A two-dimensional HEC-RAS model represents the physical conditions and characteristics of a watershed in order to simulate hydrologic and hydraulic processes. A 2D watershed modeling approach in HEC-RAS is being used increasingly to provide economical estimates of flood hazards. This approach can also be used to increase coverage of floodplain mapping and related hazard identification. The user/modeler should follow the below recommendations while preparing a 2D HEC-RAS model.
Model Naming Convention Recommendations
Consistent management of the 2D HEC-RAS input and output data is important for the reusability of the model. For better project management, the user should develop data in a format that is easily recognizable, storage efficient, and user-friendly for uploading and downloading the project data. While naming the model, the user should always remember that HEC-RAS models, their plans, and other modeling components should be consistent and self-explanatory.
For example, the general naming convention recommends the usage of lowercase characters throughout and underscores rather than spaces, etc. The objective of naming conventions is to allow users to locate and understand the modeling data components quickly, which is especially useful for large scale studies with large file sizes.
Project Coordinate System Recommendations
A project area may span multiple coordinate projection systems. The user should select one consistent coordinate system and use the same .prj file for all watershed models within the project footprint to avoid variations or mismatching of input and output data. If a single coordinate system for a project area is not possible, then the best practice is to include the coordinate system projection file within the folder that contains the model files so that the .prj file is retained with the model inputs and outputs. Refer to this article to learn more about the project coordinate system.
2D Mesh Development Recommendations
Creating a HEC-RAS 2D mesh that correctly represents the flow area being modeled can take some time due to the need to add the appropriate details to the model to represent the components and due to the terrain data to be included.
The following model mesh development practices are listed in order of importance in 2D watershed modeling:
- Watershed Model Mesh Scale
When uniform spatial precipitation is applied in a 2D model, the recommended model mesh size for most of the area is of scale 20-60 mi2. While setting up a model mesh scale with uniform spatial precipitation, the user must have an assurance that the runoff (flows) and total runoff volume computed for each stream in the watershed is representative of the desired recurrence interval.
- Watershed Model Mesh Connectivity and Delineation
Where a project area includes inflows from upstream watersheds, the relationship between inflow and in-watershed hydrologic conditions must be examined. Delineating consistent boundaries among watershed areas is also critical for modeling and mapping flood hazards across large project areas.
- Mesh Hydro-Enforcement and Refinement
One advantage of 2D watershed modeling is that the model mesh can be reconfigured by the user iteratively more easily than in 1D modeling. This iterative process is useful for using initial model results to inform mesh configuration. Once the initial model mesh is set and modeling results are available, hydro-enforcement and refinement regions should be applied.
- Hydro-Enforcing Stream Channels
The most critical step in model mesh development is to hydro-enforce the mesh so that the channel capacity of the stream can be realized during flow routing and hydraulic calculations. This includes areas where there is assumed hydraulic connectivity through the high ground (e.g., roadways, railroads, and dams) using breaklines or 2D area connections to simulate the effect of culverts, spillways, etc. The mesh breaklines should also be added along the feature of the channel bank that affects channel overflow.Effective practices for developing a hydro-enforced model mesh are listed below:
- Hydro-enforcing should be accomplished through the addition of refinement regions, breaklines, or terrain modification.
- Hydro-enforcing through v-notch, u-notch, or offset breaklines should be enforced with a cell protection radius prior to enforcing any stream centerline or channel bank breaklines so that the hydro-enforcing is preserved as the mesh is refined. Alternatively, stream centerline or channel bank breaklines can be clipped to not interfere with hydro-enforcing breaklines.
- The user should avoid manual edits to individual computation nodes and should instead use breaklines to reshape the mesh. Manual node edits cannot be reinforced in a consistent manner.
- 2D area connections or terrain modification approximations of the channel can be used to simulate flows through the embankments.
- 2D area connections should be used to represent dams. The breaklines can be more easily interfered with during model refinement and are dependent on the underlying terrain to accurately capture the spillway geometry.
- Model Mesh Cell Sizes
Typically, computation points are created with a 200’ x 200’ cell spacing. This ensures that the cell count is low enough for manageable computation times but detailed enough to pass water through the model. Refinement regions should be added to the model to define populated areas, and steep areas, typically at 50’ x 50’ cell spacing. If the entire watershed area requires smaller cell spacing, then the initial model mesh can be set at 100’ x 100’ or 50’ x 50’ cell spacing. Note that the mesh refinement areas may be desired for reasons beyond their effect on the water surface elevation.
