Dr. Nelson
Overview
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, storage occurs which can greatly reduce peak flow (depending on the size and outlet structures). This hydrologic process can be modeled in HEC-HMS and is commonly called reservoir routing. As you have already seen in HEC-HMS tutorials, the storage-elevation-discharge information about the reservoir must be defined in order to perform reservoir routing.
In this assignment we will simulate the routing with the Outflow Curve and Outflow structures methods. You should review Chapter 3 of the HEC-HMS tutorial which deals with the reservoir routing options.
Model set up
Use your WMS project file that you used to create basic Judy's 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 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 or Elevation-Storage-Discharge or Storage-Discharge curves. Selection of any one of the curves is often based on the available data.
In this assignment we will use Elevation-Storage-Discharge curve. The way you use other curves is same as Elevation-Storage-discharge method. If you remember, you were provided with the elevation-Storage-Discharge data in the HEC-HMS tutorial. But for this assignment you will generate your own storage discharge elevation curve.
Use the following model:
Watershed: Single Basin with a detention basin at the outlet
Loss Rate Method: Green and Ampt ( Initial loss = 0.3 in, Moisture deficit = 0.2, suction = 12 in, Conductivity = 0.25, Impervious = 0%)
Transform method: Clark
Lag time: Kerby Method for Overland Flow for Tc
Precipitation: 2.5 inches with a Type I temporal distribution.
Model Run: 36 hrs @ 6 mins interval
Then, follow this tutorial to define the Reservoir
Save your HMS file and open in HEC-HMS. The version of WMS 8.0 you are using does not map the Outflow Curve data properly to HMS (but it has already been fixed in the new release). So, you will enter the outflow curve data in HMS (As you had done in the HMS tutorial).
How to generate Storage-Elevation-Discharge Curve
Fortunately! WMS can create such rating curve based on the geometry of the detention basin.
Switch to the Hydrologic Modeling Module and select the outlet of your basin.
Select Calculators | Detention Basins. This will open elevation-Storage-Discharge dialog. Click the Define button.
In the new window, you can define the elevation and discharge methods. Click on Define Storage button.
Select Known geometry at the right and enter the following information about the detention basin.
Length: 300 ft
Width: 200 ft
Depth: 8 ft
Side slope: 0
Base elevation: 129 ft
Click Ok, this will populate the elevation storage-elevation data for your detention basin.
Next, click on Define Discharges button and click Add Outlet.
Change the Name of outlet to Lower level outlet, Type: Circular, Diameter 2.0 ft, Invert elevation: 129.5ft, Manning's n: 0.05, Slope: 0.005ft/ft.
Now you will add the upper outlet. Click on Add weir button. Change the Name to Upper outlet, Weir Length: 30ft, Weir elevation: 134ft and click OK.
Now you can see the storage-elevation-discharge data for the detention basin. You will cut and paste these data in HMS. It is better to save the data in Excel spreadsheet before you open HMS.
Follow these steps to enter the storage-elevation-discharge information in HMS.
Run the HMS model.
Compare the results with the results from Muskingum Cunge routing method so that you are comparing the cases with no reservoir and with reservoir.
2. Outflow structures routing method
HMS allows you to have outflow structure (s) for the detention basin through which you are routing your hydrograph. Defining output means you do not have to define the discharge curve now as you did in case of outflow curves method. In this assignment we will define 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.
Follow this tutorial to define the outflow structures.
Compare the results with the results from Outflow curves method and Outflow structures method.
3. Determine the precipitation
Go to NOAA web site for Illinois. In the frequency storm page, determine what is the return period for the precipitation of 2.5 in.
Now, keep increasing (or decreasing) the rainfall in the model you used for part 2 until the upper outlet is just at the verge of being overtopped?
Determine the rainfall depth and its return period at which both the outlets will contribute to the flow.
4. No Lower Outlet:
In the HMS model you used for part 2, now remove the lower outlet and run the model again with all other condition same.
Compare the results with the results with "With lower outlet" case.
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