As those of you who are working on development projects in Fairfax County where the
receiving stream is eroding can attest, the current adequate outfall requirements in the
Public Facilities Manual (PFM) present a major source of confusion. Although adequate
outfall requirements have not changed in many years, they have not been uniformly enforced
and interpretations as to what “adequate” means have been subject to change. In situations
where the downstream channel is found to be inadequate and no amount of on-site stormwater
detention will make it “adequate” (often due to a large watershed and small site), the only
recourse currently available in the PFM is to make the stream adequate through restoration
and/or stabilization measures. The problems with this mandate are obvious – even if stream
restoration is feasible, downstream access issues are often problematic. Historically, a
waiver would be granted or common sense requirements were imposed in this situation, but due
to new interpretations of the PFM, these options are no longer possible.
Recognizing this as an issue, a committee of County staff and private sector engineers
began meeting two years ago to develop specific and objective options that would provide an
improvement in the stream over existing conditions in lieu of a case by case “ad hoc”
approach. In summary, the primary changes would allow adequacy to be achieved through one
of three methods: 1) by application of the current allowable velocity-based protocol; 2)
through demonstration of no adverse impact based on an assessment of shear stress within the
channel; or 3) by providing significant on-site over-detention of stormwater so that there
is no increase in “energy” imparted into the stream system due to an increase in runoff
volume. Specific details of the proposed changes to the PFM are contained in two separate
documents (Part
1 and
Part
2). These proposed changes have recently been approved by the Engineering Standards
Review Committee (ESRC) and are scheduled for two public hearings – December 8th before the
Planning Commission and January 23rd before the Board of Supervisors.
For further information, please contact Mike Rolband or Frank Graziano.
