H33L-01 INVITED
Feedback Limiting the Coastal Response to Irregularities in Shelf Bathymetry
Observations and engineering studies have shown that non-uniform inner shelf bathymetry can influence longshore sediment transport gradients and create patterns of shoreline change. One classic example is from Grand Isle, Louisiana, where two offshore borrow pits caused two zones of shoreline accretion landward of the pits. In addition to anthropogenic cases, many natural situations exist in which irregularities in coastal planform are thought to result from offshore shoals or depressions. Recent studies using the hydrodynamic model Delft3D have successfully simulated the observed nearshore erosion and accretion patterns landward of an inner shelf borrow pit. An analysis of the momentum balance in a steady-state simulation has demonstrated that both alongshore pressure gradients (due to alongshore variations in wave setup) and radiation stress gradients (terms relevant to alongshore forcing) are important for forcing the initial pattern of nearshore sedimentation in response to the borrow pit. The response of the coast to non-uniform inner shelf bathymetry appears to be limited, however, because observed shoreline undulations are often rather subtle. (An exception may exist in the case of a very high angle wave climate.) Therefore, feedbacks in processes must exist such that growth of the shoreline salient itself modifies the transport processes in a way that limits further growth (assuming the perturbation in inner shelf bathymetry itself remains unchanged). Examination of the Delft3D momentum balance for an inner shelf pit test case demonstrates that after a certain degree of morphologic development the forcing associated with the well-known shoreline smoothing process (a.k.a., diffusion) counteracts the forcing associated with the inner shelf pit, producing a negative feedback which arrests further growth of the shoreline salient. These results provide insights into the physical processes that control shoreline changes behind inner shelf bathymetric anomalies (i.e. man-made dredge pits and natural bathymetric features) and are therefore relevant to the understanding and prediction of shoreline change on many coasts.
H33L-02
Determining littoral sediment transport paths adjacent to an eroding carbonate beach through net sediment grain-size trend analysis: Lanikai Beach, Hawaii.
Identifying long-term trends of sediment transport in coastal environments is a fundamental goal shared by coastal scientists, engineers, and resource managers. Historical photographic analysis and predictive computer models have served as the primary approaches to charactering long-term trends in sediment flux. Net sediment grain-size trend analysis is an empirical, sedimentologically based technique that uses physical sediment samples to identify long-term sediment transport pathways. Originally developed by McLaren and Bowles (1985), net sediment grain-size trend analysis identifies progressive trends in grain-size parameters (mean size, sorting, and skewness) in sediment samples. Ultimately, the results give an indication of long-shore sediment transport, a visualization of individual littoral cells, and a better understanding of sediment processes in the near- shore region. We applied two methodologies put forth by Gao and Collins (1992) and Roux (1994) to 214 samples collected off Lanikai Beach, Hawaii; an excellent example of a coastal environment with chronic beach erosion. The Gao methodology searches point-to-point search for the two trend types used by McLaren. The Roux methodology simultaneously searches between five adjacent points for four trend types. Despite significant differences, similar trends dominate in both sets of results. The Gao methodology produces generalized trends while the Roux methodology shows finer details of sediment transport. Long-shore transport direction is shown to be northward for the majority of the study area, implying a sediment supply to the south. Therefore erosion is instigated if the sediment supply south of Lanikai Beach is cut off. A strong onshore sediment transport trend fails to accrete a beach in an armored section of the southern Lanikai coastline, demonstrating the erosive effect of increased wave refraction from coastal armoring. Results of the sediment trend analyses agree well with tidal current models produced in Delph3D and directional current velocity data for the area.
H33L-03
Video and GPS measurements of morphology and swash flow in beach mega- cusps
The alongshore locations of beach mega-cusps appear to be directly associated with the alongshore positions of surf zone rip channels (Thornton et al., Mar. Geol. 240, 2007). Previous lab and field measurements have detected mean alongshore "counter-currents" that are shoreward of rip channel feeder currents but flow in the opposite direction (Haas et al., JGR 108-C7, 2003; MacMahan et al., draft, 2007). These counter-currents may contribute to the development and persistence of mega-cusps on a beach with rip channel bathymetry. The present analysis uses both video and GPS methods to map the flow patterns and morphology in the region shoreward of several large rip channels in southern Monterey Bay, California. High frequency rectified color images and B/W video pixel time series of the swash and surf zone are analyzed with particle imaging velocimetry (PIV) methods to indirectly measure swash flow in the cusps. The accuracy of the video-based estimates is tested against in-situ GPS measurements. The PIV-predicted surface flow field is compared with velocity vectors generated by concurrently deployed, GPS-equipped mini-drifters. Shoreline mapping techniques are applied to time-averaged video images to estimate the corresponding intertidal morphology and compared with in-situ measurements made using a GPS-equipped ATV and personal watercraft. Preliminary results indicate that nearshore counter-currents are intermittently present shoreward of rip channels and suggest that they act together with the alongshore current to influence the evolution and migration of mega-cusps.
