U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3061

June 2005

Building a Community Sediment Transport Model

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Introduction

Predictions of the transport and 

long-term fate of particles in the coastal 

ocean are needed to address issues 

related to commerce, defense, public 

health, and the quality of the marine 

environment. For example, models can 

be used to investigate waste disposal and 

the transport and fate of contaminated 

materials; burial rates for naval mines or 

archaeological artifacts; water-column 

optical properties; transport and fate of 

biological particles; prediction of coastal 

flooding and coastal erosion; impacts of 

sea-level or wave-climate changes and 

coastal development; construction and 

maintenance of navigable waterways; 

habitat for commercial fisheries; 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.

Community Approach to 

Model Development

The U.S. Geological Survey (USGS) 

is leading a collaborative effort to develop 

and test a coupled hydrodynamics and 

sediment transport model to address 

coastal issues and has implemented 

and tested algorithms for calculating 

erosion, transport, and deposition of 

multiple sediment classes and evolution 

of sediment stratigraphy and benthic 

habitats in coastal environments. The 

emerging model has been used to 

simulate grain-size changes caused by 

wave- and current-induced sediment 

transport and sorting to evaluate model 

performance and to obtain solutions 

to real-world problems such as fate of 

contaminated sediments on the Palos 

Verdes shelf (fig. 1) and pollution in 

Massachusetts Bay (fig. 2).

The computer code for the Community 

Sediment Transport Model (CSTM) 

is freely available and publicly 

accessible. Researchers can modify 

the code to address specific needs. 

Improved versions of the model are 

collated and released by a “keeper of 

the code” (http://marine.rutgers.edu/po/

index.php?model=roms).

There are many advantages to opensource, community-based software 

development. Experts can improve 

parts of the model so that other 

users can benefit. When alternative 

parameterizations of the same physical or 

geological process are included, they can 

be compared and evaluated in identical 

frameworks. Users (including scientists 

from other disciplines, students, resource 

managers, engineers, and operational 

personnel) may draw from well-tested, 

state-of-the-art algorithms. Collaborative 

work on a community model helps 

synthesize understanding of sediment 

transport, 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, 

help identify bugs and produce a robust 

model that can serve as a scientific and 

legal standard.

Coastal Ocean Modeling with 

ROMS

The community sediment transport 

model is part of the Regional Ocean 

Modeling System (ROMS). ROMS has 

been developed by a group of academic, 

industry, and federal researchers 

supported with funds from the National 

Oceanographic Partnership Program. The 

model code, as well as documentation 

and supplemental programs to prepare 

input and evaluate results, is maintained 

by Rutgers University. The USGS has 

contributed routines to calculate vertical 

mixing, erosion, transport, and deposition 

of sediments, as well as evolution of 

small-scale stratigraphy in the seabed 

(fig. 3).

Winds, waves, tides, rivers, heating 

and cooling, and offshore motions all 

influence the coastal currents responsible 

for sediment movement. Decades 

of effort have gone into developing 

numerical models for these processes, 

and ROMS incorporates some of the 

most advanced modeling technology. 

The physical oceanographic model 

is based on the Reynolds-averaged 

Navier-Stokes equations, written for a 

finite-difference grid. The horizontal 

grid is curvilinear, and the vertical grid 

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Figure 1. A pilot study toevalueate capping of 

contaminated sediments was performed off 

the Palos Verdes Peninsula (pictured above) 

by the U.S. Army Corps of Engineers. The inset 

is a side view of the sediment profile showing 

the sand cap overlying fine effluent-affected 

sediment with benthic infauna. Models are 

helping to evaluate cap effectiveness and 

predict fate of contaminates. (REMOTS© photo 

courtesy SAIC.)

