H34B-01 INVITED
Initial Modeling of the Large-Scale Interactions Between Lithology and Coastal Processes on Rocky Coastlines
The large-scale evolution of rocky coastlines involves the interactions between: 1) coastal erosion and sediment- transport processes; 2) coastline shape and topography; 3) variations in rock strength and composition; and 4) tectonics. We will present an initial model exploration of some of the interactions among the first three influences, and how those interactions shape the plan-view coastline over time. In the simplest case, with spatially uniform lithology, after a rapid sea-level rise, wave-driven coastal processes modify an initially fractal coastline. (The fractal nature results from the intersection of sea-level with a fluvially or glacially sculpted landscape.) Sediment production and alongshore transport creates pocket beaches. As sediment accumulates and headlands retreat, these pocket beaches expand and coalesce, ultimately producing long, straight sections of beach backed by cliffs where the most recent headlands existed. Southernmost California (Orange and San Diego Counties), USA, seems to exemplify an approach to this state. As sediment- transport pathways lengthen, nearshore canyons, which act as sediment sinks, strongly influence coastline evolution. Understanding how and where nearshore canyons appear and evolve over long timescales may be one of the greatest needs for exploring this style of rocky coastline evolution. Numerical modeling also suggests that alongshore heterogeneities in rock strength (maximum weathering rates) can produce a headland/pocket beach morphology, and that the cross-shore amplitude of the headlands can grow over time. However, these results are only relevant in an environment where sediment is lost to deep water at a relatively high rate—as may be expected, for example, during rapid sea-level rise on a steep coastline. Many processes not addressed in these modeling endeavors also affect rocky-coastline shape, especially on spatial scales less than kilometers; this initial exploration is only intended to illuminate some of the possible key interactions affecting the large scale evolution of rocky coastlines.
H34B-02
Modelling the Holocene Evolution of a Drift-dominated Alluvial-fan Coast
A numerical model is being developed to simulate shore evolution along an alluvial fan coast over the Holocene. The alluvial fan of the Waitaki River, on the east coast of New Zealand's South Island, was built by Pleistocene glacial advances but has been eroded back by waves over recent millennia. The retreat has left a cliffed shore fronted by a narrow beach of mixed sand and gravel and a gently sloping seabed with only a thin, patchy sand cover over the Pleistocene substrate. The study motivation is to examine the sensitivity of shoreline movements in this setting to wave-climate change, sea-level rise, and river sediment supplies. The modelling couples a profile evolution model with a shoreline model. The profile evolution model is operational and is driven by a series of coupled process models which include seabed scour, berm construction during normal waves, berm overtopping and subsequent beach-stripping and scour of the exposed substrate and cliff-toe notching by storm waves, gravity failure of the cliffs and talus construction, and beach sediment abrasion. Negative feedback regulates the rate of cliff erosion through the protection provided by the new material added into the beach from the eroding cliffs and substrate. The model is forced by two wave conditions: a normal swell and a randomly-varying storm wave. These operate for proportions of the yearly time step. The model was begun on a sloping fan surface inundated by the last stages of post-glacial sea-level rise (8000 yr BP). The initial response is for rapid growth of a gravel beach ridge fed by wave-excavation of the nearshore. As the nearshore profile nears equilibrium with the wave climate, the onshore feed wanes below the abrasion rate and the beach ridge loses volume. As sea level rises the beach ridge moves upward and landward, but its capacity to do so is limited by the rate of sediment feed from the nearshore. When the beach size reduces to a threshold at which storm waves periodically overtop and strip the beach, substrate scour commences and a proto-cliff forms. The shore then settles into a mode of episodic retreat with gradually increasing cliff height. Without any net gravel gains or losses alongshore, the cliff retreat rate is simply related to the abrasion rate and the shore profile height. The equilibrium beach width is an emergent property that also relates to the abrasion rate. The modelled profile compares reasonably with the field profile at locations where the longshore transport divergence is small. A surprising result is the importance of nearshore substrate erosion to the beach sediment budget. Preliminary runs with a shoreline model show how the alongshore patterns of cliff height and historical shoreline movement on the Waitaki Fan shore reflect the net influence of abrasion and longshore transport divergence. They also suggest that the barrier shore north of the fan may have been arrested at the first stage of the profile evolution sequence because deposition due to a waning longshore drift potential just offsets gravel losses to abrasion.
