1. Site Requirements Calculation
Enter project site details to calculate the required Stormwater Retention Volume (SWRv).
Portion of Impervious/Compacted area used by vehicles. Needed for MS4 treatment check.
2. BMP Configuration & Calculation
Add Best Management Practices (BMPs) to calculate their contribution to stormwater retention.
3. Overall Site Compliance
Check if the configured BMPs meet the site's stormwater requirements.
DOEE Stormwater Database Map
This interactive map shows the approximate locations of the Municipal Separate Storm Sewer System (MS4) and Combined Sewer System (CSS) areas, which influence detention requirements. Use this map for preliminary planning.
Map provided by DC GIS. If the map does not load, you can access it directly here.
Executive Summary: DC Stormwater Requirements
Disclaimer
This tool is for preliminary informational and educational purposes only. It is NOT a substitute for detailed site-specific engineering calculations, professional judgment, or official DOEE review/submission.
The District of Columbia Department of Energy and Environment (DOEE) mandates stormwater management for regulated development projects to protect local waterways like the Anacostia and Potomac Rivers and mitigate combined sewer overflows (CSOs).
DOEE Stormwater Management Guidebook
For comprehensive guidance on DC stormwater management requirements and design standards, please refer to the DOEE Stormwater Management Guidebook (SWMG 2020).
Key Requirements (based on DOEE SWMG 2020):
Stormwater Retention Volume (SWRv)
This is the primary performance standard, calculated in cubic feet (ft³). It represents the volume of runoff from a specific rainfall event (0.8", 1.0", or 1.2" depending on project type/location) that MUST be retained on-site through infiltration, evapotranspiration, or rainwater harvesting/reuse.
Best Management Practices (BMPs)
Structural solutions like green roofs, bioretention, permeable pavement, etc., used to achieve the SWRv and/or water quality treatment goals.
Green Roof Adjustment
When Green Roofs are used, they effectively replace impervious cover. For compliance calculation purposes, the area of the green roof can be subtracted from the site's impervious area, potentially reducing the *required* SWRv. This tool performs this adjustment during the compliance check.
On-Site Retention Minimum
For sites draining to the Municipal Separate Storm Sewer System (MS4) or certain Combined Sewer System (CSS) areas targeted for green infrastructure, at least 50% of the total site SWRv (potentially adjusted for green roofs) must typically be achieved using BMPs located on the project site.
Vehicular Area Treatment (MS4/Direct Drainage Only)
In areas draining to the MS4 or directly to waterways, at least 50% of the SWRv generated *specifically by vehicular surfaces* (parking, driveways) must be *treated* by an accepted practice removing ≥80% Total Suspended Solids (TSS). This ensures higher-pollutant runoff gets treated locally.
Treatment vs. Retention Credits
BMPs are assigned a specific *retention percentage* based on their design (e.g., Standard Bioretention = 60%, Enhanced/Infiltration Bioretention = 100% if Ksat conditions met, Green Roof = 100%). The calculated storage volume (Sv) of the BMP is multiplied by this percentage to determine its contribution to the SWRv *retention* requirement. The remaining portion of Sv (if the BMP is an accepted TSS treatment practice) may count as *treatment* volume (important for the vehicular area check).
Off-Site Compliance (Offv)
If the SWRv (or the 50% on-site minimum) cannot be met on-site due to verified constraints (Extraordinarily Difficult Site Conditions), the remaining volume (Offv) generally must be met off-site by purchasing Stormwater Retention Credits (SRCs) or paying an In-Lieu Fee (ILF). This requires a specific relief process. *Note: Projects in CSS areas reducing CSOs via tunnels have different off-site rules.*
Detention
Separate requirements exist to control the *peak flow rate* of larger storms (typically 2-year and 15-year) to prevent downstream erosion/flooding. This usually requires additional storage beyond SWRv, often managed through the same BMPs or separate structures.
This tool helps calculate your site's SWRv and evaluate how added BMPs contribute towards meeting the retention and vehicular treatment requirements, including adjustments for green roofs.
Green Area Ratio (GAR) Information
Important Distinction
GAR is a separate environmental requirement enforced by the DC Department of Buildings (DOB) through the Zoning Regulations (Title 11 DCMR), not primarily through the DOEE Stormwater Management Regulations (Title 21 DCMR, Chapter 5).
Key Points:
Purpose
GAR aims to increase the quantity and quality of environmental performance on a site by requiring a certain ratio of landscape elements to lot area.
Applicability
It applies to specific zoning districts and project types (primarily larger new construction and substantial additions). Check DCMR Title 11, Subtitle C, Chapter 6 for specific applicability.
Calculation
It involves assigning weighted values to different landscape elements (like trees, shrubs, green roofs, permeable pavement, bioretention, soil depth) and dividing the total weighted landscape area by the lot area.
