Alan W. Niedoroda

MODELING SHORE-NORMAL LARGE-SCALE COASTAL EVOLUTION

Abstract We present a model for the evolution of the shelf surface in response to marine sedimentary processes. Following morphodynamical theory, the model presupposes a characteristic configuration of the shelf surface, that in profile is a concave-up exponential curve, whose steeper inner limb is the shoreface. The profile is seen as an equilibrium response to the variables of sedimentation. The profile translates landward or seaward as sea level rises or falls, but will do so in an state of dynamic equilibrium with the shape of the profile varying according to changes in: (1) the rate of sea level change, (2) the time-averaged wave and bottom current conditions, (3) the average allochthonous sediment supply rate, and (4) the sediment grain size distribution. The governing equations for the model include an equation for time-averaged cross-shore (diabathic) sediment flux and the sediment continuity equation. p ]Simulations of continental margin profiles show that profile adjustments affect mainly the coefficient of curvature of the profile. An increase in the rate of sea level rise straightens the profile; it decreases the slope of the shoreface, but increases the shelf slope. An increase in sediment input increases profile curvature; the shoreface steepens while the shelf floor flattens. An intensification of hydraulic climate straightens the profile in a manner similar to an increase in the sea level rise rate, while an increase in grain size increases profile curvature as does an increase in sediment input.

Modeling sediment entrainment and transport processes limited by bed armoring

Abstract A sediment-transport model including a bed-armoring algorithm is used to ascertain the potential importance of bed-armoring processes. The parameters that control the bed-armoring effects on the suspension of granular sediments in the marine environment are examined using the numerical model and data from the Eel shelf. Results for the shelf indicate that bed armoring controls the total suspended-sediment load and affects the across-shelf gradients of suspended material. The armoring effects are too significant to be neglected in most analyses of shelf and nearshore sediment transport, and this is especially relevant for models that are developed to represent large-scale and long-term sedimentary regimes. A principal controlling parameter is the thickness of the active layer, and the dynamics of this layer are not well known.

Contaminant dispersal on the Palos Verdes continental margin: III. Processes controlling transport, accumulation and re-emergence of DDT-contaminated sediment particles

Abstract A large mass of DDT-contaminated sediment lies buried beneath a thin cover on the Palos Verdes shelf and slope off Los Angeles, California. Analyses, including several types of numerical simulation, have been applied to an extensive data set to evaluate the biological and physical processes controlling the fate of the strongly particle-reactive pollutants. Sequential measurements of the p,p′ -DDE (an isomer of DDT) content in cores from monitoring locations indicate that contaminated sediment particles are being re-introduced into the marine environment from the buried historic deposits at a significant rate. There is considerable scatter in the data but the trends for ongoing release over the 8 years between 1983 and 1991 are statistically significant at the 90% level. A comprehensive mathematical model of shelf sediment dynamics (Resuspension Model) has been used to explore sediment erosion and deposition during major storms of the early 1980s and of 1988. This modeling shows that storms alone cannot explain the observed losses of contaminated sediment particles from the historical deposits of the Palos Verdes shelf. On the other hand, it also demonstrates that both severe and common storm events do re-entrain some of the bottom sediment at all water depths across the Palos Verdes shelf. A numerical model (Contaminant Release Model), which couples upward biodiffusion of DDT with storm removal, satisfactorily explains the observed losses. The infaunal activity that creates biodiffusion has been explored in two separate ways. In calibrating the Contaminant Release Model, the depth-dependent biodiffusion coefficient profile is estimated by comparing the predicted and measured depth concentration distributions. A separate analysis has been conducted using the measured vertical distribution of infaunal species (Stull et al. 1995; Swift et al., 1995). The agreement between these two methods is good. Both show that bioturbation extends to the level of the high contamination. Bioturbation is now playing a significant role in redistributing the buried historic contaminants. The Contaminant Release Model, in combination with evaluations of the total sedimentation rate and the natural background sedimentation rate, clearly shows that the flux of solids from the outfall enhances the rate of accumulation of natural sediment on the Palos Verdes shelf. A sharp reduction of the flux of solids, such as would accompany conversion from partial to full secondary treatment, will result in considerably prolonging the high rates with which DDT and other particle-reactive contaminants are re-entering the marine environment.

Contaminant dispersal on the Palos Verdes continental margin II. Estimates of the biodiffusion coefficient, DB, from composition of the benthic infaunal community

Sea bed mixing has generally been modeled as a one-dimensional, vertically diffusive process in which a biodiffusion coefficient is estimated by fitting regression lines to vertical profiles of tracer concentrations, typically radioisotopes. In this paper, we describe an alternative approach to deriving time- and space-dependent biodiffusion coefficients in the sea bed, in which the coefficient is estimated directly from the composition and distribution of the benthic infaunal community. We have used a random walk model to describe diffusion driven by benthic organisms. The model is applied to an infaunal population sampled in 13 cores from the 60-m isobath, each sliced into 10 segments. The biodiffusion coefficient, DB, is evaluated for a species by means of this model as the product of characteristic length and velocity scales, L2T−1. The mixing frequency, T−1, is the product of the estimated velocity of sediment displacement and mean cross-sectional area, multiplied by the number of individuals in the core segment, and divided by the volume of the segment. The areal coefficient, L2, is equal to the mean cross-sectional area of the species. The aggregate DB curve is the sum of the species curves. Much of the bioturbation is due to the behavior of a few large species which are incompletely sampled due to the limited core volume. Mixing values for these species are redistributed through all of the core segments by probabilistic methods. The mixing coefficients computed in this manner are considered relative in view of uncertainty in determining sediment velocity, but correspond closely to values estimated independently from DDT gradients. The computations indicate that bioturbation extends through the effluent deposit in the Palos Verdes shelf, into the zone contaminated by DDT.

