H33I-01 13:40h
On ecogeomorphology of tidal landscapes
The analysis of the morphological structure of a tidal landscape is crucial for our capabilities to predict the evolutionary scenarios of wetlands, lagoons, estuarine areas and tidal environments in general. Field surveys and remote sensing are employed here to accurately characterize different morphodynamic features in a lagoonal environment, in particular probing the validity of the empirical (and vastly useful) geomorphic relationships like e.g. extensions of Jarrett's 'law' for inner tidal channels. Studies on (suitably defined) bottom shear stresses at the tidal channel heads show surprisingly narrow distributions and support views on the formation of tidal channel networks. Interestingly, we confirm, through refined analyses, earlier findings about the notable lack of scaling invariance of landforms within the tidal basin, which we discuss facing the sources of dynamic crossovers. Physical measures of eco-geomorphological diversity strongly suggest the dominance of subvertical processes in the control of the distribution of halophytic vegetation, a key morphodynamic factor. We finally discuss distinctive geomorphic indicators suitable for comparative purposes with modelling exercises of long-term evolution of tidal systems, which, somewhat apart from a significant portion of the pertinent literature, should not rely heavily on topological similarity that is deemed too lenient a measure.
H33I-02 13:55h
Flow, sedimentation, and biomass production on a vegetated salt marsh in South Carolina: toward a predictive model of marsh morphologic and ecologic evolution
A 1-D model for exploring the interaction between hydrodynamics, sedimentation, and plant community evolution on a salt marsh populated by Spartina alterniflora is developed. In the model tidally induced flows over marsh platforms are affected by S. alterniflora through drag forces. In general macrophyte characteristics are determined by a wide range of processes; here, based on field studies at North Inlet estuary, South Carolina, the biomass of the S. alterniflora on the marsh platform is simply related to their time of submergence under tidally induced flows. Additionally, field data collected at North Inlet are used to relate biomass to plant area per unit volume, stem diameter, and an empirical drag coefficient. Sedimentation is also related to biomass, through either organogenic deposition or trapping of suspended sediment particles. The morphologic evolution of simulated marshes is explored by varying the sedimentation process and the rate of sea level rise. Different sedimentation processes result in marshes with different morphologies. An organogenic marsh is predicted to evolve under a regime of steady sea level rise into a platform with a relatively flat surface, whereas a marsh developed primarily through a trapping mechanism is predicted to have a surface that slopes gently away from the salt marsh creek. As predicted by 0-D modeling studies, sea level rise may be accommodated up to a certain critical sea level rise rate, after which the salt marsh platform will drown. Marshes that accrete through sediment trapping adjust to changes in sea level more rapidly than marshes that accrete through organogenic deposition.
H33I-03 14:10h
Long-term morphological evolution of tidal embayments: Channel Newtwork incision and early development.
The long-term morphological evolution of tidal landforms in response to physical and ecological forcings is a subject of great theoretical and practical importance. Towards the goal of a comprehensive theoretical framework suitable for large-scale, long-term applications, we set up a mathematical model of tidal channel network initiation and early development, which is assumed to act on timescales considerably shorter than those of other landscape-forming eco-morphodynamical processes of tidal systems. A suitable hydrodynamic model capable of describing the key landforming features is coupled with a morphodynamic model which retains the description of the main physical processes responsible for tidal channel initiation and network ontogeny. The model is designed for the direct inclusion of eco-morphological mechanisms like (organic and inorganic) soil production and transport, vegetation dynamics, tidal meandering and relative sea-level regressions/transgressions, which are not discussed here because of their different characteristic timescales. We assume that water surface elevation gradients provide key elements for the description of the processes that drive incision, in particular the exceedence of a stability (or maintenance) suitably defined shear stress. The model describes tidal network initiation and its progressive headward extension through the carving of incised cross-sections where the local shear stress exceeds a predefined, possibly site-dependent threshold value. Capture of divides and migration of tidal watersheds are derived. Validation of the results against observed network properties, distinctive network statistics and structural heterogeneities are critically presented.
