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Models

As the importance of channel degradation to stream sediment loadings has been recognized, watershed and stream model developers have incorporated channel degradation subroutines into existing model structures. Both watershed- and reach-scale models are available with varying levels of complexity. Most models only represent fluvial erosion, calculating sediment contributions from the channel boundary based on a critical shear stress and stream sediment transport capacity. More complex models, such as CCHE-1-D, CONCEPTS, and GSTAR-1D, represent both fluvial erosion and mass wasting. Screening-level models include the watershed models AnnAGNPS, ANSWERS-2000, GWLF, HEC-6, HSPF, and WARMF and the reach-scale model SAM. Planning-level models include the watershed model SWAT 2000 and the channel models CCHE-1D and GSTAR 1-D. CONCEPTS, a reach-scale model developed by the US Department of Agriculture's Agricultural Research Service, was specifically designed to model changes in channel morphology. This research-level model permits assessment of stream restoration practices, including the influence of riparian vegetation on bank stability.

When modeling the impact of stream restoration designs, channel degradation process simulation is necessary. Simple empirical models may provide reasonable preliminary estimates of channel degradation sediment loadings, but do not afford the parameter flexibility required to model sediment movement at the reach level. Reach-scale models are required to adequately simulate channel level changes associated with stream restoration practices. Increased channel-level detail available in reach-scale models allows the user to simulate changes in riparian areas through the adjustment of multiple cross section roughness values. Detailed soil strength parameters can also be manipulated to simulate changes in root reinforcement. These detailed channel-level parameters make reach-scale models the only suitable choice if before and after affects of stream restoration are the goal of a modeling effort.

More detailed information on these models can be found in the project’s final report available for download.

For this study eight models listed below were reviewed. However, only GWLF, SWAT and CONCEPTS were evaluated as part of the case study.

AnnAGNPS
AnnAGNPS is a continuous-simulation, multi-event modification of single-event model AGNPS with improved technology and the addition of new features. The model can be used to predict non-point source pollutant loadings from agricultural watersheds. It is a tool for comparing the effects of implementing various conservation alternatives within the watershed. Cropping systems, fertilizer application rates, water and dissolved nutrients from point sources, sediment with attached chemicals from gullies, soluble nutrient contributions from feedlots, and the effect of terraced fields can be modeled (http://www3.bae.ncsu.edu/Regional-Bulletins/Modeling-Bulletin/bosch-annagnps-bulletin-manuscript.html, access November 15, 2006).

AnnAGNPS simulated fluvial erosioin using the Bagnold stream power channel degradation algorithm.

Answers2000
ANSWERS-2000 is a distributed parameter, physically-based, continuous simulation, farm or watershed scale, upland planning model developed for evaluating the effectiveness of agricultural and urban BMPs in reducing sediment and nutrient delivery to streams in surface runoff and leaching of nitrogen through the root zone. The model is intended for use by planners on ungaged watersheds where data for model calibration is not available (http://www3.bae.ncsu.edu/Regional-Bulletins/Modeling-Bulletin/answers2k-draft0.html, accessed November 15, 2006).

Answers2000 simulates fluvial erosion using critical shear stress and transport capacity channel degradation algorithms. Channel adjustment is considered for widenin after nonerodible bed layer is reached.

CCHE-1D
CCHE-1D is a one-dimensional flow and sediment transport model for channel networks. Its hydrodynamic module computes unsteady flows in channels of compound cross sections, accounting for the effects of in-stream hydraulic structures. The sediment transport module computes non-equilibrium transport of non-uniform sediment mixtures. (http://www.ncche.olemiss.edu/content/software/cche1d/applications.pdf, accessed November 15, 2006)

CCHE-1D simulates fluvial erosion and planar bank failure. Channel degradation is accounted for using the critical shear stream algorithm. Channel adjustment is considered by adjusting depth and width based on erosion rate predictions and is designed to assess scour and deposition at hydraulic structures.

CONCEPTS
The Conservation Channel Evolution and Pollutant Transport System (CONCEPTS) is a computer model created by the U.S. Department of Agriculture (USDA), Agricultural Research Service (ARS) at the National Sedimentation Laboratory (NSL) in Oxford, Mississippi. Two versions of the CONCEPTS model were created to simulate open-channel flow hydraulics, sediment transport, and channel morphology (Langendoen, 2000). A watershed-scale version was created to simulate watershed-scale processes and the connectivity of stream networks. The stream corridor version is a reach-scale model created to focus on the hydraulics, sediment movement, and channel shaping processes with increased detail in modeling in-stream processes. The CONCEPTS model was designed as a tool for the assessment of stream corridor restoration projects. When combined with watershed-scale modeling programs, CONCEPTS may be used to assess the long term effectiveness of restoration efforts and provide engineers, planners, and ecologists with quantitative simulation output useful in design implementation procedures.