Relationship to Stormwater
While many GAR elements *are also* stormwater BMPs (e.g., green roofs, bioretention, permeable pavement), GAR compliance is a *separate calculation and review process* from stormwater SWRv compliance. Meeting GAR does not automatically mean SWRv is met, and vice versa, although using multifunctional BMPs helps achieve both.
Tool Limitation
This tool does not calculate GAR. A separate process and potentially different tools or calculations are needed for GAR compliance.
For detailed information on GAR requirements and calculations, please refer to the official DC Zoning Regulations and the DOEE GAR Guidebook available on the DOEE website.
(Future versions of this tool may incorporate GAR estimation features.)
BMP Information (DOEE SWMG 2020)
Technical Reference
This section provides summarized information on selected BMPs based on the 2020 DOEE Stormwater Management Guidebook. For complete design criteria and specifications, refer to the full SWMG document.
Select a BMP type below to view detailed feasibility criteria, design requirements, and compliance calculation methods.
Green Roofs (SWMG Section 3.2)
Practices that capture and store rainfall in an engineered growing media installed over a waterproof membrane designed to support plant growth on a roof.
- • Structural Capacity: Roof must support additional weight (15-30 psf+ for extensive). Structural engineer review required.
- • Roof Pitch: Ideal is 1-2%. Max ~30% (may need baffles). Steeper (>30%) considered green wall.
- • Roof Access: Required for construction, maintenance, inspection (hatch, stairs, window).
- • Roof Type: Most surfaces suitable, but avoid treated wood or uncoated galvanized metal due to leaching.
- • Setbacks: 2-ft vegetation-free zone at perimeter recommended; 1-ft around penetrations. Avoid placement near HVAC/electrical.
- • Drainage Area: Typically limited to roof itself. Max contributing area = 1:1 ratio with green roof area (with proper distribution).
- • Building Codes: Comply with fire, wind, drain codes (ANSI/SPRI VF-1, RP-14).
- • Layers: Typically include Deck, Waterproofing, (Insulation), Root Barrier, Drainage Layer, Filter Fabric, Growing Media, Plant Cover.
- • Waterproofing: Must be 100% waterproof with long lifespan.
- • Drainage Layer: 0.25-1.5 inches+ depth, synthetic or granular (ASTM D448 #8). Must convey overflow without backups. Box drains.
- • Filter Fabric: Prevents media clogging drainage layer.
- • Growing Media: Lightweight inorganic (70-80%) mix, 30% max organic matter. Depth typically 3-8 inches (extensive).
- • Plant Cover: Succulents (Sedum), hardy perennials tolerant of drought, sun, wind, frost.
- • Irrigation: Required for establishment. Permanent irrigation prohibited for extensive roofs, reduces retention credit by 50% if used.
Green roofs receive 100% retention value (Sv) for the calculated storage volume.
- Sv = Storage Volume (cf)
- SA = Green Roof Area (sf)
- d = Media Depth (in, min 3)
- MWR₁ = Media Max Water Retention (verified or 0.10 default)
- DL = Drainage Layer Depth (in)
- MWR₂ = Drainage Layer Max Water Retention (verified or 0.0 default)
- IF = Irrigation Factor (1.0 if not irrigated, 0.5 if irrigated)
Considered an accepted TSS treatment practice.
Bioretention (SWMG Section 3.6)
Practices that capture and store runoff, passing it through an engineered filter media composed of sand, soil, and organic matter.
- • Space: Surface area typically 3-6% of CDA.
- • Hydraulic Head: Need 4-5 ft generally from inflow to outflow invert (less if no underdrain).
- • Water Table: Bottom must be ≥ 2 ft above seasonally high water table.
- • Soils/Underdrains: Usable in most soils. Underdrain required if underlying Ksat < 0.5 in/hr. Impermeable liner required for hotspots or contaminated soils.
- • Drainage Area: Max CDA generally 2.5 acres (traditional) or 1 acre (small-scale), can be adjusted with proper hydraulic controls (forebay, etc.).
- • Setbacks: Typically 10 ft from foundations (unless lined/waterproofed). Avoid utility conflicts.
- • Geometry: Maximize flow path from inlet to outlet.
- • Ponding Depth: Typically 6-12 inches (max 18 inches). Shallower (6-12") for streetscape/planters. Min 3" average.
- • Filter Media: Specific mix (80-90% sand, 10-20% fines, 3-5% organic). Depth min 18" (Standard), min 24" (Enhanced). Max depth 6.5 ft (see Table 3-20).
- • Underdrains: Required if Ksat < 0.5 in/hr. Perforated pipe (4-6") in gravel layer (min 12" deep, #57 stone). Must drain within 72 hrs (Standard) or 48 hrs (Enhanced sump).
