Grand Canyon Monitoring and Research Center 

Sediment Transport Modeling 

Review Workshop 

Review Panel Report 

February 19, 2007 

U.S. Geological Survey Field Center 

Flagstaff, AZ
FINAL REPORT OF THE EXTERNAL REVIEW PANEL, SEDIMENT 

TRANSPORT MODELING REVIEW WORKSHOP 

WORKSHOP DATES: February 15–16, 2007

MEETING LOCATION: USGS Pacific Science Center, 400 Natural Bridges Drive, Santa 

Cruz, CA 95060 

SEDIMENT TRANSPORT MODELING PEER REVIEW PANEL: 

 CHAIR: Francisco J. Simões, U.S. Geological Survey, Geomorphology and Sediment 

Transport Laboratory, 4620 Technology Drive, Suite 400, Golden, CO, phone 303-278-7956, fax 

303-279-4165, frsimoes@usgs.gov 

Peter Goodwin, University of Idaho, Center for Ecohydraulics Research, 322 East Front 

Street, Suite 340, Boise, ID, 83702, phone 208-364-6164, pgoodwin@uidaho.edu 

Daniel M. Hanes, U.S. Geological Survey, Coastal and Marine Geology Program, 

Pacific Science Center, 400 Natural Bridges Drive, Santa Cruz, CA, 95060, phone 831-427-

4718, fax 831-427-4748, dhanes@usgs.gov 

Chris Sherwood, U.S. Geological Survey, Coastal and Marine Geology, 384 Woods 

Hole Road, Woods Hole, MA 02543-1598, phone 508-457-2269, fax 508-457-2310, 

csherwood@usgs.gov 

David H. Schoellhamer, U.S. Geological Survey, Placer Hall, 6000 J Street, 

Sacramento, CA, 95819-6129, phone 916-278-3126, fax 916-278-3071, dschoell@usgs.gov 

Kees Sloff, Delft WL | Delft Hydraulics, P.O. box 177, 2600 MH Delft, The Netherlands, 

phone +31 (0)15 285 87 52, fax: +31 (0)15 285 85 82, kees.sloff@wldelft.nl
Sediment Transport Modeling Review Workshop 

1. OVERVIEW 

Construction of Glen Canyon Dam in 1963 has resulted in drastic reduction of sediment supply 

to the Colorado River ecosystem (CRE) in Glen Canyon National Recreation Area and Grand 

Canyon National Park (GC). The ensuing sediment-depleted regulated flows affected many of 

the natural resources vital to the riparian ecosystem. Of particular concern in this report is the 

substantial erosion of sand bars and sandy beach habitat that has taken place throughout Grand 

Canyon (Wright and others, 2005). 

The Glen Canyon Environmental Studies (GCES) were established in 1982 in order to determine 

the long-term impact of river regulation on the CRE. The GCES was the precursor to the present 

Grand Canyon Monitoring and Research Center (GCMRC) which serves as the science provider 

to the Glen Canyon Dam Adaptive Management Program (GCDAMP), a Federally chartered 

advisory committee to the Secretary of the Department of the Interior. One of the most important 

and unanswered strategic science questions facing the GCDAMP is if there is “a ‘Flow-Only’ 

operation (non sediment augmentation) that will rebuild and maintain sandbar habitats over 

decadal time scales” (Wright 2007, Melis and others, 2007; Melis and others, 2006). In 2006, the 

Protocols Evaluation Panel (PEP-SEDS III) convened in Flagstaff, AZ, as part of an ongoing 

peer review of the monitoring and research efforts carried out by the GCMRC (Wohl and others, 

2006). PEP-SEDS III was charged with appraising current research activities and long-term 

monitoring of physical resources below Glen Canyon Dam, and to provide recommendations for 

future work. 