CONCEPTS simulates fluvial erosion and planar bank failure. Channel degradation is accounted for using the critical shear and transport capacity algorithms. Channel adjustment is considered by width and depth.

Langendoen, E. J. 2000. CONCEPTS – Conservational Channel Evolution and Pollutant Transport System. United States Department of Agriculture – Agricultural Research Service National Sedimentation Laboratory. Oxford, MS.

GSTAR-1D
GSTAR-1D (Generalized Sediment Transport for Alluvial Rivers – One Dimension) is a hydraulic and sediment transport numerical model developed to simulate flows in rivers and channels with or without movable boundaries (http://www.usbr.gov/pmts/sediment/model/gstar1d/index.html , accessed November 15, 2006).

GSTAR-1D simulates fluvial erosion and planar bank failure. Channel degradation is accounted for using critical shear stress and angle of repose adjustment. Width and depth channel adjustment is considered.

GWLF
Researchers at Cornell University developed the Generalized Watershed Loading Function (GWLF) model for predicting sediment loadings to ungaged streams. The program was designed to use readily available data for the prediction of sediment and nutrient movement over time periods ranging from a few months to years. GWLF is suitable for the simulation of small homogeneous watersheds, but may be applied to larger watersheds on a subwatershed by subwatershed basis.

GWLF using an empirical approach for channel degradation as a function of monthly flow volume, % development, animal density, curve number, and channel slope.

HEC-6
HEC-6 is a one-dimensional sediment transport model that calculates water surface and sediment bed surface profiles by computing the interaction between sediment material in the streambed and the flowing water-sediment mixture. The total sediment load is computed for each cross section along with the trap efficiencies for clays, silts, and sands. The change in bed elevation, water surface elevation, and thalweg elevation are also computed for each cross section. Dredging can be simulated and reservoir deposition can be analyzed with the model http://www.hec.usace.army.mil/software/legacysoftware/hec6/hec6.htm, accessed November 15, 2006).
 
HEC-6 simulates fluvial erosion using the Exner equation, bed sorting, critical shear stress and transport capacity for channel degradation. Vertical adjustment for user defined erodible channel is considered.

HSPF
The HSPF model simulates nonpoint source runoff and pollutant loadings, performs flow routing through streams, and simulates in-stream water quality processes. HSPF estimates runoff from both pervious and impervious parts of the watershed and stream flow in the channel network.

HSPF simulates fluvial erosion using critical shear stress, Parthenaiades’ equation, and Krone’s formula.

SAM
SAM is an integrated system of computer programs developed under the Flood Damage Reduction and Stream Restoration Research Program sponsored by the U.S. Army Corps of Engineers. It is intended to be used primarily as an aid in the design of stable channels.  The SAM package enables the user to evaluate the hydraulics, sediment transport, and sediment yield for representative stream cross sections. The programs are not considered to be a model in the sense of evaluating the hydraulics and sediment transport characteristics of an entire stream reach. The sediment transport algorithms in SAM do not compute bed elevation change (erosion and deposition), only sediment transport capacity based on computed hydraulics (http://chl.erdc.usace.army.mil/library/publications/chetn/pdf/chetn-vii-7.pdf, accessed November 15, 2006).

SAM accounts for fluvial erosion using stream power and bed material transport equations.

SWAT2000
The Soil and Water Assessment Tool (SWAT) was developed by Dr. Jeff Arnold of the Grassland, Soil, and Water Research Laboratory in Temple, Texas for the USDA Agricultural Research Service (Neitsch et al., 2002). SWAT is a physically based watershed-scale model created to predict impacts of “land management practices on water, sediment, and agricultural chemical yields in large complex watersheds with varying soils, land uses, and management conditions over long periods of time.

SWAT2000 simulates fluvial erosion using the Bagnaold stream power channel degradation algorithm.

WARMF
To facilitate TMDL analysis and watershed planning, WARMF was developed under sponsorship from the Electric Power Research Institute (EPRI) as a decision support system for watershed management. The system provides a road map to calculate TMDLs for most conventional pollutants (coliform, TSS, BOD, nutrients). It also provides a road map to guide stakeholders to reach consensus on an implementation plan. The Engineering Module is a GIS-based watershed model that calculates daily runoff, shallow ground water flow, hydrology and water quality of a river basin (http://www.epa.gov/ATHENS/wwqtsc/html/warmf.html, accessed November 15, 2006).

WARMF simulates fluvial erosion using critical velocity and transport capacity algorithms.

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