Ocean the symbiont-bearing benthic foraminifera (single celled marine organisms getting size of a few millimeters) are the main producers of calcium-carbonate grains deposited at carbonate beaches. The proportion of larger foraminifera tests on sand grains ranges from 20 to 95 %. This amount depends on the two factors productivity and transport, since the larger foraminifers do not live on or in the sandy bottom near the beaches, but prefer firm substrate in high-energy environments close to the front of coral reefs. Therefore, foraminiferal tests are produced in extreme numbers on the reef crest that is covered by filamentous macroalgae or in the transition zone between the crest and the reef moat. After the release of the test by reproduction or death, empty tests are entrained through the high water energy acting at the reef crest and transported. This transport depends on the direction and intensity of currents connecting the production area (reef crest and transition to the moat) with the deposition area (central moat, lagoon, beach). Devastation of the production area and/or the interruption or diversion of water flow hinders the accumulation of larger foraminifera tests at the beach. Understanding the ecology of larger foraminifera in combination with the transport of empty tests is thus necessary to preserve the equilibrium between deposition and removal of sand at carbonate beaches.

The Chesapeake Bay Estuarine System consists of a variety of shorelines that vary from low upland banks and marshes to beaches and dunes to high bluffs. Erosion of these shorelines becomes significant when fetch exposures exceed a few miles and becomes severe when shorelines are exposed to fetches exceeding 16 kilometers or more. Critical erosion has been defined as that erosion that immediately threatens upland improvements and infrastructure no matter what the fetch exposure. The use of headland breakwaters coupled with beach fill to create stable pocket beaches for shoreline management has become somewhat common place. Over the last 15 years research and project installations respectively by the authors have paved the way for widespread usage. Utilizing the empirical database of 14 representative installations, design parameter relationships are derived for headland breakwater/pocket beach systems. Breakwater systems with a bimodal wave exposure have a breakwater length to breakwater gap ratio (Lb:Gb) of about 1:1.0 to 1:1.5. When headland breakwater systems are sited in more unidirectional settings, the Lb:Gb ratio can approach 1:1.7 to 1:2.0 particularly within embayed coastal settings that usually have an appreciable amount of natural littoral sands. Previous research by the authors have shown a relationship between the breakwater gap to pocket beach depth or indentation (Gb:Mb) ratio to be about 1:1.65. Further analysis shows us that for a unidirectional project setting that the Gb:Mb ratio can average 1:1.9. For a bimodal wave climate the Gb:Mb ratio reduces to 1:1.5.

A characteristic peculiarity of mineral composition of bottom sediments in the Sea of Azov is their high carbonate content. The sea has features characteristic to weakly mineralized basins both of arid and humid zones. Accumulation of calcium carbonate in bottom sediments of the Sea of Azov is characterized by spatial irregularity of its distribution. A study of chemical composition of bottom sediments and CaCo₃ content in them has confirmed the absence of connection between the general amount of calcium carbonate and granulometric composition of sediments. CaCo₃ percentage in surface layer of bottom sediments varies from 1 to 90%. But its natural increase is observed in the regions with the highest benthos biomass. An intensity of biogenic carbonate accumulation in the Sea of Azov is not a constant, and it is an integral reflection of the mollusks' productivity. As the research has shown, removal of continental run-off and, as a consequence, increase of sea water mineralization, promotes an essential change of structure and productivity of benthos biocenosis. As a whole, carbonate accumulation in the Sea of Azov is characterized by an extraordinary dynamics. A supply of carbonate matter of benthos organisms to the sea bottom is determined first of all by ecological criteria. Hence, before considering hydrodynamic conditions under which shells of mollusks and other carbonate-forming organisms are carried out of the area of habitation, we must ascertain reasons of their death. Since the 60-ies till now the Sea of Azov basin and its carbonate beaches suffer from a great anthropogenic impact. It is in the discharge into rivers of different pollutants including rather toxic ones for hydrobionts; erection of water storage basins and hydroengineering complexes on large rivers; construction of ports, jetties and other hydraulic systems in the nearshore zone. The influence of these factors resulted in increasing of sea water salinity and in reducing of solid run-off. As a result, here occurs dislodgment of biocenosis from the usual zones of their habitation, which is accompanied by the death of a part of mollusks. The process of the coastal erosion began, living conditions for hydrobionts became worse, the fish-productivity of the basin fell in tens times. On the whole, this region is a good example of an irrational use of natural resources.

