RWSA dredging committee interviews two firms; selection to come Wednesday
By Julia Glendening & Sean Tubbs
Tuesday, August 4, 2009
Two firms with ties to dredging projects in Northern Virginia were interviewed today by a committee of the
Rivanna Water and Sewer Authority
(RWSA). The committee will make a selection between HDR Engineering and F.X. Browne Incorporated at 2:00 PM on Wednesday, August 5 at the Albemarle County Service Authority’s headquarters.
Each firm was given 30 minutes to make a brief presentation, followed by an open session in front of the public. Then the Committee went into executive session to ask further questions, allowing the firms’ representatives to speak candidly about specific business practices.
Officials with HDR Engineering went first. The company was founded in 1917 and now has close to 8,000 employees throughout the world, including five offices in Virginia. Project Manager Carey Burch said his company offers a full range of dredging services as well as reservoir management services. He brought the three members of his team who would oversee the various studies if HDR were selected.
Burch said HDR would use an “iterative process” when approaching the feasibility study, but admitted that they have so far not studied the
South Fork Rivanna Reservoir
in much depth. He did say that in the end, the feasibility of dredging the reservoir would be determined by what the community is willing to pay for.
One of the most important tasks to be conducted in the studies will be the bathymetric study to determine the contours of the reservoir’s floor. HDR Engineer Peter Berrini said the firm would use a “dual frequency transducer” to measure both the original bottom of the reservoir as well as the upper end of the sediment deposits. This would produce a 3-D model. HDR would also conduct a series of cross-sections called transects which would be performed using both hand-held equipment as well as GPS devices that can measure with an accuracy of less than one meter.
Next, Berrini outlined how HDR would manage dredged material by building a series of confined dike facilities to allow for dewatering. He said given the size of the reservoir, a “sizeable land area” would be required. Berrini said the ideal site would be clear and flat and within the reservoir’s watershed so water from the material could return back to the water supply. Berrini said that HDR could also construct dewatering fields on multiple levels to conserve space. On one project, he said they were able to store 1 million cubic yards of dredged material on less than 26 acres.
Berrini introduced a concept which has so far not entered the community’s conversation on dredging. He said HDR could use special chemicals to speed up the dewatering process by causing the solid materials in the slurry to bind together. This process, referred to in the dredging industry as flocculation enhancement, may render the fill less desirable for use as topsoil but could improve its ability to be used for structural fill.
Robin Bedenbaugh would handle permitting duties for HDR. He said from his experience, regulators are concerned with the impacts that occur from the discharge of spoiled material, as well as the dangers of allowing forebays from turning into wetlands. Bedenbaugh said HDR has addressed a wide range of regulatory challenges and used HDR’s work on
in Fairfax County as one example of how they were able to go from design to permit. Lake Accotink is owned by the Fairfax County Park Authority, and it was dredged to increase recreational access. In that case, HDR was able to get a permit to dredge about 200,000 cubic yards in part because they were able to locate a nearby dewatering and storage site that was accessible via an abandoned railroad line.
HDR Engineer Pieter Dahmen discussed the firm’s familiarity with methods to control future sediment from entering the reservoir. Dahmen was again using the Lake Accotink work as an example. However, HDR went over the 30-minute time limit for the public presentation.
Members of the committee did have the chance to ask clarifying questions. Rebecca Quinn, representing the citizens of Charlottesville on the committee, asked Burch if he thought the long, riverine quality of the South Fork Rivanna Reservoir would make it less likely a forebay would be effective in stopping sediment from getting in. Burch said it would not necessarily stop HDR from recommending one, but that a forebay would need to be large enough to slow down the velocity of the river so that sediment could settle before entering the reservoir. He added that a forebay would need to be dredged every few years or it would fail.
Tom Frederick, Executive Director of the RWSA, asked how HDR calculates land costs. Burch said they would investigate sites only within a 3 mile radius. After identifying several candidates, they would turn to local realtors and then have HDR’s right-of-way acquisition experts get to work.
, the Director of Community Development for Albemarle County, asked what experience HDR had with dealing with noise and odor complaints from nearby residents. Berrini said dredging in the water does not make a lot of noise, and residents near one project HDR was hired for actually allowed dredging to be conducted around the clock because the low rumbling was considered to be soothing.
