Runoff is an important aspect of the hydrologic cycle. Understanding the amount of water that will be conveyed to hydraulic structures such as storm drains, pipes, culverts, or waterbodies is critical to designing effective stormwater infrastructure. For this reason, it is important that hydrologic and hydraulic (H&H) modelers accurately depict the relationship between rainfall and runoff. There are a number of ways to model stormwater runoff. A common example of an empirical method for calculating runoff is the SCS Curve Number Method. There are also a number of software programs that take into the spatial variability of stormwater runoff.
Traditionally, hydrologic and hydraulic modeling have been somewhat separate exercises in the sense that two modeling programs are required. For example, HEC-HMS may be used to perform hydrologic calculations and generate a hydrograph that would serve as the input for an HEC-RAS model. However, rain-on-grid modeling combines hydrologic and hydraulic modeling by applying rainfall directly to a 2D mesh. The following blog post describes rain-on-grid modeling using the Hydrologic Engineering Center’s River Analysis System (HEC-RAS).
Comparison of HEC-RAS Rain on Grid Modeling and HEC-HMS
The Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) is a program developed by the United States Army Corps of Engineers to model hydrologic processes. The predecessor of HEC-HMS, HEC-1, performs similar computations. Some engineers still prefer to use HEC-1 because it allows for more control in some ways. However, HEC-1 does not have a graphical user interface (GUI).
Before HEC-RAS added the Precipitation boundary condition, which allows users to perform rain-on-grid modeling, hydrologic and hydraulic (H&H) modelers needed to develop a separate hydrologic model using HEC-HMS or HEC-1. The output of such models would serve as the input for a hydraulic model developed in HEC-RAS. Although HEC-RAS rain-on-grid modeling affords H&H modelers the opportunity to capture hydrology and hydraulics in one model, the two programs do not perform calculations in the same way. The table below outlines the differences between rain-on-grid modeling in HEC-RAS and hydrologic modeling in HEC-HMS.
| Calculation Description | HEC-RAS 2D | HEC-HMS |
| Infiltration is the process of water entering the soil profile through the ground surface. | Infiltration Calculation Methods Available: – Deficit and Constant – SCS Curve Number – Green and Ampt | Infiltration Calculation Methods Available: – Deficit and Constant – Exponential – Green and Ampt – Gridded Deficit Constant – Gridded Green and Ampt – Gridded SCS Curve Number – Gridded Soil Moisture Accounting – Initial and Constant – Layered Green and Ampt – SCS Curve Number – Smith Parlange – Soil Moisture Accounting |
| Routing | Water is conveyed through cell faces | Basins are routed together using generalized calculations for each basin |
| Hyetographs which is rainfall intensity over time. | – The user can only input a single hyetograph per flow area | – Can develop hyetographs from rainfall depth input |
| Results | – Flows, velocities, and flow depths are calculated for each cell face. | – Generates a single hydrograph output per model element |
Why Rain-On-Grid Modeling?
Many engineers and modelers are accustomed to developing hydrologic models well. With so many robust hydrologic modeling tools available, why even bother with rain-on-grid? There are some reasons to apply precipitation to your HEC-RAS 2D mesh.
Rain-on-grid may be a good option for modeling urban areas. This is because traditional one-dimensional (1D) modeling does not fully capture the complex flow patterns that are typically seen in urban areas. Using the rain-on-grid modeling approach is also useful when building a model that contains multiple undefined flow paths. It can be difficult to assign boundary conditions to watercourses with highly bifurcated flows.
Adding a Precipitation Boundary Condition
To apply rainfall to a two-dimensional (2D) mesh in HEC-RAS, first navigate to the Unsteady Flow Data Editor.
Next, click the Add SA/2D Flow Area button, which is circled in red on the image below.
Select the applicable 2D flow area and use the arrow in the middle of the screen to move it to the right side of the box. Finally, click Ok.
Next, click the Precipitation boundary condition button. The window shown in the image below will appear. Simply populate the Precipitation column with your hyetograph. It is important to note that the precipitation values you enter in the Precipitation Hydrograph dialog box should represent excess precipitation unless your terrain has an infiltration grid associated with it.
