James G. Flocks

Quaternary Geomorphology and Modern Coastal Development in Response to an Inherent Geologic Framework: An Example from Charleston, South Carolina

Abstract Coastal landscapes evolve over wide-ranging spatial and temporal scales in response to physical and biological processes that interact with a wide range of variables. To develop better predictive models for these dynamic areas, we must understand the influence of these variables on coastal morphologies and ultimately how they influence coastal processes. This study defines the influence of geologic framework variability on a classic mixed-energy coastline, and establishes four categorical scales of spatial and temporal influence on the coastal system. The near-surface, geologic framework was delineated using high-resolution seismic profiles, shallow vibracores, detailed geomorphic maps, historical shorelines, aerial photographs, and existing studies, and compared to the long- and short-term development of two coastal compartments near Charleston, South Carolina. Although it is clear that the imprint of a mixed-energy tidal and wave signal (basin-scale) dictates formation of drumstick barriers and that immediate responses to wave climate are dramatic, island size, position, and longer-term dynamics are influenced by a series of inherent, complex near-surface stratigraphic geometries. Major near-surface Tertiary geometries influence inlet placement and drainage development (island-scale) through multiple interglacial cycles and overall channel morphology (local-scale). During the modern marine transgression, the halo of ebb-tidal deltas greatly influence inlet region dynamics, while truncated beach ridges and exposed, differentially erodable Cenozoic deposits in the active system influence historical shoreline dynamics and active shoreface morphologies (block-scale). This study concludes that the mixed-energy imprint of wave and tide theories dominates general coastal morphology, but that underlying stratigraphic influences on the coast provide site-specific, long-standing imprints on coastal evolution.

Predictions of barrier island berm evolution in a time‐varying storm climatology

Low-lying barrier islands are ubiquitous features of the worlds coastlines, and the processes responsible for their formation, maintenance, and destruction are related to the evolution of smaller, superimposed features including sand dunes, beach berms, and sandbars. The barrier island and its superimposed features interact with oceanographic forces (e.g., overwash) and exchange sediment with each other and other parts of the barrier island system. These interactions are modulated by changes in storminess. An opportunity to study these interactions resulted from the placement and subsequent evolution of a 2 m high sand berm constructed along the northern Chandeleur Islands, LA. We show that observed berm length evolution is well predicted by a model that was fit to the observations by estimating two parameters describing the rate of berm length change. The model evaluates the probability and duration of berm overwash to predict episodic berm erosion. A constant berm length change rate is also predicted that persists even when there is no overwash. The analysis is extended to a 16 year time series that includes both intraannual and interannual variability of overwash events. This analysis predicts that as many as 10 or as few as 1 day of overwash conditions would be expected each year. And an increase in berm elevation from 2 m to 3.5 m above mean sea level would reduce the expected frequency of overwash events from 4 to just 0.5 event-days per year. This approach can be applied to understanding barrier island and berm evolution at other locations using past and future storm climatologies.

Sediment Characterization and Dynamics in Lake Pontchartrain, Louisiana

Abstract Lake Pontchartrain in southeastern Louisiana is the largest of several shallow estuaries that together cover over 15,000 km2. Wetlands, forests, and large urban areas surround the lake. Primary transport mechanisms of sediments to Lake Pontchartrain include urban runoff, major diversions of the Mississippi River, discharge from streams along the north and west shores, and tidal circulation. Sediments deposited in Lake Pontchartrain are subjected to resuspension and mixing by natural and human activities. Bioturbation and water turbulence throughout the lake are the major mixing agents, and mechanical shell dredging has reworked much of the lake bottom over the last century. Sediment characterization through direct sampling and geophysical surveys indicates that these processes continually rework the top meter of sediment. The lake receives discharge from roadways and industrial and agricultural sources. Contaminants from these sources accumulate in the lake sediments and are an important contributor to the degradation of the estuary. Decline in populations of various benthic organisms, such as shrimp and clams, has been documented in the lake. To characterize the health of this important estuary, the U.S. Geological Survey (USGS) conducted a comprehensive evaluation of the geology, geomorphology, coastal processes, and environmental condition of the Pontchartrain Basin from 1994 to 1997. This report presents an assessment of sediment distribution and quality using a multidisciplinary approach to characterize the influence of various physical and chemical parameters: nearsurface stratigraphy, major trace metal concentrations (Cu, Pb, Zn, and Ni), and short-lived radionuclides (210Pb, 7Be, and 137Cs). The results are compared with water-circulation patterns to determine high-resolution sedimentation patterns in the lake. The data show a significant increase in trace metals in the top 1 m of lake sediments. Above this horizon, pollen analysis indicates a correlation with land clearing in the area, a proxy for increasing human development of the surrounding landscape and an increase in surface run-off. The data also show that the top meter of sediment undergoes frequent resuspension during high-energy circulation events and via circulation gyres in the lake. This regular turnover does not allow stratification of recently deposited sediments, restricting the sequestration of contaminated material that enters the lake.