As a result and follow-up of the part of the PEP-SEDS III recommendations related to the 

modeling program, the GCMRC organized a Sediment Transport Modeling Review Workshop in 

February 2007. This report summarizes the recommendations of the review panel assembled for 

the workshop concerning four aspects of the GCMRC’s sediment modeling program: the coarse 

sediment modeling program, the fine sediment modeling program, the multi-dimensional 

modeling efforts, and the sandbar stability modeling program. In particular, the panel is charged 

to provide recommendations “toward developing a modeling approach for simulating the fate of 

sandbars over decadal time scales for various dam operational scenarios” (Wright, 2007). 

2. OVERALL RESEARCH PROGRAM

A clear statement of the research objectives, anticipated products, and required data and models 

should be formulated to guide the research and ensure that all of the components are well 

integrated. This panel reaffirms the PEP-SEDS III panel recommendation (Wohl and others, 

2006) for the development of a unified vision for the overall research program that incorporates 

the different research components and that places them within the broader scope of the physical 

resources program.. 

3. COARSE SEDIMENT TRANSPORT MODELING PROGRAM 

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The geomorphology of the CRE in the GC is generally dominated by the coarse sediment 

(cobbles and boulders), and particularly by the debris fans originated at distinct points along the 

steep topography encircling the river corridor (Melis, 1997). These dominant geomorphic 

features define the step-eddy-pool morphology in the CRE that controls fine sediment 

(principally sand) distribution and storage. The coarse sediment modeling program is, therefore, 

a tool to investigate the long-term evolution of the CRE longitudinal profile. 

Preliminary Studies 

The current coarse sediment transport program, as presented by Chris Magirl, has depended on 

one-dimensional (1D) GSTARS and HEC models and calibrated cross sections (i.e., cross 

section profiles that were adjusted to provide agreement between simulated and measured 

backwater profiles). The channel roughness – represented by a Manning’s n – was maintained 

constant during this approach. This is contrary to the normal practice of calibrating models in 

which the best estimate or interpolation of the cross-section is obtained and then the roughness 

used as the calibration coefficient. However the approach by Dr. Magirl was adopted because the 

roughness at the reach scale was expected to show little variation and the entire bathymetry was 

based on just a few cross-sections that were available at the time this modeling was conducted. 

This program is focused on the fate of coarse sediment in the Colorado River, where “coarse” 

refers to cobbles and larger-sized particles. For this purpose, the current research efforts to refine 

and build upon past accomplishments and knowledge discovery should continue. The objective 

of the coarse-sediment modeling is to improve our understanding of how the river evolves in 

response to delivery and transport of debris fans. The model could be used as a management tool 

to investigate “what-if” scenarios involving new debris fans; the magnitude and duration of 

effects on river navigation and resources associated with postulated new debris fans could help in 

formulating management responses. It is also envisaged that the program yields tools for 

predicting the hydraulics of the rapid and pool system (e.g., how the hydraulic controls modify 

with different flood conditions), and the response of geomorphic features controlled by coarse 

sediments to synthetic (man-made) floods. 

The 1D flow modeling program appears to have reached a stage in which it could continue to be 

refined and better calibrated, but it is unlikely that there will be any significant new 

developments. There is the question of whether there is an advantage in using a full unsteady 

model (such as the one of Wiele and Smith, 1996) instead of the step backwater model used so 

far, but this point is not critical for the questions addressed to date. Instead, it is more important 

to broaden the scope of the program to include more detailed modeling to the smaller scales, i.e., 

multi-dimensional modeling around individual debris fans. 

In addition to the previous studies of Dr. Magirl, some future application of the one-dimensional 

model might include: 

• Simulation of flood wave propagation through the entire canyon; 

• Setting local boundary conditions for local higher order models of sub-reaches or specific 

features of concern; and 

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• Water quality and temperature modeling through the entire Canyon or as boundary 

conditions for local models. 

