LIDS / BMP Calculator (beta)
Read This First
- All infiltration must occur on-site. No LID features may be located off property (i.e. in the Town right of way or other private property). The only exception to this rule is upsloping lots with BMPs in the right of way that treat only runoff from the portion of the driveway in the right of way.
- Infiltration features must be located a minimum of 10' from the building foundation.
- If you navigate away or refresh the page you will start over. The Clear button in the menu bar will also cause you to start over.
- Grayed out fields cannot be edited, they are instead filled out automatically by the calculator.
- You will need the property APN to continue, click here to look it up.
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All units are as follows:
Length = feet | Area = square feet | Volume = cubic feet
- If gutters and downspouts are not used then 3" deep dripline armoring is required. The width must be 18" + 6" for each additional story.
- If you do not wish to take credit for the saturated hydraulic conductivity of on-site soil you may hit the "Bypass KSAT" button in the green menu bar. The maximum KSAT value allowed by the Town is 2.4
- Maintain a 2% downward slope away from the building for 10'
- If you would like to make two areas into one DMA simply combine the areas in the DMA section.
- A green checkmark should appear next to each of your proposed Drainage Management Areas. If no green check mark appears in the Volume Check column, verify correct inputs in all cells.
- This tool is available on mobile at the same web address, though you will need to have some form of PDF handling setup.
- Click here for the fact sheets containing more information on the LID BMP treatments and requirements.
Project Info
Drainage Management Areas (DMAs)
- Design Storm Volume is impervious surface area multiplied by volume of the 85th percentile 24hr storm event, identified above. As the design storm depth is reported in inches about, the formula includes a 1/12 adjustment to convert to feet.
- Formula: Vret = Aimp * V85 * (1/12)
- Retention Volume of BMP must be equal to or greater than Design Storm Volume of corresponding Drainage Management Area (DMA).
- Tired of scrolling back and forth? Try the "Toggle Sticky Table" button below.
| DMA # | (Aimp) Impervious Area (sf) | (Vret) Storm Volume (cf) | Volume Check |
|---|
Infiltration Trenches (IT), Dry Wells & Subsurface Retention
- Retention Volume is calculated using the actual void volume of the retention area and the Saturated Hydraulic Conductivity denoted as Ksat below.
- Formula w/ Ksat: Vret = (L*W*D*Vv) + (L*W*Ksat/12) + 2/3(L*D*Ksat/12)
- Formula no Ksat: Vret = (L*W*D*Vv)
- Max Void Volume for Gravel Fill is 40% and must use clean drain rock.
- Max Void Volume for Subsurface Retention Capsules is 94% and must be documented by providing manufacturer's cutsheet with calculations summary.
- Dry wells should be deeper than they are wide.
- BMP# - This number must correspond with associated DMA. If multiple BMPs are to be used for one DMA each BMP should have the same number.
| Associated DMA # | (Vv) Void Volume (0.4 = 40%) |
(L) Length (ft) | (W) Width (ft) | (D) Depth (ft) | (Vret) Retention Volume (cf) |
|---|
Infiltration Basins
- Retention Volume is the actual volume capacity.
- BMP# - This number must correspond with associated DMA. If multiple BMPs are to be used for one DMA each BMP should have the same number.
| Associated DMA # | (Vret) Retention Volume (cf) |
|---|
Naturally Vegetated Area (NVA)
- Naturally vegetated areas under 10% grade may be used as an infiltration area with a 2:1 ratio of impervious to pervious area being required (ie 500 sf of NVA can treat 1000 sf of impervious surface area). The total retention volume (Vret) is calculated as follows:
Formula: Vret = Aper * Dstorm * 1/12 * 2
- Area must be naturally vegetated, undisturbed and not landscaped.
- Not appropriate for slopes over 10%
- Must exclude areas used for other infiltration measures.
- BMP# - This number must correspond with associated DMA. If multiple BMPs are to be used for one DMA each BMP should have the same number.
| Associated DMA # | (Aper) Pervious Area (sf) | (Vret) Retention Volume (cf) |
|---|