Geologic Controls on Regional and Local Erosion Rates of Three Northern Gulf of Mexico Barrier-Island Systems

ABSTRACT Twichell, D.C.; Flocks, J.G.; Pendleton, E.A., and Baldwin, W.E., 2013. Geologic controls on regional and local erosion rates of three northern Gulf of Mexico barrier island systems. 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. 32–45, Coconut Creek (Florida), ISSN 0749-0208. The stratigraphy of sections of three barrier island systems in the northeastern Gulf of Mexico (Apalachicola, Mississippi, and Chandeleur) have been mapped using geophysical and coring techniques to assess the influence of geologic variations in barrier lithosomes and adjoining inner shelf deposits on long-term rates of shoreline change at regional and local scales. Regional scale was addressed by comparing average geologic characteristics of the three areas with mean shoreline-change rates for each area. Regionally, differences in sand volume contained within the part of the barrier lithosome above sea level, sand volume on the inner shelf, and to a lesser extent, sediment grain size correlate with shoreline change rates. Larger sand volumes and coarser grain sizes are found where erosion rates are lower. Local scale was addressed by comparing alongshore variations in barrier island and inner shelf geology with alongshore variations in shoreline change. Locally, long-term shoreline change rates are highest directly shoreward of paleovalleys exposed on the inner shelf. While geology is not the sole explanation for observed differences in shoreline change along these three coastal regions, it is a significant contributor to change variability.

Offshore Sediment Character and Sand Resource Assessment of the Northern Gulf of Mexico, Florida to Texas

Abstract The Gulf of Mexico (GOM) continental shelf, extending approximately 1600 km from the Florida west coast to the U.S.–Mexico border, is a large sedimentary basin that has been the focus of much geologic study and surveys during the past 70 years, related mostly to oil and gas exploration. Relatively little attention has been focused on mapping and assessing offshore sediment character and resources, such as sand. It is increasingly recognized, however, that baseline scientific information on seafloor sediment character and composition is needed for managing and protecting natural resources and for providing information on sand availability and quality for potential use in a variety of coastal restoration and protection projects in all five of the states from Florida to Texas. The geomorphologic character and shallow sedimentary stratigraphy of the GOM shelf has been determined over geologic time by sediment inputs from rivers; sea-level fluctuations up to 120 m, resulting in transgressions and regressions of the shore; and frequent storms. These processes have resulted in deposition, reworking, and preservation of a variety of sand bodies, both on the seafloor and in buried, ancestral stream channels. Sand bodies of highly varying grain size, sorting, color, and composition are present throughout parts of the GOM inner shelf, varying greatly in size and number and often overlain or admixed with finer-grained, muddy sediment. The shelf sand bodies tend to be fine grained and are often mixed with muddy or organic detritus as well as carbonate shell material. The GOM shelf is mantled with sand mostly off the Florida shore, and sediments become progressively finer and muddier westward across the Alabama, Mississippi, Louisiana, and Texas shelf regions. The shelf off each state contains shoals that represent drowned paleoshoreline and buried, ancestral, stream-channel features that originated when sea level was lower than at present and the shore was farther seaward. These shoals offer the best promise as potential sand resources; however, further study is needed to refine these findings based on reconnaissance-scale work.

Abstract: Holocene Geologic Framework of Lake Pontchartrain Basin and Lakes of Southeastern Louisiana

