Spreadsheet: 2128d001-83e4-4dcb-88d0-1a337a0ecf0c
Sheet: Infiltration Basin

INFILTRATION BASIN CALCULATOR
This calculator sizes an infiltration basin to treat the Water Quality Volume (WQv).
CALCULATOR
Infiltration Basin Calculator - by Water Quality Volume
1 | Water Quality Volume - WQv |  |  | [NAME]
2 | Infiltration rate - f |  |  | inches/hour (generally require f>0.5"/hr)
3 | Safety Factor | 2 |  | recommend 2
4 | [NAME] drawdown time - t | 48 |  | hours (recommend between 24 and 48 hours)
5 | Base Area Calculator |  |  | square feet
6 | Side Slope |  |  | feet/feet (ratio of horizontal to vertical, for example, 3 for 3H:1V)
7 | [NAME] of Basin Bottom |  |  | feet
8 | Length of Basin Bottom |  |  | feet
9 | Depth at WQv - d |  |  | feet (generally between 1 and 3 feet, recommend 2 feet)
10 | [NAME] of Basin at WQv Depth |  |  | feet
 | Length of Basin at WQv Depth |  |  | feet
11 | Storage Volume |  |  | [NAME] (adjust depth in Step 9 until storage volume is greater than WQv)
12 | Check of Volume
 | Est. Impervious DMA |  |  | ft2
 | Sizing Ratio (BMP/DMA)
INSTRUCTIONS
Follow the steps below to determine the required size of your basin.
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 | Enter the infiltration rate of the underlying soils. This should be determined by geotechnical testing or, for conceptual studies, soil infiltration maps.
3 | A recommended safety factor of 2 will be applied to the infiltration rate to account for the inherent uncertainties in rate measurement and the decrease in infiltration rate over time.
4 | The recommended maximum drawdown time for the infiltration basin is 48 hours.
5 | The minimum base area of the infiltration basin is calculated here based on infiltrating the entire WQv within the maximum drawdown time.
6 | Enter the side slope of the basin. 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 sidewalls.
7 | Enter the width of the basin bottom.
8 | The length of the basin bottom is calculated based on the base area and width.
9 | Enter the design water depth of the infiltration basin (i.e., the water depth when the basin has completely captured the WQv event). [NAME] are typically between 1 and 3 feet. This depth DOES NOT include freeboard, which is typically 1 foot above the design depth (freeboard should be added in actual design).
10 | The width and length of the basin at the design water depth (i.e., [NAME]) are calculated here.
11 | The storage volume in the basin is calculated here. If the volume is less than the WQv entered in Step 1, increase the basin depth.
12 | This step checks that the storage volume is greater than the WQv entered in Step 1.
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.
 | Infiltration Rate (f) | The rate of water entry into the soil, generally expressed as inches/hour, and determined by geotechnical tests or estimated from soil maps. It equals the hydraulic conductivity of the soil times the hydraulic gradient.
 | 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.
 | Freeboard | The vertical distance between the maximum design water depth and the top of the basin embankments.