DPWES Letters to Industry


Land Development Services
12055 Government Center Parkway
Fairfax, Virginia 22035-5503
703-324-1780, TTY 711

April 25, 2003
03-10

Outfall Analysis and Pre-development Conditions Narratives

Issue:

To improve compliance with existing regulations relating to adequate outfall and stormwater detention by enhancing consistency and clarity in the subject narratives.

Outfall Analysis:

Fairfax County Public Facilities Manual (PFM) Section 6-0203 (you may also view the full version of Section 6-0203) and the State's Minimum Standard 19 require a detailed outfall analysis for all development plans. While most plans submitted for review include language from the submitting engineer or surveyor stating an opinion on the adequacy of the receiving channel, often no supporting calculations are provided.

All future plan submissions must contain supporting computations as justification for the conclusions contained in the outfall narrative. For consistency, the following items are to be included:

  1. Site-specific narrative with a description of the elements of the storm drainage system and adjoining properties. A sample outfall narrative can be found in Appendix 3 of the Northern Virginia Adequate Outfall Reference Manual (1998) published by the Engineers and Surveyors Institute.

  2. Outfall location(s) map with the contributing drainage areas and detailed hydrologic and hydraulic calculations.

  3. Three to five cross-sections, at a minimum, at each critical location to verify the outfall adequacy. Cross-sections shall be based on a 2-foot field run contour interval and additional spot elevations in the vicinity. The cross-sections shall have the same vertical and horizontal scales.

  4. Extent of outfall analysis in accordance with Section 6-0203.6 of the PFM.

  5. Description of the outfall channel and permissible velocity. Manning's roughness coefficient shall be supported by soil classification, cover material and/or channel lining.

  6. Design velocities be compared with the permissible velocities of the existing channel. To reduce peak flow rates in the channels an extended detention for the 1-year storm event (in addition to 10-year storm detention) could be useful. If protection measures are provided necessary design details shall be shown and supported by calculations.

  7. Drainage easement when channel improvements are proposed.

  8. Engineer's statement regarding the adequacy of outfall.

Pre-development Site Conditions Narrative:

Often it is problematic to establish the true predevelopment site conditions, particularly in the review of redevelopment projects. Most plans submitted for approval lack pre-development site information. There is a need for a comprehensive narrative to substantiate the pre-development conditions on which a stormwater detention or water quality control waiver request is based.

All future plans and associated waiver requests must provide a factual pre development land-use narrative. In addition, computations of weighted runoff coefficients and derivation of the time of concentration shall be provided. As stipulated in Sections 6-0803.1 through 6-0804.5 D and 6-0805 (you may also view the full version of Section 6-0800) of the PFM, these details are essential for fair evaluations of the waiver requests and adequacy of outfalls.

Your compliance with the above will assist the county in ensuring that its erosion and sediment control program remains consistent with the State's requirements and its own goals.

If you have any questions concerning the information above or need additional information, please contact Qayyum Khan, Chief Stormwater Engineer, Environmental and Site Review Division at 703-324-1720, TTY 711.


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