As part of the U.S. Geological Survey's multi-disciplinary Coral Reef Project addressing the health and geological variability of coral reef systems, sediment components and their distribution along the fringing reef on the south shore of the Hawaiian island of Molokai are being examined. Particular interest is being paid to the types and origin of sediment found on the reef. The south shore of Molokai is sheltered by one of the largest fringing reefs in the US. At approximately 50 km in length, up to 1.5 km in width, and covered by 90% live coral in many locations, the reef seemingly should be able to provide ample sediment for large carbonate beaches. However, siliciclastic grains supplied by erosion of the basaltic uplands of Molokai are often the most conspicuous individual nearshore sediment type. Coralline algae and coral are the most common carbonate components of the beaches. On the nearshore reef-flat, chemically-altered carbonate grains, particularly coralline algae, are the most abundant component. Molluscs and Halimeda may be common in specific locations, but are usually minor components. Sediment calcium carbonate levels increase to the west from a minimum at Kamalo, and are high along the east shore of Molokai. However, these general island-scale trends may be overridden by local influences, such as protected stream mouths or high carbonate growth rates. Additionally, trends seen on the beach and nearshore environments may not reflect trends a few hundred meters offshore since shore normal trends are more pronounced than shore parallel ones.

People commonly associate carbonate sand beaches with low-latitude regions, typically between 20° south latitude and 20° north latitude. The warmer water and greater abundance of calcium producing organisms in low-latitude regions help promote this belief. Carbonate sand beaches, however, can also be found at various high-latitude locations from Europe to Alaska to Maine. Along the coast of Maine, carbonate sediments dominate several island beaches. Sand Beach, on Mount Desert Island, is the largest of these beaches, and is well known due to its location in Acadia National Park. The accumulation of carbonate sediments on Sand Beach is due to such factors as the high local population of calcareous organisms (barnacles, echinoids, mussels, and others), the shortage of terrigenous sediments, the local wave climate, the nearshore bathymetry, and the absence of longshore transport. These factors are common at many of the carbonate beaches in Maine and, at Sand Beach, have resulted in a beach composed of relatively young (less than 800-years old) carbonate sediments with concentrations of CaCO₃ approaching 80%.

This study was prompted by the scarcity of quality offshore borrow areas and increased environmental concerns associated with offshore dredging, primarily along the South Florida coast, and the possible use of alternate materials and methods for beach nourishment. Oolitic aragonite is present in large deposits and is located approximately 60 miles offshore of the lower east coast of Florida in the Caribbean, and is considered to be an excellent candidate as a material for beach nourishment. This paper documents a series of laboratory experiments designed to compare the evolution and transport characteristics of quartz and oolitic aragonite sediments. The three experiments conducted during this study include a beach nourishment experiment, sediment transport experiment, and a settling velocity test. For the beach nourishment experiment, the wave basin was partitioned into two beach segments with a quartz project in one beach segment and an oolitic aragonite project in the other segment. Each project consisted of a trapezoidal planform configured to induce rapid planform evolution. Both nourishment projects were constructed on an otherwise quartz beach. The nourishment sand sizes for the two projects were well matched. The results revealed that the average profile, shoreline, and volumetric density equilibration of the oolitic aragonite nourishment are comparable with, and somewhat superior to, compatible quartz nourishments. The proportion of fill material remaining within project limits was determined to be, on average, 10% higher for the oolitic aragonite nourishment section. The sediment transport experiment was also carried out in a partitioned configuration of the wave basin with one segment containing a quartz beach and the other an oolitic aragonite beach. Analysis of the sediment transport experiment demonstrated that the alongshore sediment transport magnitudes for oolitic aragonite were substantially lower than those for quartz, and the direction of cross-shore sediment transport was found to be landward for the oolitic aragonite and seaward for the quartz test sections, respectively. These results were attributed primarily to an initial unintentional oolitic aragonite profile slope which was considerably milder than the quartz test section, and increased fall velocities of the oolitic aragonite sediment associated with a slightly larger grain size (0.30 mm) than that of quartz (0.25 mm) resulting in less transportability. The settling velocity experiment, performed in a rapid sand analyzer, was conducted by sieving quartz and oolitic aragonite samples into narrow size ranges and subjecting these fractions to subsequent fall velocity tests. Based on the settling velocities, oolitic aragonite was determined to perform as a comparable quartz equivalent that is 1.11 times its initial diameter. The results of the laboratory experiments conducted during this study demonstrate that oolitic aragonite is comparable to quartz sand and is suitable for future use in beach nourishment projects. The effectiveness and the ability of oolitic aragonite to perform as a nourishment material has been established and should exceed somewhat the predictions of nourishments employing comparable quartz sediments.

