DPWES Letters to Industry
Land Development Services
12055 Government Center Parkway
Fairfax, Virginia 22035-5503
703-324-1780, TTY 711
October 3, 2005
Additional Information Required on Construction Plans to Assist in the Assessment of Adequate Drainage and Erosion and Sediment Controls
Issue: Enforcement of Section 11-0109.5 of the Public Facilities Manual (PFM) with respect to the provision of temporary/permanent detention during construction.
Background: At the request of the Board of Supervisors, this agency created a committee to develop recommendations for enhancing the county’s erosion and sediment control program and review of adequate storm drainage to try to further minimize the potential for impacts on adjoining properties. The committee was comprised of both public and private sector representatives. Among other things, the committee recommended that enforcement of Section 11-0109.5 of the Public Facilities Manual be reviewed to determine the necessity for clarification about enforcement requirements. This recommendation was referred to the Adequate Outfall Committee for implementation. The Adequate Outfall Committee is a sub-committee of the Fairfax committee and is developing recommendations for amendments to the PFM provisions regarding adequate drainage. The guidance provided in this letter is the result of the efforts of the Adequate Outfall Committee.
Procedures: Effective December 5, 2005, first submission construction plans proposing land disturbing activity, as defined in Chapter 104 of the Code of the County of Fairfax, Virginia, will be required to address the adequacy of drainage based on the drainage patterns and outfalls created by the installation of the Phase I erosion and sediment controls and after the site has been cleared. The assessment is to be incorporated in the erosion and sediment control narrative and shall be provided in addition to the outfall narrative and supporting analysis that are required by the provisions of Section 6-0200 of the PFM. The assessment shall include a description of each outfall, including concentrated and sheet flow situations, and an analysis/assessment of the downstream system's ability to accommodate the increased runoff caused by the changed soil and surface conditions of the cleared site. The analysis shall take into account increases in volume and peak flows, as well as the creation of concentrated discharges. The storm frequency to be used in the analysis shall be based on the type of outfall facility (natural channel, pipe system, man-made channel) in conformance with Section 6-0200. If an outfall at the Phase I stage is at the same location as an outfall in the permanent storm drainage system, a comparison of the runoff quantities during and post construction to show that the quantities during construction are lower than those post development and a reference to the outfall narrative will suffice as the analysis. Temporary/permanent detention shall be proposed with the installation of Phase I controls as necessary to ensure the adequacy of downstream systems during construction.
If you have any questions or need additional information, please call Michelle Brickner at 703-324-1780, TTY 711.