H34C-01 16:00h
Autogenic Shoreline Responses to Fluvial Change
We present experimental shoreline-migration data that show high-frequency autogenic variability superimposed on low-frequency allogenic shoreline responses. The experiment, which was performed in the Experimental EarthScape (XES) facility at St. Anthony Falls Laboratory, used (1) a linear-hinge type subsidence profile for which the subsidence rate was kept constant in time, (2) a constant overall sediment supply, and (3) base-level variation on two time scales that were first applied separately and then superimposed. Autogenic signals in shoreline trajectory imprinted on the allogenic signatures are generally thought of as local "noise". However, the variability in the experimental shoreline data persists even when the shoreline migration is averaged laterally. The autogenic signal in the shoreline migration rate (i.e., a high-frequency variability of the rate) is strongest during relative base-level rise and weakest during relative base-level fall. Base-level change, which is the only externally imposed time-variable parameter in the experiment, can work either with or against the sediment transport regime and thus can magnify or diminish autogenic processes. We use a 1-D geometric model to model the autogenic signals present in the experiment. With the model, we investigate whether periodic changes in the fluvial slope could explain the effects of storage and release of supplied sediment. By storing and releasing sediment, these slope changes can cause strong pulses of sediment discharge to pass through the shoreline position, causing high-frequency change in the shoreline position against a background of relatively long-term shoreline migration. A series of test results, using different magnitudes of the slope change, different frequencies of sediment-release events, and different types of release events, suggests that the storage and release model can explain much of the shoreline response.
http://www.geo.umn.edu/orgs/seds/
H34C-02 16:15h
Modeling Shoreline Erosion and Channel Migration in a Highly Dynamic Bay Entrance: Willapa Bay, WA
Shoreline erosion rates of up to 50 m/yr are well documented for the northern shoreline of Willapa Bay, a large estuary on the Washington coast. Despite this knowledge, there is not a good understanding of the large-scale morphodynamics of the bay entrance. Historical shoreline change calculated from T-sheets dating back to 1871 and aerial photographs from 1942 to 2002 show a decreasing trend in erosion rates. Historical bathymetric maps document migration of the 25-m deep main channel that parallels the shoreline erosion history during that same time period. To better understand the processes responsible for these decreasing channel migration and shoreline erosion rates and to help predict future erosion we developed a process-based morphological model of the bay and adjacent coast. A process-based numerical model of flow, waves, sediment transport, and bed-level updating (Delft3D) is used to understand and predict the morphological response of the estuary entrance to tidal and storm forcing. In order to conduct long-term morphological simulations (up to 5 yrs long), we employed a number of input reduction techniques, including calculating a representative tide, discretizing the wave climate into 19 classes, and employing a variable morphological factor to scale-up from a processes-based time scale to the morphological response. Model validation with field data occurs at two time scales. Model results are compared with measurements of water levels, currents, waves, and suspended-sediment concentration collected over 3 months from November 2002 to January 2003. In addition, net erosion and sedimentation from multi-year hindcasts are compared with measured bathymetric changes. Numerical simulations suggest a net export of sediment through the deep channel and a net import across the shallow entrance shoal. Wave breaking over the shoal induces a net circulation that drives this residual sediment transport. Residual transport patterns are also strongly controlled by the initial bathymetry. During the 1940s northeasterly net sediment transport across the entrance shoal forces the northward migration of the main channel and a high rate of shoreline erosion. By the 1990s the ebb delta is elongated to the north and the net sediment transport is more northwesterly, reducing the transport of sediment into the main channel and easing the northward migration of the channel.
H34C-03 16:30h
Coastal System Interactions on Shackleford Banks, NC
An in-depth study of the beach-dune-nearshore system of Shackleford Banks, an undeveloped North Carolina barrier island located in the southern Outer Banks, has yielded insights about some of the complex interrelationships between these components. Although Shackleford is a small (~15 km long) island, it has a wide variety of beach-dune morphologies that respond differently to both `normal' conditions and storm events. Detailed monthly RTK-GPS surveys and several years of LIDAR data provide information about morphology and changes in the beach, dunes, and overwash. In general, the character of the island changes from a wider, taller, more stable western half to a narrow, overwash-dominated eastern half subject to frequent change. Sections with a continuous row of foredunes tend to experience repeated scarping of the beach and/or dunes. The thinnest portion of the island is also one of the least stable, and appears to be the location of a reversal in the dominant direction of longshore sediment transport. Sidescan and CHIRP data indicate that the nearshore of the eastern half of the island is reworked and sediment-starved, while the western half has numerous channel fill deposits and greater sediment availability. The outcropping of relict peat deposits (~300-900 years BP) and abundant fossil shell material provide further evidence of a lack of modern sediment cover in eastern sections. The far ends of the island differ from adjacent stretches of beach, with much of the western end experiencing rapid erosion, most likely due to inlet dynamics, while the eastern end of the island is accreting. The sheltering effect of Core Banks, Cape Lookout and Power Squadron Spit create typically calm conditions in this area, allowing sediment to accumulate. This is in contrast to adjacent areas, which experience greater wave energy and receive less sediment. The semi-annual simultaneous deployment of three electromagnetic current meters at western, central and eastern locations show that current velocities and wave heights and direction differ at the three areas, in spite of their proximity and shared wind climate.
