Routing Through Detention Ponds

The procedure for routing a hydrograph through a detention pond is quite similar to using any of the other processing functions like the Junction, Channel Reach or Divert. You simply select an inflow hydrograph (one you’ve already created) and click the [Route] button. However, in this case, there will need to be an existing pond to route through before clicking [Compute]. See Creating Detention Ponds for a complete discussion on creating and designing a detention  pond.

Even though you may have used the Trial Route feature in the Pond Designer, you’ll need to perform the real routing back here at the Main Window. Once you have set up at least one pond, you can route any existing hydrograph through it. You may specify only one inflow hydrograph. If more than one needs to enter the pond, combine them using the Junction process first.

To route a hydrograph, select the inflow hydrograph first by dragging a rectangle around it with your mouse or alternatively, click on the icon.

Then click the [Route] button on Ribbon Toolbar.

A Pond Route icon will be added to your model similar to the following. Note that the Pond Route icon represents the pond-routed outflow hydrograph, not the actual pond. The Pond is simply one of its input parameters.

Next, click on the Route icon to populate its input window.

Enter a Name, select an inflow hydrograph, select a pond and click [Compute].

Required Data

The following is a description of each of the required input items.

Name Prefix
You can include a prefix (“Pre”, “Post” or nothing to your hydrograph Name field with a single mouse click. This saves precious keystrokes when doing a pre- and post-development study and you want to identify those hydrographs that are “Pre” or “Post” development. The chosen prefix will be added to the front of the Name label on the Basin Model and throughout the software, for example, Tables, Charts, Reports, etc.

Enter any descriptive name for this hydrograph. It will appear on the printed reports as well as the Basin Model.

Inflow Hydrograph
The inflow hydrograph has already been selected but can be edited at any time. Just select from the drop-down list.

Pond Name
Select the pond you wish to use for this routing (Ponds 1 through 25) from the drop-down list. You can edit an existing pond or create a new one directly by clicking the adjacent Pond Edit button.

Your Routing Options

Hydrology Studio offers four separate routing options.

1. Standard (default)
This option will perform a standard routing.

2. Interconnected Pond Routing
Use this option to perform an interconnected pond routing where an Upper pond flows into a Lower pond. (See Interconnected Pond Routing below for more information.) 

Here you will be asked to specify the Upper and Lower Ponds as well as an optional additional inflow hydrograph into the Lower pond.

Upper Pond
Select an existing Upper Pond from the drop-down list box.

Lower Pond
Select an existing pond for the Lower pond. This pond cannot be the same as the Upper Pond.

Additional Inflow (applicable to Interconnected ponds only)
You can choose an additional inflow hydrograph to be added to the Lower Pond as long as this hydrograph was previously added to your Basin Model. Select the hydrograph from the drop-down list.

The additional inflow hydrograph must have a number greater than the primary inflow hydrograph and less than the outflow, routed hydrograph. In the example shown below, Inflow Hyd is Hyd No. 1, Add In is Hyd No. 2 and the Inter Connected is Hyd No. 3. Hydrograph Numbers must increase as you work downstream.

3. Wet Pond Routing
This feature allows you to route through a pond which has a pre-set amount of water in it. Remember that when creating this pond you should describe the entire pond, from Stage zero up to the Top. Not just the part that’s above the wet pond water surface.

Select the wet pond water surface elevation from the drop-down list. Note that there cannot be outflow from the pond at this chosen wet pond elevation.

4. Variable Tailwater
This option allows you to specify fixed tailwater elevations for each active return period. Simply enter the tailwater elevation in the input grid. Note that any tailwater elevation that you may have used while setting up your pond will not be carried over to the final routing. You must specify your tailwater elevation(s) on this screen.

Click [Compute] when finished.

A Rational method hydrograph routed through a pond

Interconnected Pond Routing

Use the Interconnected routing feature when you have two separate detention ponds and you want to connect them. Hydrology Studio can route any hydrograph through two connecting, or “interconnected” ponds.

When the Downstream Pond is Below the Upstream Pond

Keep in mind that a routed outflow hydrograph from an upstream pond can simply be used as the inflow hydrograph into the second pond. You simply do two separate routings and not employ the interconnected pond routing option. Do this when the Lower pond is sufficiently below the Upper pond and has no backwater affects on the Upper. The Basin Model will look like this:

Figure 1.
Lower pond has no affect on Upper pond

Downstream Pond is Near Same Elevation as the Upstream Pond

However, if the downstream Lower pond is at or near the same bottom elevation as the Upper pond, that can affect the stage-discharge relationship of the Upper pond. This in turn will affect the outflow hydrograph from the Upper pond. This scenario rarely affects the final outflow hydrograph from the Lower pond but this case requires an interconnected routing procedure.

In the Basin Model shown below, the Interconnected Hydrograph icon represents both routings (Upper pond through the Lower Pond).

Interconnected Pond Routing

Figure 3.

Lower pond affects the Upper pond

Here is a Blog post about when and when not to use this feature. Routing Interconnected Ponds. Should You or Shouldn’t You?

Interconnected Pond Calculation Procedure

The routing process is accomplished by dynamically linking the two ponds together. The routing calculation procedure is done in single time increments (equal to the project Time Interval). During this procedure, the outflow from the upper pond, over one time increment, is in turn routed through the lower pond. The computed stage in the lower pond is then used as a tailwater for the upper pond for the next calculation increment. Its stage-discharge curve is recomputed, no interpolation from the original Stage-Discharge curve. A new outflow is computed from the upper pond and the process is repeated for the remaining time increments.

Thousands of detailed computations take place during this procedure. Hydrology Studio’s fast and highly structured source code is demonstrated.

Although the program computes two hydrographs during this procedure, only the Lower Pond outflow hydrograph is available for further downstream processing. The Upper Pond hydrograph is attached to the lower (primary) hydrograph and will appear on all reports, graphs etc.

Only two ponds can be interconnected at once.

The Upper pond’s outflow hydrograph is shown as a dotted line. The final outflow hydrograph is the solid red line.