Figure 2. Model grid for Massachusetts Bay 

simulation showing horizontal grid with 

bathymetry in meters.
is terrain following. Currents are driven 

by winds, water-level gradients, and 

density gradients under the influence of 

friction and the Coriolis effect. Sub-gridscale mixing is represented with one of 

several turbulence models with one or 

two equations that describe transport of 

turbulence quantities. Highly accurate 

third- and fourth-order advection schemes 

are used to calculate movement of water, 

salt, heat, sediment, and biologically 

important quantities. The effect of waves 

on near-surface and near-bottom mixing 

and shear stress is included, and ROMS 

can be coupled with wave-evolution 

models, including the public domain 

model SWAN. Heating and cooling 

across the air-sea interface can make 

use of data from detailed atmospheric 

models. Tides can be imposed at the 

boundaries, and rivers or other sources 

can provide inputs of water and sediment. 

The model is written in modern 

FORTRAN90 and can take advantage 

of multiple processors using either 

shared-memory or distributed-memory 

computer architectures. It runs in most 

computing environments from laptops to 

supercomputers.

Sediment Dynamics

Coastal sediment ranges in size from 

cobbles to fine mud. Finer materials can 

remain in suspension for long periods and 

be transported great distances by currents. 

Coarser, faster settling sediment moves 

only intermittently, often in response to 

wave motions, and is transported more 

slowly. Small-scale seabed stratigraphy 

and biological mixing of bottom 

materials are important in determining 

what type of sediment is available for 

transport. Near-bottom currents critical to 

sediment transport are affected by smallscale bottom topography, like ripples. 

Various components of the sediment 

transport model have been developed 

to address these and other processes. 

Parameterization of these processes 

is a topic of active research, and the 

modeling community will incorporate 

improvements as they develop. The model 

maintains a record of how much of each 

kind of sediment is in each layer of the 

bottom and updates the stratigraphy 

when erosion, deposition, or sediment 

mixing occurs. These procedures allow 

the model to represent natural processes 

like winnowing of fine material and 

development of graded beds under waning 

transport conditions. Both bedload 

and suspended sediment transport are 

included. To date, wave-driven circulation 

has not been simulated, but the physics of 

nearshore processes will be improved in 

the future.

Regional Applications

The CSTM is being applied to several 

coastal systems. In Massachusetts 

Bay, the model has been used to study 

changes in bottom-sediment distribution 

caused by winter storms. Under storm 

conditions, large waves and currents 

generated by winds and tides combine 

to winnow fine sediment from shallow 

regions and deposit it in relatively quiet 

deeper regions. When the simulations are 

started with an idealized uniform blanket 

of mixed sediments, the bottom sediment 

distribution evolves toward one that 

resembles actual sediment-distribution 

maps (fig. 4). This confirmation of patterns 

of erosion and deposition suggests that the 

model is capable of accurately modeling 

transport pathways for fine sediment and 

associated contaminants.

CSTM results from the Hudson River 

Estuary are being compared with field 

data obtained by scientists from Woods 

Hole Oceanographic Institution. These 

comparisons allow evaluations of the 

vertical mixing algorithms in the model 

that are critical to estuarine circulation.

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Figure 3. Schematic illustration of bed evolution during erosion (top panel) and deposition (bottom panel) in community sediment 

transport model.

Figure 4. Sediment-size distribution in Massachusetts Bay obtained by simulating waves, 

currents, and sediment transport during winter 

storms.

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An extensive data set has been 

collected from the Adriatic Sea, located 

in Europe between Italy and Croatia. 

The Adriatic presents an excellent test 

case because it is a largely enclosed sea, 

forced mostly by local weather and river 

input. A large, multi-institution research 

effort during the winter of 2002-2003 

has produced an outstanding data set that 

has been used to quantify water transport 

in the coastal current. The comparison 

demonstrates that the model has 

predictive skill over long times (several 

months) and great distances (hundreds of 

kilometers).

For additional information, please 

contact:

Christopher R. Sherwood

Richard P. Signell

John C. Warner

U. S. Geological Survey

384 Woods Hole Road

Woods Hole, MA 02543-1598

(508) 548-8700

E-mail: csherwood@usgs.gov

rsignell@usgs.gov

jcwarner@usgs.gov