Mobility of Very Large Particles and Influence on Local Bed Conditions 

The science plan to determine the mobility of very large particles under different hydraulic 

conditions should continue. The current state-of-the-art of sediment transport modeling has very 

limited information about the transport of large boulders, and usually that information pertains 

only to rip-rap stability, rather than to rates of transport. This work should be expanded and 

applied in the future to include modeling the wear, abrasion, and communition of very large 

particles because the slow degradation of very large particles (or lack of it) will affect the longterm evolution of river morphology. Further investigations are also warranted to determine the 

effect of these very large particles on local transport of adjacent large particles. In particular, it 

would be important to understand under what conditions these very large roughness elements act 

to recruit other large and intermediate particles and when the local flow field acts to repel or 

transport intermediate particles. 

Debris Fans 

The data collection of flow and turbulence near debris fans should also continue. This data is 

needed to understand the flow characteristics near areas where strong turbulence is generated 

and/or dissipated—e.g. relations between velocity, depth, roughness and channel geometry. 

Turbulence has a direct impact on sediment resuspension and transport, which constitutes 

necessary information for the fine sediment transport modeling program. Particle Image 

Velocimetry (PIV) is a technique that could perhaps provide good flow estimates in the CR and 

should be considered in the current research program. This issue has important management 

implications. Following a tributary ‘blow-out’, how long will an alluvial fan persist under certain 

flow conditions and is intervention necessary to avoid unacceptable risks to life? Both the large 

particle mobility and flow and turbulence measurement programs should be designed to gather 

information required to develop a multidimensional model of debris fan creation and subsequent 

long-term response. This effort should be coordinated with the fine sediment and multidimensional programs described next. 

Summary 

For the objectives of the coarse modeling study, the adopted approach has served a useful 

purpose. The model was developed during a time of poor spatial coverage of channel 

bathymetry. The two primary calibration parameters used were Manning’s n (the roughness) and 

channel cross-section. In this application the Manning’s n was held constant and the channel area 

varied to match water surface profiles. The problem associated with varying the area rather than 

n is that it is difficult to use the model in a predictive mode at discharges other than the 

calibrated conditions. Experience in the CRE and other systems allow rough estimates of how 

the resistance will vary at different discharges and this relationship can be calibrated with water 

surface elevation surveys or velocity profiles. Technologies have improved rapidly in the past 

few years so that it is now possible to obtain detailed bathymetric data (a complete coverage of 

the GC bathymetry is expected in the coming months). Further, the technologies will be available 

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to measure typical roughness elements allowing more accurate representation of spatial variation 

in resistance. Thus further refinement of this model should be considered using channel 

bathymetric surveys and in the light of the integrated modeling framework described in Section 

7. 

4. FINE SEDIMENT TRANSPORT MODELING PROGRAM 

An Innovative and Experimental Approach 

Recent fine sediment modeling efforts by the GCMRC comprise the work of Wiele and others 

(2007, hereinafter WWG). The objective of the WWG model is to provide a practical 

management tool to allow the Bureau of Reclamation to design future artificial floods with a 

goal of maximizing bar creation. This initiative started with a conceptual model that identified a 

framework of the primary processes that influence the transport and deposition of fine sediment 

in a sediment depleted system. The review committee recognized the value of this approach and 

it is intended by WWG that the algorithm for each of these processes could be refined or 

calibrated as more field data becomes available in future years. 

From a management perspective a 1D model may be the most feasible and practical approach. 

The WWG model uses a simple 1D reach-averaged description of the river processes, but 

includes several novel components. These are 1) the influence of coarse particles on interstitial 

and bed sand storage; 2) adjustment of near-bed suspended-sand concentrations when sand 

transport occurs over a boulder bed; 3) provision for sand storage in eddy deposits and 

parameterization of the exchange of sand between eddy deposits and the main channel; and 4) a 

characterization of channel geometry that incorporates reach-averaged properties, including the 

distribution of pools. In this approach, pools are defined as sub-reaches with depths more than a 

standard deviation greater than the mean depth. Efforts to calibrate the model using the 2004 

beach habitat building flow (BHBF) test data were also reported by Scott Wright. 

The relative simplicity of the WWG model makes it easy to evaluate model response to varying 

inputs and model sensitivity to varying coefficients. In this way, it is a tool for understanding the 

sand storage response to fluctuations in water and sediment supply. A disadvantage to this 

simplified approach is that at least some of the model coefficients must be set by empirical 

calibration with a (limited) data set (rather than being set by theory, laboratory data, or detailed 

modeling). This means that the model, in its present formulation, is best used as a diagnostic tool 

that identifies the relative importance of different processes. Currently, it does not have reliable 

predictive capability beyond the range of calibration experience. The recent calibration efforts by 

Scott Wright indicate that the WWG model can be tuned to match some observational data, but 

his best results were achieved with unrealistically high values for the bed roughness coefficient. 

These high values are not surprising because they indicate that the model lumps several physical 

processes into the roughness coefficient and the high value compensates for the over-simplified 

1D and reach-averaged assumptions. 

We view the present version of the WWG model as a proof of concept: it indicates that the 

parameterizations of sand storage in a boulder bed and sand storage in eddy deposits are useful 

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and provide reasonable results in the context of a simplified model domain. Future research 

should be directed toward evaluating and improving the model. A few suggestions are provided 

below. 

Next Steps 

Sensitivity studies should be performed, and the calibration effort should be continued. Tests 

should be performed with the calibrated model and objective measures of the model performance 

should be developed using data that were not used for calibration, such as the 1996 controlled 

flood test (first test of the BHBF concept) and the 1965 channel cleaning flow release. These 

performance measures should be established from the perspective of the appropriate accuracy to 

address the questions being posed by the managers of the CRE. 

The most essential and easily attained model improvement is to continue and complete the 

calibration of the model and validate each of the conceptual components. A one-dimensional 

model contains many assumptions so accurate specification of physical parameters does not 

necessarily result in accurate results. Thus, calibration is essential and the GCES Program is 

blessed with a wealth of data for calibration and validation. The pool geometries used in the 

model should be considered in the calibration process because these seem to be the least certain 

component. Validation provides a measure of the confidence that can be placed in the predictive 

ability of the model and demonstrates the uncertainty of the model to scientists and managers. 

This is a necessary step in developing management decisions that depend on model predictions. 

Another relatively easy and fast way to improve the WWG model is to refine the reaches and to 

use a more accurate and detailed description of the eddy systems along the river. This work only 

involves the refinement of the statistical analysis presented in WWG and would require no reprogramming of the code (no reparameterization of the eddy systems are required at this stage). 

A further refinement of the fine sediment modeling program might be achieved by abandoning 

the reach-averaging approach of the WWG model and adopting a 1D model with shorter reaches 

and more detailed cross sections—similar to the approach followed by the coarse sediment 

program. At the earliest stage of this process, the pool and eddy storage approach would remain 

similar to that of the WWG model. A further step to this effort, which this review panel 

recommends, is to better integrate the fine and coarse sediment programs. It seems that the 

coarse modeling efforts run parallel to the fine sediment modeling efforts and there are no 

attempts to integrate both programs (i.e., the presentations by Chris Magirl and Peter Wilcock 

had no intersection or cross-referencing). 

Finally, there is the need to evaluate the algorithms that model the pool and eddy storage/scour 

processes. The present algorithms in WWG were derived using a 2D, single particle-size model. 

However, recent research indicates that sorting and gradation in a sand bar are important factors 

for long term simulation. Furthermore, there is no strong evidence that a 2D model is sufficient 

to capture accurately the sand exchange processes of interest—i.e., there is no definitive study 

that indicates what level of model dimensionality and complexity is necessary for these 

circumstances. 

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Evaluation of the WWG sand exchange algorithms can be accomplished by the use of laboratory 

experiments, field monitoring campaigns that target the flow structure and sediment transport, 

and/or by using multi-dimensional models that accurately capture the physics of the processes—

such as followed by WWG. The use of multi-dimensional models for this purpose is addressed in 

the next section. 

