Pohick Creek Watershed: Dam Rehabilitation Dredging and Lake Restoration Sept. 14, 2013

1. Green Breakfast
Pohick Creek Watershed
Dam Rehabilitation
Dredging and Lake
Restoration
September 14, 2013
Stormwater Management

2. Pohick Creek PL-566 Facilities
Dam Rehabilitation
Dredging and Lake Restoration

3. PL-566 Facilities Locations

4. 4
Pohick Creek PL-566 Dams
 The Natural Resources Conservation Service (NRCS) designed and
built six dams in the Pohick Creek Watershed between 1970 and 1985,
under the authority of Public Law 83-566, the Watershed and Flood
Prevention Act
 The Northern Virginia Soil and Water Conservation District
(NVSWCD) and Fairfax County were the project sponsors
 Fairfax County owns and maintains the six facilities
 The Pohick dams have helped mitigate downstream flood damages and
provide improved water quality, wildlife habitat, recreational
opportunities, and economic benefits to the residents of Fairfax
County, and are a significant asset to the residents who live near the
lakes

5. PL-566 Dams Auxiliary Spillway Erosion Study
• In 1995, partly as a result of the White Oak dam failure, Fairfax County initiated a
spillway erosion study on four of the PL-566 dams (Royal Lake, Woodglen Lake, Lake
Barton, and Huntsman Lake)
• The studies indicated remedial measures were required to preclude excessive
headcutting and possible dam failure during the design flow event (PMF)
White Oak Dam in Madison County after June 1995 rainfall event
(~18” in 24 hours)

6. NRCS Technical and Financial Assistance
• Fairfax County and NVSWCD as the original project sponsors requested NRCS
assistance to rehabilitate the four PL-566 facilities (starting with Royal Lake) in
2005
• NRCS developed a rehabilitation plan for Royal Lake in 2006, working
collaboratively with Fairfax County, NVSWCD, and a citizen’s task force set up
by the Braddock District Supervisor’s office
• After adoption of the rehabilitation plan, NRCS asked Fairfax County and
NVSWCD to consider taking the lead on the design and construction effort,
with NRCS continuing to provide technical and financial assistance
• NRCS also asked Fairfax County and NVSWCD to provide assistance with the
Hydrologic and Hydraulic (H&H) studies required to develop rehabilitation
plans for the other facilities (Woodglen Lake, Lake Barton and Huntsman Lake)

7. PL-566 Facility Rehabilitation – Royal Lake
• Challenges
– First project – significant learning curve for county
and consultant in understanding and utilizing
NRCS design standards and specifications
– Tropical Storm Hanna impacts

8. PL-566 Facility Rehabilitation – Royal Lake

9. 9
Tropical Storm Hanna (September 6, 2008)
Rainfall Amounts in Fairfax County

10. 10

11. Royal Lake
September 7 and 8, 2008

12. 12
Royal Lake
ACB installation

13. Royal Lake
Post-Construction

14. PL-566 Facility Rehabilitation – Woodglen Lake
 NRCS received ARRA funding which accelerated the design schedule
somewhat
 Unsuitable subgrade material encountered at the end of the new
auxiliary spillway exit channel resulted in a field design change for the
installation of a subsurface drainage system

15. 15
PL-566 Facility Rehabilitation – Woodglen Lake

16. PL-566 Facility Rehabilitation – Woodglen Lake

17. PL-566 Facility Rehabilitation – Lake Barton
 Preferred concept (ACB armoring) in original NRCS rehabilitation plan was
changed at the request of landowners from whom easements were required. A
new concept utilizing cutoff walls that resulted in a much smaller tree loss was
moved forward
 The sediment pool did not meet the minimum 50-year life requirement at the
completion of rehabilitation, and the rehabilitation plan recommended raising
the permanent pool elevation by 0.8 feet to achieve the required sediment
storage
 Because of resident concerns and land rights issues, Fairfax County decided to
dredge Lake Barton to meet the sediment pool capacity requirements
 NRCS also received ARRA funding for this project which significantly accelerated
the design schedule

18. 18

19. PL-566 Rehabilitation Projects: Cost-Share Funds Received
• 2007/2008: Royal Lake Dam, $1.8 million
• 2009/2010: Woodglen Lake Dam, $1.4 million
• 2010/2011: Lake Barton Dam, $2.8 million
• 2013: Huntsman Lake, $1.8 million (obligated)
Fairfax County and NVSWCD as project sponsors have received approximately
$6 million to date for rehabilitation of the PL-566 facilities:

20. Huntsman Lake Dam Rehabilitation Project
Background
• Huntsman Lake Dam is regulated by the Virginia Department of
Conservation and Recreation (DCR), Division of Dam Safety
• Huntsman Dam is operating under a conditional operation and
maintenance (O&M) certificate from DCR because it’s auxiliary spillway
can only convey about 50 percent of the flows from the design storm

21. Huntsman Lake Dam Rehabilitation Project
Summary of Rehabilitation Plan

22. Huntsman Lake Dam Rehabilitation Project
Major Design Elements

23. Huntsman Lake Dam Rehabilitation Project
Riser Replacement

24. Huntsman Lake Dam Rehabilitation Project
Fish Relocation (May 2013)

25. Lake Dredging and Restoration
Lake
Sediment
(CY)
Sediment
Pool
Capacity
(CY)
% sediment
pool
expended
Annual Load
(CY)
Annual Load
(CY/AC)
Lake Barton 32,500 75,000 43.33% 1,100 2.04
Huntsman Lake 65,000 295,000 22.03% 2,968 2.00
Woodglen Lake 32,500 160,000 20.31% 1,244 1.69
Lake Royal 105,000 416,000 25.24% 4,411 1.88
Lake Mercer* 37,000 550,000 6.60% 1,650 0.41
Lake Barton Example

26. Sources of Sediment

27. Lake Restoration and Dredging Projects
Project Goals
• Restore sediment pool capacity
– Downstream water quality benefits
– Aesthetics
• Improve lake water quality and habitat
• Provide long term maintenance improvements

28. Dredging Alternatives
1. Mechanically dredge deposited
sediment and haul wet
2. Drain lake and dry deposited
sediment in place before hauling

29. Lake Barton Example: Dredging in Progress

30. Disposal Site – Sediment Reuse
• Topsoil for landfill cover
• Topsoil product for resale

31. Lake Restoration
• In-lake forebay
• Wetland planting
• Shoreline stabilization
• Fish habitat
• Fish stocking
Lake Barton: Fish stocking by Virginia Game and Inland Fisheries
Fish habitat created from salvaged trees

32. Shoreline Protection
Options

33. Huntsman Lake Facts
• Drainage area = 1,482 acres (2.3 square miles)
• Lake surface area = 28.6 acres
• Sediment pool capacity = 295,000 cubic yards (CY)
• Current sediment in lake = 65,000 CY
• Estimated annual sediment load = 2,970 CY
32.5 feet
65,000 CY

34. 2011
Inactive
Dewatering
Basin

35. Sediment Thickness

36. Water Depth

37. Huntsman Lake - Forebays
bank stabilization
aquatic bench
forebay
forebay berms
maintenance access
sanitary sewer
dam
embankment

38. Woodglen Lake

39. Woodglen Lake: Sediment Thickness

40. Woodglen Lake: Water Depth

41. Royal Lake Facts
• Lake surface area = 38 acres
• Sediment pool capacity =
417,000 cubic yards (CY)
• Current sediment in lake =
105,000 CY
• Estimated annual sediment load
= 4,400 CY
• Phased
– 55,000 base dredge
– 50,000 additional
55 feet
105,000 CY

42. Royal Lake

43. Royal Lake: Sediment Thickness

44. Royal Lake: Water Depth

46. Proposed Schedule
• Huntsman Spillway
Rehabilitation and Dredging:
Fall 2013
• Woodglen:
Winter 2013
• Royal:
Summer 2014

47. Lake Accotink – Fairfax County Park Authority

48. Lake Accotink Dredge Disposal Site – Concrete Plant

49. Lake Accotink Dredging Disposal Site
at Virginia Concrete
Disposal site during dredging – March 2008
(198,000 cubic yards of dredge material was disposed at this site)
Disposal site 3 months after final site reclamation – July 2009
(Dredging and disposal were completed in 2008. Site reclamation was performed in spring 2009)

50. Reclaimed Old Concrete Disposal Site
Lake Accotink Dredge Disposal Site
2013
concrete
plant

51. Lake Accotink
2009

52. Lake Accotink
2010 with lake drained for
spillway repairs

53. Project Contacts
Stormwater Planning Division
(703-324-5500)
• Matt Meyers
• Dipmani Kumar
• Martin Chang
Partners: