HydroCAD® Stormwater Modeling - Since 1986
Detention time is a measure of how long a given molecule of water resides within a pond before being discharged. HydroCAD provides two techniques for evaluating the detention time, as described below. These values are automatically calculated for all pond routings, and the results are displayed in the pond summary report.
Note: Detention time calculations are not routing procedures. They are a separate analysis that is performed after the actual hydrograph routing has been completed by the specified routing procedure.
Center-of-Mass detention time
The center-of-mass method is a basic procedure for calculating detention time. It works by calculating the time difference between the center-of-mass of the inflow and outflow hydrographs.
The center-of-mass detention time is automatically calculated by HydroCAD-7 and later, and appears near the top of the pond summary like this:
The values in parenthesis are the center of mass times for the outflow and inflow hydrographs, and may be useful when verifying the results.
Although the center-of-mass method has the appeal of an intuitive graphical solution, it cannot fully account for variations in the shape of the inflow hydrograph, or for the effects of so-called "dead storage".
Plug-flow detention time
A better approach is the plug flow method, which can be used to determine the theoretical average detention time within any pond. In essence, this technique indicates the average length of time that a given molecule of water resides within the pond.
This is a theoretical maximum value, in that it assumes first-in-first-out flow within the pond. This means that any any pre-storm volume (such as "dead storage" that is pre-filled with water) will be discharged before any of the new inflow, resulting in an increased detention time. Depending on the layout of the pond and the location of the inflow relative to the outlet, some mixing and short-cutting will occur, resulting an an actual detention time that is somewhat less than the theoretical maximum. (See this sample pond for one solution.)
The plug flow detention time is automatically calculated by HydroCAD-5 and later versions. The plug flow report will be found in middle of each pond summary. For further details, see Section 23 of the HydroCAD Owner's Manual.
My detention time seems too low!
Look at the hydrograph and make sure the time span is long enough to cover the entire time range of the inflow and outflow hydrographs. An extended time span is often needed to include the entire outflow volume. When the time span is adequate, you will see the outflow hydrograph decrease very close to zero, as shown in the illustrations above. The outflow volume (as reported on the pond summary) should also be very close to the inflow volume.
How do I increase my detention time?
In general, detention time can be increased by using a smaller outlet device with the appropriate amount of storage. In some cases (such as low-volume water quality events), this may require an unreasonably small outlet, which is prone to blockage. A common solution is to raise the outlet above the bottom of the pond, and provide enough storage capacity so that the water quality event can be fully retained, thereby providing an infinite detention time. The draw-down of the pond can then be allowed to occur over an extended period, often by infiltration. In some cases, a "wet pond" may also provide a suitable detention-time, based on the first-in-first-out criteria used by the plug flow method, as described above.
Other ways to specify detention time
Since the plug-flow and center-of-mass calculations are not practical to perform by hand, local stormwater regulations may use simplified techniques that are easier to evaluate. They may also use different measures of "detention time" that give different results than the center-of-mass or plug-flow methods described above. A careful reading of the regulations is required to discern any such differences. Detention time may also be specified in terms of "first-inch" calculations.
Is this the same as the draw-down time?
No. To determine the time required to empty the pond, see the elevation column of the tabular hydrograph. This will allow you to determine the exact time when the pond level falls to any given elevation. To see the elevation in graphical form, right-click the hydrograph plot and select Report Items|Select|Elevation from the menu.
With HydroCAD 8.5 and later, you can also tabulate the cumulative discharge (volume) vs. time. (Right-click the tabular hydrograph and use the "Select" menu.)
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