5. MULTI-DIMENSIONAL MODELING 

Within the scope of the present workshop review meeting, the primary motivation for a multidimensional modeling plan stems from the need to validate and improve the WWG model. Of 

particular concern is the algorithmic approach followed to describe the interaction between the 

conveyance of sediment by the river’s main channel and the role played by the eddy and pool 

systems in sediment storage. WWG used a 2D model to reach the current form of the parameters 

used by the algorithms. Unfortunately, the degree of complexity needed by a model to capture, 

with sufficient accuracy, the processes incorporated in WWG is unknown. Therefore there is no 

evidence that a 2D depth-averaged model is adequate for the task. 

The panel recommends that an evaluation of 2D, pseudo-3D and 3D models, using single and 

multiple grain-sizes, be conducted in order to determine a suitable computational platform to 

carry out simulations of eddy/bar sediment exchange under different hydraulic conditions. Rather 

than developing new models, the emphasis should be placed into existing models, especially 

those that are easily accessible by the GCMRC and that have good technical support. During this 

evaluation it is suggested that nesting of models or other approaches that will allow extensive 

reaches to be simulated, but with local fine mesh resolution capturing one or two eddy features. 

This approach would allow guidance for the grid size necessary to simulate the exchange 

between the main flow and eddies, as well as the magnitude of expected errors in the coarser 

grid. 

A critical issue is the availability of appropriate experimental data for the purpose at hand. 

Existing data sets must be re-evaluated to determine if they are appropriate for multi-dimensional 

model validation. Additional field work and laboratory experiments may have to be undertaken. 

Consideration should be given to classifying the typical sand bars and selecting a series of test 

sites that cover the range of bar types (perhaps 3 or 4 sites) rather than all 10 considered by 

WWG. These sand bar/eddy complex sites could then be instrumented under different hydraulic 

conditions. Measurement stations would be set above and below an eddy/bar complex specially 

chosen for the experiment, allowing a detailed local mass balance to be performed. Specifically 

the key assumptions of the conceptual model should be verified. As an example, one important 

assumption is that the eddies scour on the rising limb of the hydrograph, whereas eddy 

deposition occurs on a more continuous basis (section 2.4 of WWG). This should be proven with 

field data to ensure that the significant changes in pool morphology do not occur on a more 

episodic basis. 

The GCMRC, however, should not leap too quickly into a new data collection campaign without 

a detailed assessment of the existing database. Prior to collecting new eddy data, a multiPage 8 of 14 
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dimensional model should be used to perform some initial simulations that would be used to help 

design a measurement study. The collected data would then be used to refine the model. 

Finally, multi-dimensional modeling activities have been undertaken by GCMRC, but they have 

been limited mostly to two-dimensional models, such as the one of Wiele and others (1996). 

Current multi-dimensional modeling efforts using a three-dimensional model with large eddy 

simulation (LES) turbulence was presented by Mark Schmeeckle. 3D LES modeling has the 

potential to provide numerical simulations with the finest degree of detail that is currently 

practical—and possible—for the flows and channel dimensions in the GC. Current efforts should 

continue, perhaps with emphasis to bring down computational cost (i.e. CPU time) using 

techniques such as parallelization of the code. Also, what are the differences between the code 

being developed and other existing codes? Is a new code needed? Is the cost/benefit ratio best 

served by a new code or by application of an existing code? One possible way forward is to 

collect the field data and then invite different modeling teams to simulate the bar building and 

erosion process. The purpose would not be to select the perfect model, but rather to develop a 

community dialogue and evaluate the importance of different processes that are or are not 

captured in the algorithms. This would then facilitate improvement of the transfer functions in 

WWG. 