The Lake Pontchartrain Basin is a 12,170-km2 (4,700-mi2) watershed in southeastern Louisiana, stretching from the State of Mississippi on the north and east to the Mississippi River on the west and south, and to Breton Sound at the Gulf of Mexico. The Pontchartrain Basin is about 200 km along strike and 75 km along dip with modern lakes (Maurepas, Pontchartrain, and Borgne) covering the southern portion of the basin. Lake Pontchartrain and its adjacent lakes form one of the largest estuaries in the United States. Nearly 1.5 million people (one-third of the entire population of Louisiana) live in the 14 parishes within the Lake Pontchartrain Basin. The basin is bounded by incised Pleistocene terraces to north, the Mississippi River delta plain to the south/southwest, and the Pine Island barrier shoreline to the south/southeast. Over the last 150 years, urban growth of New Orleans and the north shore communities and associated exploitation of natural resources have severely altered the environmental quality of the basin. In 1994, the USGS began a multidisciplinary evaluation of the geology, geomorphology, coastal processes, and environmental quality of the Pontchartrain Basin for use by Federal, state and local officials in coastal management and restoration planning. Existing geological information has been integrated with newly acquired high-resolution seismic profiles (>700 line km), 76 vibracores, and more than 1000 samples for geochemical data to develop a geologic history and record of sediment distribution of the basin (Fig. 1). The Pontchartrain Basin has a complex depositional history controlled by sea-level change. Lake Pontchartrain originated as a result of sedimentary processes. During the late Wisconsin lowstand, the region was entrenched by rivers. A buried incised channel of the ancestral Mississippi River, identified from seismic profiles (see Fig. 2 A-A’), underlies the southern margin of Lake Pontchartrain. The incision is three to four km across, was cut to a depth of 40 m, and can be traced from west to east into Lake Pontchartrain (Fig. 3). Sea-level rise during deglaciation truncated the filled paleochannel and the surrounding region of the Pontchartrain Basin. The ravinement surface is a sharp contact with siderite nodules and has been identified from vibracores as the Pleistocene-Holocene contact. The late Pleistocene unit is typically described as a stiff, olive-gray to light grayish-yellow clay that is highly bioturbated. The burrows are fill with oxidized organics or sand and silt. The structure contour map (Fig. 3) of the Pleistocene surface shows the contact is shallow (near the sediment surface, 2 m below sea level) in northeast Lake Pontchartrain and deeper (20 m below sea level) to the southwest. Cross section B-B’ (Fig. 3) illustrates how the Pleistocene contact crops out along the northeast shore and dips to the southwest. Sea-level rise flooded the “Pontchartrain Embayment” and

Review of the Geologic History of the Pontchartrain Basin, Northern Gulf of Mexico

Abstract The Pontchartrain Basin extends over 44,000 km2 from northern Mississippi to the Gulf of Mexico and includes one of the largest and most important estuarine systems in the United States. The basin supports a variety of environments, from woodlands in the north to wetlands in the south, and a growing socioeconomic infrastructure that has led to rapid development of the southern half of the basin over the past two centuries. To properly administer this infrastructure, managers need to understand the complex geologic framework of the basin and how it will respond to continued sea-level rise, variable rates and magnitudes of land subsidence, and human alteration of the landscape. This article summarizes the body of work that describes the regional evolution and stratigraphic architecture of the Pontchartrain Basin. The northern two-thirds of the basin is underlain by a stratigraphy of undifferentiated sands and clays deposited throughout the Plio-Pleistocene by glacially influenced rivers. These deposits were weathered and incised by rivers during sea-level low stands, forming a series of terraces that increase with age from south to north. The southern third of the basin is composed of estuaries formed during the Holocene, while shoreline processes created a series of sandy barriers that restricted communication to the Gulf of Mexico. The Mississippi River completed the geologic development of the basin by building a sequence of subdelta lobes along this southern margin over the past 5000 years, further sealing it from the open Gulf of Mexico. Presently, the modern Mississippi River bypasses the estuarine environment and only contributes sediments during flood events when the river overtops the levee system. Sea-level rise, subsidence within the Holocene delta-plain deposits, and movement along numerous fault systems are the active natural processes that continue to affect basin geomorphology.

Wave Scenario Results of Proposed Sediment Borrow Pit 3 on the Nearshore Wave Climate of Breton Island, LA

Provided here are the SWAN wave model input of grid 4 with pit 3 configuration and output of significant wave height, dominant wave period, and mean wave direction resulting from simulation of wave scenarios at the Breton Island, LA, as described in USGS Open-File Report 2015 1055 (https://doi.org/10.3133/ofr20151055). There are 128 individual scenarios that are based on significant wave height (H) and mean wave direction (D). There are 8 bins for significant wave height (H1-H8) that range from 0 meters to 6 meters in intervals of 0.5 meters (0 2 meters) and 1 meter (2 6 meters). There are 16 bins for mean wave direction (D1-D16) that range from 0 degrees to 360 degrees in intervals of 22.5 degrees. Of the 128 scenarios, 12 had no data and therefore are absent from the list below. For more information on scenario discretization readers are referred to Daylander and others (2015; https://doi.org/10.3133/ofr20151055).

