Pohick Creek Watershed: Dam Rehabilitation Dredging and Lake Restoration

Green Breakfast

Pohick Creek Watershed Dam Rehabilitation Dredging and Lake Restoration September 14, 2013

Stormwater Management

Pohick Creek PL-566 Facilities Dam Rehabilitation

Dredging and Lake Restoration

PL-566 Facilities Locations

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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

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)

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)

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

PL-566 Facility Rehabilitation – Royal Lake

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

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Royal Lake September 7 and 8, 2008

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Royal Lake ACB installation

Royal Lake Post-Construction

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

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PL-566 Facility Rehabilitation – Woodglen Lake

PL-566 Facility Rehabilitation – Woodglen Lake

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

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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:

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

Huntsman Lake Dam Rehabilitation Project Summary of Rehabilitation Plan

Huntsman Lake Dam Rehabilitation Project Major Design Elements

Huntsman Lake Dam Rehabilitation Project Riser Replacement

Huntsman Lake Dam Rehabilitation Project Fish Relocation (May 2013)

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 2.04Huntsman Lake 65,000 295,000 22.03% 2,968 2.00Woodglen Lake 32,500 160,000 20.31% 1,244 1.69

Lake Royal 105,000 416,000 25.24% 4,411 1.88Lake Mercer* 37,000 550,000 6.60% 1,650 0.41

Lake Barton Example

Sources of Sediment

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

Dredging Alternatives

1. Mechanically dredge deposited sediment and haul wet

2. Drain lake and dry deposited sediment in place before hauling

Lake Barton Example: Dredging in Progress

Disposal Site – Sediment Reuse

• Topsoil for landfill cover • Topsoil product for resale

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

Shoreline Protection Options

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

2011

Inactive Dewatering

Basin

Sediment Thickness

Water Depth

Huntsman Lake - Forebays bank stabilization

aquatic bench

forebay

forebay berms

maintenance access

sanitary sewer

dam embankment

Woodglen Lake

Woodglen Lake: Sediment Thickness

Woodglen Lake: Water Depth

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

Royal Lake

Royal Lake: Sediment Thickness

Royal Lake: Water Depth

Proposed Schedule

• Huntsman Spillway Rehabilitation and Dredging: Fall 2013

• Woodglen: Winter 2013 • Royal: Summer 2014

Lake Accotink – Fairfax County Park Authority

Lake Accotink Dredge Disposal Site – Concrete Plant

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)

Reclaimed Old Concrete Disposal Site

Lake Accotink Dredge Disposal Site

2013

concrete plant

Lake Accotink

2009

Lake Accotink

2010 with lake drained for spillway repairs

Project Contacts

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