Mark R. Byrnes

GEOMORPHIC RESPONSE-TYPE MODEL FOR BARRIER COASTLINES : A REGIONAL PERSPECTIVE

Based on quantitative documentation of historical changes in shoreline position between 1847 and 1991, eight geomorphic response-types were established for classifying megascale changes along barrier coastlines: (1) lateral movement, (2) advance, (3) dynamic equilibrium, (4) retreat, (5) in-place narrowing, (6) landward rollover, (7) breakup, and (8) rotational instability. Long-term (decades to centuries) monitoring of shoreline position over a spatial scale of 10 to 100 km provides a scientific basis for documenting process-response relationships that shape regional coastal morphodynamics. Although megascale shoreline change studies often are lacking, this type of information is critical for developing realistic research and management strategies regarding form/process relationships in coastal depositional systems. The spatial distribution of geomorphic response-types is delineated along the barrier coastlines of Louisiana, Mississippi, and southern Georgia/northern Florida. At megascale, the rate of relative sea level rise along these barrier coastlines appears to be one of the major factors controlling the occurrence of geomorphic response-types; however, sediment supply exerts significant influence on shoreline response as well.

Large-scale sediment transport patterns on the continental shelf and influence on shoreline response: St. Andrew Sound, Georgia to Nassau Sound, Florida, USA

Abstract Regional sediment transport patterns on the continental shelf seaward of Cumberland Island, Georgia and Amelia Island, Florida are documented using historical shoreline position and bathymetry data. Spatial variability in the net rate of shoreline change is considerable due to jetty construction at St. Marys Entrance in the early 1900s. Net average shoreline progradation is documented for both islands (1.5 m/yr for Cumberland and 0.4 m/yr for Amelia), however, localized areas of shoreline retreat are recorded along Amelia Island, especially for the southernmost 5 km of beach where erosion has been chronic since 1871. Rapid shoreline progradation adjacent to the jetties accompanied sediment deposition by longshore sediment transport. Simultaneously, a large quantity of sediment from the natural ebb-tidal delta was reworked and transported offshore in response to jetty construction and channel dredging, creating the modern ebb-tidal delta. Patterns of sediment movement at this inlet and throughout the study area indicate a dominant direction of drift to the south-southeast. Sediment losses and gains were quantified to evaluate long-term coastal change within the framework of a sediment budget. Qualitative descriptions of net sediment transport were integrated with quantitative results to produce a model of large-scale coastal evolution for the study area. From this analysis, net sediment transport in this coastal compartment is controlled by inlet and shelf hydraulics, and littoral zone processes have minimal impact on net long-term coastal change.

The U.S. Minerals Management Service Outer Continental Shelf Sand and Gravel Program: Environmental Studies to Assess the Potential Effects of Offshore Dredging Operations in Federal Waters

Abstract The U.S. Department of the Interior, Minerals Management Service, provides policy direction relative to the development of all marine mineral resources located beneath Federal waters of the United States. Over the last ten years or so, geological studies encompassing the collection and analysis of seismic, vibracore, and grain size data have been conducted in partnership with coastal States in the Atlantic and Gulf of Mexico to locate suitable sources of compatible sand for beach and coastal restoration. Environmental studies have been initiated to provide biological, physical, and other pertinent information for decisions regarding leasing and use of this resource. Aggregate dredging studies also have been conducted in the event that an offshore aggregate mining operation is proposed in the future. A symposium was held in New Orleans in January 2002 to report results from several studies completed over the past 2 years. The papers prepared for this Special Issue summarize the findings of recently completed environmental studies.

Effects of sand mining on physical processes and biological communities offshore New Jersey, U.S.A.

Abstract Physical processes and biological data were collected and analyzed for eight sand resource areas on the New Jersey Outer Continental Shelf to address environmental concerns raised by the potential for mining sand for beach replenishment. Nearshore wave and sediment transport patterns were modeled for existing and post-dredging conditions, with borrow site sand volumes ranging from 2.1 to 8.8 × 106 m3. Wave transformation modeling indicated that minor changes will occur to wave fields under dominant directional conditions and selected sand extraction scenarios. Localized seafloor changes at borrow sites are expected to result in negligible impacts to the prevailing wave climate at the coast. At potential impact areas along the New Jersey coast, wave height changes averaged approximately ±3 to 15% when compared with wave heights for existing conditions. For all selected sand borrow sites offshore New Jersey, average variation in annual littoral transport was approximately 10% of existing values. Because borrow site geometries and excavation depths are similar to natural ridge and swale topographic characteristics on the New Jersey OCS, infilling rates and sediment types are expected to reflect natural variations within sand resource areas. Infaunal distribution and abundance correlated best with the relative percentages of gravel and sand in surficial sediments. In addition to sediment regime, other physical environmental differences between northern and southern portions of the study area also may have affected infaunal community patterns. Impacts to the benthic community are expected from physical removal of sediments and infauna. Based on previous studies, levels of infaunal abundance and diversity may recover within 1 to 3 years, but recovery of species composition may take longer. The nature and duration of benthic effects may differ with location of mined sites, due to physical and biological differences between northern and southern portions of the New Jersey shelf.