Single-Beam Bathymetry Data Collected in 2015 Nearshore Dauphin Island, Alabama

Dauphin Island, Alabama is a barrier island located in the Gulf of Mexico that supports local residence, tourism, commercial infrastructure, and the historical Fort Gaines. During the past decade the island has been impacted by several major hurricanes (Ivan, 2004; Katrina, 2005; Isaac, 2012). This, in addition to ongoing sea level rise, presents a continued threat to island stability. In an effort to properly restore and provide longer-term island resilience to future storms and sea-level rise, state and federal managers are taking a scientific investigative approach to identify the best options available to help formulate and implement a long-term plan for the Island s protection. Island morphology, including current seafloor and shoreline bathymetry data, is one of several aspects being investigated and funded through a grant from National Fish and Wildlife Foundation Gulf Environmental Benefit Fund. In August 2015, the United States Geological Survey Saint Petersburg Coastal and Marine Science Center (USGS SPCMSC) in cooperation with the United States Army Corps of Engineers (USACE) and the state of Alabama conducted bathymetric surveys of the nearshore waters surrounding Dauphin Island. This data release only includes the single-beam data collected by the USGS. This USGS data release provides 1,165-line kilometers (km) of processed single-beam bathymetry (SBB) data collected by the USGS SPCMSC under the Field Activity Number (FAN) 2015-326-FA. This FAN encompasses four subFANs each of which represents one survey vessel; the RV Sallenger (15BIM10), the RV Jabba Jaw (15BIM11), the RV Shark (15BIM12), and the RV Chum (15BIM13). This data release provides SBB point data (x,y,z) in three datums: 1) the International Terrestrial Reference Frame of 2000 (ITRF00) and ellipsoid height (-47.04 meters (m) to -29.36 m); 2) the North American Datum 1983 in the CORS96 realization (NAD83 (CORS96)) for the horizontal and the North American Vertical Datum 1988 (NAVD88) for the vertical (-0.24 m to -17.33 m); and 3) NAD83 (CORS96) for the horizontal, and Mean Low or Lower Water (MLLW) for the vertical (-0.12 m to -17.93 m). Additional files include trackline shapefiles, respective digital and handwritten field logs, and a comprehensive 50-meter Digital Elevation Model (DEM).

Effects of proposed sediment borrow pits on nearshore wave climate and longshore sediment transport rate along Breton Island, Louisiana

As part of a plan to preserve bird habitat on Breton Island, the southernmost extent of the Chandeleur Islands and part of the Breton National Wildlife Refuge in Louisiana, the U.S. Fish and Wildlife Service plans to increase island elevation with sand supplied from offshore resources. Proposed sand extraction sites include areas offshore where the seafloor morphology suggests suitable quantities of sediment may be found. Two proposed locations east and south of the island, between 5.5–9 kilometers from the island in 3–6 meters of water, have been identified. Borrow pits are perturbations to shallow-water bathymetry and thus can affect the wave field in a variety of ways, including alterations in sediment transport and new erosional or accretional patterns along the beach. A scenariobased numerical modeling strategy was used to assess the effects of the proposed offshore borrow pits on the nearshore wave field. Effects were assessed over a range of wave conditions and were gaged by changes in significant wave height and wave direction inshore of the borrow sites, as well as by changes in the calculated longshore sediment transport rate. The change in magnitude of the calculated sediment transport rate with the addition of the two borrow pits was an order of magnitude less than the calculated baseline transport rate. Introduction North Breton Island, located at the southern end of the Chandeleur Islands, Louisiana, and part of the Breton National Wildlife Refuge (BNWR), provides important habitat for nesting colonies of brown pelicans. Loss of subaerial island extent can affect this species through reduction of nesting area. Due to storm impacts, relative sea level rise, and diminished sediment supply from dredging of the Mississippi River Gulf Outlet (MRGO), island area has been reduced by 93 percent since the 1920s (Martinez and others, 2009). In an effort to preserve Breton Island (fig. 1), the southernmost extent of the BNWR, the U.S. Fish and Wildlife Service (FWS) plans to nourish the island by restoring island elevation using nearby offshore sand resources. FWS requested that the U.S. Geological Survey (USGS) evaluate the potential effects of mining offshore sand on the wave climate and longshore sediment transport at Breton Islands; results of that evaluation are presented in this report. Studies have shown that sediment deposits within BNWR suitable for shoreline nourishment are rare (Twichell and others, 2009) and are confined to buried distributary channels, terminal spits, and tidal deposits (Flocks and others, 2009). Analyzing the seafloor morphology offshore of Breton Island, potential relict spit and tidal deposits have been identified in approximately 3–6 m of water. Dredging borrow pits in these deposits will change the seafloor morphology, which could alter the wave transformation and result in changes in the wave climate locally and around the island. If effects on the wave climate extend to nearshore regions around the island, the breaking wave characteristics (significant height and peak wave direction), which dictate alongshore sediment transport magnitude and direction, could be altered. Changes in sediment