H33I-04 14:25h
Emergent Behavior of Coupled Barrier Island - Resort Systems
Barrier islands are attractive sites for resorts. Natural barrier islands experience beach erosion and island overwash during storms, beach accretion and dune building during inter-storm periods, and migration up the continental shelf as sea level rises. Beach replenishment, artificial dune building, seawalls, jetties and groins have been somewhat effective in protecting resorts against erosion and overwash during storms, but it is unknown how the coupled system will respond to long-term sea level rise. We investigate coupled barrier island - resort systems using an agent-based model with three components: natural barrier islands divided into a series of alongshore cells; resorts controlled by markets for tourism and hotel purchases; and coupling via storm damage to resorts and resort protection by government agents. Modeled barrier islands change by beach erosion, island overwash and inlet cutting during storms, and beach accretion, tidal delta growth and dune and vegetation growth between storms. In the resort hotel market, developer agents build hotels and hotel owning agents purchase them using predictions of future revenue and property appreciation, with the goal of maximizing discounted utility. In the tourism market, hotel owning agents set room rental prices to maximize profit and tourist agents choose vacation destinations maximizing a utility based on beach width, price and word-of-mouth. Government agents build seawalls, groins and jetties, and widen the beach and build up dunes by adding sand to protect resorts from storms, enhance beach quality, and maximize resort revenue. Results indicate that barrier islands and resorts evolve in a coupled manner to resort size saturation, with resorts protected against small-to-intermediate-scale storms under fairly stable sea level. Under extended, rapidly rising sea level, protection measures enhance the effect of large storms, leading to emergent behavior in the form of limit cycles or barrier submergence, depending on the relative rates of resort recovery from storms and sea level rise. The model is applied to Ocean City, Maryland and neighboring undeveloped Assateague Island National Seashore. Supported by the National Science Foundation, Geology and Paleontology Program, and the Andrew W. Mellon Foundation
H33I-05 14:40h
Carbonate Beaches: A Balance Between Biological and Physical Processes
Carbonate beaches are a unique example of the interaction between biological processes, creating the sediments, and physical processes, moving and often removing the sediments. On the sediment supply side, carbonate sediments are born, not made. They exist in dynamic equilibrium between production and destruction. Following the creation of carbonate sediment in coral reef and lagoon environments, the sediments are moved shoreward to the beach, transport along the shore and sometimes, eventually lost offshore, often as the result of tropical storms. Comprehensive studies of the balance between the supply and loss of carbonate sediments and beach dynamics have been completed for the islands of Mauritius and Barbados. Field studies and remote sensing (Compact Airborne Spectrometry Imaging) have been applied to develop carbonate sediment production rates for a range of reef and lagoon conditions. Using GIS, these production rates have been integrated to determine sediment supply rates for different segments of the coastline. 1-D and 2-D models of waves, hydrodynamics, sediment transport and morphodynamics were set-up and tested against observed beach response to storm events or a sequence of storm events. These complex deterministic models are not suitable for application over periods of decades. However, it was possible to characterize storm events by the extent of sand loss, and relate this to key descriptive factors for groups of storm events, thereby encapsulating the erosion response. A long-term predictive tool for evaluating beach erosion and accretion response, over a period of several decades, was developed by combining the supply rates for carbonate sediment and the encapsulated representation of the loss rates through physical processes. The ability of this predictive tool was successfully tested against observed long term beach evolution along sections of the coast in Barbados and Mauritius using air photo analysis in GIS for shoreline change over periods of 40 years. The long-term predictive tool for carbonate beach evolution provided valuable support to developing coastal zone management policy and actions to preserve the beaches in their natural form, minimizing the need for artificial nourishment of the beaches. Many models of sediment movement on shorelines are derived from clastic examples, and fit carbonate coastlines only with difficulty. We have combined field surveys of benthic biota, estimates of sediment production from skeletal growth and bioerosion, and sediment destruction by comminution and dissolution with dynamic models of sediment movement in the littoral zone, achieving improved understanding of coastal processes of erosion and deposition. Mauritius is fringed by shallow lagoons, often with luxuriant stands of Acropora. The offshore region is exhumed Pleistocene-all the sediment on the beaches comes from the lagoons. From surveys of coral cover, and estimates of sediment production from reef, sand and hardground areas, we produced dynamic models that faithfully hindcast shoreline dynamics for decades, and allowed identification of regions especially vulnerable to erosion. On the south coast of Barbados, one of the main issues in stabilising and rehabilitation the coastline is the balance between sediment from longshore drift and local sources. By identifying localised areas of characteristic sediment-producers (e.g., the foraminiferan Homotrema rubrum, the green alga Halimeda), we were able to determine the balance between proximal and distal sediment sources. The resulting model hindcasts the coastline through all the major hurricanes of the past 30 years.
