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Development in Dam Break Inundation Zones

Any proposed development that will occur within mapped dam break inundation zones of state-regulated dams must be identified on all Preliminary Plats, Subdivision Plan, Site Plan and Minor Site Plan submissions, effective July 1, 2009. A dam break inundation zone refers to the area downstream of a dam that would be inundated by the failure of the dam.

As part of the plan review process, the county will notify dam owners and the Virginia Department of Conservation and Recreation (DCR) of any proposed development within the boundaries of a dam break inundation zone of a state-regulated dam. DCR is required to make a determination of the potential impacts of the proposed development on the spillway design flood required of the affected dam and notify the dam owner and the county of its determination. If the proposed development will change the spillway design flood standards of the impounding structure, the development cannot be approved unless it is modified or the developer contributes a payment for the necessary upgrades to the impounding structure. In addition, when any part of the land proposed for subdivision lies in a mapped dam break inundation zone, this fact must be noted on the Final Subdivision Plat for the development.

The requirements apply only to proposed development downstream of a dam for which a dam break inundation zone map is on file with the county at the time of the official submission of a plan to the county. Additional details on requirements for proposed developments are explained in Land Development News Letter to Industry 09-10.

State-Regulated Dams in Fairfax County

To determine if a proposed development will occur within a mapped dam break inundation zone, locate the development area on the countywide locator map. Those that have an inundation zone map on file have a link from the dam name in the chart below to a detailed map.

Dam	Tax Map	Dam Operator	Current Web Map
Barcroft Dam	61-4	Private	June 2009
Burke Centre 11B Dam	77-4	DPWES	April 2011
Burke Lake Dam	87-4	DGIF	March 2012
Carrington Regional Pond Section 1A	28-2	DPWES	April 2011
Chantilly Country Club Dam **	43-4	Private	n/a
Crippen Dam (also called Stump Dump Landfill Dam)	7-3	Private	n/a
Dulles Station at Dulles Corner Regional Pond	15-4	DPWES	February 2012
Fairfax Center Regional Pond D77	56-2	DPWES	April 2011
Fairview Lake Dam (also called Holmes Run 2A)	49-4	DPWES	April 2011
Fox Lair Regional Pond	78-2	Private	May 2011
Fox Lake Dam	37-3	Private	n/a
Hampton Forest Section 4 Regional Pond	55-4	DPWES	July 2011
Hunter Mill Estates Regional Pond	27-2	DPWES	March 2012
Kings Park West Section 18 Dam	68-4	DPWES	May 2011
Kingstowne Lake Dam	91-2	Private	n/a
Kingstowne Pond 4	91-3	DPWES	June 2011
Lake Accotink Dam	79-2	FCPA	June 2009
Lake Anne Dam	17-2	Private	n/a
Lake Audubon Dam (also called Lower South Lake)	27-1	Private	n/a
Lake Fairfax Dam	18-1	FCPA	June 2009
Lake Newport Dam	11-4	Private	August 1984
Lake Thoreau Dam (also called Upper South Lake)	27-1	Private	n/a
Lehigh Dam **	80-2	Private	n/a
North Twin Lakes Dam	66-1	Private	June 2009
Northern Virginia Community College Annandale Campus Dam	59-3	Private	September 2002
Occoquan Reservoir - Lower Dam *	112-2	FCWA	June 2009
Occoquan Reservoir - Upper Dam *	106-3	FCWA	June 2009
Piney Run Dam **	13-3	Private	n/a
Pohick Creek Dam #1 (also called Lake Mercer)	97-2	DPWES	April 2011
Pohick Creek Dam #2 (also called Lake Barton)	77-2	DPWES	April 2011
Pohick Creek Dam #3 (also called Woodglen Lake)	77-1	DPWES	April 2011
Pohick Creek Dam #4 Map 1 (also called Royal Lake)	78-1	DPWES	May 2011
Pohick Creek Dam #4 Map 2 (also called Royal Lake)	78-1	DPWES	May 2011
Pohick Creek Dam #7 (also called Lake Braddock)	78-1	DPWES	October 2011
Pohick Creek Dam #8 (also called Huntsman Lake)	88-4	DPWES	April 2011
Pulte McLean Pond D67	29-1	DPWES	May 2011
Reston Northern Sector Pond 1	11-2	DPWES	May 2011
Reston Town Center Western BMP Dam	17-1	Private	n/a
South Twin Lakes Dam	66-3	FCPA	June 2009
Upper Occoquan Dam (also called Polish Pond)	74-1	UOSA	n/a
West Ox Road Regional BMP Dam	25-3	DPWES	April 2011

Key:
* Currently regulated by Federal Energy Regulatory Commission
** Not on countywide locator map

BMP - Best Management Practices
DPWES - Department of Public Works and Environmental Services
DGIF - Virginia Department of Game and Inland Fisheries
FCPA - Fairfax County Park Authority
FCWA - Fairfax County Water Authority
UOSA - Upper Occoquan Sewage Authority

Please note that where dams are located in proximity to one another, their inundation zones may overlap and be plotted on a single map. Also, information provided on the maps is compiled from an inventory of dams owned and operated by Fairfax County, information on file with the Fairfax County Office of Emergency Management and a list of state-regulated dams provided by the DCR Division of Dam Safety. Dam break inundation zones for many state regulated dams are not currently available.

For more information about dam break inundation zone maps, email the Stormwater Planning Division, 703-324-5500, TTY 711. For information about development in dam break inundation zones, contact the Stormwater and Geotechnical Section of the Site Development and Inspection Division at 703-324-1720, TTY 711.

State-Regulated Dams

All dams in Virginia are subject to the Dam Safety Act and Dam Safety Regulations unless specifically excluded. A dam is excluded if it:

is less than six feet high;
has a maximum capacity less than 50 acre-feet and is less than 25 feet in height;
has a maximum capacity of less than 15 acre-feet and is more than 25 feet in height;
is used primarily for agricultural purposes and has a maximum capacity of less than 100 acre-feet or is less than 25 feet in height ( if the use or ownership changes, the dam may be subject to regulation);
is owned or licensed by the federal government;
is operated for mining purposes under 45.1-222 or 45.1-225.1 of the Code of Virginia;
is an obstruction in a canal used to raise or lower water levels;
the height of a dam is defined as the vertical distance from the streambed at the downstream toe to the top of the dam;
the maximum capacity of a dam is defined as the maximum volume capable of being impounded at the top of the dam.