Unmanaged stormwater runoff contributes to localized flooding and erosion of property and stream channels, especially in urbanized watersheds and established neighborhoods. The cumulative impact of even small increases in infill lot imperviousness contributes to these problems. Because drainage and outfall problems are prevalent throughout the county, the director of Land Development Services has determined mitigation of increased imperviousness through onsite stormwater detention is a reasonable and effective strategy to be used for most residential infill lot development scenarios.
The county adopted a Stormwater Management Ordinance (SWMO), effective July 1, 2014, that meets the local ordinance requirements mandated by the Commonwealth of Virginia. Since Fairfax County is a Chesapeake Bay Locality, the land disturbance threshold that triggers stormwater management requirements is 2,500 square feet. Infill Grading Plans must demonstrate compliance with the SWMO. The SWMO allows exemptions from the ordinance for residential infill development meeting the criteria in County Code § 124-1-7.3. Projects that are not exempt must include measures to mitigate impacts to water quality (phosphorus reduction) and water quantity (downstream outfall analysis, meet channel and flood protection criteria, onsite stormwater detention) in accordance with the methods and criteria in the SWMO.
Optional Simple Infill Detention Method
The county issued Technical Bulletin 22-06: Localized Flooding Mitigation Policy for Residential Infill Development-Detention Requirements, containing the county policy and methodology for mitigating increases in runoff generated from residential infill lot development. SIDM, the acronym for Simple Infill Detention Method, is a standardized detention facility design method that meets water quantity compliance and is specifically for single-family detached residential infill lot development projects. Designers may continue to analyze downstream networks for adequacy of the outfall system or may presume inadequate outfall conditions and apply the detention requirements of SIDM. Fairfax County has created a web application showing locations of specific issues within the stormwater conveyance network for use by plan preparers identifying locations of known flooding and erosion problems. The County issued Technical Bulletin 22-05: Inadequate Outfall which elaborate on the requirements for assessing increases in sheet flows and provide the link to the web application. SIDM is the recommended compliance method when detention is required on infill lot developments. When SIDM is appropriately applied to the project the design professional need not conduct a channel protection and flood protection outfall analysis, nor the routing analyses for the proposed detention facilities, nor provide a water quantity narrative.
SIDM Design Information
The county has developed design calculation tools and standard detail sheets for three acceptable detention practices that may be used with SIDM:
- Urban bioretention planter box (UPB1 and UPB2)
- Bioretention-detention rain garden (RG), and
- Underground pipe detention (UPD).
The proper use of, and strict adherence to these tools and details will result in the streamlined design and review of detention facilities that can reliably be expected to be acceptable to the County with the first plan submittal. A complete submission using the SIDM will include the following:
- Standard Design Calculations Spreadsheet for each facility type proposed
- Standard Design [Plan] Sheet for each facility type proposed
- Standard Pretreatment/Outlet Protection Details [Plan] Sheet
- Standard General Site SWM/BMP Data [Plan] Sheet
Technical Bulletin 22-06: Localized Flooding Mitigation Policy for Residential Infill Development-Detention Requirements establishes the county policy and provides information on the SIDM methodology.
The SIDM User’s Guide and Reference user’s manual is recommended for design professionals as a "how to" manual for using SIDM.
Links to SIDM Calculation and Design Sheets
| # |
Facility Type |
Design Sheet Format |
Calculation Sheets |
| 1 |
UPB1, Urban Bioretention Planter Box |
.pdf |
See below1 |
| 2 |
UPB2, Urban Bioretention Planter Box |
.pdf |
See below1 |
| 3 |
RG, Bioretention-Detention Rain Garden |
.pdf |
See below1 |
| 4 |
UPD, Underground Pipe Detention |
.pdf |
See below1 |
| 5 |
Pretreatment / Outlet Protection Details Sheet |
.pdf |
See below1 |
| 6 |
General Site Data Sheet |
.pdf |
See below1 |
1DWG file packages and Microsoft Excel versions of the facility calculation sheets may be obtained by filling out your email. You will automatically receive a copy of those files.
