Working Group Presentations
Preface: The following summary is copyright 2002 American Geophysical Union and is printed in Eos, Transactions of the American Geophysical Union, 83(50):605.
Toward a Community Coastal Sediment-Transport Modeling System: Report of the Second Workshop
Models for transport and long-term fate of particles in coastal waters are essential for a variety of applications related to commerce, defense, public health, and the quality of the marine environment. Examples include: analysis of waste disposal and transport and the fate of contaminated materials; evaluation of burial rates for naval mines or archaeological artifacts; prediction of water-column optical properties; analysis of transport and the fate of biological particles; prediction of coastal flooding and coastal erosion; evaluation of impacts of sea-level or wave-climate changes and coastal development; planning for construction and maintenance of navigable waterways; evaluation of habitat for commercial fisheries; evaluation of impacts of natural or anthropogenic changes in coastal conditions on recreational activities; and design of intakes and outfalls for sewage treatment, cooling systems, and desalination plants.
All of these issues require an understanding of the transport and fate of sediment under varying hydrodynamic, geological, and biological conditions. Numerical models of coastal sediment transport are valuable tools for synthesizing and applying scientific understanding to particular environments, for framing and testing hypotheses, and for making predictions for scientific, tactical, or resource-management purposes. Models are also necessary for interpreting data from ocean observatories, providing virtual laboratories for testing emerging theories and algorithms, and creating realistic numerical settings for biological and geochemical models.
The goal of an effort funded in fiscal year 2002 by the National Oceanographic Partnership Program (NOPP), entitled "Planning for a National Community Sediment-Transport Model," was to promote development of an advanced numerical modeling system for coastal sediment transport that is scientifically sound, expertly coded, well tested, and suitable for use in both research and practical applications. A modeling system encompasses more than the model itself, and should include infrastructure for managing development, testing, improvement, documentation, and distribution; test cases ranging from simplistic to prototypical; tools such as pre- and post-processing software; and a forum for exchange of scientific information, code, and tools.
About 50 scientists and engineers from academia, government agencies, private consulting firms, and European and Australian research institutions attended a workshop in Williamsburg, Virginia to complete a NOPP planning effort that had been initiated by a small group of investigators after an initial workshop, held in Woods Hole, Massachusetts in June 2000 (see Sherwood et al., 2000). Since then, test cases have been developed and posted on the Web (http://woodshole.er.usgs.gov/project-pages/sediment-transport/); code for two models that might serve as a foundation for a community model (ROMS and EcomSED) have been placed on the Web for public access (http://marine.rutgers.edu/po/index.php and http://www.hydroqual.com/ehst.html); and a Town Meeting has been held at the AGU/ASLO Ocean Sciences in Hawaii, February 2002.
The Williamsburg workshop benefited from participation of researchers associated with the ongoing Nearshore NOPP project, which includes surf zone sediment transport models [Kirby, 1999], and the developing community sediment model for large-scale continental margin evolution (http://instaar.colorado.edu/deltaforce/workshop/csm.html).
Critical questions addressed in Williamsburg were: (1) Why do we need a coastal community sediment transport model? (2) What are the biggest challenges to modeling coastal sediment transport? (3) What key elements should be included in a community modeling program? (4) How should the effort be balanced between modeling, laboratory, and field programs? (5) How many models should there be? (6) How modular should the model(s) be?
Attendees listed applications for coastal sediment-transport models (see above) and identified important advantages to a community approach. A common set of models and modeling tools captures and integrates scientific knowledge and facilitates transfer of that information. A community effort enables inclusion of a broad range of processes and scales that are not feasible for individuals or small groups. Scientists and engineers may contribute according to their expertise, and users (including scientists from other disciplines, students, resource managers, engineers, and operational personnel) may draw from well-tested, state-of-the-art algorithms. Incorporation of alternative parameterizations for similar processes allows them to be compared in identical frameworks. Collaborative work on a community model helps identify key research and modeling issues, and efficiently focus research efforts, minimizing duplication and preventing critical components from being overlooked. Wide use and broad participation in model development, along with extensive testing and peer review, produces a robust model that can serve as a scientific and legal standard.