Beach sediments of the Florida Keys are of calcium carbonate origin. As part of a state program to manage the beaches of Florida, critical erosion areas have been identified and beach management strategies developed for the Florida Keys.

In the last twenty years, Negril beach in Jamaica has seen more frequent and longer lasting periods of beach erosion. Sediment samples collected in 1980 and 1999 were analyzed and grain size and sediment composition determined. Grain size data indicate that there has been little change in the grain size of sediment along Negril beach between 1980 and 1999. Sediment grains were identified using thin sections and a petrographic microscope. The non-skeletal grains are dominated by amorphous and crystalline grains, while the skeletal grains are characterized by abundant foraminifera with smaller amounts of red algae, bivalves and Halimeda. Grains derived from corals are notably absent from the beach sediment. Comparisons between the 1980 and 1999 datasets suggest that there may have been a small decline in the percentage of foraminifera. The skeletal grains present indicate that the carbonate sediment was produced in the shallow water inner shelf characterized by Thalassia meadows and carbonate sand substrates. Recent development in the Negril area has seen an increase in nutrient influx to the marine system that has resulted in the decline in the coral reefs. A similar decline in the Thalassia meadows with the replacement of Thalassia by fleshy algae due to nutrient influx, coupled with the removal and/or damage of sea grasses by the Tourism industry is probably leading to smaller areas of sea grass cover. This loss in sea grass area will lead to a reduction of carbonate sediment production and consequently the increased likelihood of beach erosion.

The geologic evolution of tropical oceanic, intraplate volcanic islands is reasonably well understood and follows a succession from island initiation through volcanism followed by denudation, subsidence, and reef formation. The end-member island form is the atoll. There is also a general progression in the development of beach systems as the islands grow and decay. Using examples from Hawaii and Micronesia this paper discusses: a) the variation in beach system size with island age (and reef development), and, b) a brief description of typical beach systems. The main Hawaiian Islands increase in age from Hawaii towards Kauai. Associated with this increase in island age is a related increase in percentage of shoreline fronted by beach, beach volume per kilometer of coast, and total beach sand reservoirs. A similar trend is evident between islands in Micronesia, however, there are additional variations caused by reef development and vertical tectonic history. For example, within the Federated States of Micronesia, the Kosrae-Pohnpei-Chuuk island chain increase in age towards Chuuk but beach system development on Pohnpei is limited because the wide barrier reef and deep lagoon prohibit reef sediment from reaching the coast. Uplifted islands with fringing reefs, such as Guam and Saipan, display beach systems analogous to slowly subsiding islands with fringing reefs. Although the beach systems examined are Holocene in age, it is clear from the examination of a wide variety of islands that the geologic framework is critical to beach system development. It takes longer than 6,000 years (i.e. the Holocene) to develop extensive beach systems. There are a number of distinct physiographic settings where beaches occur in Pacific Islands. These include: drowned river valley with bayhead beach; structural embayment with beach; pocket beach; coastal plain/cuspate foreland beach; delta beach; fringing beach; and, perched storm-beach. In addition to these beach systems of high islands, there are atolls which are almost entirely encompassed by beaches along their shorelines. Atoll islet beaches can be further classified by islet type and exposure.