6. SANDBAR STABILITY 

The emphasis of the GCMRC program has been on bar building with less effort placed on bar 

erosion and flow management to preserve built bars. It is unclear what the relative contribution 

of bank caving and subsequent mass slumping is in the overall sand bar erosion process. In order 

to provide accurate predictions of sand bar losses and long term forecast of sand storage, the 

importance of this erosion mechanism in the GC system must be better understood. Methods of 

approach to achieve this goal are in situ measurements, laboratory experiments, and numerical 

modeling. Local measurements have the potential to provide the most relevant data and should 

be pursued when possible. 

Past difficulties encountered when instrumenting sand bars during high releases from the Glen 

Canyon Dam indicate that obtaining data during future BHBF tests may be difficult. However, 

this panel recommends that the subject be revisited by the GCMRC, at least from a feasibility 

point of view. A suitable and representative sand bar should be identified for instrument 

placement, and data should be collected at different discharges. The particular data of interest 

here concerns the slumping of sand due to bank collapse, modes of failure, and relative 

importance of mass wasting (due to bank caving) when compared to basal toe erosion. However, 

the collection of this type of data should be coordinated and synchronized with the other data 

collection efforts suggested in previous parts of this report. 

The numerical modeling efforts, currently represented by the work of Brent Travis and Mark 

Schmeeckle of Arizona State University showed considerable promise and should continue. This 

work may provide the answer sought, but it must be further refined and validated using 

experimental data. Laboratory experiments may have to be performed to support the numerical 

modeling program. Future model development should be focused into two aspects. The first is to 

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be able to answer the question whether or not erosion due to bank failure is a significant process 

when compared to toe scour. The second, if it is determined that bank failure is important, is then 

to provide a quantitative way to predict bank erosion for different BHBF tests (i.e., different 

discharges and flow ramping profiles). In the second stage of the work, attention must be given 

to the integration of the bank model with the other sediment transport modeling efforts sponsored 

by the GCMRC. 

Before the present modeling work of Travis and Schmeeckle is adopted as part of the science 

program for bank erosion and channel morphological evolution prediction, it must be placed 

within the current state-of-the-science. There is extensive work on bank erosion processes—

including geotechnical failure, interaction with water table level variations and with vegetation, 

cohesive sediment effects, etc.—and it is unclear if new models are needed. There must be a 

concerted effort to review existing models and assess their suitability for application to the 

conditions in the GC. Before potentially investing in the development of new models, a process 

that is expensive and takes a long time, a solid justification that new software is needed must be 

made. 

7. MODEL INTEGRATION 

The research team has identified numerous sediment issues and developed a suite of models to 

further the fundamental understanding of sediment processes through Grand Canyon and to assist 

in management decisions. In the future, there will undoubtedly be other geomorphic and 

sediment questions that have not been considered yet. It is recommended that some thought is 

given to an integrated modeling system that would be flexible and allow researchers and 

scientists to support the GCMRC efforts over the coming decade that would have flexibility for 

different models or new algorithm development within existing models. Clearly major steps have 

already been made in establishing databases and GIS coverage. However, it was unclear how the 

various models might be developed to interface and enhance one another in the future. One 

possible structure of an integrated modeling system is: 

1. Include tributary/alluvial fan inputs; 

2. Set-up a one-dimensional model of the entire reach. The objectives are, among others, to 

evaluate large scale channel controls, to determine the possible migration of channel controls and 

to provide sediment inflow and hydrodynamic boundary conditions for more detailed local 

models. This model could also be used to help address other concerns that include aquatic 

habitat, temperature and water quality; 

3. Incorporate the reach-averaged sediment model described in section 4 above. 

4. Set-up detailed 3-d or pseudo 3-d models of specific features of interest, such as bar-pool-eddy 

combinations. 

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This model structure could be integrated with data and information flow between the different 

scales. For example, item #4 could lead to improvements of item #3, item #1 improves item #2, 

item #2 would set boundary conditions for items #3 and 4, etc. 

