Often times your detention pond designs will involve extended detention times. 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. Draw down times are computed at each Stage shown on the output Table where you enter your outlet devices. It uses the following equation:
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. It has a lot of storage compared to the small outflows, Total Q.
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.