Proving Adequate Outfall Gets Tougher in Fairfax County   In a Letter to the Industry dated April 25, 2003, Fairfax County announced it is now requiring all future plan submissions to provide additional documentation of how land development projects comply with state and county requirements regarding demonstration of adequate outfall. This will require significant additional work on the part of land development engineers. This is significant because without approval of "adequate outfall" the site cannot be developed. Existing state and county regulations (Minimum Standard 19 in the Virginia Erosion and Sediment Control Handbook and PFM Section 6-0203) require submissions demonstrate that stream channels located below development projects will be able to adequately convey stormwater flows. Currently as standard procedure, narratives from submitting engineers are provided to comply with this regulation. However, according to county staff, supporting data are often lacking. Therefore, all future plan submissions are required to include supporting data to “justify the conclusions contained in the outfall narrative”. Specific supporting data that will now be required are: " 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." (Letter to Industry, April 2003) Also included in this Letter to the Industry are details of the data the county will now be requiring for all design plans and associated stormwater waiver requests regarding pre-development site conditions. As outlined, these requirements include a factual pre-development land-use narrative and computations of weighted runoff coefficients, as well as the derivation of the time of concentration. For further information, refer to Fairfax County's April 25, 2003 Letter to the Industry or if you have specific questions on this topic, contact: Frank Graziano at fgraziano@wetlandstudies.com or Bill Nell at bnell@wetlandstudies.com.