Lindino Benedet

Interpretation of Seabed Geomorphology Based on Spatial Analysis of High-Density Airborne Laser Bathymetry

Abstract Airborne laser bathymetric (ALB) systems rapidly acquire large, high-quality datasets via variable swath widths that are independent of water depth. Laser bathymetric survey tools have applicability in clear coastal (Case II) waters down to −70 meters depth. Deployed along the southeast Florida (Palm Beach, Broward, and Miami-Dade Counties) coast, an advanced ALB system provided a continuous dataset for 160 kilometers of coast from onshore to 6 kilometers offshore. Digital terrain models developed from this high-density bathymetric data permitted recognition of numerous seafloor features and bathymetric patterns from different image formats. Bathymetric analysis of the 600-km2 survey area on the narrow continental shelf shows inherited lithologic features that are partly covered by surficial sediments. Primary parabathic provinces include: (1) nearshore rocky zones dominated by the Anastasia Formation, (2) coral-algal reef systems (Florida Reef Tract [FRT]), and (3) marine terraces. Secondary sedimentary subprovinces include shoreface sands, inter-reefal sedimentary infills (coral rubble in basal sequences and near reef gaps), and finer-grained materials seaward of the FRT. Tertiary topographic features include: (1) longshore bar and trough systems, shoals, sand sheets, and diabathic channels; (2) reef crests and ledges, forereef spurs and grooves, sediment ramps in large reef gaps, and incised paleo-river channels; and (3) drowned karst topography. Hierarchical organization of these bathymetric features is now possible for the first time because of the increased accuracy and density of bathymetric data obtained by ALB systems.

Processes Controlling Development of Erosional Hot Spots on a Beach Nourishment Project

Abstract Beach erosion, a problem along most sandy shores, can be caused by man-induced interventions to the coast or natural processes. Remediation of beach erosion (i.e., beach restoration) along eroding developed beachfronts is commonly practiced in the United States by periodic beach renourishment with or without coastal structures. Rates of erosion within beach fills generally vary greatly, and areas that erode faster than the nourishment average are commonly termed erosional hot spots (EHSs). Delray Beach, located on the southeast coast of Florida, was renourished for the fourth time on December of 1992 with about 914,000 m3 of sand dredged from offshore and placed along 2.7 km of beach. About 448,000 m3 of the fill had eroded away by 2001, about eight and a half years after initial construction. Two beach segments with erosion rates higher than the nourishment average were identified based on analysis of annual beach profile data. About 40% of the eroded volume accrued from one of these beach segments, a 600-m long EHS located on the downdrift end of the nourishment. We evaluated hypotheses to explain EHS development; these included the influence of nearshore features (reefs and borrows) on nearshore wave propagation, variability of grain size alongshore, and changes in shoreline orientation induced by the placement of fill. The nearshore reefs have little to negligible influence on the nearshore waves and are not the cause of the EHSs. Borrow areas significantly influence nearshore waves along the beach. Grain-size differences alongshore were also not the cause of increased erosion of EHS segments since grain sizes are not persistently finer where higher erosion is observed or vice versa. Change in shoreline orientation in the south end of the fill (EHS segment) causes an acceleration of the alongshore currents and an increase in sediment transport potential. Shoreline orientation effects appear to play a relatively more significant role in the development of the EHS in the south end of the fill than the other processes evaluated.

Submarine Geomorphology of the Continental Shelf off Southeast Florida Based on Interpretation of Airborne Laser Bathymetry

Abstract Airborne laser bathymetry (ALB) is a new laser bathymetric survey tool that has applicability in clear coastal (Case II) waters to depths of −70 m. The new sounding technique features rapid acquisition of large, high-quality data sets via variable swath widths that are independent of water depth. This advanced ALB system was deployed along the continental shelf of southeast Florida (Palm Beach, Broward, and Miami-Dade Counties), providing a contiguous data set for 160 km of coast from onshore to 6 km offshore. Image enhancement of the ALB digital data facilitates recognition of numerous seafloor features and bathymetric patterns. Bathymetric analysis of the 600-km2 survey area on the continental shelf shows inherited lithologic features that are partly covered by surficial sediments. Primary parabathic (shore-parallel) provinces include: (1) nearshore rocky zones dominated by the Anastasia Formation, (2) coral-algal reef systems (Florida Reef Tract [FRT]), and (3) marine platforms (terraces). Sedimentary subprovinces include shoreface sands, interreefal sedimentary infills (coral rubble in basal sequences and near reef gaps), and finer-grained materials seaward of coral reefs. Tertiary topographic features include: (1) longshore bar and trough systems, shoals, sand sheets (flats), and diabathic (cross-shore) channels; (2) reef crests and ledges, forereef spur-and-groove topography, sediment ramps in large reef gaps, and incised paleo-river channels; and (3) drowned karst topography. Hierarchical organization (classification and mapping) of these bathymetric features is now possible for the first time because of the increased accuracy and density of ALB data. These data and related maps allow, for the first time, assessment of links between the influence of seabed morphology on wave transformation patterns and beach morphodynamics in southeast Florida.

Appreciation of static bay beach concept for coastal management and protection

Abstract Sandy bay beach in static equilibrium is the only physiographic feature that could remain stable without sediment supply in a persistent swell environment anywhere around the world. With a continuing trend of erosion to sandy beaches worldwide, a conservative and practical hypothesis is not to consider the mobile dynamic wave parameters but to safeguard against a sandy beach under vanishing supply in the future. To achieve this, an empirical bay shape equation is strongly recommended as one of the best coastal management options. Combining with MEPBAY, software derived for implementing the static bay beach concept on a display unit, this empirical bay shape equation is readily applicable to verify the stability of numerous headland bay beaches in natural and man-made condition. It is also adequate for coastal managers to apply this empirical equation as a valuable tool to aid in the creation of headland bay beaches for shore protection, recreation, and coastal management before embarking on sophisticated numerical modeling.

Fluvial Sand Sources for Barrier Island Restoration in Louisiana: Geotechnical Investigations in the Mississippi River

Abstract Coastal land loss in the Mississippi River delta region, related to degradation of wetlands and erosion of barrier islands, contributes to loss of valuable habitat, endangerment of infrastructure and socioeconomic systems, and coastal flooding hazards. Restoration of these ecosystems is thus a primary activity that requires large volumes of sand to rebuild beach–dune systems and restore wetland habitats. Sand sources have traditionally been sought offshore in the marine environment, but there are problems associated with setbacks from oil and gas infrastructure, presence of muddy overburden, and limited reserves of beach-quality sediments. Fluvial sand sources in channel and point-bar deposits become an attractive alternative for barrier island restoration because of large volumes of relict deposits and because active sand waves are renewable. Results of preliminary geophysical and geotechnical investigations in the lower Mississippi River (south of New Orleans) along a 32-km stretch of the river indicate the presence of at least 23 million cubic meters of usable sediments in seven potential borrow sites. Caveats to assessment and exploitation of river sands include interpretation of vibracores and seismic reflection profiles that include evidence of lateral translation-type bank failures along bendways, lateral slumps, and postdepositional disturbance of strata above −30 m. Reliable extraction of good quality sediment should occur down current from bendways.

Sediment Ridges on the West Florida Inner Continental Shelf: Sand Resources for Beach Nourishment

Abstract Beach nourishment requires large volumes of sand from offshore and new sources are constantly sought for development. The sediment-starved continental shelf off the central-west coast of Florida has traditionally supplied beach-quality sediments from ebb-tidal shoals and nearshore sand sheets, but as these supplies dwindle, sand searches increasingly look farther offshore for resources. Widely spaced sediment ridges, interspersed by karstified limestone seafloor (hard grounds), offer potential as sand resources that can be exploited by dredging to renourish eroded beaches for shore protection. The sand ridges, late Holocene in age, are generally shoreface detached, sediment starved, and clustered in “ridge fields.” Six sediment ridge fields identified along 285 km of coast (Anclote, Sand Key, Sarasota, Manasota, Captiva, and Collier) contain about 1.4 billion cubic meters of sediments that are potentially available for dredging. Evaluation of these sediment sources, within the purview of the USMinval Code, requires the determination of resources, reserves, and level of certainty of assessment applied to a rating of resource potential. Present research is attempting to identify the overall resource potential with an eye toward eventually determining sand volumes in reserves, which will be much less than the total sand resource volume.