H34C-04 16:45h
Swaying and Shaking of Sea Cliffs: Fatigue and Failure Mechanisms on Rocky Coasts
Wave energy is imparted to sea cliffs in discrete, concentrated bundles every 4-25 seconds. While the resulting episodic sea cliff failure occurs when internal strength of sea cliff material is exceeded by forces associated with waves, the detailed mechanics of sea cliff failure processes are poorly understood. Failure events are difficult to predict and challenging to instrument. Ground motions associated with nearshore wave energy, microseisms, directly measure the response of sea cliffs to assailing waves. Through several microseismic field deployments from 2000 - 2003, we documented both the high-frequency shaking of sea cliffs, an instantaneous ringing response to the strike of waves against the sea cliff face, and a low-frequency (10 to 20 sec period) swaying of sea cliffs. This latter signal we interpret to reflect ground motion associated with water from the broken wave bore loading the wave cut platform that fronts the sea cliff face. We integrate microseismic velocities to obtain continuous swaying displacements of 50 micron and 10 micron amplitudes in horizontal and vertical directions, respectively. Shaking displacements, although discontinuous and less frequent, can be significantly greater ($>$100 microns) than swaying displacements. Displacement ellipsoids reveal simultaneous downward and seaward sea cliff motion with each incoming broken wave bore; time-registered video footage corroborates the downward sea cliff flex in response to the imposed water load on the wavecut platform. Gradients in displacement amplitudes in a direction orthogonal to the shoreline, documented using three seismometer locations, suggest longitudinal strain of the flexing sea cliff ranging up to 2.5 microstrain and shear strain ranging up to 3.5 microstrain during each wave loading cycle. Assuming an average wave period of 10 seconds, sea cliff flexure occurs approximately 3 million times annually. This continuous cyclical loading, and the associated incomplete recovery, may fatigue the sea cliff rock through development and lengthening of micro-cracks. Local sea cliff retreat rates of 10 cm/yr imply a given parcel of rock is flexed through roughly one billion cycles of increasing strain before exposure to direct wave attack at the cliff face. We suggest that this history of strain will make this material more susceptible to erosion upon reaching the cliff face, where high frequency shaking from direct wave impacts does the work.
H34C-05 17:00h
Nearshore Sediment Budget: Correlating Volume to Shoreline Change, Outer Banks, North Carolina
Though the importance of understanding the exchange of sediment between the shoreline and the nearshore has long been recognized, data capable of addressing the three-dimensional character of the system are lacking. The response of traditional cross-shore profiles to forcing is not representative of the entire beach because of alongshore variability of the beach and nearshore sandbar. Measuring changes in shoreline position along the coast permits analysis of alongshore shoreline variability, but does not account for the actual volume of sediment lost from and restored to the beach. Temporally and spatially variable erosional hotspots identified by List and Farris (1999) along the Outer Banks, North Carolina further confound the study of coastal sediment exchange in the region of interest. A geophysical survey (from Duck to Oregon Inlet) was carried out in order to understand the role of geological characteristics in the transport of sediment between the shoreline and nearshore. Contrary to the assumptions of many shoreline and shoreface change models, the modern sand layer observed over the 40-km study area is not infinitely thick and is highly variable. Modern sediment thicknesses were calculated to a continuous seismic reflection surface that spanned the survey area, the average being 0.42m $\pm$ 0.19m. Higher standard deviations (variability) in sediment thickness seem to be loosely related to the presence of shore-oblique sandbars as described by McNinch (in press). Nearshore sediment volume was calculated and relationships to nearshore morphology were explored. Though shore-oblique bars may not represent a large percentage of the total sand regionally (7.9%), locally their influence is much greater. Of the three areas in which bars were identified, the volume of sediment contained within the bars represented 44% of the total volume in the largest bar field, and 14% and 11%, respectively, in more minor bar fields. Nearshore sediment volumes correlated well (correlation coefficient $\sim$0.60) with long-term shoreline change data (50 year data set, NC Division of Coastal Management) suggesting that long-term trends in shoreline change may be related to the total amount of available sediment in the nearshore. Correlation analyses with short-term shoreline change data will also be explored. These preliminary data suggest that volume calculations considering the total amount of nearshore sediment above a continuous, non-sandy seismic reflection surface may be useful in the prediction of long-term shoreline change trends. The utility of this approach in predicting changes over shorter temporal and spatial scales is the subject of current research.