Presentation from F.X. Browne
Frank Browne, founder and CEO of F.X. Browne, was the sole presenter for his firm, which he said his firm has been involved with over 25 dredging projects in 7 states. F.X. Browne conducted the bathymetric study for Fairfax County’s Lake Accotink project. Compared to HDR, F.X. Browne is a smaller company with only 23 employees. However, Browne said his company’s first project was helping the RWSA create a watershed management plan beginning in 1975. F.X. Browne also conducted the very first bathymetric study of the South Fork Rivanna Reservoir in 1976.
Browne said that he was aware of the
desire on the part of City Council to reevaluate the role dredging might play in the community water supply plan
, and said his firm could offer cost estimates that would establish whether it would be worthwhile. Browne said that dredging itself is cheap, but the expense comes from disposing of the sediment.
Browne provided an overview of the many potential ways to dredge the reservoir.
He compared the differences between mechanical and hydraulic dredging. The first would use a backhoe or crane with a clamshell that would scoop material off of the reservoir. He said this would result in drier sediment. Hydraulic dredging would involve getting a dredging boat connected to a discharge pipe that would carry dredged material to a dewatering site and holding area. Browne recommended that this pipe be limited to one mile or less, even with booster pumps. Browne said although hydraulic dredging would be more suitable for the South Fork Rivanna Reservoir, he would want to be flexible before making a firm recommendation depending on what the RWSA Board ultimately decides upon.
In his presentation, Browne promoted the benefits of mechanical dewatering, where the dredged material is pushed through a series of high-pressure belt filter presses. This has the effect of producing clean water as well as drier sediment. Depending on the situation, Browne would also use special chemicals for flocculation. Mechanical dewatering would mean that less trucks would be needed to carry the material to its final resting place.
Browne said his company’s approach would begin with a review of all previous studies as well as a new bathymetric study. He said his engineers needed to know the volume of original storage capacity that could be recovered, and that figure must be as accurate as possible in order to provide accurate cost estimates. A device called a GPS fathometer would be used to determine the contours of the reservoir floor. After that, a pre-dredge survey would be conducted to determine if stumps and other hard objects would present barriers to dredging in key locations. A volume analysis would then be conducted to determine the cost of sediment removal, as well as to identify areas where it might be done more cheaply.
Browne said he was familiar with the way water flows into the South Fork Rivanna Reservoir. He said in looking over his firm’s previous reports, he is aware that the reservoir has a large ratio of drainage area to reservoir area of 364 to 1. That means a very large forebay would be needed to make sure the sediment could settle before flowing into the reservoir. He was skeptical that an effective forebay could be installed given the large drainage ratio. In concluding his presentation, Browne said a final report could be ready within 11 months of being awarded the bid for the feasibility study.
During the question and answer period, Rebecca Quinn asked Browne what experience his company had in evaluating sediment to see if it could be reused. Browne said it was doubtful that there would be much of a market to use the South Fork’s sediment for topsoil given that the organic materials which provide nutrients to plants separate out and float over the top of the dam. He said he thought that the material might also be too gritty to use in highway fill, but a full study of the sediment would determine that for sure.
Tom Frederick said other firms had not been concerned about the one mile limit that Browne recommended for hydraulic dredging. Browne responded that his recommendation comes in part because there is often no right of way to place long pipes overland.
Russell Peery, Charlottesville’s second citizen representative on the committee, noted that F.X. Browne’s proposal used the phrase “selective dredging” in its report. He asked if the use of the word “selective” indicated a prejudice on F.X. Browne’s part towards not dredging the reservoir for water supply purposes.
Browne said that when he wrote the proposal, he followed the report of the
Rivanna Stewardship Task Force
, which recommended selective dredging. He said that in many lakes, there are simply areas where the costs of dredging are prohibitive because of the logistics of getting the dredged material to dewatering sites. However, Browne said he had not worked in the Rivanna watershed for 11 years and was not prejudiced one way or the other.
After Browne’s presentation, the Committee spent over an hour and a half in closed session. At 5:05 PM, the Committee returned to the public meeting room and Tom Frederick announced that the RWSA would conduct reference checks.
Their decision will be announced on Wednesday at 2:00 PM
. Their recommendation will be forwarded on to the RWSA Board of Directors, who will make a decision at a future meeting.