Beach erosion studies on tropical islands are often problematic because of the lack of quantitative data. Historical maps and charts are usually of too poor resolution for use in shoreline change measurements, and aerial photographs are sometimes similarly constrained. Furthermore, the high reflectance of the white carbonate sand makes it difficult to properly interpret the high-water line as the shoreline indicator on aerial photography. While hurricanes can cause dramatic changes in beach dimensions on tropical islands within the tropical storm belt, the long-term trend is often quite small (e.g., frequently one foot or less of erosion per year).

The southern and southwestern Australian coast is the largest temperate carbonate province in the world, extending for over 6000 km. The carbonates are derived from two major sources. On the more exposed open coast shelf carbonates are produced at depths to 130 m and are swept landward by both the sea level transgression and the energetic wave climate to supply extensive beach and dune systems, with average volumes of 10000 m³ m^–1. In the more protected gulfs and bays, extensive seagrass meadows and their epi-fauna supply shell detritus to build wide sand flats and backing beach ridge systems. In addition, lithification of barrier deposits during glacial low sea level periods has produced thousands of kilometers of beach and dune calcarenite, which may be exposed as calcarenite sea cliffs up to 150 m high, and as islands and reefs. These deposits not only lock up carbonate sediments but have a major impact on wave shoaling and induce a calcarenite-controlled realignment of the modern shoreline and redistribution of wave energy, which in turn controls Holocene coastal evolution.

The location of offshore sand resources for beach nourishment is a high priority coastal management issue in the Southeast region of the United States. As such, beach nourishment has become a major driving force in the development of the Ocean Planning Information System (OPIS). OPIS is a regional, Web-based, ocean resource and policy mapping tool that provides the coastal management community with access to on-line maps, spatial data, and regulatory information. Several key natural resource, coastal hazards, and conflicting use spatial data layers, among others, provide the foundation for a wide range of coastal and ocean analyses. However, the types of analyses associated with the advanced planning of beach nourishment activities required the acquisition of additional geological, physical, and socioeconomic data layers. Although countless sediment texture and grain size analyses had been conducted to satisfy a wide range of initiatives within the Southeast, these data were distributed among several agencies in a variety of formats and were typically very localized. Still, the regional depiction of available sand resources was considered critical to overall product functionality. Therefore, the NOAA Coastal Services Center selected those data sets that were most appropriate for inclusion and, based on this list, created a geographic representation of offshore sand resources for the Southeastern U.S.

The transport of neritic carbonate sediments suspended by strong sustained winds has been observed from land and from low-altitude aircraft. In March 1996, following a sustained gale-force wind event, aerial photographs of offbank sediment transport plumes from the Bahamas Banks were obtained, and these photographs were correlated with Advanced Very High Reflectance Radiometer (AVHRR) reflectance images which showed significantly increased reflectance on the Banks due to suspended sediments. Earlier manuscripts by the authors suggested that new visible-light oceanographic remote sensing satellites, notably the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS), should be capable of detecting similar sediment plumes induced by meteorological events, despite inherent difficulties of satellite remote sensing. In 1999, Hurricanes Floyd and Gert induced sediment plumes on the Bahamas Banks (Floyd) and Bermuda (Gert) that were observed by SeaWiFS. These observations validated the concept of observing neritic carbonate transport from satellites, and also provided important information about the magnitude of the meteorological events that are required to produce plumes large enough to be observed from space. Increased reflectance due to sediment suspension (i.e., turbidity) in the shallow waters overlying carbonate platforms will likely be accompanied by some amount of offbank sediment transport. Therefore, satellite observation of "increased reflectance events", which is significantly easier than observation of nearshore sediment plumes, can provide data on transport occurrence frequency. Such an observational program can contribute to improved estimates of the mass of neritic carbonate that is transported from shallow carbonate platforms. The 1999 interaction of Typhoon Dan with a shallow carbonate platform in the South China Sea (Tungsha Tao) demonstrates the feasibility of monitoring candidate environments.