It is important to note that you should proceed with caution if you choose to adjust the time step based on the Courant Number. This is because HEC-RAS will look at your highest velocities, which might not be in your channel if your 2D mesh contains areas with steep slopes. As result, you might end up running your 2D model for much longer than you would if you used a fixed time step.
Adjusting the 2D Flow Area Properties for Rain on Grid Modeling
The default values may not allow for enough detail to depict the very shallow depths that need to be calculated when preparing a rain-on-grid HEC-RAS model. Thus, the default tolerances shown in the table below should be decreased by one or two orders of magnitude. This will produce cell face property tables that compute volume at more discrete intervals so that really shallow depths are not overly simplified.
Adding Infiltration to a Rain-on-Grid HEC-RAS Model
HEC-RAS allows the user to create an Infiltration Layer in RAS Mapper for the purpose of estimating losses. The Infiltration Layer defines the method that will be used to estimate losses from a precipitation event. The methods available in HEC-RAS include the SCS Curve Number Method, the Deficit and Constant Method, and the Green-Ampt method. A list of the data required for each of these methods is provided below.
Data required for the SCS Curve Number Method:
- Curve number for each land cover/hydrologic soil group combination.
- Initial abstraction ratio for land cover/hydrologic soil group combination.
- Minimum infiltration rate for each land cover/hydrologic soil group combination.
Data required for the Deficit and Constant Method:
- Maximum deficit (in).
- Initial deficit (in).
- Potential evapotranspiration (ET) (in/hr).
- Potential infiltration rate (in/hr).
Data required for the Green-Ampt Method:
- Wetting front suction (in).
- Saturated hydraulic conductivity (in/hr).
- Initial soil water content.
- Saturated soil water content.
- Soil Potential Evapotranspiration (ET) (in/hr).
You can import an Infiltration Layer created outside of HEC-RAS. To create an Infiltration Layer in RAS Mapper, the user must first generate a land cover dataset and Soils Layer. The following sections will describe how to create a land cover dataset and Soils Layer in RAS Mapper, and how to use those layers to create an Infiltration Layer that can be used in your rain-on-grid model.
Creating a Land Cover Dataset
To create an infiltration layer in HEC-RAS, you must first create a land cover dataset. Creating a land cover dataset is also helpful for defining a Manning’s n for each land cover type. You can create your own land cover dataset in GIS and import the resulting shapefile or raster layer into HEC-RAS. Alternatively, you create a land cover dataset in HEC-RAS using data from the National Land Cover Dataset (NLCD). This method is useful if you are modeling a large area and it is impractical to delineate land cover manually using aerial imagery. After importing the NLCD data, use Classification Polygons to further refine your land cover dataset. After importing your land cover dataset and finalizing the Classification Polygons, right-click your land cover data and select Edit Land Cover Table as shown in the image below.
The following table will appear. Populate the Manning’s n column of the table. The Percent Impervious column is optional depending on what infiltration method you select for your model.
Creating a Soils Layer
Next, you will need to add a Soils Layer to your HEC-RAS model that will classify soils into hydrologic soil groups (HSG). In the United States, this information can be obtained from the Natural Resources Conservation Service (NRCS) Soil Survey Geographic (SSURGO) Database. Simply download the soil data associated with your study area from the NRCS Web Soil Survey. Then select Create a New RAS Layer and Soils Layer from the Project menu in RAS Mapper (see image below).
Then navigate to the geodatabase (.gdb) file you downloaded from the Web Soil Survey. After the data loads, use the dropdown menu to select whether your Soils Layer will be based on hydrologic soil group, texture, or both.
Finally, click the Create button and HEC-RAS will generate a raster layer of the soils dataset.
Creating the Infiltration Layer
Finally, to create the final infiltration layer, select Create a New RAS Layer and Infiltration Layer from Land Cover / Soils Layers from the Project menu in RAS Mapper (see image below).
The dialog box shown below will appear. You can then select the applicable infiltration method, Land Cover Layer, and Soils Layer. You do not have to select both a Land Cover Layer and Soils Layer. The time between rainfall events to establish initial loss and soil moisture is also an option. It is useful if you have a multi-peaking event. Finally, click the Create button to generate the infiltration layer.
After creating the infiltration layer, right-click your infiltration dataset in RAS Mapper and click Edit Infiltration Data Table as shown below.
A table like the one below will appear and allow you to populate relevant infiltration parameters.