H33L-04
Mutli-Temporal Analysis of Beach Morphology on Fire Island, NY and the Impacts of Human Alterations Within the System
Fire Island is a barrier island that lies along the south shore of Long Island, New York. Fire Island National Seashore comprises the majority of the island and a number of private communities are located within the boundary of the Seashore. The beach-front houses within the communities are generally built on or just behind the primary dune. A series of severe storms in the early 1990s resulted in widespread erosion and prompted many communities to begin a program to create protective, artificial dunes by bulldozing sand from the berm to the back beach area: a practice known as beach scraping. The National Park Service grants the permits for scraping, and there is some concern as to whether morphologic alterations to the beach and dune from scraping may be permanently impacting the natural resources in the park, both in the scraped and adjacent non-scraped and undeveloped areas. A study is currently underway to characterize the geomorphology of the beach/dune system on Fire Island and is presently focused on assessing beach and dune change at storm-event, seasonal, and decadal time scales, based on both field data and existing lidar, beach profile and shoreline data. Semi-annual topographic beach surveys are being conducted to map seasonal variations. These data, along with lidar, are being used to calculate seasonal and storm volumetric beach changes as well as assess shoreline change. The volumetric change analysis from 1998 to 2007 indicates that the beach and dune are in a dominantly erosional state. However, shoreline change analyses over both the long-term (30-year) and short-term (seasonal and storm) indicate a relatively uniform pattern of erosional and accretional cells that range from approximately 0.8 to 1.0 km in length. These correspond to alongshore undulations in beach width that are clearly visible in aerial photography. The long-term and seasonal patterns of shoreline change suggest that the cells occur as pulses of sediment that rapidly move in a longshore, westward direction. Previous shoreline change studies, conducted, at a broader spatial scale, show larger cells ranging from 8 to 10 km in length. These larger undulations appear to remain somewhat stationary over decadal timescales, and thus result in chronic erosion hotspots which lead to increased requests for permits to alter the beach. The reduction in the elevation of the beach from scraping, and the relocation of material from the berm to the foredune, where it is less likely to be mobilized during seasonal events, exacerbates the erosion, and slows the pulses of material that naturally move alongshore.
H33L-05
Morphodynamic Estuarine Evolution Using a 2D Numerical, Process-based Approach
Introduction. The morphodynamics of estuaries is complex and subject to an interactive system of different spatial scales and time scales. Although some sort of equilibrium can be observed over decades (ie the location of intertidal flats), longer term processes might cause ongoing evolution. Focus of extensive modeling research has been on specific aspects like the characteristics of tidal hydrodynamics within embayments and their assumed impact on the morphology, 1D morphodynamic models and 2D morphodynamic models focusing on stability analysis and initial pattern formation. Aim The current research aims to investigate long-term estuarine evolution characteristics, including effects of pattern formation on the development of the longitudinal profile in a rectangular embayment. Special emphasis is put on evaluating the model results in terms of energy dissipation and validating the results against empirical relationships. Methodology Use is made of a 2D process-based numerical model (Delft3D), which is based on a relatively simple formulation using the shallow water equations, the Engelund-Hansen sediment transport formulation and bed slope effects. Use is made of an advanced morphological update scheme that allows for bed level updates every time step. Wetting and drying of intertidal flats is allowed by a water level criterion. This means that cells are extracted from hydrodynamic and morphodynamic calculations when the water level becomes lower than a certain threshold value (0.1m) and are re-activated when the water level in the surrounding cells exceeds this value again. A special procedure allows for bank erosion and erosion of (dry) intertidal flats. This is done by assigning the calculated erosion in a wet cell to the adjacent dry cell. Model configuration The focus of the research is on different model configurations, ie, a 2.5 km wide basin of respectively 20 km, 80 km and 400 km long. The model domain is extended seaward 20 km so that the seaward boundary is not disturbed by morphodynamic processes and a harmonic water level boundary is prescribed with an amplitude of 1.75 m and a period of 12 hours. The landward boundary and the banks of the embayment are defined by a zero flux (no water flow, no sediment transport) boundary. The sediment is uniform of size (0.24 mm). Grid size is uniform over the domain with a size of 100 by 200 m. The computational time step is 2 minutes and the morphological acceleration factor is 400. Results Model results show the formation of a channel-shoal system within the first 100 years. The pattern becomes less dynamic on longer timescales of millennia. Empirical equilibrium relations (like between the tidal prism and cross sectional area) are fulfilled and the basin wide energy dissipation decreases at decreasingly lower rate.
