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

INFILTRATION TRENCH/DRY WELL CALCULATOR
These calculators size an infiltration trench or drywell to treat the Water Quality Volume (WQv), or calculate the volume of water treated given user entered available dimensions.
CALCULATOR
Infiltration Trench/Dry Well Calculator - by Water Quality Volume
1 | Water Quality Volume - WQv |  |  | [NAME]
2 | Porosity - n | 0.35 |  | recommend 0.35 for coarse aggregate
3 | Infiltration Rate - f |  |  | inches/hour (generally require f ≥0.5"/hr)
 | Safety Factor | 2 |  | recommend 2
 | Fill time - T | 2 |  | hours (1 to 2 hours, recommend 2)
 | [NAME] | 48 |  | hours (recommend 48)
4 | Depth to GW/Bedrock |  |  | feet
5 | [NAME] |  |  | feet
 | Design Trench Depth - d |  |  | feet (between 3 to 8 feet)
 | Check Depth
6 | Base Area Calculator |  |  | square feet
7 | [NAME] of Trench |  |  | feet (should be less than 25 feet)
8 | Dimension Calculator
 | Depth |  |  | feet
 | [NAME] |  |  | feet
 | Length |  |  | feet
 | Dry Well Diameter |  |  | feet
 | Estimated Drainage Area |  |  | sf
 | BMP Area/Drainage Area
Infiltration Trench/Dry Well Calculator - by Available Dimensions
9 | Available Length (or Diameter for Dry Well) |  |  | feet
10 | Available Width |  |  | feet (should be less than 25 feet)
11 | Porosity - n |  |  | recommend 0.35
12 | Infiltration Rate - f |  |  | inches/hour (generally require f ≥0.5"/hr)
 | Safety Factor | 2 |  | recommend 2
 | Fill Time - T | 2 |  | hours (1 to 2 hours, recommend 2)
 | [NAME] | 48 |  | hours (recommend 48)
13 | Depth to GW/Bedrock |  |  | feet
14 | [NAME] |  |  | feet
 | Design Trench Depth - d |  |  | feet (between 3 to 8 feet)
 | Check Depth
15 | Treated Volume - Infiltration Trench |  |  | [NAME]
 | Treated Volume - Dry Well |  |  | [NAME]
INSTRUCTIONS
Use either steps 1 through 8 to determine the required size of your trench. Or use steps 9 through 15 to check what volume of water is treated given the available dimensions of your trench.
To Determine The Required Size of the Infiltration Trench/Dry Well
Step
1 | Enter the Water Quality Volume (WQv) for the drainage management area (DMA). The WQv is calculated in Step 14 of the Water Quality Volume and Water Quality Flow Rate Calculator.
2 | Adjust the porosity of the trench/dry well backfill material if necessary. Recommend using a default value of 0.35. For a dry well without backfill use 1.
3 | Enter the infiltration rate of the underlying soils. This should be determined by geotechnical testing or, for conceptual studies, soil infiltration maps.  A 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. The default value for fill time is 2 hours. The default drain time is 48 hours. These values should not be changed unless explanation is provided by user.
4 | Enter the lesser value of the depth to groundwater and depth to bedrock at the site. The bottom of the trench should be at least 4' from bedrock and the high groundwater table.
5 | The maximum allowable depth of the trench/[NAME] is calculated here. In the next row, enter a design trench depth less than or equal to the [NAME] allowable depth.
6 | The trench/dry well surface area calculator determines the base surface area of the trench/dry well that is required to treat the design WQv.
7 | To calculate actual trench dimensions, enter a desired width for the trench. Generally this will be between 3 and 8 feet, and not exceed 25 feet.
8 | The required dimensions for your WQv trench or dry well are determined here.
To Determine the Volume of Water Treated by Available Dimensions
9 | Enter the design length of your trench. For a Dry Well enter the design diameter, and skip Step 11.
10 | Enter the design width of your trench.
11 | Adjust the porosity of the trench backfill material if necessary. Recommend using a default value of 0.35. For a dry well without backfill use 1.
12 | Enter the infiltration rate of the underlying soils. This should be determined by geotechnical testing or, for conceptual studies, soil infiltration maps.
 | A safety factor of two will be applied in the calculations to account for the decrease in infiltration rate over time due to accumulation of solids in the trench.
 | The default value for fill time is 2 hours. The default drain time is 48 hours. These values should not be changed unless explanation is provided by user.
13 | Enter the lesser value of the depth to groundwater and depth to bedrock at the site. The bottom of the trench should be at least 4' from bedrock and the high groundwater table.
14 | The maximum allowable depth of the trench/[NAME] is calculated here. In the next row, enter a design trench depth less than or equal to the [NAME] allowable depth.
15 | The volume of water treated by your trench or dry well design is shown here. If this volume is less than the required WQv then additional treatments will likely be required.
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.
 | Porosity (n) | Volume of void space to total volume.  This is expressed as a ratio, e.g. 0.35 means that one cubic foot of gravel contains 0.35 cubic feet of empty space.
 | 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.
 | Fill Time (T) | The time taken for the trench/ dry well to fill with runoff. Use recommended values or calculate iteratively with runoff hydrographs and trench volumes.
 | Drain Time (t) | The time for the trench/dry well to drain the water quality volume.
 | Depth (d) | The depth of the trench/dry well from the surface of the gravel backfill material.
 | Length | The longitudinal length of the trench.
 | [NAME] | The width of the trench.
 | Base Area | The area of the base of the trench or dry well.