H34B-03
Coastal Evolution of the Mississippi River Chenier Plain: A Geomorphic Process-Response Model
Using 28 topographic profiles, air-photo interpretation, and historical shoreline-change data, coastal processes were evaluated along the Mississippi River Chenier Plain to explain the occurrence, distribution, and geomorphic hierarchy of primary landforms. The Louisiana Chenier Plain, classified as a low-profile, microtidal, storm- dominated coast, is located west and downdrift of the Mississippi River deltaic plain. This late-Holocene, marginal-deltaic environment is 200 km long, less than 30 km wide, and composed of mud deposits capped by marsh interspersed with thin sand- and shell-rich ridges ("cheniers") that are less than 4 m in elevation. Most Chenier-Plain ridges represent open-Gulf paleoshorelines. Past shoreline morphodynamics allow ridges to be classified as transgressive (cheniers), regressive (beach ridges), or laterally accreted (spits). Geomorphic zones that contain two or more regressive, transgressive, or laterally accreted ridges are termed complexes. Consequently, we further refine the Chenier-Plain definition by Otvos and Price (1979, Marine Geology) and define Chenier Plain as containing at least two or more chenier complexes. As such, a geomorphic hierarchy of landforms is devised relative to dominant coastal process. The Chenier Plain is defined as a first-order feature (5000 km2) composed of three second-order features (30 to 300 km2): chenier complex, beach ridge complex, and spit complex. Individual ridges of each complex type were further separated into third-order features: chenier, beach ridge, and spit. To understand long-term evolution of the Chenier Plain, modern tidal-inlet processes operating at Sabine, Calcasieu, and Mermentau river entrances were also examined relative to the inlet-stability ratio. Prior to human modification and stabilization efforts, the Mermentau River entrance is classified as wave-dominated, Sabine Pass as tide-dominated, and Calcasieu Pass as tide-dominated to mixed. Hoyt (1969, American Association of Petroleum Geologists Bulletin) presented the first depositional model for chenier genesis and mudflat progradation. However, Hoyt's model oversimplifies Chenier-Plain evolution because it omits ridges created by non-transgressive processes. Thus, the geologic evolution of the Chenier Plain is more complicated than Mississippi River channel avulsions, and it involved not only chenier ridges (transgressive), but also beach ridges (regressive) and spits (lateral accreted). A six-stage geomorphic process-response model is presented to describe Chenier-Plain evolution primarily as a function of: 1) the balance between sediment supply and energy dissipation associated with Mississippi River channel avulsions, 2) local sediment reworking and lateral transport, 3) tidal-entrance dynamics and sediment trapping, and 4) possibly higher-than-present stands of Holocene sea level. Hence, the development of transgressive, regressive, and laterally-accreted ridges typically occurred contemporaneously along the same shoreline at different locations.
