Benjamin T. Gutierrez

Cross-Shore Variation of Wind-Driven Flows on the Inner Shelf in Long Bay, South Carolina, United States

and exceeded 0.1 N/m 2 , vertical mixing occurred, the two-layered flow pattern disappeared, and currents were directed alongshore with the wind at all sites and throughout the water column. In the fall, near-bed flows close to the shore (water depth <7 m) were often reduced compared to or opposed those measured farther offshore under southwestward winds. A simplified analysis of the depth-averaged, alongshore momentum balance illustrates that the alongshore pressure gradient approached or exceeded the magnitude of the alongshore wind stress at the same time that the nearshore alongshore current opposed the wind stress and alongshore currents farther offshore. In addition, the analysis suggests that the wind stress is reduced closer to shore so that the alongshore pressure gradient is large enough to drive the flow against the wind.

Using a Bayesian network to predict barrier island geomorphologic characteristics

Quantifying geomorphic variability of coastal environments is important for understanding and describing the vulnerability of coastal topography, infrastructure, and ecosystems to future storms and sea level rise. Here we use a Bayesian network (BN) to test the importance of multiple interactions between barrier island geomorphic variables. This approach models complex interactions and handles uncertainty, which is intrinsic to future sea level rise, storminess, or anthropogenic processes (e.g., beach nourishment and other forms of coastal management). The BN was developed and tested at Assateague Island, Maryland/Virginia, USA, a barrier island with sufficient geomorphic and temporal variability to evaluate our approach. We tested the ability to predict dune height, beach width, and beach height variables using inputs that included longer-term, larger-scale, or external variables (historical shoreline change rates, distances to inlets, barrier width, mean barrier elevation, and anthropogenic modification). Data sets from three different years spanning nearly a decade sampled substantial temporal variability and serve as a proxy for analysis of future conditions. We show that distinct geomorphic conditions are associated with different long-term shoreline change rates and that the most skillful predictions of dune height, beach width, and beach height depend on including multiple input variables simultaneously. The predictive relationships are robust to variations in the amount of input data and to variations in model complexity. The resulting model can be used to evaluate scenarios related to coastal management plans and/or future scenarios where shoreline change rates may differ from those observed historically.

Relative sea-level rise and the development of valley-fill and shallow-water sequences in Nantucket Sound, Massachusetts

Valley-fill sequences offshore southern Cape Cod, Massachusetts, record Late Pleistocene and Holocene subaerial and submarine sedimentation in a post-glacial setting. Complications arise because some of the lows that contain valley-fill sediments represent variations in glacial drift thickness, whereas others reflect channel incision into glacial sediments and subsequent infilling. The valley-fill preserved in topographic lows in the presently submerged glacial terrain is composed of Late Pleistocene glaciolacustrine silts and clays covered with Holocene sand units. In contrast, the valley-fill sequence preserved in incised valleys, which are thought to have been caused by groundwater seeps in the Late Pleistocene, comprises a transgressive-systems tract composed of a consistent sequence of coastal embayment and nearshore marine facies that succeeded one another in response to Holocene relative sea-level rise. This transgressive valley-fill sequence started to accumulate approximately 7600 BP and reflects the submergence and consequent onshore migration of coastal embayment and shoreline environments along Late Pleistocene spring-sapping valleys. The Holocene transgressive-systems tract that rests unconformably atop the lake deposits is limited to the upper meter of the sequence. The absence of coastal embayment and other paralic facies in these topographic lows suggests that the infilled lows did not accommodate protected coastal embayments, indicating that they were most likely filled by glacial processes prior to the Holocene transgression. The occurrence of both topographic-low and incised-valley fill sequences in Nantucket Sound reveals the depositional complexity of the deposits associated with glacial recession in the region.