Pohick Creek Watershed: Dam Rehabilitation Dredging and Lake Restoration

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

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  • 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 citizens task force set up by the Braddock District Supervisors 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 its 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 45. Proposed Schedule Huntsman Spillway Rehabilitation and Dredging: Fall 2013 Woodglen: Winter 2013 Royal: Summer 2014 46. Lake Accotink Fairfax County Park Authority 47. Lake Accotink Dredge Disposal Site Concrete Plant 48. 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) 49. Reclaimed Old Concrete Disposal Site Lake Accotink Dredge Disposal Site 2013 concrete plant 50. Lake Accotink 2009 51. Lake Accotink 2010 with lake drained for spillway repairs 52. Project Contacts Stormwater Planning Division (703-324-5500) Matt Meyers Dipmani Kumar Martin Chang Partners:

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