H34B-04
When Might Barrier Island Chains 'Collapse'? An Initial Model Investigation
There has been recent speculation that, in response to the accelerated sea-level rise and intensified storms expected over the coming century, barrier island chains such as those found on the US Atlantic and Gulf coastlines, could develop large (10-kilometer-scale) gaps in their most narrow stretches, or might disappear completely (Riggs, S. R., 2001). Such a collapse along the North Carolina Outer Banks barrier island chain, for example, would leave the mainland vulnerable to direct hits from Western Atlantic storm systems, and also would dramatically alter the estuarine system it encloses with potentially devastating effects to marine life. Concern for the future of the Outer Banks is also motivated by the decimation of the Chandeleur Islands in 2005 from Hurricane Katrina. We will present a series of initial numerical modeling experiments addressing how barrier island morphodynamics respond to the sudden creation of kilometer-scale gaps. Large-scale barrier island evolution is influenced by sea-level rise and barrier island overwash, alongshore sediment transport, tidal currents, and the availability of mobile sediment. Barrier islands transgress towards the mainland in response to sea-level rise through overwash: ocean-facing shorelines provide sediment that is transported onto the island to maintain its subaerial height and behind the island to maintain its width, while gradients in alongshore sediment transport typically dictate the large-scale shape of a coastline over long time frames (decades to millenia). Tidal currents also tend to scour inlet channels; the relative strength of this effect depends in part upon the width of the inlet channel. Our exploratory model includes both a one-line alongshore transport component and a cross-shore overwash component, as well as representations of underlying geology (weathering rates and material compositions). In our modeling experiments, we test the effects of perforating a 30 km barrier island chain with variable-sized gaps, ranging between 2.5-10 km. In preliminary model experiments, where we do not limit the availability of mobile sediment nor include tidal inlet dynamics, large gaps tend to close under all of the erosion rate scenarios and gap sizes. The ends of barrier islands extend to fill in the gaps and recurve landward. The rate of closure of gaps is unaffected by sea-level rise rates even under the most extreme cases; alongshore sediment fluxes exceed those associated with sea-level rise as highly curved isolated islands migrate rapidly landward before coalescing into an island chain again at a new location. In a natural setting, the overwash and spit-growth that maintain sub-aerial islands and tend to knit them back together (respectively) could be inhibited by a lack of mobile sediment. The shoreface of the Outer Banks, for example, consists of sometimes patchy Holocene sands perched atop a semi-lithified, sometimes more muddy Pleistocene substrate. Weathering of the Pleistocene substrate over long timescales generates mobile sediment consisting of both sands and muds. The fine-grained material, however, is typically lost to the nearshore system. The shoreface may not be able to weather fast enough to keep up with rapidly migrating islands. This effect, combined with that of substrate composition, will tend to limit the rate that sediment can be liberated, and, in turn, could prevent island-chain recovery. We conduct a series of model experiments to determine the combinations of geological parameters (weathering rates, composition) and forcing parameters (rate of sea-level rise, frequency of storms) that prevent barrier-island-chain recovery.
H34B-05
Using Paleocoastal Features to Model Epeirogenic Uplift History and Karstification of North Florida
Pleistocene beach ridges oriented sub-parallel to the modern U.S. Southeast Atlantic coastline reflect a rich history of nearshore sediment accumulation and sea level oscillation. These features are preserved at elevations up to ~50 meters above modern sea level suggesting regional uplift has occurred, despite the tectonically quiescent setting. Previous studies have proposed the combination of near surface karstification of the upper portion of the Florida carbonate platform and isostatic response as a mechanism to explain regional uplift. Measurements of dissolved solids in Florida's springs correspond to a carbonate rock dissolution rate sufficient to uplift the Florida platform at a rate of ~0.03 mm/yr. We present a simple numerical model that combines (i) sea level oscillation, (ii) precipitation induced- karstification, and (iii) isostatic response to change in bulk crustal density to calculate the elevation history of a prominent beach ridge (Trail Ridge) and coastal marine terrace (Penholoway Surface) in North Florida from the early Pleistocene to present. Using the assumption that beach ridge formation occurs during sea level highstand events, the model enables us to test various combinations of paleo-precipitation history and karstification function for a known eustatic sea level history. Preliminary model simulations verify the hypotheses that (1) paleo-precipitation history is positively correlated with sea level fluctuation (higher precipitation during warm climate intervals), as indicated by lake core studies, and (2) carbonate rock dissolution rates may be lower now than in the past due to more efficient flushing of groundwater undersaturated with respect to carbonate through well-developed secondary porosity in karst terrain.
