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Nearshore and Inner Shelf Controls on Regional-Scale Coastal Erosion


Project Title: Nearshore and Inner Shelf Controls on Regional-Scale Coastal Erosion
Mendenhall Fellow: David Stolper, (508) 457 2356, dstolper@usgs.gov
Duty Station: Woods Hole, Massachusetts
Start Date: June 9, 2002
Education: Ph.D. (2002), Marine Science, University of Sydney
Research Advisors: Jeff List, (508) 457 2343, jlist@usgs.gov; Rob Thieler, (508) 457 2350, rthieler@usgs.gov

Project Summary: The main goal of my Mendenhall project was to develop a new morphological-behaviour model capable of simulating large-scale evolution of coastal morphology. The necessity of the USGS to develop a capability for predicting long-term rates of shoreline change coastal erosion was made clear at the USGS sea-level rise workshop (2002) attended in the third month of my Fellowship period. Here it was discussed how global increases in sea-level rise are likely to exacerbate the chronic coastal erosion already occurring on much of the United States coastline. It was at this workshop that I presented the initial model parameterisation and received feedback from scientists throughout the agency. The bulk of my research time during the fellowship period was spent programming the model and then applying it to contrasting sites on the Atlantic and Pacific coasts of the USA.

“GEOMBEST” (Geomorphic Model of Barrier, Estuarine, and Shoreface Translations), encapsulates qualitative principles drawn from established geological concepts that are parameterized to provide quantitative predictions of morphological change on geological time scales (order 103 years), as well as shorter time-scales applicable for long-term coastal management (order 101 to 102 years). Changes in sea level, and sediment volume within the shoreface, barrier and estuary, drive model behaviour. Further parameters, defining substate erodibility, sediment composition and time-dependent shoreface response, constrain the evolution of the shoreface towards an equilibrium profile

GEOMBEST was developed to improve understanding of large-scale coastal evolution and stratigraphic package formation and also to provide quantitative prediction of shoreline change at timescales applicable to coastal management (several decades to centuries). The use of a stratigraphically-calibrated numerical model to predict future shoreline change is considered valid since the stratigraphy records the result of sediment transport over time-periods long enough to distinguish mean depositional trends from fluctuations associated with storms and other unpredictable occurrences.

Simulation results confirmed that GEOMBEST is able to recreate coastal stratigraphy in the low-gradient autochthonous setting of North Carolina and the steep allochthonous setting of the Washington shelf. These applications both show how GEOMBEST can be used as a quantitative tool to aid understanding of coastal evolution at geological timescales. The use of the model forces a synthesis of data and quantification of all assumptions involved in reconstructing the geological history. This approach provides a more rigorous link between the data and knowledge than a qualitative reconstruction provides. GEOMBEST also adds value to the data by reconstructing aspects of coastal evolution that are not obvious from the data alone. For example, antecedent shoreline positions and morphological states can be estimated in cases where direct evidence of these aspects has been removed by shoreface transgression.

Two GEOMBEST simulations reconstructed possible late-Holocene evolution of the Currituck region of North Carolina's Outer Banks (Figure 1). These simulations, involving a positive sediment budget and a neutral sediment budget, were both able to reproduce the modern coastal morphology (Figures 2 and3). Each simulation, however, created distinct stratigraphic results characteristic of the different possible evolutionary histories. Further data collection, aimed at determining the stratigraphy beneath the subaerial barrier in this location is required to determine which scenario, and therefore evolutionary history is more likely.

Location 
        map of the North Carolina study site.   Figure 1. Location map of the North Carolina study site.

Simulation reproducing the evolution of the Currituck tract during the last 8000 years. The modern morphology was reproduced with the assumption of a balanced sediment budget and a rate of estuarine accretion set at 69% of the rate of sea level rise.
Figure 2. Simulation reproducing the evolution of the Currituck tract during the last 8000 years. The modern morphology was reproduced with the assumption of a balanced sediment budget and a rate of estuarine accretion set at 69% of the rate of sea level rise.
Simulation of the Currituck tract during the last 4500 years. The modern 
        morphology was recreated by adding sediment to the barrier at a constant 
        rate, while setting the rate of estuarine accretion at 69% of the rate 
        of sea-level rise sea level rise.
Figure 3. Simulation of the Currituck tract during the last 4500 years. The modern morphology was recreated by adding sediment to the barrier at a constant rate, while setting the rate of estuarine accretion at 69% of the rate of sea-level rise sea level rise.