- Levee Systems
Where there are levees in a watershed model, two model meshes should be created: One with the mesh updated to account for crest elevations of the levee systems; and one with the mesh updated to eliminate any effects of the levee systems (natural valley) as shown below. Applying this process for each system within the project footprint provides insight into the range of possible flood hazard outcomes for each event.
- Model Mesh Along Streams and Roadways
Applying refinement regions and breaklines to stream channels typically helps better capture terrain features in the conveyance calculations. Aligning mesh cell edges along channel banks can be important to properly account for the transfer of water between the channel and floodplain, especially if those channel banks are perched or raised relative to the adjacent overbanks.If after refinement, the density of cells within the stream channel banks is more than two, the HEC-RAS solver should likely be switched to the full momentum equation; otherwise, the velocity values within the channel can be unrealistically high. The full momentum equation should be used in these cases because the diffusion wave equation disregards certain components of fluid dynamics, such as local acceleration of velocity with time, advective acceleration, and viscosity terms that are important for modeling flow separations and eddies between 2D cells within the channel. Refer to this article in our knowledge base to learn more about HEC-RAS 2D flow area modeling.
Unsteady Flow Data Recommendations
Unsteady flow data are required to perform an unsteady flow analysis. Unsteady flow data consists of both external and internal boundary conditions. External boundary conditions are required to run an unsteady model. External boundary conditions must be established at all the open ends of the river system being modeled. These are the boundary conditions that the user must add to the upstream and downstream ends of each reach (or 2D flow area). Internal boundary conditions are optional and allow the user to define gate operations and add flow within a river reach. Refer to this article in our knowledge base to learn more about unsteady flow data.
Unsteady Flow Computation Recommendations
GeoHECRAS software provides some default computational options and tolerances for 1D and 2D unsteady flow models. The tolerances are used in the solution of unsteady flow equations. In general, it is recommended that the default computation options and tolerances be maintained. However, the user can override the default computational options to achieve model stability while maintaining computational accuracy. Extra care should be taken while overriding the default calculation tolerances as it could result in computational errors in the water surface profile. Refer to this article in our knowledge base to learn more about unsteady flow computational options.
Unsteady flow computations can also be performed using 2D floodplain encroachment and unsteady flow floodplain encroachment methods. These methods automate the placement and analysis of floodplain encroachments along the river reach when performing a 2D steady or unsteady flow computation. This allows the user to have a better understanding of the true flow effects when attempting to determine the floodway in a complex flow situation. The user can also examine the DxV (Depth x Velocity) factor, as this directly indicates greater flood hazard and hydraulic importance.
Model Validation and Calibration
Validation and calibration of rainfall-runoff models provide assurance of accuracy in model results. While the flow routing component of 2D HEC-RAS is much more detailed than a typical rainfall-runoff model, the placement of breaklines and the development of Manning’s n values need to be validated and calibrated (where possible) to confirm realistic results.
Users should inspect the maximum velocities along the stream channels and near hydraulic structures to confirm the reasonability of the results. Typically, velocity issues can be resolved by the following:
- Adjusting 2D area connections
- Adding breaklines
- Using full momentum solver, etc.
Peak Flow Validation
The 2D HEC-RAS modeling is an iterative process. After each simulation, the results should be analyzed and adjustments should be made to verify that the model produces the expected results. The breaklines can significantly influence flow routing and should be modified to allow water to pass through (i.e., hydro-enforcing) the model in a realistic way.
For example, if water is known to pond behind the high ground, this behavior should be consistent with how the mesh and terrain allow the area to drain. After each iteration of mesh refinement, stage and flow hydrograph results in HEC-RAS should be compared to flow and water surface elevation data calculated with USGS gages, regression equations, and rating curves for validation.
Viewing 2D HEC-RAS Model Results
After defining the 2D HEC-RAS model and the associated parameters, the analysis should be computed with the incorporated changes in the model. The results for a 2D model can be viewed using the following options:
- Flood Maps
Refer to this article in our knowledge base to learn more about flood maps.
- Stage & Flow Hydrographs
Refer to this article in our knowledge base to learn more about stage and flow hydrographs.
- Profile Line Plots
Refer to this article in our knowledge base to learn more about profile line plots.