Reduced seasonality of Holocene climate and pervasive mixing of Holocene marine section: Northeastern Gulf of Mexico shelf

The large porcelaneous foraminifers Cyclorbiculina compressa, Parasorites orbitolitoides , and Peneroplis proteus are conspicuous in death assemblages from Holocene marine sediments of the Alabama and Florida panhandle shelf. The species inhabited the northeastern Gulf of Mexico in the Holocene (ca. 6.4–1.9 ka) but do not live in the region today. These foraminifers require warm, clear waters, and thus are important paleoclimatic and paleoenvironmental indicators. They apparently were derived from sublittoral seagrass habitats and indicate reduced seasonality in the region during the middle to late Holocene. In addition, a mixed foraminiferal fauna, a hydrodynamically reworked macrofaunal assemblage, and stratigraphic disorder in foraminiferal 14 C dates indicate extensive reworking of the entire Holocene marine transgressive package. Evidence that mollusks are indigenous but large foraminifers are transported supports the generalization that out-of-habitat transport of macrofauna is negligible in most marine settings.

Physical and Biological Effects of Sand Mining Offshore Alabama, U.S.A.

Abstract Physical processes and biological data were collected and analyzed at five sand resource areas offshore Alabama to address environmental concerns raised by potential sand dredging for beach replenishment. Nearshore wave and sediment transport patterns were modeled for existing and post-dredging conditions, with borrow site sand volumes ranging from 1.7 to 8.4 × 106 m3. Wave transformation modeling indicated that minor changes will occur to wave fields under typical seasonal conditions and sand extraction scenarios. Localized seafloor changes at borrow sites are expected to result in negligible impacts to the prevailing wave climate at the coast. For all potential sand excavation alternatives at borrow sites offshore Alabama, maximum variation in annual littoral transport between existing conditions and post-dredging configurations was approximately 8 to 10%. In general, increases or decreases in longshore transport rates associated with sand mining at each resource area amounted to about 1 to 2% of the net littoral drift, distributed over an approximate 10 km stretch of shoreline. Because borrow site geometries and excavation depths are similar to natural ridge and swale topographic characteristics on the Alabama Outer Continental Shelf, infilling rates and sediment types are expected to reflect natural variations within sand resource areas. Impacts to the benthic community are expected from physical removal of sediments and infauna. Based on previous studies, levels of infaunal abundance and diversity may recover within 1 to 3 years, but recovery of species composition may take longer. Western areas can be expected to recover more quickly than eastern areas because of opportunistic life history characteristics of numerically dominant infauna west of Mobile Bay.

Historical Sediment Transport Pathways and Quantities for Determining an Operational Sediment Budget: Mississippi Sound Barrier Islands

ABSTRACT Byrnes, M.R.; Rosati, J.D.; Griffee, S.F., and Berlinghoff, J.L., 2013. Historical sediment transport pathways and quantities for determining an operational sediment budget: Mississippi Sound barrier islands. In: Brock, J.C.; Barras, J.A., and Williams, S.J. (eds.), Understanding and Predicting Change in the Coastal Ecosystems of the Northern Gulf of Mexico, Journal of Coastal Research, Special Issue No. 63, pp. 166–183, Coconut Creek (Florida), ISSN 0749-0208. Historical shoreline and bathymetric survey data were compiled for the barrier islands and passes fronting Mississippi Sound to identify net littoral sand transport pathways, quantify the magnitude of net sand transport, and develop an operational sediment budget spanning a 90-year period. Net littoral sand transport along the islands and passes is primarily unidirectional (east-to-west). Beach erosion along the east side of each island and sand spit deposition to the west result in an average sand flux of about 400,000 cy/yr (305,000 m3/yr) throughout the barrier island system. Dog Keys Pass, located updrift of East Ship Island, is the only inlet acting as a net sediment sink. It also is the widest pass in the system (about 10 km) and has two active channels and ebb shoals. As such, a deficit of sand exists along East Ship Island. Littoral sand transport decreases rapidly along West Ship Island, where exchange of sand between islands terminates because of wave sheltering from the Chandeleur Islands and shoals at the eastern margin of the St. Bernard delta complex, Louisiana. These data were used to assist with design of a large island restoration project along Ship Island, Mississippi.