The Washington simulations examined a section of the Columbia River Littoral Cell (CRLC) in southern Washington, USA (Figure 4). This region of the Pacific Northwest coast differs from North Carolina, and most of the US coast, by having a shelf that has accumulated a huge volume of sediment during the Holocene sea-level rise. The simulations confirm that GEOMBEST is able to reproduce complex coastal stratigraphy involving regressive and transgressive phases of coastal evolution (Figures 4 and 5). Results demonstrate that three phases of sedimentation were required to reproduce the Washington stratigraphy. The first phase (12900 - 12400 bp) involving the rapid addition of sediment attributed to the Missoula Floods, resulted in shoreface regression during a period of rapid sea-level rise. This was followed by a period of rapid shoreface transgression when little or no sediment was added to the system (12400 - 9100 bp) and a final period when sediment was added at a moderate rate (9100 bp - present). The reconstruction presented in Figure 7 is the simplest given the available data and the assumption of constant sedimentation rates for each period. The simulation was able to reproduce the geometry of the stratigraphic packages, the orientation of the seismic reflectors and the late-Holocene shoreline progradation.

Location map showing the Modeled cross section of the Washington coast.   Figure 4. Location map showing the Modeled cross section of the Washington coast.

Data 
        model of the Long Beach section morphology and stratigraphy.   Figure 5. Data model of the Long Beach section morphology and stratigraphy.

Part of the Long Beach seismic section showing the clinoform 
        package with highlighted reflectors.   Figure 6. Part of the Long Beach seismic section showing the clinoform package with highlighted reflectors.

Successful reconstruction of the Long Beach Tract. 
        Time lines are every 50 years for the first 500 years of the simulation 
        period to and every 500 years thereafter.   Figure 7. Successful reconstruction of the Long Beach Tract. Time lines are every 50 years for the first 500 years of the simulation period to and every 500 years thereafter.

Comparative results from Washington and North Carolina show how substrate slope controls coastal evolution in settings with non-erodible substrates. It is widely recognized that substrate slope affects the rate of barrier transgression, with gentler slopes associated with higher transgression rates (Pilkey and Davis, 1987). Results from my research suggest that as barrier islands translate over a steepening lithified substrate the estuary backing them will narrow, then the backbarrier will narrow and finally the subaerial barrier will cease to exist and a subaqeous deposit is formed. This evolutionary sequence can be retarded, however through the addition of sediment volume to the barrier

Estuarine infill reduces barrier transgression rates by filling the accommodation space behind the barrier (Cowell et al., 1995). GEOMBEST simulation results demonstrate that the potential for estuarine infilling to retard shoreface transgression depends on the slope of the underlying substrate. Settings with gentle slopes, such as North Carolina, are associated with wide estuaries, in which a large volume of estuarine sediment can potentially accumulate. This makes barrier transgression rates in low-gradient settings very sensitive to variations in the rate of estuarine infill. Steep settings, in contrast, are associated with narrow or non-existent estuaries offering limited potential for estuarine infilling and therefore a limited potential for estuarine infilling to affect the rate of barrier transgression. Simulations in North Carolina also demonstrate how substrate composition affects the rate of barrier transgression. Transgression over an erodible substrate provide a potential source of sediment to the barrier island, which slows the rate of transgression relative to transgression over a lithified substrate.

References Cited

Cowell, P.J., Roy, P.S., Jones, R.A., 1995. Simulation of large-scale coastal change using a morphological-behaviour model. Marine Geology, 126, 45-61.

Pilkey, O. H., Davis, T. W., 1987, An analysis of coastal recession models: North Carolina coast in Nummedal, D., Pilkey, O.H., Howard, J.D., eds., Sea-level Fluctuation and Coastal Evolution: SEPM (Society for Sedimentary Geology) Special Publication No. 41, Tulsa, Oklahoma, p. 59-68.


Original Project Description: Coastal management and engineering requires identification of low-order (large-scale) coastal change to determine whether shoreline and seabed movements involve systematic trends. Such trends may cause chronic problems that require long-term planning and major engineering interventions. Morphological change entailing temporary fluctuations may cause acute problems but these generally require only local remedial measures. Coastal management strategies are very different for the two types of problems and usually involve different levels of expense. If estimates of the long-term change cannot be quantified then it seems unlikely that predictions will discriminate adequately between chronic and acute coastal change.

Dave's research focuses on the development of a three-dimensional morphological-behaviour model to simulate the evolution of coastal sand bodies to variation in sea level, longshore sediment transport and estuarine deposition. The model will initially be used to simulate the evolution of the barrier coastline from Cape Henry to Cape Hatteras (North Carolina). Existing geological data will be integrated with the model to determine mean trends of barrier evolution during the post-glacial marine transgression and to predict shoreline change resulting from anticipated rates of sea-level rise.

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Last modified: 16:08:33 Thu 13 Dec 2012