- • Infiltration Sump (Enhanced): Optional gravel layer below underdrain invert to promote infiltration. Must drain within 48 hrs.
- • Surface Cover: Mulch (2-3"), river stone, or turf (depending on design).
- • Plants: Native, water-tolerant species appropriate for sun/shade conditions and moisture zones.
- • Standard Design: 60% retention value for Sv.
- • Enhanced Design (Infiltration Sump or Ksat ≥ 0.5 in/hr): 100% retention value for Sv.
- Sv = Storage Volume (cf)
- SAbottom = Bottom Surface Area (sf)
- dmedia = Filter Media Depth (ft)
- ηmedia = Media Porosity (~0.25)
- dgravel = Gravel Layer Depth (ft)
- ηgravel = Gravel Porosity (~0.40)
- SAaverage = Average Surface Area (sf)
- dponding = Ponding Depth (ft)
Considered an accepted TSS treatment practice.
Detention Requirements Walkthrough (Simplified)
Important Note
This tool currently focuses on SWRv compliance. Detention requirements are separate and typically require hydrologic modeling. The information below provides guidance on when detention may be required.
Detention controls the peak discharge rate of stormwater runoff for larger, less frequent storm events to prevent downstream channel erosion and reduce localized flooding.
When is Detention Required?
General rules for determining if detention is needed for your project
Detention controls the *peak discharge rate* of stormwater runoff for larger, less frequent storm events (typically the 2-year and 15-year, 24-hour storms in DC) to prevent downstream channel erosion and reduce localized flooding. It is a requirement *in addition* to meeting the SWRv (which focuses on volume retention/treatment for smaller, more frequent storms).
General Rules (Refer to SWMG Section 2.6, 2.7 & Appendix I for full details):
- • 2-Year Storm Detention: Required if the *post-development peak discharge rate* (calculated using an Adjusted Curve Number method that accounts for SWRv retention) exceeds the *pre-development peak discharge rate* (assuming "meadow in good condition"). This applies primarily in areas draining to the MS4.
- • 15-Year Storm Detention: Required if the *post-development peak discharge rate* exceeds the *pre-project peak discharge rate* (based on existing site conditions before the project). This applies primarily in areas draining to the MS4 or certain parts of the CSS.
- • Exemptions: Major Substantial Improvement projects and projects consisting entirely of reconstruction within the existing Public Right-of-Way (PROW) are generally exempt from the 2-year and 15-year storm detention requirements (SWMG Section 2.12.1). Specific direct discharge conditions may also exempt sites (SWMG Section 2.6).
MS4/CSS Map: You can view the approximate locations of MS4 and CSS areas, which influence where the 2-Year (Pre-Development Meadow) and 15-Year (Pre-Project) detention requirements apply, using this map:
DOEE Stormwater Database MapDetermining if detention is needed requires hydrologic modeling (like TR-55 or SWMM) for both pre- and post-project conditions, incorporating the volume reduction effects of the SWRv BMPs.
How is Detention Volume Calculated?
The process for determining required detention storage
If detention is required, hydrologic and hydraulic (H&H) modeling is necessary. This involves:
- 1 Developing inflow hydrographs (runoff over time) for the post-development site for the 2-year and/or 15-year design storms.
- 2 Determining the target maximum outflow rates (based on pre-development or pre-project conditions).
- 3 Designing an outlet structure (usually orifices and/or weirs) for a detention facility (e.g., dry pond, underground tank, oversized pipe, or extra storage in a retention BMP like bioretention or a wet pond).
- 4 Performing stage-storage-discharge routing calculations to determine the storage volume needed behind the outlet structure to reduce the peak outflow rate to the target level.
Commonly used tools include:
- • NRCS Methods (TR-55, WinTR-55): Can provide peak flows and runoff volumes. Appendix I details an approximate routing method based on TR-55 charts.
- • Hydrograph Methods (SWMM, HEC-HMS, HydroCAD, etc.): Required for detailed routing calculations, especially for complex sites or outlet structures.
The required detention volume is the temporary storage needed during the storm event and is typically located *above* the permanent SWRv storage level in combined facilities.
Future Tool Enhancements
Planned improvements to support detention calculations
This tool currently focuses on SWRv compliance. Future enhancements could include:
Detention Check
Implementing the TR-55 Adjusted Curve Number method and peak flow calculations (using qpSWRv from Appendix I) to *indicate* if detention is likely required (though full H&H modeling would still be needed for design).
Approximate Detention Sizing
Incorporating the TR-55 graphical routing method (Figure I.1) to provide a rough estimate of detention volume needed.
Integration with Models
Allowing import/export of data relevant to H&H models like HydroCAD or SWMM.