Evaluating Shoreline Response to Offshore Sand Mining for Beach Nourishment

Abstract An analytical approach that incorporates analysis of nearshore wave transformation and wave-induced longshore sediment transport was developed to quantify the significance of potential physical environmental impacts associated with offshore sand mining. Calculation of longshore sediment transport potential for a series of wave cases provided a method for determining the extent and magnitude of alterations to nearshore processes, but the magnitude of change alone did not provide enough information to determine the significance of changes for a particular coastline. This paper documents a method for evaluating the significance of borrow site impacts that incorporates temporal and spatial variations in the incident wave field. Example applications of this method are presented for borrow sites offshore Oregon Inlet, North Carolina; Martin County, Florida; and Corsons Inlet, New Jersey. As a management tool, this methodology holds several advantages over methods previously employed to assess the significance of borrow site impacts, including: 1) a model-independent component (observed shoreline change) is used to verify model results; 2) impacts associated with borrow site excavation can be directly related to their potential influence on observed coastal processes; 3) site-specific temporal variability in wave climate and sediment transport potential is calculated as part of the methodology; and 4) the procedure accounts for spatial and temporal variability in wave climate, as well as provides a means of quantifying significance of impacts relative to site-specific conditions.

Late Holocene record of community replacement preserved in time-averaged molluscan assemblages, Louisiana chenier plain

Late Holocene relict shorelines of the southwestern Louisiana chenier plain contain molluscan assemblages that vary greatly in taxonomic composition and bioclast preservation. Taxonomic composition varies with ridge age: older ridges are oyster rich, whereas younger ridges are dominated by infaunal bivalves. Taphonomic features can be separated into those caused by biostratinomic and those caused by pedogenic processes. Pedogenic alteration generally increases as ridge age increases, whereas biostratinomic alteration reflects the prevalence of reworked bioclasts in assemblages. These molluscan assemblages are extensively time averaged, causing temporal overcompleteness of depositional units (i.e., amount of time averaging for bioclasts within a unit is much greater than the time it took for that unit to form). Chenier-plain progradation over the last 3,000 years both caused and preserved the observed trend in community composition. This trend was caused by community replacement related to changing substrate stability and by changes in the source of reworked bioclasts, both of which operated in response to progradation. Net progradation also allowed this trend to be preserved because time averaging occurred episodically and shorelines were effectively separated into discrete generations. Although coastal deposits are not typically viewed as ideal sites for high-resolution paleoenvironmental studies, millennial-scale community trends can be detected in this setting.

Geophysical Techniques for Evaluating the Internal Structure of Cheniers, Southwestern Louisiana

ABSTRACT Historically, Louisiana chenier plain studies have consisted of surficial mapping, sediment analyses, cores, and direct observation of limited exposures from borrow pits. Ground penetrating radar (GPR), a shallow geophysical technique, provides high resolution profiles of contrasts in electrical permittivity that can be correlated with sedimentary layers in sand and gravel depositional environments not effected by salt water. Reconnaissance vertical electrical soundings (VES) were performed at several sites to determine the applicability of GPR on the chenier plain. Based on resistivity data, three field sites were targeted for GPR profiling. GPR profiles were collected using a Geophysical Survey Systems, Inc. Subsurface Interface Radar 10A unit with a 500MHz antenna on a grid designed to assess three-dimensional variability in subsurface characteristics of each chenier. Topographic corrections were applied for each processed profile to adjust for surface elevation differences that effect reflection orientation. At selected chenier locations, subsurface reflections reveal a complex internal structure within these clastic deposits. However, at the interface with adjacent or subsurface fine-grained deposits, clay and/or conductive pore water limit penetration. Electrical resistivity proved to be a valuable reconnaissance tool for determining applicability of GPR, and VES models correlated directly with vibracore data. Preliminary interpretations of radar profiles suggest cheniers evolve through multiple processes in different environments. Radar facies observed in dip-oriented profiles at two sites are described as landward stepping foresets and may be interpreted as transgressive washover deposits. Bedset orientation of strike-oriented profiles indicates a westward component of migration. At a third site landward and seaward dipping reflections, interpreted as upper shoreface and washover deposits, indicate shifting shoreline position related to transgression and regression associated with variations in sediment supply.