8. CONCLUDING REMARKS 

One of the most important and unanswered strategic science questions facing the GCDAMP is: 

Will any ‘Flow-Only’ operation (non sediment augmentation) rebuild and 

maintain sandbar habitats over decadal time scales? (Wright, 2007, Melis and 

others, 2006). 

For this purpose, a review workshop was organized and this review panel was charged with 

providing a road map for a numerical modeling program that can be useful to the GCMRC for 

the next few years. This program has the objective to improve the GCMRC’s current capability 

for predicting the long-term fate of sand in GC under the various operational strategies for Glen 

Canyon Dam. The following points summarize the recommendations presented in the previous 

sections. 

1. Formulate an integrated research plan that anticipates future research and management needs, 

and ensures that the modeling and monitoring components are complementary. 

2. Further calibrate and test the WWG model in its present form in order to determine its 

immediate applicability to situations of interest. Through sensitivity and uncertainty analyses, 

quantify the level of confidence that can be placed in the model for management decisions. 

3. Gradually revise and refine the ansatz used by WWG. This should be undertaken in stages, 

starting by refining the level of discretization, then by increasing the complexity of the modeling 

techniques used. Continue this process until significant improvements are no longer cost 

effective. 

4. The GC is a very difficult location to conduct field work due to accessibility, permit 

restrictions and the very difficult field conditions. All of the USGS teams are to be commended 

for their professional approach and adoption of new experimental equipment (for example, the 

Pitot tube from New Zealand). Given the high profile research and testing conditions, the Review 

Panel recommends that the GCMRC is encouraged to continue this innovative work and that the 

resources are made available to understand the detailed flow structures. 

5. Use data and multi-dimensional modeling tools (2D and 3D) to improve the understanding of 

the sediment exchange in eddy and bar systems. Rather than developing new multi-dimensional 

models, the emphasis should be placed into existing modeling tools, especially those that are 

easily accessible by the GCMRC and that have good technical support. The primary use of data 

will mostly likely be that of numerical model calibration and validation. A preliminary model 

should be used to optimize design of a data collection study. 

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6. Revisit the existing data, from the point of view of the multi-dimensional modeling effort, and 

determine the need to complement it with field measurements and laboratory experiments. 

Detailed local experiments and observations of sand bar deposition and erosion are likely to be 

needed to guide the development of a useful bar-scale model. 

7. Analyze the uncertainty of the model predictions following the uncertainty of input (lack of 

data, measurement error, stochastic conditions) and of the algorithm assumptions and 

approximations. A probabilistic approach, in which the effect of uncertainties in input data and 

model algorithms on the predictions can be computed, is considered relevant for evaluating the 

value of the predictions. 

The panel recognizes that the WWG model constitutes a significant step towards the 

development of a management tool to answer some of the critical GCDAMP science questions. 

The work plan outlined in the previous sections has the objective of honing the approach used in 

WWG, to bring it to a stage where its predictions can be reliable and accurate enough for the 

management purposes that it is intended for, to reduce and quantify remaining uncertainty. There 

may be parallel complementary models used. For example the WWG may be employed as the 

practical management tool for decision making and more complex higher dimension models 

could be used for reducing uncertainties in WWG by refining the constituent algorithms over 

time and to address particular local complexities. 

This review panel stresses the need for the development of a whole-system view to guide future 

modeling efforts. There are different management questions that most likely will require 

different model needs—i.e., different resolution levels, diverse time and spatial scales, etc. 

Anticipating future needs at an early stage enables the development of a unified approach in 

which models may complement and communicate with each other. This may result in significant 

economy of resources later. For example, a few pertinent questions are: 

1. Are there any efforts to interface other models (e.g., the temperature modeling efforts 

currently under way) with the models developed for the geomorphology studies? 

2. How can the effects of vegetation be incorporated in the modeling effort? Vegetation has 

significant impact in hydraulic and aeolian erosion. 

3. How will the hydraulic/sediment transport models interact with water quality, habitat, and 

ecology models in the future? 

Some of these questions require the modeling program to anticipate and seek integration with 

other program activities and to address issues in other disciplines. 