H33L-06
Stable states and catastrophic shifts in tidal eco-morphodynamics
Changes in relative sea level, nutrient and sediment loading, and ecological characteristics expose tidal landforms and ecosystems to responses which may or may not be reversible. On this basis alone predicting the response of tidal geomorphology is important in view of the ecological, cultural and socio-economic importance of endangered tidal environments worldwide. Here we present a point model of the joint evolution of tidal landforms and biota including the dynamics of intertidal vegetation, benthic microbial assemblages, erosional and depositional processes, local and general hydrodynamics, and relative sea-level change. Alternative stable states and punctuated-equilibrium dynamics emerge, characterized by possible sudden transitions of the system, governed by marine transgressions or regressions, vegetation type, disturbances of the benthic biofilm and sediment availability. Multiple equilibria are the result of the interplay of erosion, deposition and biostabilization, highlighting the importance of the coupling between biological and sediment transport processes in determining the evolution of a tidal system as a whole. Hysteretic switches between stable states may arise because of differences in the threshold values of relative sea level rise inducing transitions from vegetated to unvegetated equilibria and viceversa. A similar hysteretic mechanism is also found to link successive periods of increased and decreased sediment availability.
H33L-07
Simplified Marsh Platform Flow Model of Flood and Ebb Informed By Platform Length Scale and Biomass
Tidal salt marsh ecosystems are maintained by macrophytes that regulate platform elevation by inorganic and organic sedimentation, as well as modify the flow within the canopy. In an effort to understand and simplify the ecological and physical interactions on tidal salt marsh platforms, here we explore a two-dimensional hydrodynamic field that incorporates velocity solved as a function of the drag coefficient and Reynolds number. The flow model, based on depth-integrated conservation of mass and momentum equations, is governed by the assumption of a balance between the drag force generated by macrophyte stems and the gravitational force due to the slope of the water surface. The form drag exerted by a single infinite cylinder is parameterized through a drag coefficient at transitional Reynolds numbers. At large scales the flood and ebb can be described using diffusion-like equations; however, at smaller scales a bathtub approximation is more appropriate. Here we quantify the lengths scales and fractional volume of the flow domain occupied by plants appropriate for a bathtub- like flooding description. Because the rate of platform flow on short length scales is a function of biomass, flooding translates to frequency and duration of inundation with elevation. The resulting simplified marsh platform flow and mapping of mean high tide can be used directly with a zero-dimensional model for biomass production as a function of elevation.
H33L-08
Vegetation, sea level rise, and the morphological stability of wetland landscapes.
Tidal wetland landscapes -extensive, vegetated marshes and the channel networks that wind through them- provide a striking example of a geomorphological system in which the physical and biological processes cannot be addressed separately. Vegetated marsh platforms and intertwining channel networks are thoroughly coupled; the dynamic marsh elevation relative to high-tide level determines the volume of tidal flow through the channels, and channels distribute the water and sediment that facilitates plant growth and therefore marsh accretion. We have developed a numerical model that couples biologically influenced platform accretion with hydrodynamically driven channel network erosion, and have used it to simulate marsh system response to sea-level rise. In the model, increased inundation on the platform stimulates biomass productivity and therefore tends to increase: sediment deposition rates, resistance to channel bank erosion, and the elevation of vegetated surfaces relative to sea level. Establishment of vegetation plays a critical role in developing, and maintaining, a horizontal platform incised by a well defined channel network. In model simulations with intact vegetation, a 10-fold increase in the rate of sea-level rise results in a stable platform, and a channel network morphology that is undetermined by platform elevation and the volume of water flowing through the channels. Temporary disturbance to vegetation, however, leads to rapid and widespread erosion of the channel network. In some model experiments, an increase in the intensity of disturbance converts broad and expansive marshland into highly dissected platforms characterized by numerous marsh islands and interior ponds. In this case, temporary disturbance in one part of the system (i.e. the vegetated platform) leads to a change in physical conditions in another part of the system (i.e. channel edges), and an irreversible loss of vegetation. Vegetated portions of an episodically disturbed platform accrete more rapidly than rates of relative sea level rise, giving submerging marshland the appearance of maintaining elevation relative to sea-level.