Carl H. Hobbs Iii

Geology of the Wrightsville Beach, North Carolina shoreface: Implications for the concept of shoreface profile of equilibrium

Abstract Nearly 300 km of 3.5 kHz subbottom profile and 100 kHz sidescan-sonar data, a suite of over 100 short (~2 m) percussion cores and vibracores have been collected on the shoreface and inner continental shelf off Wrightsville Beach, North Carolina. Sidescan-sonar images were analyzed for acoustic backscatter to delineate the surface sediment distribution. Groundtruth data for the sidescan-sonar interpretations were provided by surface grab samples. Cross-shore sediment transport by combined waves and currents is the predominant sedimentologic signature on this shoreface. The shoreface is dominated by a shore-normal system of rippled scour depressions that begin in 3–4 m water depth and extend to the base of the shoreface about 1 km offshore, at 10 m depth. The depressions are 40–100 m wide, and up to 1 m deep. They are floored by coarse, rippled shell hash and gravel; some are separated by rock-underlain fine sand ridges. On the inner shelf, the bathymetric and sedimentary fabrics become shore-oblique, due to a series of relict ridges with 1–2 m of relief. The ridges are coarse on their landward sides and covered on their seaward flanks by thin veneers of fine sand. Field evidence from the Wrightsville Beach shoreface demonstrates that a shoreface equilibrium profile as defined by Dean (1991) and others does not exist here. For example: (1) the grain size varies widely and inconsistently over the profile; (2) shoreface profile shape is controlled predominantly by underlying geology, including Tertiary limestone outcrops and Oligocene silts; and (3) sediment transport patterns cannot be explained by simple diffusion due to wave energy gradients, and that transport occurs seaward of the assumed engineering “closure depth” of 8.5 m. This has several implications for the application of equilibrium profile-based numerical models used to investigate coastal processes and design coastal engineering projects at Wrightsville Beach. The most important practical implication is that a number of assumptions required by existing analytical and numerical models (e.g., Dean, 1991; genesis ; sbeach ) used for the design of shore protection projects and large-scale coastal modeling over decadal time scales cannot be met.

Ancient channels of the Susquehanna River beneath Chesapeake Bay and the Delmarva Peninsula

Three generations of the ancestral Susquehanna River system have been mapped beneath Chesapeake Bay and the southern Delmarva Peninsula. Closely spaced seismic reflection profiles in the bay and boreholes in the bay and on the southern Delmarva Peninsula allow detailed reconstruction of each paleochannel system. The channel systems were formed during glacial low sea-level stands, and each contains a channel-fill sequence that records the subsequent transgression. The trunk channels of each system are 2 to 4 km wide and are incised 30 to 50 m into underlying strata; they have irregular longitudinal profiles and very low gradients within the Chesapeake Bay area. The three main-stem channels diverge from the head of the bay toward the southeast. The channels are rarely coincident, although they commonly intersect. All three main channels pass beneath the southern Delmarva Peninsula, forming an age progression from north (oldest) to south (youngest) beneath the Peninsula, and from east (oldest) to west (youngest) beneath Chesapeake Bay. Southward progradation of the tip of the Delmarva Peninsula during interglacial high sea-level stands caused southward migration of the mouth of the bay, so that the next generation of channels were incised progressively further towards the southwest. The youngest paleochannel is clearly of late Wisconsinan age, about 18 ka, and the intermediate one appears to be late Illinoian in age, or about 150 ka. The age of the oldest paleochannel is not well constrained, but it is in the range of about 200 to 400 ka. The three paleochannel systems imply a dynamic coastal-plain environment and at least two previous generations of the Chesapeake Bay. Both the Chesapeake Bay and the southern Delmarva Peninsula have changed considerably in the past half million years.

Assessing anthropogenic and natural impacts on ghost crabs (Ocypode quadrata) at Cape Hatteras National Seashore, North Carolina.

Abstract This study explores impacts of off-road vehicles on ghost crab populations as a measure of impact from recreational beach use on two beaches of the Cape Hatteras National Seashore and assesses the effectiveness of several alternatives for the use of off-road vehicles (ORVs) on the beach. Ghost crab population size and density have been used as indicators of the environmental quality of beaches and dunes. Data on the creation of an “ORV corridor” in which ORVs can drive on the landward portion of the berm, but not on the beach crest, indicate that it may be possible to preserve ghost crab populations on the beach while still permitting the use of ORVs. Closing the beach crest 24 hours a day may be the optimal solution for preservation of ghost crab populations. High-energy weather events, however, resulted in larger changes to the population dynamics of the ghost crabs. After storms, ghost crabs were able to (re)inhabit areas where their numbers previously had been reduced by the operation of ORVs. While temporary closures of the beach crest may be used to reduce the short-term impacts of ORVs on ghost crab populations on the outer banks of North Carolina, long-term impacts ultimately are controlled by the strength and frequency of storms that reset the system.