Specify Geometry Associations
After creating the datasets described above, right-click Geometries in RAS Mapper and click Geometry Associations. The following table will appear. Use the dropdown arrows to associate your land cover and infiltration layers with the appropriate geometry file.
Adding Gridded Precipitation Data to HEC-RAS
After preparing the geometry files as described above, it is time to work on preparing your flow data in the Unsteady Flow Data Editor. In the Unsteady Flow Data Editor, there is now (in HEC-RAS version 6) a tab called Meteorological Data. By default, Precipitation/Evapotranspiration is disabled. Simply, use the dropdown menu to enable the meteorological variables. Some options will then appear under Precipitation. The following sections will describe each of the three modes available (e.g., Point, Gridded, Constant).
Point
You can also add precipitation data to your HEC-RAS model as point gauge data. HEC-RAS interpolates the point gauge data and converts it to a gridded format. To add this kind of precipitation data to your HEC-RAS model, first, click on Create/Edit Stations to enter the name and coordinates of each gage as shown below (Figure 4-12 from the HEC-RAS 2D User’s Manual).
Next, select the interpolation method (Thiessen Polygon, Inverse Square of the Distance, Inverse Distance Squared (Restricted), Peak Preservation). Finally, upload the DSS file for each rain gage.
Gridded
The Gridded option allows you to enter precipitation files as either a DSS file or a raster. The DSS option is useful if you used another program to get the precipitation data into that format. The United States Army Corps of Engineers has a program, which is freely available, that can help you do this. It is called HEC-MetVue. You can check out that program using the link below.
If the GDAL Raster Files option is selected, that data must be in either the NetCDF or the GRIB file formats. These are National Weather Service (NWS) file formats for NWS gridded data.
Constant
The Constant option allows the user to enter a constant rainfall intensity as shown below.
Evapotranspiration and Wind Data
Evapotranspiration (ET) data is only used for the Deficit and Constant and Green and Ampt infiltration methods. Even then, it is optional and is only useful when modeling dry periods between rainfall events. ET is added to HEC-RAS similar to precipitation. It can be entered as gridded data, point gauge data, or just a constant rate.
Wind forces can be incorporated in both 1D and 2D unsteady flow modeling. Like precipitation and ET data, wind data (speed and direction) can be entered as either gridded data or point gauge data.
Adjusting Rendering Mode for a Rain on Grid Model
After running your model with a Precipitation boundary condition, you may notice that the model results appear disjointed. In order to give your model results a smooth appearance, it is important to modify the render mode. In HEC-RAS version 6.2, you can do this by navigating to RAS Mapper and selecting Options under the Tools menu. Then click Render Mode under Project Settings.
Simply adjust the Plot Tolerance to only plot water depths greater than the value indicated. You may also want to consider checking Use Depth-Weighted Faces (“Precip Mode”). This option gives cell faces with deeper water depths more weight than those with shallower water depths. It is important to note that changing the way that adjusting the options in Render Mode does not change the HEC-RAS results/computations. Rather, it just adjusts the way the results are displayed.
Limitations of Rain-On-Grid Modeling
The following section will help you understand some of the limitations of rain-on-grid modeling in HEC-RAS.
All models are wrong, but some are useful
George E. P. Box
Terrain Data
Like a 2D hydraulic model, rain on grid models that contain terrain data with a coarser spatial resolution may not capture the hydraulics associated with smaller terrain features and thus, will not be as accurate as a model with better terrain data. It is also important to ensure that modeler adds breaklines to capture roadway crowns and berms that may influence flow patterns.
Computation Time
Rain on grid HEC-RAS models takes a very long time to run because every cell within the 2D mesh is wetted by rainfall. You also have to include the entire catchment area to generate accurate results and the simulation time needs to be long enough to capture the time of concentration for the entire drainage area.
Subsurface Drainage Systems
HEC-RAS cannot simulate subsurface drainage systems.
Limited Infiltration Calculations
HEC-RAS can model infiltration over a 2D flow area. However, infiltration is subtracted from the precipitation hyetograph. The program is not capable of performing infiltration calculations based on water depth. HEC-RAS can estimate infiltration using three methods: the Deficit and Constant Loss method, the SCS Curve Number Method, and Green and Ampt.