Coupling Geological Concepts with Historical Data Sets in a MIS Framework to Prospect for Beach-Compatible Sands on the Inner Continental Shelf: Experience on the Eastern Texas Gulf Coast1

Abstract Chronic erosion of beaches along the eastern Texas barrier island coast is increasingly mitigated by renourishment efforts that periodically place large volumes of sand onshore. Location of beach-quality sands on the inner continental shelf is challenged in an environment where terrestrial rivers deposit fluvial sediments in back bays and lagoons instead of offshore and by shelf areas that are dominated by muds. The search for beach-quality sands thus requires understanding of the coastal geological framework and morphodynamic processes that accompanied late Quaternary evolution in the northern Gulf of Mexico. The occurrence of surficial sand deposits as positive bathymetric features on the seafloor (ridges, shoals, banks) and presence of sands buried in paleovalley (drowned channels) infill sequences makes for complicated search procedures that must accurately differentiate a range of sedimentary settings by geophysical and geotechnical surveys. Compilation of vast amounts of data from historical core logs and newly acquired information in a marine information system (MIS) permits spatial analyses in a format that is compatible with development of a sand search model. The resulting differentiated investigative sand-search methods, that comprise part of the Texas Sand Search Model (TSSM), are able to target potential borrow areas in ebb-tidal shoals, low-relief ridge deposits, high-relief banks, and in mud-covered paleovalley sequences.

SAND BORROW AREA DESIGN REFINEMENT TO REDUCE MORPHOLOGICAL IMPACTS: A CASE STUDY OF PANAMA CITY BEACH, FLORIDA, USA.

The coastline of Panama City Beach, Florida (FL) has been stricken by several hurricanes during the last decades, especially after 1995. In 1998, beach nourishment projects started being implemented to address the impacts of the hurricanes on the coast. Sources of sand for that purpose are commonly from borrow areas located just offshore of the nourishment site. Impacts of these nearshore dredge pits on adjacent coasts will depend on incident wave conditions, nourishment sediment characteristics and some features of the borrow pit (distance from the shore, depth of cut, cross-shore extent, alongshore extent and orientation - Bender & Dean, 2003; Benedet & List, 2008). The practical goal of the current study was to mitigate for the negative potential effects by discovering the less impactful design of dredge pit geometries on the Borrow Area S1 in Bay County-FL. Five different cut widths and excavation depths within the permitted limits were herein evaluated. Evaluation of morphological impacts on adjacent beaches was carried with the processed-based morphodynamic model Delft3D, calibrated and simulated for a period of 13 years. Results were evaluated in terms of beach volume changes compared against a baseline simulation (no action).Switching from Alternative 1 (6,260,000 m³) to Alternative 2 (5,380,000 m³) does not result in a substantial reduction of the borrow area’s projected impact. The cut depth is still deep, and the surface area is unchanged. Alternative 3 (3,555,000 m³) is able to provide more substantial reductions in the borrow area’s impact. By reducing the acreage of the borrow area and switching to a uniform cut depth, the projected impact of the borrow area decreases 39% for 1.56 km along the downdrift beach. Under Alternatives 4 (3,060,000 m³) and 5 (2,755,000 m³), the impacts of the borrow area are projected to be less than 3.75 m³/m/yr. While both alternatives are viable, Alternative 5 minimizes potential impacts, and has a uniform cut depth and a volume that still satisfies the project’s requirements. Given these considerations, Alternative 5 is the preferred alternative. Additionally, all the alternatives increase the net-accretion along 6.5 km of Shell Island between 0.25 to 1 m³/lm/yr., a valuable side effect in a region with high net erosion. By conducting various simulations an optimal borrow area design has been identified that reduces its effects on the adjacent beaches.

Seawall impacts on adjacent beaches: separating fact from fiction

Studies of coastal morphodynamics increasingly focus on quantification of relationships between processes, form, and function of dynamic beach systems because wave climates and beach sediments interact to collectively produce distinctive types of beaches. This classifies beach types in terms beach morphology, field investigations and interpretation of aerial photography and bathymetric data. Regionalization of coastal morphodynamic patterns in turn facilitates compartmentalization of hazard zones (risks to coastal infrastructure associated with extreme meteorological events such as northeasters, tropical storms, and hurricanes) and beach safety to swimmers. The approach adopted here classifies morphological and geographical variability of beach types and indicates coastal hazards designations. Results indicate that the Ω parameter is strongly influenced by cross-shore selective sorting of bimodal sediments. General beach morphology is approximated by the Ω parameter but hardgrounds and coastal structures induce variability to beach morphology that is not detected by the Ω calculations. The east Florida coast is divided into five morphological compartments and 24 sub-segments with distinct hazard levels. The higher hazard level, in reference to coastal flooding due to storm impact, was the dune-less dissipative beaches of the Daytona coastal segment. The least hazardous beaches were undeveloped, intermediate beaches of the Cape Canaveral and Sebastian Inlet State park area.