H33I-06 14:55h
River Delta Morphodynamics: Examples From Danube Delta
Morphodynamics of the open-coast Danube delta has been largely determined by the interaction between fluvial deposition and the strong southward wave-induced longshore transport. Morphological and facies asymmetry displayed by the marine lobes of the Danube delta indicate that a strong and sustained southward-directed longshore transport has been a persistent process along the delta shore. Coastal evolution on the adjacent non-deltaic coast is also strongly coupled to the delta morphodynamics via the longshore transport. Analysis of recent deltaic progradation of the youngest open-coast lobes of Danube delta indicates that river mouth morphodynamics is highly non-linear, involving multiple feedbacks between subaerial deltaic progradation, deposition on the subaqueous delta, current and wave hydrodynamics as well as wave-current interactions. First, a feedback loop is activated by the hydraulic groin effect of the river plume that leads to a mutually-sustained progradation of the updrift coast and subaqueous delta at the mouth. Second, the development of a shallow subaerial delta platform, strongly offset to the downdrift direction, helps dissipate waves reaching the platform, leading to entrapment of sediment on the platform. Third, increased flood-induced deposition on the subaqueous delta platform, followed by wave reworking, leads to recurrent emergence of barrier islands at its offshore edge; longshore transport is then channeled (i.e., intensified and guided) by the new coast along the barrier, leading to a rapid alongshore expansion of the subaqueous delta in the downdrift direction. Although the sedimentation processes are complex, the resulting morphology at the mouth exhibits a tendency to self-organize that is reflected and preserved by the facies architecture of wave-influenced lobes.
H33I-07 15:10h
Implications of Small-Scale Superimposed Bedforms on the Morphodynamics and Sediment Transport Patterns on the Lower Shoreface in the Southern North Sea
Shoreface connected ridges (length up to a few kilometers, height up to a few meters) on the shoreface of the Island of Spiekeroog in the southern North Sea have been shown to be rather stable over decades and possibly hundreds of years (Flemming and Davis 1994). Recent high-resolution multibeam bathymetric surveys have revealed patterns of small-scale bedforms (length and height up to 10 m and 0.3 m, respectively) superimposed on the ridge-trough morphology across the entire lower shoreface. In general, the flood current velocities, measured at 1 m above the bottom are as much as 47% higher than ebb counterparts (Antia et al., 1995). Exceptions are the landward flanks of and troughs between the shoreface connected ridges where higher ebb velocities have been recorded. The superimposed megaripple-scale bedforms imply a net shoreward sediment transport on the seaward flanks of the shoreface connected ridges at a low angle to the ridge crest. Sediment reaching the crest and being pushed over to the landward side is then transported seaward by occasionally dominating ebb currents there. Storm events, having generally an erosive effect on the shoreface, tend to increase the amplitude of the shoreface connected ridges, i.e. amplify the morphodynamic feedback mechanisms. Recovery of the seabed after ordinary winter storms occurs in time scales less than a month. Hence, the ridge maintenance seems to be controlled by the fair-weather tidal currents that are competent to transport fine to medium sands during a considerable part of the tidal cycle. The suggested sediment transport circulation mechanism is consistent with the long term stability of the shoreface connected ridges as well as high sediment mobility on the seabed. References Antia, E.E., Flemming, B.W. and Wefer, G. 1995. Calm-weather spring and neap tidal current characteristics on a shoreface-connected ridge complex in the German Bight, southern North Sea. Geo-Marine Letters 15, 30-36. Flemming, B.W. and Davis, R.A. Jr. 1994. Holocene Evolution, Morphodynamics and Sedimentology of the Spiekeroog Barrier Island System (Southern North Sea). Senckenbergiana maritima 24, 117-155.
H33I-08 15:25h
Applications of Dune Erosion Models to Barrier Island Breaching
Two engineering models for dune erosion (EDUNE and SBEACH) are tested for their capacity to predict barrier island breaching, through hindcasts of recent occurrences. In the two most recent hurricane seasons, barrier island breaching was observed in September 2003 when Hurricane Isabel cut a 500 meter wide inlet on Hatteras Island in North Carolina, and in August 2004 when Hurricane Charley opened a 450 meter wide breach on North Captiva Island on Florida's west coast. Each of these occurrences was documented with airborne lidar systems, providing large-scale, high-resolution data on the morphologic changes forced by the associated storm system. This investigation will examine the capability of two numerical models to accurately hindcast breaching events, focusing on the September 2003 inlet cut by Hurricane Isabel. EDUNE and SBEACH are both two-dimensional models that use input wave and storm surge time series to calculate excess energy dissipation across the underwater profile, and subsequently force sediment transport. Whereas previous tests have shown that both models have the ability to predict dune scarping and minor erosional events associated with extreme storms, this study attempts to apply the models to situations of extreme morphologic change. The models are applied to multiple cross-shore profiles, evenly spaced in the longshore, encompassing the breaches as well as adjacent areas that did not experience inundation. This analysis provides a unique look at the ability of the models to couple hydrodynamic forcing with highly variable morphologic response over a longshore distance of approximately one kilometer.