Evaluating Reservoir Routing in HEC-HMS

Dr. Nelson


If a watershed contains a reservoir or a detention basin, then the flood hydrograph is routed through this reservoir. As the flood wave passes through the reservoir, some of the runoff is stored in the reservoir.  This can greatly reduce the peak flow (depending on the size and outlet structures). This hydrologic process can be modeled in HEC-HMS and is commonly called reservoir routing. For HEC-HMS models storage-elevation-discharge relationships for the reservoir or detention basin must be defined.

In this assignment you will learn how to define detention basin routing using both the "Outflow Curve" and "Outflow Structures" methods available in HEC-HMS. We will practice with Judy's Branch and then review using the Park City Model

Model set up

Use your WMS project file that you used to create the basic Judy's Branch Project. In this model, the flood hydrograph will pass through the reservoir and get routed to the outlet. Your schematic detention basin will be like this.

The detention basin will have two outlets. One is closer to the bottom and is called the lower level outlet and the other is at the upper level and called the spillway (see the schematic below).

1. Outflow curve routing method

In this method, the response of the reservoir to the flood wave is expressed in terms of the Elevation-Area-Discharge, Elevation-Storage-Discharge, or Storage-Discharge curves. Selection of one of these curves is based on the available data.

For this part we will use the Elevation-Storage-Discharge curve. The way you use other curves is the same as the Elevation-Storage-Discharge method.

Use the following model:

 Then, follow the following steps to define the Reservoir

Define the reservoir in HMS model

1. Switch to Hydrologic Modeling Module and select HEC-HMS. Then click on the "Select Outlet" tool

2. Select the outlet by clicking it.

3. Right-click on the outlet and select Add | Reservoir

Setting up the Reservoir Routing Parameters

In order to define reservoir routing with HEC-HMS you should define elevation vs. storage (storage capacity curve) and elevation vs. discharge rating curves. You can enter values directly, or enter hydraulic structures and compute the values; in this exercise you will use both methods. You will use the same elevation values for both curves.

1. Double-click on the reservoir outlet point (it is now represented as a triangle since you have defined a reservoir at this location)

2. Change the Reservoir name to Judys.

3. Set the Method drop down to be Storage-Elevation-Discharge

You need to define the initial conditions of the reservoir. The initial condition can be defined as an elevation, a discharge, or a volume. For this example we will set the initial condition to an elevation.

4. Set the Initial drop down to be Elevation

5. Enter 129.5 ft for the Initial Value.

6. Select OK.

Generate Storage-Elevation-Discharge Curve

Fortunately WMS can create a rating curve based on the geometry of the detention basin.

Compare the results with the results from Muskingum Cunge routing method so that you are comparing the cases with no reservoir and with reservoir. See how the reservoir has attenuated the peak.

2. Outflow structures routing method

HMS allows you to define the outflow structure(s) for the detention basin through which you are routing your hydrograph.  Defining outflow structures means you do not have to define the discharge curve now as you just did in part 1 above.  Now we will define the two structures in HMS itself.

In HMS  open your model which you used for Part 1 (Outflow curve method) and save it as with a new name so that the previous file remains unchanged.

Defining the outflow structures in HMS for reservoir routing

1. In WMS save the same HMS file that you used for part 1 of this assignment (HMS model with Outflow curve method) with a distinguishing name.  We will modify this file so that you do not have to develop the same model from scratch.

3. Open the project in HMS and expand the Basin model, Meteorological model, Control specification, and Paired data in the watershed explorer window on the left.

 4. Select the reservoir by clicking the reservoir icon.

5. Then in the parameter edit window, change the Method to “Outflow Structures” (It should have "Outflow Curve" as the default) and click Yes when prompted.

6. Change the storage method to Elevation-Storage

7. For the "Ele-Stor" function, select the "ElevationStorage" data from the drop down list which you defined in the previous model. If you can't find the "ElevationStorage" data, you need to define it again as you did in the previous exercise.

8. Set the initial condition to Elevation and enter 129.5 ft as the "Initial Elevation."

9. For number of outlets, enter 1.

10. For number of spillways, enter 1.

11. Now you can see "Outlets" and "Spillways" are added to the reservoir in the watershed explorer. See the figure to the right. Expand the Outlets folder and select Outlet 1.

12. In the properties edit window, set the center elevation to 130.5ft, area to 3.142 square feet and coefficient to 0.6.

13. Similarly, expand Spillways and select Spillway 1.

14. In the properties edit window set  the elevation to 143 ft, length to 30 ft and coefficient to 3.33.

With these two outlets, the first outlet will discharge until the water surface reaches an elevation of 143 ft and after that, both the outlets will work simultaneously.

15. Now save your project.

16. Select Compute | Select Run | Run 1

17. Run the model by clicking the Run macro. Now you can see the results in the graphical form as well as the time series table.

Compare the results with the results from Outflow curves method and Outflow structures method.

3. No Lower Outlet:

In the HMS model you used for part 2, save another copy, remove the lower outlet and run the model again with all other conditions the same.

 Compare the results with the results with the "With lower outlet" case. Discuss why the area under the two hydrographs and thus the runoff volumes (with and without reservoir) are different.

To turn in Now: