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May 07, 2008

Dredging Consultants Recommend a Feasibility Study

Reader comments (2) On May 5, 2008, a dredging consulting firm told an audience of about 60 Charlottesville and Albemarle residents that for about $275,000 it could produce a study detailing the ultimate cost and feasibility of dredging the South Fork Rivanna Reservoir.  

20080505-Gahagan1
Representatives from Gahagan & Bryant spent two days touring the reservoir and its surroundings to make a preliminary assessment of the dredging opportunities which would help maintain the reservoir as part of the area’s water supply system.  The firm was invited by Citizens for a Sustainable Water Plan and the consultants made their presentation at the CitySpace meeting room in the Market Street Parking Garage.

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Gahagan’s Vice President for Operations, Chris Gibson, shared his firm’s qualifications which include consulting on the dredging of other reservoirs used as drinking water sources.  However, in the presentation, Gibson could not offer a firm estimate on the total cost of dredging the South Fork Rivanna Reservoir and admitted their expertise was on dredging and not water supply issues (like treatment and safe yield at times of drought). 

“Dredging is a fancy word for digging dirt under water,” said Gibson.  Gibson gave a brief “Dredging 101” presentation which highlighted equipment options that could be used by the actual dredging contractors that his firm might recommend.  The actual equipment selected would depend on the speed with which the community wanted the project completed, the nature of the dredge material, and the location of the disposal site.  “Typically what we’re going to be dealing with on the Rivanna is pipeline dredges,” said Gibson.  “I have seen material pumped as far as 70,000 feet which is about 14 miles….When you get out [that far] it’s very expensive to do and can be problematic....Distance means horsepower, horsepower means fuel, fuel means expense.”

Gibson said his firm could help the community address the key questions about a dredging project, most importantly, what to do with the material that is removed. “My first question is, ‘Do you have a disposal site?’  Dredging is easy, getting rid of the material, that’s the hard part, and not only is it the hard part, it is what controls the whole project.”  Disposal sites that cannot be reached by pipeline may require trucks to haul sediment.  Small disposal sites may require more time for the dewatering process.  If the dredging removes water from the reservoir, plans may need to be made to return water to the source so the water supply is not diminished.  “Ultimately, disposal site selection is the cornerstone of any good project,” said Gibson

One site that has been proposed by some residents is the Charlottesville-Albemarle Airport about 2 miles from the reservoir.  The airport expects to need fill material for a future runway expansion.  After his tour, Gibson recommended taking that site off the table as a target for a dredge pipeline.  “Realistically, because of the elevation and topography of that site, a direct pump to the airport is probably not going to be feasible… a direct pump is probably not economically viable.”

Gibson said his best guess was that the community would need a 50-100 acre site near the reservoir to handle the long term needs for depositing the dredge material removed by hydraulic dredging equipment.  Gibson was invited to tour Panorama Farms as one potential site.  Gahagan engineer Rob Kite said he saw a lot of promise in Panorama Farm as a potential disposal site.  “The places that I saw out there, and the open acreage that I saw…that is a perfect site,” said Kite.

20080506-SFRR1 Gibson advised the community to get a good baseline picture of the condition of the reservoir.  He recommended a physical and geotechnical site investigation, the use of sidescan sonar to capture a sonographic picture of the bottom of the reservoir, and “vibracore samples” of the dredge material.  In other words, a complete dredge feasibility study which would also examine the multiple disposal options.

Gahagan & Bryant was asked by reporter Jeremy Borden of The Daily Progress to assess the previous dredging price estimates of Gannett Fleming, the firm retained by the Rivanna Water and Sewer Authority (RWSA).  “I saw the estimates that were done in 2004 as well as the updated ones recently in 2008,” said Gibson.  “From a dredging cost [estimate], the dredging scenario was reasonable….The estimates were that it would be $7 a [cubic] yard to dredge the material and get it out of the lake.  That’s a reasonable baseline number.”  Gannett Fleming updated its cost estimates for dredging in March 2008 and estimated hydraulic dredging would be $7 a cubic yard and total $35 million for material removal.

What concerned Gibson were the projected costs of moving the material once it was taken out of the reservoir.  “Seventy-five percent of those cost estimates is in handling the material once it is out of the lake, it’s not in the actual dredging costs.”  Gibson suggested the actual costs for transporting the material could be significantly lower than the Gannett Fleming estimates.  Gibson also said until you do a feasibility study and bid the project out to specific contractors who will do the work, there is no way to know the actual cost of dredging.  According to the Daily Progress, the RWSA is standing behind its estimate that dredging will cost “between $159 million and $178 million, with a 25 percent contingency bringing it to $223 million.”

The night after this briefing to the community, Gibson and Kite presented their findings to the Charlottesville City Council.  Charlottesville Tomorrow will share coverage of that work session in an upcoming story.  City Council will have a public hearing on the Community Water Supply Plan on May 19, 2008 [event details].

Brian Wheeler

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