RMNA

See Also
SMALL BITES: A New Look at Dredging the SFRR by Richard Lloyd

Dredging Feasibility Reports; February 2010

Summary of reports issued by HDR are found below.
For the full reports, click here

DREDGING FEASIBILITY STUDY SUMMARY
A Powerpoint presentation of the dredging study and conclusions.

DEWATERING PROCESSING ALTERNATIVES REPORT
This report describes the alternatives to dewater sediment material removed from the Reservoir by hydraulic dredging. The typical (and generally least cost) method for dewatering the sediment slurry is to pump the material to a confined dike facility constructed on an upland location near the waterway being dredged. HDR developed five conceptual layouts and ballpark cost estimates for constructing confined dike facilities at four sites. The storage capacity and estimated cost for each site is summarized in Table S-1 of this report.

DREDGING ALTERNATIVES REPORT
The purpose of the South Fork Rivanna Reservoir Dredging Feasibility Study is to evaluate the feasibility of restoring the water supply capacity of the South Fork Rivanna Reservoir (Reservoir) to as near its original contours and water storage volume as practical by removing accumulated sediment. This report presents a proposed two-part dredging plan for the Reservoir.

POTENTIAL BENEFICIAL REUSE OF DREDGED MATERIAL
This report describes various potential beneficial reuse applications for dredged material from the South Fork Rivanna Reservoir. The need to restore the capacity of South Fork Rivanna Reservoir and subsequent availability of valuable dredged material provides opportunity for creative partnerships to meet both environmental and economic objectives of the community.
If dredging the Reservoir moves forward, the dredged sediment would be suitable for most types of beneficial reuse.

BATHYMETRIC SURVEY OF THE RESERVOIR
HDR conducted a bathymetric survey of the Reservoir between December 14 and 18, 2009, using a dual frequency precision depth sounder deployed aboard a small, outboard-powered boat.

HDR’s volume estimate based on our 2009 bathymetric survey data shows total existing Reservoir volume is 1,389 million gallons (MG) (assuming a dam crest elevation of 382 feet). The dead storage (i.e. storage below the lowest intake elevation of 367 feet) is 401 MG.

Restoring the Reservoir to its original total volume of 1,700 MG would require dredging approximately 1,742,567 cubic yards of sediment. If only the usable volume (i.e. the volume of water above the intake) of the Reservoir is considered, the required dredging volume would be significantly less. The total dredging volume to restore the Reservoir to its original usable volume is estimated to be 918,391 cubic yards.

Sediment deposition has reduced the total capacity of the Reservoir by 22%, while the usable capacity has been reduced by 17%.

SEDIMENT CHARACTERIZATION STUDY
HDR conducted a sediment characterization study of the Reservoir to determine sediment characteristics that could affect dredging feasibility. The sediment characterization results are typical of similar riverine reservoirs and watersheds. There are no findings in the sediment characterization results that preclude or inhibit dredging the Reservoir.

The Reservoir sediments do not contain harmful levels of contaminants or metals; removing and reusing the sediment will not require special conditions or restrictions beyond those typically imposed on dredging projects.

PRE-DREDGE SURVEY REPORT
HDR conducted a pre-dredge survey of the Reservoir which evaluated natural and man-made conditions that could impede or influence future dredging activity, including in-water areas with wetlands, rock shoals, low bridges, and utility right-of-ways (ROWs), as well as publically available Reservoir access for equipment.

Observed conditions and characteristics of the Reservoir that could influence dredging activity include:

  1. Wetlands
    Limiting the disturbance of specific wetland types simplifies the permitting process and reduces project costs. However, limiting wetland disturbance also reduces the restoration of water supply capacity. The tradeoff between overall project cost and desired Reservoir capacity is typically addressed during final design of a dredging project.
  2. Threatened and Endangered Species
    Based on this survey, there should be no restrictions on dredging due to the James spinymussel.
  3. Highway Bridges:
    There are three highway bridges crossing the Reservoir; of these, only the Hydraulic Road1 (Rt. 676) bridge at the mouth of Ivy Creek poses an impediment to dredging.
  4. Utility Crossings:
    There are several utility crossings (power lines and a natural gas line) of the Reservoir. The overhead power lines do not preclude movement of equipment beneath them, provided care is taken to ensure adequate clearance. The natural gas line crosses the Reservoir upstream of the Reas Ford Road Bridge, and is therefore outside of the area being considered for dredging.
  5. Access:
    The two existing boat ramps are inadequate for dredge mobilization, although the ramps could be used to launch and service work boats in support of the dredge. It may be possible to launch some types of dredging equipment from the Hydraulic Road or the Earlysville Road bridges using a crane. Alternatively, mobilization and staging areas may be available on private property adjacent to the Reservoir.

WETLANDS ASSESSMENT REPORT
Federal and state permits will be required for any dredging activity within the Reservoir. These permits will consider impacts to water quality and wetlands, as well as other environmental factors. Wetlands/waters impacts will be unavoidable if the Reservoir is to be dredged. Some wetlands/waters impacts can be avoided or reduced through establishment of wetland buffer zones where no dredging activity would occur.

The amount of wetland compensation required will be determined by the permitting agencies following submission of a permit application showing the extent of wetland impacts.