DPWES Letters to Industry
Land Development Services
12055 Government Center Parkway
Fairfax, VA 22035-5503
703-324-1780, TTY 711
February 15, 2006
Drainage Divides, Stormwater Outfall and Notification Amendments
The Board of Supervisors adopted amendments to the Public Facilities Manual regarding drainage divides and stormwater outfall on February 6, 2006. Amendments to the Subdivision Ordinance and the Zoning Ordinance regarding required notification were adopted as well. Synopses of the amendments are below.
Drainage Divides Amendment
The amendment to the Public Facilities Manual (PFM) clarifies the current requirements pertaining to existing drainage divides. The amendment permits the administrative approval of diversions of surface drainage in some instances as long as such a diversion does not negatively impact the downstream drainage system and provides for an improved site design as detailed in the amendment
Pursuant to the amended provisions, if a diversion is requested, the submitting engineer is required to perform an engineering analysis of the downstream drainage systems in order to demonstrate that the diversion does not create adverse impacts. Further, the engineer must be able to demonstrate that the diversion results in an improved site design that either improves the outfall characteristics, preserves green space, provides for better stormwater management, or addresses a site’s topographic constraints to minimize impacts due to steep slopes and/or runoff to adjoining properties. If the analysis identifies a conflict with other relevant county policies and provisions, a diversion would not be approved.
Stormwater Outfall Amendment
The amendment to the PFM revises the requirements of stormwater outfall from a development site, including clarification of the required extent of review and the addition of options that require demonstration of no adverse impact with a proportional improvement.
The amendment clarifies the extent of downstream review to the following:
- To the point that is 150 feet downstream of the point where the receiving pipe or channel is joined by another that has a drainage area that is at least 90% of the size of the first drainage area; or
- To the point where the drainage area is 100 times greater than the contributing drainage area of the development site; or
- To the point that is at least 150 feet downstream of a point where the drainage area is 360 acres or greater.
Unless the drainage area is 100 times the contributing site area, the analysis must be to a point where all the cross-sections are adequate in the most downstream reach of 150 feet. A minimum of three cross-sections must be provided in this 150-foot reach. In addition, the proposed amendments retain the Director’s ability to require review farther downstream if known problems exist. The amendment allows for alternative methods to show that there will be no adverse impact to the outfall from the development and that the conditions of the stream will be proportionally improved after the development. The alternative methods include the following: shear stress method; channel capacity method; and detention method. The option to prove that the existing outfall is adequate is retained.
If the outfall is inadequate due to erosive velocity, the amendment allows for the use of the shear stress method to show that the erosive work that the outfall is subjected to after development is proportionally less than the pre-development erosive work. The proportional reduction in erosive work is based on the amount of imperviousness the development site is contributing to the drainage area.
If the outfall is inadequate due to over-bank flooding, the amended provisions allow for the use of the channel capacity method to show that the amount of time that the outfall will be subject to over-bank flooding will be less than the pre-development over-bank flooding. The amended provisions allow the use of the detention method to address adverse impacts due to erosive velocity or over-bank flooding. The detention method requires the following: 1) that detention is provided so that the 1-year storm volume is detained for a minimum of 24 hours; and 2) that the 2-year and 10-year peak flows at the development site are reduced below the relative peak flows of the site in a “good forested” condition. The reduction is based on the ratio of the cumulative forested runoff volume to the proposed cumulative development runoff volume, and provides a proportional improvement to the condition of the outfall. Leadership in Energy and Environmental Design (LEED) certification includes the option of showing that the 1.5-year peak flow has been reduced to a “good forested” condition. Consequently, the amended provisions allow the option of providing the 1.5-year peak flow calculations. Provision of the expanded detention is sufficient for the presumption of no adverse impact and proportional improvement; however, in the case of a natural watercourse, the design professional will still be required to demonstrate that the outfall discharges into a defined channel (bed and bank).
The amendments include the following provisions:
A Subdivision Ordinance requirement for notification of adjoining property owners with subdivision construction plans for proposed subdivisions that are not required to submit preliminary subdivision plats. Currently, notification is provided with the preliminary subdivision plat or with the final plat when a preliminary subdivision plat is not required. The proposed amendments will change the timing of notification of adjoining property owners for proposed subdivisions subject to a proffered generalized development plan, proffered or approved final development plan, approved special exception plat for a cluster subdivision, or waiver of minimum lot size by requiring that notification of adjoining property owners be provided with the subdivision construction plan instead of the final plat. This change will ensure that notification occurs prior to the start of construction.
An expansion of the extent of notifications for site and subdivision plans to include all property owners within 500 feet of the proposed development and an increase in the minimum number of property owners required to be notified from five to 25.
A new requirement for site and subdivision plans to notify at least one civic association in the immediate vicinity of the proposed development.
Subdivision Ordinance and Zoning Ordinance requirements for additional information to be provided in site and subdivision plan notices including:
- A description of the proposed development;
- A description of the location of the proposed development;
- Contact information for a representative of the applicant;
- A reduction of the plan or plat of the proposed development;
- A statement that the proposed construction may alter storm drainage from the site;
- A statement that comments on proposed plans may be submitted to DPWES.
Grandfathering of Plans
The drainage divides, stormwater outfall and notification amendments became effective on February 7, 2006, at 12:01 a.m. However, certain plans were grandfathered in the following manner:
Drainage Divides and Stormwater Outfall – Subdivision Plans (excluding Preliminary Plats), Site Plans, Minor Site Plans, Public Improvement Plans and Grading Plans submitted prior to 12:01 a.m. on February 7, 2006, provided that such plan obtains final approval no later than close of business on August 7, 2006.
For Site Plans and Public Improvement Plans, final approval will be considered obtained when the plan has been recommended for final approval and all outstanding items for project approval have been completed for the plan, except for bonding of public improvements and the escrowing of conservation funds. For Subdivision Plans, Minor Site Plans, and Grading Plans, final approval will be considered obtained when the plan has been signed off by the Director as final approved.
Notifications – Site Plans, Preliminary Subdivision Plats, Final Subdivision Plats when a Preliminary Subdivision Plat and a Construction Plan are not required, Subdivision Construction Plans when a Preliminary Subdivision Plat is not required, provided that such plan complies fully with the notification provisions in effect at the time of the mailing, and the postmark date on all white receipts for the certified mailing is prior to 12:01 a.m., February 7, 2006. If the mailing is incomplete (i.e. missing properties, wrong addresses, or in some other way not in compliance with the notification provisions in effect at the time of the mailing), the plan will no longer be grandfathered regarding the new notification requirements.
Please call Judy Cronauer, Code Analysis, at 703-324-1808, TTY 711, if you have general questions regarding the amendments. Questions about the application of the amendments to a site-specific situation should be referred to the assigned review engineer in the Environmental and Facilities Review Division. Review engineers may be reached at 703-324-1720, TTY 711.