Spreadsheet: 28edd9bb-ba73-4940-8e20-af1b7e7cbb1e
Sheet: Detention Pond

DETENTION POND CALCULATOR
This calculator sizes volumetrically a detention pond such that it treats the Water Quality Volume (WQv).
CALCULATOR
Detention Pond Calculator - by Water Quality Volume
1 | Water Quality Volume - WQv |  |  | [NAME]
2 | Forebay Volume Calculator |  |  | [NAME]
3 | Dimensions Calculator
4 | Depth at WQv - d |  |  | feet (generally between 2 and 5 feet, recommend 3 feet)
5 | [NAME] to [NAME] (L/W) |  |  | generally between 2 and 4
6 | Pond Side Slope |  |  | feet/feet (ratio of horizontal to vertical, for example, 3 for 3H:1V)
7 | [NAME] of Pond Bottom |  |  | feet (adjust width until storage volume in Step 10 is greater than WQv)
8 | Length of Pond Bottom |  |  | feet
9 | [NAME] of Pond at WQv Depth |  |  | feet
 | Length of Pond at WQv Depth |  |  | feet
10 | Storage Volume |  |  | [NAME] (must be greater than WQv)
11 | Check of Volume
12 | Drain time - t | 48 |  | hours (recommend 48 hours)
 | Surface Area at Mid Elevation |  |  | square feet
 | Orifice Area |  |  | square feet
13 | Approx. Orifice Diameter for Drain Time |  |  | inches (refine outlet control sizing and configuration during design)
14 | Flow Rate thru Orifice |  |  | gpm (flow rate when basin is at WQv depth)
 | Est. Impervious DMA |  |  | ft2
 | Sizing Ratio (BMP/DMA)
 | DP + Forebay
INSTRUCTIONS
Follow the steps below to determine the required size of your detention pond.
Step
1 | Enter the Water Quality Volume (WQv) for the DMA. The WQv is calculated in Step 14 of the Water Quality Volume and Water Quality Flow Rate Calculator.
2 | The required volume for the forebay is shown here (typically 10 to 25% of the WQv).
3 | Use the following steps of the Dimensions Calculator to determine the configuration of the detention pond.
4 | Enter the design water depth of the detention pond (i.e., the water depth when the basin has completely captured the WQv event). [NAME] are typically between 2 and 5 feet. This depth DOES NOT include freeboard, which is typically 1 foot above the maximum design depth (freeboard should be added in actual design).
5 | Enter the length to width ratio of the detention pond.  A ratio between 2 and 4 is recommended.
6 | Enter the side slope of the pond. The slope of earthen side walls should be no steeper than 3:1 (i.e., at least 3 feet horizontal for each 1 foot vertical). Enter 0 for vertical hardscaped sidewalls.
7 | Enter the width of the pond bottom at the invert of the detention pond.
8 | The length of the pond bottom is calculated here.
9 | The width and length of the pond at the design water depth (i.e., [NAME]) are calculated here.
10 | The storage volume in the pond is calculated here. Adjust the basin depth and/or width (entered in Steps 4 and 7, respectively) until the volume is greater than or equal to the WQv entered in Step 1.
11 | This step checks that the storage volume is greater than the WQv entered in Step1. If the volume is less than the WQv, increase the pond width or depth.
12 | The recommended design drain time for the basin is 48 hours.
13 | The diameter of the orifice needed to drain the WQv in 48 hours is shown here. This planning level estimate of the required outlet control should be revised during design. The pond outlet must be designed such that less than 50% of the WQv is emptied during the first third of the design drain time (i.e., within 16 hours for a 48 hour design drain time). Minimum allowable orifice diameter is 1-inch.
14 | The estimated flow rate through the orifice when the water level is at the WQv depth is calculated here.
DEFINITIONS
 | Water Quality Volume (WQv) | The runoff volume to be managed by the stormwater BMP such that treatment requirements are met. The WQv is calculated by the Water Quality Volume and Water Quality Flow Rate Calculator.
 | Freeboard | The vertical distance between the maximum design water depth and the top of the basin embankments.
 | Orifice | An orifice is a flow restrictor. It drains the pond slowly to improve water treatment through settling of pollutants.
 | Weir | A weir is a designed overflow facility, and is often the crest of the pond/wetland spillway.
 | [NAME] | A spillway is a structure to provide the control release of extreme flows, in the event the capacity of the pond/wetland is overwhelmed.
 | Drain Time (t) | The time for the storage area to drain the water quality volume.
 | Depth at WQv (d) | The depth of the basin from the design water level to the invert of the basin. Does not include freeboard.