Using the Minimum Drain Time calculator in PondPack

Product(s): PondPack
Version(s): V8i, CONNECT Edition
Area: Modeling

Problem

How can I quickly determine the minimum drain time (minimum time to drain a pond from full to empty with a given outlet structure)

How does the Minimum Drain Time calculator work in PondPack?

Solution

Minimum drain time (MDT) is one of several performance parameters used to grade a pond's effectiveness in mitigating water quality impacts. Frequently, best management practice criteria is to design a pond outfall so that the time to completely drain a full pond is held above a specified value, for example, 12 hours.

The Minimum Drain Time tool is available in PondPack as a quick way to determine this time. In the CONNECT Edition, you can find it under Analysis > Calculators > Minimum Drain Time.

This tool computes the amount of time it takes water to completely drain, starting from a user specified water surface elevation in the pond. Because the pond's depth and outflow vary over time, this calculation actually requires a pond routing.

PondPack automatically computes the MDT by routing a dry hydrograph through a pond defined by its outlet works and volumetric rating. A dry hydrograph is a hydrograph that, at every time step, discharges zero flow. You supply the starting water surface elevation, pond size, outlet structure and the analysis time step. 

The time it takes the pond to completely drain is the minimum drain time, since zero inflow was used. For any inflows greater than zero, drain time would be longer. Meaning, the results you get from this tool will not be the same as computing your model and viewing the pond water surface elevation, since your model will route any upstream flow (such as attached catchments) into the pond. To see the same results, you would need to remove all inflows into your pond, set the starting water surface elevation in the pond properties to be equal to the value used in the tool, then compute the model and graph elevation for the pond.

The following figure shows the time versus water surface elevation curve for an example minimum drain time analysis. Note that the beginning water surface elevation is defined at time = 0 and that detention time is defined where the water surface recedes to the invert of the pond.

The relationship of volume to drain time is very important when using minimum drain time for water quality criteria. The image above shows the total drain time to be about 27 hours, but the image below shows that 74 percent of the volume drains out in only six hours.

If the minimum drain time criteria are not specified based on percentage of total volume versus time, a pond could be designed to meet the minimum drain time for total volume but still not achieve the desired water quality effects.

For instance, the pond in the following figure would meet a total volume minimum drain time requirement of, say 24 hours, but detains only 26 percent of the total volume for longer than 6 hours. If, however, the minimum drain time criterion was that this pond detain 50 percent of the total volume for at least 24 hours, the pond would not meet the requirement.


Below is a list of the Minimum Drain Time calculator tabs and input fields.

Minimum Drain Time: This tab consists of the following controls:

  • Elevation: Set the starting elevation of the water surface.
  • Start: Enter the time at which calculations begin. This value establishes the base time for the calculations. PondPack does not adjust the starting water surface elevation as if that amount of time has already passed. 
  • Increment: Enter the time step for the calculations. Make sure an appropriate increment is selected for the situation. For example, if you have a small pond with a large outlet structure, the time increment needs to be small enough to account for the large fluctuations in water surface elevation that could occur over a small period of time.
  • End: Enter the time at which calculations end.
  • Pond Node: Select the Pond you want to use from the list, or click the Select From Drawing button to select the desired element in the drawing pane.
  • Outlet Structure: Select the Outlet Structure used by the pond. Click the ellipsis (...) button to open the Composite Outlet Structures dialog.

Notes: This text field that allows you to enter descriptive notes that will be associated with the currently highlighted list pane entry.

Results: This tab displays the calculated results for the list pane entry that is currently highlighted. Select "Minimum Drain Time [Elevation vs. Time]" from the dropdown list to see the elevation over time. 

You can also click the Report button at the top to produce a text output report.

See Also

Wiki Article: Detention Times for a pond in PondPack

Help Topic: Minimum Drain Time

Help Topic: Minimum Drain Time Dialog Box

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