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HEC-HMS Part 1 CE 421, Fall 2005
Background
For the Bethlehem Township project, it will be necessary to estimate peak flows and hydrographs at the Santee Road area. We will also have to determine the effectiveness of existing detention facilities by “routing” the hydrographs through them. Because WinTR- (used in CE 372) is not very flexible for routing calculatins, we will be using a different program, the Army Corps of Engineers Hydrologic Modeling System (HEC-HMS) software (http://www.hec.usace.army.mil/software/hec- hms/hechms-hechms.html). HEC-HMS is not a specific method, rather it is a modeling framework that allows the user to choose how each component of the rainfall-runoff process (rainfall, infiltration, hydrograph transformation, baseflow, reach routing, reservoir routing) will be modeled. For example, the SCS methods that are specified by local ordinance can be incorporated into HEC-HMS.
The current version of HEC-HMS is 2.2.2 although there is a beta version 3.0 out. You may also use HEC-GeoHMS, which is supposed to work with ArcView and Spatial Analyst (I have never tried it). I forwarded you an email from Becky Rosenbauer describing how to install the program.
Setting up the Model
Upon starting the program (hms.exe icon), a Project Definition window will appear (as shown below) which lists the current project name and description, and lists the associated “Basin Model”, “Precipitation Model”, and “Control Specifications”. (note that multiple basin models, precipitation models, and control specs can exist under the same project file).
Since you are starting a new model, under the File menu, click "New Project" (or "Open Project" for an existing one)
When you create a new project, there will be boxes for naming and describing the project, and to create a project folder under c:\hmsproj. So that different groups working on the same computer are not altering the same set of project files, switch to another directory (on student or scratch) to save your project files. Enter OK
A new Project Definition window will now appear.
A recommended next step to save you time later is to go to the File-Project Attributes menu to set up the default units and hydrologic methods you would like to use throughout the project. For this project, you will use:
Basin Defaults : Loss Method = SCS Curve Number, Transform = SCS UH, Baseflow = No Baseflow, Channel Routing = Muskingum Met Defaults : Precipitation = SCS Storm, Evapotranspiration = no Units = U.S. Customary
Hit OK and you return to a front Project Definition window.Under the Component menu, you now create the Basin Model , the Meteorologic Model , and the Control Specifications
Basin Model
Select New to create your basin model. A New Basin Model window will appear will similar boxes as when you created the new project. Name it something useful like “Santee”. The directory path should be the same as for your project files. Enter OK
A Basin Model window will appear. This window is used to create a symbolic representation of the watershed. Icons for subbasins, reachs (channels), reservoirs, and junctions are clicked and dragged to the screen. For consistency with the existing modeling in the Nancy Run Watershed Act 167 Storm Water Management Plan , I recommend setting up the same sub-basins as in the plan (10 – 15). Use some logical naming system, not the default names – change names by double-clicking on the icon (you will then see other required information, like subbasin area, SCS curve number, etc.)
First drag all the necessary icons to the screen - you will need six sub-basins, four reaches (channels or flow paths that connect the subbasins), and a junction to represent the outlet. Reservoirs can later be used to represent detention basins.
Use the right click and “Connect Downstream” to connect the icons to the corresponding downstream elements. If you connect to the wrong icon, you can always delete the connection and do it over. The figure below shows a HEC-HMS representation of the Santee Road watershed. No detention basins (represented by reservoir elements) have been added yet.
Meteorologic Model
Select New for creating your meteorologic model. A New Meteorologic Model window will appear will similar boxes as when you created the new project and new basin. Name the model something useful like “SCS 2-yr Type II”. The directory path should be the same as for your project files. Select OK
A Meterologic Model will appear. Select SCS Hypothetical Storm, Type II, and add the storm depth in inches. This will depend on what 24-hr storm you wish to model (1-yr, 2-yr, 10- yr, 25- yr, etc).
When you select OK you will be returned to the Project Definition window. Note that the current selection will have a check- mark next to it.
A second handout on HEC-HMS will cover “Control Specifications”, the Run Manager, running the model, and model outputs.