Structure, age and origin of the bay-mouth shoal deposits, Chesapeake Bay, Virginia

Abstract The mouth of Chesapeake Bay contains a distinctive shoal complex and related deposits that result from the complex interaction of three different processes: (1) progradation of a barrier spit at the southern end of the Delmarva Peninsula, (2) strong, reversing tidal currents that transport and rework sediment brought to the bay mouth from the north, and (3) landward (bayward) net non-tidal circulation and sediment transport. Together, these processes play a major role in changing the configuration of the estuary and filling it with sediment. The deposits at the mouth of the bay hold keys both to the evolution of the bay during the Holocene transgression and to the history of previous generations of the bay. The deposit associated with the shoals at the mouth of the bay, the bay-mouth sand, is a distinct stratigraphic unit composed mostly of uniform, gray, fine sand. The position and internal structure of the unit shows that it is related to near-present sea level, and thus is less than a few thousand years old. The processes affecting the upper surface of the deposit and the patterns of erosion and deposition at this surface are complex, but the geometry and structure of the deposit indicate that it is a coherent unit that is prograding bayward and tending to fill the estuary. The source of the bay-mouth sand is primarily outside the bay in the nearshore zone of the Delmarva Peninsula and on the inner continental shelf. The internal structure of the deposit, its surface morphology, its heavy-mineral composition, bottom-current studies, comparative bathymetry, and sediment budgets all suggest that sand is brought to the bay mouth by southerly longshore drift along the Delmarva Peninsula and then swept into the bay. In addition to building the southward- and bayward-prograding bay-mouth sand, these processes result in sand deposition tens of kilometers into the bay.

Potential Impacts of Sand Mining Offshore of Maryland and Delaware: Part 1—Impacts on Physical Oceanographic Processes

Abstract In an effort to assess the possible changes to physical oceanographic processes that might result from alteration of bathymetry as a result of dredging or sand mining, we evaluated the differences in the output of various numerical models run with the natural and hypothetical post-dredging bottom conditions. Fenwick and Isle of Wight Shoals offshore of the Delaware-Maryland border of the mid-Atlantic continental shelf served as the test site. We considered two dredging scenarios, a one-time removal of 2 × 106 m3 of sand from each of two shoals and a cumulative removal of 24.4 × 106 m3, but only the larger appeared significant. The study of wave transformation processes relied upon a series of runs of the REF/DIF-1 model using sixty wave conditions selected from analysis of the records from a nearby, offshore wave gauge. The model was tuned and calibrated by comparing measured near-shore wave conditions with data calculated using the same measured offshore waves that generated the real near-shore conditions. The modeled, post-dredging data indicated an increase in wave height of up to a factor of two in the area between the dredged shoals and the shore and, in some locations, a lesser increase in breaking wave height and a decrease in breaking wave height modulation. The model results also may help explain the existing pattern of erosion and relative stability. Application of the well-known SLOSH model (Sea, Lake, and Overland Surges from Hurricanes) for storm surge and POM (Princeton Ocean Model) for tidal currents indicates that the likely dredging related changes in those processes are negligible.

Postimpact Deformation Associated with the Late Eocene Chesapeake Bay Impact Structure in Southeastern Virginia

Upper Cenozoic strata covering the Chesapeake Bay impact structure in southeastern Virginia record intermittent differential movement around its buried rim. Miocene strata in a graben detected by seismic surveys on the York River exhibit variable thickness and are deformed above the crater rim. Fan-like interformational and intraformational angular unconformities within Pliocene–Pleistocene strata, which strike parallel to the crater rim and dip 2°–3° away from the crater center, indicate that deformation and deposition were synchronous. Concentric, large-scale crossbedded, bioclastic sand bodies of Pliocene age within ∼20 km of the buried crater rim formed on offshore shoals, presumably as subsiding listric slump blocks rotated near the crater rim.

Side-scan sonar as a tool for mapping spatial variations in sediment type

Studies with increasing sample densities over a common area demonstrate that the distribution of estuarine sediment types varies on several scales. The variations and accompanying steep gradients in the characteristics of the sediments may lead to different interpretations of the dynamics of estuarine depositional processes depending on the scale examined. Field work in parts of Chesapeake-Bay and the York River Estuary, Virginia indicates that the spatially continuous data on the nature of the bottom surface available with side-scan sonography offers a means of assessing the continuity of sediment type.

Shoreline erosion: a problem in environmental management

Abstract Finding the most satisfactory solution to a shoreline erosion problem, particularly one affecting several owners of private property, is a complex task. In addition to the expected technical‐physical analysis of the shore, an economic assessment of the direct and indirect costs of and benefits accrued due to each of several alternative measures or options is required. The efficiency of shore protection is significantly enhanced if entire shoreline reaches are managed with a coordinated strategy rather than with unrelated actions.

An economic analysis strategy for management of shoreline erosion

Abstract An economic framework for making management decisions on tidal shoreline erosion is provided. It provides for evaluation of the impacts of coastal erosion on shoreland property and structures. Coefficients for estimating erosion‐induced losses to property and dwellings were determined. Lots which contained a dwelling decreased in value by an average of

Mining Sand on the Continental Shelf of the Atlantic and Gulf Coasts of the U.S.

0.24 for each square foot of soil lost to erosion. Decrease in value of dwelling due to erosion‐induced loss of distance between dwelling and shoreline was an average decrease of