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Summary Staff Report:
The most significant change
in Chapter 2 is the removal of the Northern Virginia BMP
Handbook as one of the design criteria. The Northern
Virginia BMP Handbook was originally adopted for use by
communities with drainage into the Occoquan Reservoir. (These
communities include the Counties of Fairfax, Prince William
and Loudoun and the incorporated areas within.) Since
approximately 1/3 of Fauquier County drains into the
Occoquan, the decision was made with the adoption of the
stormwater management (SWM) ordinance in 2002 to apply these
standards to the entire county. Thus, they were adopted in
conjunction with the Virginia Stormwater Management Handbook
(VSMH). With this change, stormwater management ponds will
only be designed to the VSMH standards, consistent with the
majority of Virginia communities. This eliminates the often
confusing application of two diverse sets of standards. In
addition, the elimination of the Northern Virginia BMP
Handbook standards will now allow the water quality volume
in a pond to be credited in the calculation of the required
quantity control amount, thus reducing the size (and
potential oversizing) of SWM facilities.
In simplifying the DSM,
duplicative state standards were removed from Chapter 2,
while retaining references to the applicable state
standards. The VSMH Technical Bulletins have also been
deleted from this chapter. This eliminates the need to
coordinate the content of this chapter with state revisions,
thus saving time and housekeeping effort in the future.
This change has condensed the chapter thereby making it more
user-friendly. (The deleted Technical Bulletins, as well as
deleted checklists and worksheets associated with the
Northern Virginia BMP Handbook, were located in the Appendix
of the DSM and, for the sake of clarity in the presentation
of the changes, are not shown as strikethrough pages in the
amended Chapter 2 document.)
Due to the large amount of
soil having a high water table characteristic in Fauquier
County, there is a need to design appropriate SWM facilities
when such facilities are located in these soils. Over time,
conflicts between theoretical analytical practice and
in-field experience have arisen when dry facilities are
designed in wet soils. Standards for determining a high
water table and permeability testing for SWM facilities
using infiltration have been included in the revisions. New
section A204.1(24) provides detailed procedures to be
followed for determining the seasonal high water table and
for the accurate testing for soil permeability necessary for
infiltration facilities. This does not add an extra testing
requirement as a developer would normally undertake this
testing anyway. By outlining and agreeing to an acceptable
procedure, we minimize potential interpretive conflicts (as
well as time and money for the developer) down the road in
the review process. These were added at the encouragement
of the DSM Committee to clarify expectations and ensure
predictability in the review process.
There was considerable
discussion with the DSM Committee on how to prevent in the
future the significant drainage problems we are currently
experiencing in some subdivisions. As we did not want the
added requirement for on-lot grading plans, the consensus
was to increase the overland relief standard from the
10-year storm to the 100-year storm for areas located within
service districts. The DSM Committee felt this was a valid
health, safety, welfare and property value concern. The
revised standard is consistent with surrounding areas
including Loudoun County, Prince William County, Stafford
County and Fairfax County.
To further simplify the DSM
document, checklists from the Appendix have been removed and
placed on the County website with references to them in the
Chapter. This will assist in keeping the latest version of
the checklist available to the public and design community
while reducing the need for changes to the Design Standards
Manual. In addition, terminology used in the chapter has
been revised so that it is consistent with the defined terms
in the Virginia Stormwater Management Handbook and several
new definitions have been added.
As most of the constructed
stormwater management ponds are currently maintained by
homeowner’s associations (unlike many of the surrounding
more developed, suburban communities that have assumed
public maintenance for these facilities), standards
reflecting generally acceptable design practice have been
incorporated to help reduce future maintenance problems of
SWM/BMP ponds. Standards employed include elimination of
gabion weirs for SWM forebays, providing a minimum orifice
size for outflow pipes and the inclusion of anti-clogging
devices. Having such minimum standards is important because
most homeowner’s associations have very limited knowledge
about the maintenance of these structures, and the potential
for problems to arise due to a lack of maintenance is high.
Other changes and refinements
to Chapter 2 include:
-
Eliminating a specific
requirement for gravity flow drains in wet ponds, making
it easier to develop appropriately-sized wet ponds;
-
Providing consistency in
defining drainage channels, as opposed to numerous terms
such as drainage swales, drainageways, etc., that imply
all depressions in the ground are equal;
-
Providing flexibility in
the location of drainage easements on larger lots
(currently focused towards the edge of the property) and
consideration for lots less than ˝ acre provided 75% of
the available area for building is contiguous and usable
as determined by the Zoning Administrator;
-
Deleting time lines for
the review of SWM plans as such plans are reviewed in
conjunction with the overall site plan or construction
plan and would follow the defined time lines for those
review processes;
-
Eliminating the
requirement for a SWM concept plan (with engineering)
for rezonings;
-
Adding flexibility to the
required 2% minimum slope for newly graded areas – down
to 1% on commercial, business and industrial sites and
to follow accepted industry standards for recreational
sports fields and conveyance channels;
-
Clarifying the
appropriate use of high density polyethylene (HDPE)
pipe;
-
Requiring inspection and
certification of the correct installation of underground
stormwater management facilities;
-
Incorporating flexibility
in the standards for consideration of the use of
innovative new technologies for low impact development;
and
-
Reducing setbacks for
ponds with smaller dam embankments.
This DSM Chapter also
includes the requirements for Erosion and Sediment (E&S)
Control. A comprehensive review of the E&S provisions was
made and adopted in December 2006. No substantive changes
have been made to the E&S controls within this Chapter.
Revision Process
Between January 22, 2008 and
April 21, 2009, members of the DSM Committee met on a
semi-monthly basis to review the provisions of the DSM
drainage chapter. Clarifications were proposed by both
Community Development staff and the DSM Committee members,
often prompting in-depth discussion of appropriate design
practices. The Committee expended numerous hours in
refining this ordinance and their willingness, expertise,
considerable input and patience through the process is
greatly appreciated.
The members of the Committee
include several private sector engineering and surveying
design firms, including: Chris Hritsik with Bohler
Engineering P.C.; Jim Carson, Rick Ashley and Paul Bernard
(formerly with Bury Partners) with Carson-Ashley and
Associates; Dave Hall and John Orr with DRH Design Group,
Inc.; Bruce Reese and Mike Webb with The Engineering Groupe,
Inc. and Fred Ameen with Patton Harris Rust and Associates,
P.C. Also participating in the Committee from Fauquier
County were Kimberley Fogle, Assistant Director of Community
Development; Christer Carshult, County Engineer and Saundra
O’Connell, Senior Planner. Jim Sawyer, County Soil
Scientist, provided significant input into the sections
dealing with seasonal high water tables and soil testing. |