Finally, the GCMRC should be commended on the breadth of expertise that has contributed to 

the program so far from across agencies and academia such as John Hopkins University. The 

problems being addressed are very complex and the consequences associated with management 

decisions are high. This inclusive philosophy is consistent with the Community Science

approaches adopted by the National Science Foundation in the development of Environmental 

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Observatories such as LTER, the WATERs Network (CUASHI and CLEANER) and NEON and 

it is recommended that this approach be continued. 

LIST OF CITATIONS 

Melis, T.S., 1997. Geomorphology of debris flows and alluvial fans in Grand Canyon National 

Park and their influence on the Colorado River below Glen Canyon Dam, Arizona, Ph.D. 

dissertation, University of Arizona, Tucson, 490 p. 

Melis, T.S., Wright, S.A., Ralston, B.E., Fairley, H.C., Kennedy, T.A., Andersen, M.E., Coggins, 

L.G. Jr., and Korman, J., 2006. 2005 Knowledge assessment of the effects of Glen Canyon 

Dam on the Colorado River ecosystem: an experimental planning support document, U.S. 

Geological Survey, Flagstaff, Arizona 86001, 82p. 

Melis, T.S., Topping, D.J., Rubin, D.M. and Wright, S.A., 2007, Research Furthers Conservation 

of Grand Canyon Sandbars: U.S. Geological Survey Fact Sheet #2007-3020, 4 p. 

Wiele, S.M., Graf, J.B., and Smith, J.D., 1996. Sand deposition in the Colorado River in the 

Grand Canyon from flooding of the Little Colorado River, Water Resources Research, 

32(12):3579-3596. 

Wiele, S.M., and Smith, J.D., 1996. A reach-averaged model of diurnal discharge wave 

propagation down the Colorado River through the Grand Canyon, Water Resources 

Research, 32(5):1375–1386. 

Wiele, S.M., Wilcock, P.R., and Grams, P.E., 2007. Reach-averaged sediment routing model of 

a canyon river, Water Resources Research, Vol. 43, W02425, doi:10.1029/2005WR004824. 

Wohl, E., Bennett, J.P., Blum, M.D., Grant, G.E., Hanes, D.M., Howard, A.D., Mueller, D.S., 

Schoellhamer, D.H., and Simoes, F.J., 2006. Protocols Evaluation Program (PEP-SEDS III): 

Final Report of the Physical Resources Monitoring Peer Review Panel, U.S. Geological 

Survey, Flagstaff, Arizona, 86001, 25p., available at 

http://www.gcmrc.gov/library/reports/PEP/Wohl2006.pdf

Wright, S., 2007. White Paper on Sediment Transport Modeling, Colorado River below Glen 

Canyon Dam; support document for a Sediment Transport Modeling Workshop, Canyon 

Monitoring and Research Center, Flagstaff, Arizona, 9p. 

APPENDIX 

Strategic Science Questions 

Strategic Science Questions developed cooperatively by scientists and managers as a 

result of the Knowledge Assessment Workshops in 2005. (Source: Wohl and others, 2006.) 

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Sediment Transport Modeling Review Workshop 

4.1 Physical Resources 

4.1.1 Is there a “Flow-Only” (non sediment augmentation) operation that will restore and 

maintain sandbar habitats over decadal time scales? 

4.1.2 Is there an optimal strategy for BHBF implementation to manage tributary inputs on an 

annual to inter-annual time scale? 

4.1.3 What are the short-term responses of sandbars to BHBFs? 

4.1.4 What is the rate of change in eddy storage (erosion) during time intervals between 

BHBFs? 

4.1.5 How does the grain-size distribution of the deposits affect sandbar stability? Main 

channel turbidity? 

4.1.6 What are the effects of ramping rates on sediment transport and sandbar stability? 

4.1.7 Can we develop a relationship between suspended sediment concentration and turbidity 

to support fisheries research? 

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