H34B-06
Tectonic and geomorphic controls on littoral sediment distribution and transport along an emergent, rocky coastline: Northern Monterey Bay, California
Two hydrographic surveys produced high-resolution images of the inner shelf of northern Monterey Bay, Santa Cruz County, CA. Interferometric side-scan swath bathymetric sonar data were combined with sub-bottom seismic profiles, aerial imagery, high-resolution topography, and local geology to better understand how coastal geomorphology, lithology, and tectonics influence the distribution and transport of littoral sediment in the nearshore and inner shelf along a rocky shoreline. The survey data showed that the inner shelf along an emergent, high-energy rocky coastline is extremely varied in nature, being characterized by a mosaic of flat sandy areas, boulder fields and complex bedrock ridges that support rich marine ecosystems. We interpret the distribution of littoral sediment along this coastline to be primarily controlled by the northwest-trending structure of the region. Because of the structural barriers to littoral transport, high-energy wave events are required to mobilize sediment to a great enough height in the water column so that it can be transported over the bedrock highs between adjacent pocket beaches. Accordingly, the timing and magnitude littoral sediment transport along rocky coasts appears to be dictated by the episodic nature of storm activity, more so than along linear, sandy coasts where these structural barriers are much smaller or do not exist.
H34B-07
Possible Association of Oyster Terrain Mound Features in the Peconic Estuary on Long Island, NY With 8.2ka Meltwater Pulse?
Mound features revealed by multibeam bathymetry and seismic profiles are associated with an oyster terrain that covers an extensive area of the Peconic Estuary on Long Island, NY. These mounds may capture high resolution paleoclimate records of the 8.2ka melt water pulse. One of the puzzles of the 8.2ka event is the lack of a strong signal in the salinity or δ180 near the suggested outburst path via the Hudson Strait and Labrador Sea. The Peconic Estuary is situated where one might find evidence of this outburst, downstream from the modern currents exiting the Labrador Sea and to the north of the signature reported to be found off of Cape Hatteras near the present Gulf Stream North Wall. By examining the characteristics of these features based on the multibeam backscatter and bathymetry data obtained as part of an ongoing project we can distinguish relatively high backscatter regions associated with a mound morphology typified by exposed mounds of approximately 2m in height. The mounds' surfaces are associated with stained unarticulated oyster shells in grab samples, but no living oysters. These oysters are far thicker with many growth bands than the thin young oysters associated with modern Long Island aquaculture or harvested oyster beds and are often full of pits and holes. The mounds topped by oyster shells are hypothesized to be oyster bioherms that started to form around the time of the 8.2ka meltwater pulse (mwp), and as such should serve as proxies of climate around this time period. Examination of seismic profiles over this mound terrain reinforces the idea that these are indeed older features by revealing buried mounds under at least 3m of sediment that fall within the exposed mound range of 6m to at least 18m. It also reveals even greater relief of most exposed mound morphologies below the surface with many buried and exposed mounds reaching greater than 4m in relief. It would have required thousands of years to bury mounds at a steady low sedimentation rate. Cores and dates of the buried mound features are needed to confirm this hypothesis. If these mounds indeed started around that time and continued for several hundred years to millennia after, we may be able to find evidence of a freshening from meltwater pulse sources upstream related to the 8.2ka event in the North Atlantic as well as changes in interannual variability by examining oyster shells from this region.
H34B-08
Evolution of a North Slope barrier island (Narwhal Island, North Arctic Alaska) 1955- 2007
In 1955, Narwhal island was a 4 km long and 30 to 200 m wide barrier island, located at 145 30' W; 70 24' N, about 20 km offshore of the North Slope coast by Foggy Island Bay and near Prudhoe Bay, Alaska. According to available aerial photography, by 1979, the island had been breached in 4 locations creating a five island chain. By 1984, the chain consisted of 3 pieces indicating a reformation process. In subsequent years, the chain appears to have gone through a couple more cycles of breakup and reformation. The island is subject to wind waves, sea-ice impacts, and storm surges. Preliminary GIS analysis and recent GPS surveys indicate that, in the past 50 years, the western end of the island had migrated about 200 m to the west consistent with the direction of sea-ice movement and consistent with the frequent east winds during the summer (open water) period. The rate of migration is consistent with the findings of earlier studies. In addition to the island's westward migration, the northern (seaward) side of the island has retreated landward by about 5 m/year during the past decade. Here, the details of the GIS and GPS work are described. In addition, a preliminary wave (SWAN) and sediment transport model is presented that explains the morphodynamic changes. Considering continued sea ice retreat consequent to global warming, we speculate about future morphodynamic changes.