The workshop participants noted that in order to develop a robust modeling system, specific advances are needed in understanding many processes fundamental to sediment transport, such as (1) water-column processes including turbulence parameterization, particle dynamics, and fluid mud behavior; (2) bottom boundary layer hydrodynamics, especially factors controlling near-bed turbulence, bottom stress; and hydraulic roughness; (3) processes that occur near the fluid/sediment interface, including sediment erosion and deposition, bedload transport, sheet flow, behavior of mixed sediments, consolidation, bioturbation, diagenesis and evolution of critical shear stresses; and (4) influences of biological and geochemical processes on sediment transport mechanics. The general conclusion reached at the workshop was that a community modeling system helps address these problems by providing useful and extendable formalism for accumulating, comparing, evaluating, and transferring our communal understanding.
A number of technical challenges were identified and evaluated at the workshop. One of the most important challenges is extension of the model scale both upwards to incorporate larger space and time-scales, and downwards, to correctly parameterize sub-grid-scale phenomena that possess inherently short time or space scales.
Other technical challenges involve balancing computational efficiency with understandable, portable, and maintainable code, and choosing appropriate grid systems that allow accurate and efficient computations with complex shorelines and topography. Testing the model will be a major challenge because there are few analytical tests for sediment-transport processes and relatively few data suitable for critical evaluation of model components. Selection of test data and the execution of laboratory and field experiments to verify model results will be essential activities.
There are also organizational challenges associated with maintaining an open-source and evolving code. An official "keeper of the code" is required at an institution with a mandate for, and interest in, long-term support. A yet-to-be-agreed-upon process is desirable for the selection of the initial modeling system and for future contributions and changes. While there were proponents for building sediment transport modules around specific existing hydrodynamic models, others argued that evolving but less-established hydrodynamic models might offer the best framework in the future.
Most attendees agreed that for practical reasons, only one modeling system could be supported by the program over the long term, but the program would benefit by including researchers working on a range of diverse model components to facilitate comparison and scientific exchange. It was agreed that newly developed sediment-transport algorithms could be transferred to other model frameworks if they are found to be successful. Strong scientific leadership will be required to steer model development, define test cases, and guide model applications. Existing links with the Nearshore NOPP project and the community sediment model for large-scale continental margin evolution must be maintained. There are also great opportunities for synergy with emerging coastal observation systems, as well as agency programs that would benefit from model applications, and scientific projects that may provide an opportunity for model evaluation.
Workshop results have been reported to members of the NOPP agency working group (which includes representatives from the Office of Naval Research, the National Science Foundation, the Environmental Protection Agency, the U.S. Army Corps of Engineers, USGS, the Mineral Management Service, the U.S. Coast Guard, and U.S. Department of Energy, among others). Scientists and managers may encourage these agencies to establish a NOPP-funded program to begin in fiscal year 2004, through which groups can propose efforts to build, test, maintain, distribute, and use a community coastal sediment transport modeling system.
The NOPP workshop was held 29 September 29 - 2 October, 2002.
The workshop was funded by NOPP and the U.S. Geological Survey.
Christopher R. Sherwood, U.S. Geological Survey, Woods Hole, Mass.
Courtney K. Harris, Virginia Institute of Marine Science, Gloucester Point, USA
W. Rockwell Geyer, Woods Hole Oceanographic Institute, Woods Hole, Mass., USA
Bradford Butman, U.S. Geological Survey, Woods Hole, Mass.
Kirby, J.T., 1999, Nearshore community model development: Eos, Transactions, AGU, Fall Meeting Supplement, v. 80, F512.
Sherwood, C.R., Signell, R.P., Harris, C.K., and Butman, Bradford, 2000, Workshop discusses community models for coastal sediment transport: Eos, Transactions, AGU, v. 81, p. 502.