H34C-06 17:15h
Correlation of Shore-Oblique Bars and Nearshore Gravel Outcrops With Shoreline Change in North Carolina
The interaction between the surf zone and the subaerial shoreface is important for data interpretation, holistic understanding of coastal zone dynamics, and the development of predictive engineering models. Previous studies have noted the existence of beach-surf zone interactions, but in general, relationships between seafloor features and coastal change are poorly quantified. This study demonstrates the physical concurrence of shore-oblique bars and gravel outcrops observed in the surf zone along the northern Outer Banks of North Carolina. It then details, both qualitatively and quantitatively, the relationships between these subaqueous features and shoreline change on a range of temporal and spatial scales. The surf zone and nearshore region of the Outer Banks are predominantly planar and sandy, but there are several discrete regions with shore-oblique bars and interspersed gravel outcrops. These bar fields are several kilometers wide alongshore and were relatively stationary over a 1.5-year survey period. Cross-covariance analysis quantifies the spatial relationships between the locations of offshore features including gravel outcrops, shore-oblique bars, and the subaerial beach response, represented by shoreline change calculated from 100-year, 28-year, and monthly datasets. Results demonstrate that areas with shore-oblique bars and gravel outcrops are strongly correlated with areas of both high short-term shoreline variability and high long-term shoreline change trends. High shoreline erosion occurs onshore of shore-oblique bar fields, but not all of the shore-oblique bars are associated with high long-term shoreline change. On a regional scale, bathymetry (as measured by the distance between the shoreline and the 9 m contour) is correlated most highly with shoreline variability at 220 m lag and with long-term change at a lag of 280 m, a very small distance considering the width of the bars (hundreds of meters) and their oblique angles to shore. Presented results will demonstrate high correlation between shoreline change on several scales and the adjacent surf-zone bathymetry and sediment distribution.
H34C-07 17:30h
A Combined Wind Wave-Tidal Model for the Venice Lagoon, Italy
A numerical model that combines wind waves with tidal fluxes in a tidal basin is presented and validated. The model couples a hydrodynamic finite element module based on the shallow water equations with a finite volume module that accounts for the generation and propagation of wind waves. The wave module solves the wave action conservation on the same triangular mesh used in the hydrodynamic module, thus correctly reproducing the physical relationships between waves and tide propagation. The combined wind wave-tidal model is applied to the Venice lagoon, Italy. The highly irregular bathymetry of this intertidal environment, characterized by deep channels, emergent salt marshes, and extensive tidal flats, suggests the introduction of ad hoc hypotheses that simplify the governing equations with a noteworthy increase in efficiency and robustness of the algorithm. Particular attention is devoted to the dissipation of wave energy at the steep boundaries between channels, tidal flats, and salt marshes. Simulations of wave fields generated under specific wind conditions are presented and discussed. The model results are compared, with good agreement, to field data collected in two different stations inside the lagoon of Venice. Finally, evidence of the complementary effect of tidal currents and wind waves on sediment resuspension is presented using the results of different simulations.
H34C-08 17:45h
Long term net exchange of sand through Venice inlets
Venice lagoon has undergone through the centuries significant morphological changes due to several natural events as well as antropic actions: the diversions of rivers discharging into the lagoon, the construction of long jetties bounding the inlets, sea level rise and subsidence. As a result, the lagoon has progressively deepened and it is claimed nowadays that a loss of roughly one million cubic meters of sediments is experienced by the lagoon each year! Relatively sophisticated models of the inlet hydrodynamics and morphodynamics may be used in order to estimate the net exchange of sediments between lagoon and sea, however they are still computationally demanding to allow for "long term" predictions. In the present work, we formulate a simple model of the inlet hydrodynamics which allows us to estimate the net exchange of sediments associated with the sequence of tidal events recorded for several years. The exercise proves instructive. Firstly, it turns out that the present configuration of each of the inlets is far from the static-equilibrium limit, defined as the condition such that the maximum speed is equal to the critical speed for sediment motion. Secondly, we obtain an estimate of an upper bound for the loss of sediments experienced by each inlet through the years assuming that the amount of sediments carried by both flood and ebb currents is determined by their transport capacity: results suggest that the yearly loss of sand experienced by Venice Lagoon is an order of magnitude smaller than usually claimed. Thirdly, we formulate a simple framework to account for the effect of the sediment overload induced by resuspension due to breaking waves during storm events in the far field: assuming the flood current to be modelled as plane and irrotational, we are able to determine the flow pattern driven by the tidal oscillations and littoral currents in a neighbourhood of the lagoon inlets. Having determined the flow field, the convection-diffusion equation for sediment concentration has been solved numerically and allows us to explain the tendency to inlet siltation associated with sediment resuspension during storm events. Finally, the implications of the above results for the interpretation of the recent morphodynamic evolution of Venice lagoon are discussed.