Often times your detention pond designs will involve extended detention times that require the pond become empty within a certain time, say 24 hours. This readies the pond to be available for any subsequent rain events.
Typical requirements can be met by providing a 12 or 24-hour drain time for a portion of a water quality volume. Hydrology Studio automatically computes drawdown times for each pond and tabulates it on the Stage-Storage-Discharge Table.
So what exactly is Pond Drawdown? In this case, imagine the pond being X% full. Then pull the plug so-to-speak. The drawdown time is the time it takes for the pond to completely empty from X%. The plug is your outlet structure.
Hydrology Studio uses the “average head/average discharge” method to compute drawdown time. Drawdown times are computed at each Stage shown on the Output Table where you enter your outlet devices. It uses the following equation:
Where:
Time = incremental drawdown time from Stage n to Stage n-1
dStor = change in storage from Stage n to Stage n-1
Qave = average outflow from Stage n to Stage n-1
As shown in the table below, it would take 3.49 hours for this particular pond to drawdown from Stage 0.35 feet.
Hydrology Studio computes time to empty until the flow, Qave, reaches 0.01 cfs. Once the outflow in the pond gets below 0.01 cfs, the drain time calculations stop and it is assumed the pond is, for all practical purposes, empty.
Stage vs. Drain Time Plot
The Stage vs. Drain Time plot is available at Step 3 – Add Outlets in the Pond Designer. This chart provides an excellent visual guide as shown below. Just make adjustments to your outlet devices until the desired drain time is achieved.
For example, if you have chosen to include a water Quality Volume (WQv) in Step 1, the Stage vs. Drain Time plot will reveal the elevation in the pond that represents that volume as well as its time to drain. In the example below, it shows that the WQv fills the pond up to elevation 102.5 and it takes around 15.4 hours to completely drain.
Retention Pond Recovery in Shallow Aquifers
Looking to calculate the recovery times or mounding analysis (unsaturated and saturated) for retention ponds in shallow sandy aquifers? Check out Studio Express Groundwater feature.