Controls on the degree of fluvial incision of continental shelves

During sea-level low stands continental shelves were dissected by a network of channels somewhat resembling todays coastal plain streams. The network was subsequently buried or erased by marine processes during sea-level transgression, so that only some tracts are still conserved in the geological record. Herein we use a numerical model to study the effect of base level change by sea-level fall on the total channel incision. We find that four factors control the total incision on the shelf: (i) the presence of convex deposits; (ii) the evolution of the rivers towards equilibrium (graded) conditions; (iii) geometrical differences between coastal plain and shelf; and (iv) the exposure of the continental slope. The conceptual model is then applied to the Adriatic Sea, Italy. Simulations show that incisions in the Adriatic shelf develop in high stand fluvial deposits in the early stages of sea-level fall. At lower sea level, fluvial incision occurs in the mid-Adriatic due to the regrading of the Po River after the capture of the Apennine streams in its drainage system.

Numerical Simulations of Coastal-Tract Morphodynamics

A finitv difference nttrnedcal model has been developed to represent the large-scale morphodynamics of complex coastal systems. The model is based on the Coastal System Tract (CST) concept. This model contains representations of coastal features such as tidal inlets, bays and rivers, beach systems (including littoral drift and profile migration), as well as the offshore zone comprised of the shoreface, shelf and upper continental slope. The purpose of the model is to diagnose the large-scale morphodynamical interactions between the components of the CST that have previously only been represented as separate or incompletely link dements. The results show that there are important forcing factors that result from different characteristic response times of the independent system components. This means that the system components are never properly tuned to respond in unison to perturbations in the large-scale forcing parameters. INTRODUCTION The need to plan and engineer coastal facilities, such as ports or offshore artificial islands that do not upset area-wide coastal processes, has focused attention on a need for long-term and large-scale quantitative representations of large coastal systems morphodynsmics, Over the past decade it has become increasingly apparent that representing the complex processes that shape open-ocean coastline with mathematical models provides for quantitative diagnosis of the relationship between environmental forcing and morphological response. Because coastal systems are complex assemblages of subeomponents and because many available models operate with time steps that resolve wave motions (i.e. seconds), it has been common practice to consider relatively short-term processes (less than a decade) and to isolate portions of overall coastal systems for study (e.g. the surf zone, inlets or the shoreface). More recently, models have been developed to represent large-scale and long-term processes and to link some of the components of overall open-ocean coastal systems. The time-, and length-scales of interest are decades to centuries and tens to hundreds of kilometers. This paper reports on the development and application of a large-scale numerical model representing an entire coastal system tract. It allows exploration of interactions between the shoreline sediment sources and sinks (e.g. river mouth, tidal inlet), the surf zone littoral sediment Vice-President, URS, 3676 Hartsfield Road, Tatlahassee. FL 32303 USA. Alan niedoroda@urscorp.com z Sefflor Project Scientist, URS, 3676 Hartsfield Road, Tallahassee, FL 32303 USA. Chris reed@urscorp.com Pro~ssor, Delf University of Technology, Faculty of Civil Engineering and Geosciences, Delf Hydraulics, (P.O. Box 177, 2600 MH) Delft, The Netherlands. Marcel.stive@wdelft.nl 4 Professor, Institute of Marine and Ocean Sciences, University of Sydney, NSW 2006, Australia. P.cowell@csu.usyd.edu.au

Developing Engineering Design Criteria for Mass Gravity Flows in Deep Ocean and Continental Slope Environments

A series of developments that led to both new understandings of mass gravity flows in the marine environment and in the techniques to quantitatively analyze them. Methods have been developed that exploit the newly emerging technical capabilities to perform very precise and detailed field investigations in deep continental slope settings that support numerical models which simulate the flows and their deposits. This combination of technologies provides a means to create engineering design criteria for mass gravity flows. In this paper, the methods that have been developed for these analyses are described and illustrated with examples.

A Model for Simulating Barrier Island Geomorphologic Responses to Future Storm and Sea-Level Rise Impacts

ABSTRACT Dai, H.; Ye, M., and Niedoroda, A.W., 2015. A model for simulating barrier island geomorphologic responses to future storm and sea-level rise impacts. This paper presents the Barrier Island Profile (BIP) model, a new computer code developed to simulate barrier island morphological evolution over periods ranging between years and decades under the impacts of accelerated sea-level rise and long-term changes in the storm climate. The BIP model is a multiline model that represents the time-averaged dynamics of major barrier island features from front beach to backshore. Unique contributions of BIP to coastal modeling include a dynamic linking of interacting barrier island features and consideration of both future sea-level rise and storm climate impacts. The BIP model has the built-in capability of conducting Monte Carlo (MC) simulations to quantify predictive uncertainty caused by uncertainty in sea-level rise scenarios and storm parameters. For a series of barrier island cross-sections derived from the characteristics of Santa Rosa Island, Florida, BIP was used to evaluate their responses to random storm events and five potential accelerated rates of sea-level rise projected over a century. The MC simulations using BIP provide multiple realizations of possible barrier island morphologic responses and their statistics, such as mean and variance. The modeling results demonstrate that BIP is capable of simulating realistic patterns of barrier island profile evolution over the span of a century using relatively simple representations of time- and space-averaged processes with consideration of uncertainty of future climate impacts.