The dredging option28 October 2010
Is dredging the solution to sedimentation problems at South Fork Rivanna reservoir in the US? The answer isn’t as clear-cut as it seems. Suzanne Pritchard reports
Built in 1966 the South Fork Rivanna reservoir and dam are located in the US state of Virginia. Along with the Sugar Hollow, and Upper and Lower Ragged Mountain reservoirs, the facilities are operated by the Rivanna Water and Sewer Authority (RWSA). They supply drinking water and treat sewage in designated areas of Albermarle County.
Water demand within the urban service area is fast approaching the safe yield of water supplies contained within the three reservoirs. Consequently RWSA is pursuing its Community Water Supply Plan (CWSP) to ensure that adequate drinking water is available over the next 50 years.
After many years of studies, consultation and public involvement, Gannett Fleming Engineers and Vanassae Hangen Brustlin assisted RWSA with the development of the final plan. The essence of which was to expand the existing Ragged Mountain reservoir to provide a five-fold increase in raw water storage within this system. In addition a raw water pipeline from South Fork Rivanna reservoir will facilitate the transfer of water to the enlarged Ragged Mountain facility. These were considered as the least environmentally damaging and practicable alternatives to safeguard future water supplies.
However, in 2009 the South Fork Rivanna Reservoir Stewardship Task Force requested that RWSA undertake a study of the reservoir. This was to assess whether options such as dredging and siltation prevention could maintain and enhance the aquatic health and water quality of the facility. The aim is to ensure that the reservoir remains a long-term, valuable water resource for the benefit of the community. This is in addition to the CWSP, which left the idea of dredging open as a benefit to recreational or water quality benefits for the South fork reservoir, but did not recommend dredging as the solution to the future water supply need.
Since it was built South Fork Rivanna reservoir has lost 22% of total water supply volume above the water intake elevation of 112m. Sedimentation is the root of the problem.
River flow into the reservoir is from a drainage area of approx 668km2. Large portions of this are forested (73%). The majority of the remainder is agriculture, with developed areas making up almost half of the remaining total. Soil erosion from natural events; from land use in the agricultural area; from land disturbances in the developed areas; and from re-suspension of flood plain deposits created during the 19th century (stream bank erosion), are the likely causes of significant amounts of sediment becoming trapped within the reservoir.
With the approved water supply plan, and without additional community funds for dredging, the task force fears that the benefits of South Fork reservoir to the community will diminish. Although Ragged Mountain will serve as the major water storage facility once expanded, some argue that there will still be short-term storage capacity benefits that South Fork can fulfil until completion of the expansion in 2021. The suggestion is that a modest level of reservoir dredging will make this possible.
In October 2009 HDR Engineering was awarded a US$344,000 contract by RWSA to assess the feasibility of restoring water supply capacity at South Fork Rivanna reservoir. Its aim is to bring capacity as near to its original contours and water storage volume as practical, through the removal of accumulated sediment.
Restoring the reservoir to its original levels would require removal of approximately 1.3Mm3 of deposited material. However dredging the entire reservoir is not practical: it is not recommended in sensitive wetlands, vegetated islands, or close to bridges or steep banks along the shoreline. Subtracting these recommended no-dredge areas reduces the total estimated dredging volume.
Dredging 860,000m3 would restore all of the useable water supply volume plus 56% of the water supply volume below the water intake elevation of 112m in the Upper Main Stem and Ivy Creek portions of the reservoir. Overall, a dredging volume of 860,000m3 represents 79% of the 1Mm3 maximum dredging target and 65% of the total deposited material.
HDR has identified a two-part dredging approach to reach the favourable 860,000m3 dredging volume:
• Part I would dredge reservoir segments 1-3 (the uppermost portion of the Upper Main Stem), dewater the sediment using mechanical dewatering equipment, and sell or reuse the recovered materials to off-set the cost of dredging.
• Part II would dredge reservoir segments 4–9 (the remainder of the Upper Main Stem) and/or Ivy Creek and dewater the sediment using three confined dike facilities. Dredged sediments removed in Part II would remain in the confined dike facilities and there would be no recovery of material.
The two part dredging approach could be conducted simultaneously or in sequence, depending on available resources.
Full of character
Part of HDR Engineering’s investigations also focused on a sediment characterisation study of the reservoir; determining what type of sediment is in there, and if there are any contaminants which could affect how the sediment is handled or reused. Ultimately this characterisation could determine whether reservoir dredging goes ahead or not.
Sediment is generally characterised by particle size (sand, silt, or clay) and through chemical analyses. Sediments composed largely of sand are generally easy to dredge and de-water. Sediment composed largely of silt is easy to dredge, but can be harder to de-water, while sediment composed of clay is generally more difficult to dredge and dewater. Knowing what type of sediment is in the reservoir is important in determining a feasible approach to dredging, de-watering the sediment, and identifying potential sediment re-use options.
HDR collected five sediment core samples from different areas of the reservoir, from the top 1.5-1.8m of sediment. This sampling depth represents the extent to which handheld equipment can penetrate the sediment layers. Furthermore, this depth also provides a reasonable representation of sediment conditions throughout the reservoir. The five samples were then subjected to:
Contaminant analyses – this included a toxicity characteristic leaching procedure for 40 contaminants to see if these could be washed from the sediment into the water column. Tests were also carried out for detection of the presence of metals such as arsenic, barium, lead and mercury. If there are elevated levels of contaminants or metals, the sediment will require special handling.
Physical analyses – this addressed particle size, bulk density and total nutrients in order to determine the composition of the sediment.
Settleability testing – this determines how long it takes suspended sediment to settle out of the water column.
HDR’s report concluded that there are no findings in the results to preclude or inhibit dredging the reservoir. It summarised that:
• The reservoir does not contain harmful levels of contaminants or metals.
• The reservoir sediments are a mixture of layers with varying levels of density, from fairly compact to relatively fluid. Appropriate dredging equipment capable of effectively removing the varying densities of the sediment should be selected. These would include spud-based, cutterhead type dredges that can impart sufficient excavation force necessary to cut into the dense material. Similarly, if mechanical dredging is utilised, then a barge mounted excavation system capable of imparting sufficient force to cut into the denser material would be required.
• The sediment in the Ivy Creek area and the middle and lower reaches of the main body of the reservoir are largely fine grained, and require relatively extensive time to naturally settle once in suspension in the water column.
If dredging is conducted in the Ivy Creek area and the middle and lower reaches of the main body of the reservoir using a hydraulic suction dredge, use of a cationic polymer is recommended to increase settling rates and effectiveness.
Land application of the dewatered sediment could provide beneficial soil qualities for agricultural applications.
Other areas that HDR Engineering focused on included an assessment of the potential benefits of reusing dredged material from the reservoir. Its report stated that the availability of valuable dredged material provides the opportunity for creative partnerships to meet both environmental and economic objectives of the community.
If dredging moves forward the sediment would be suitable for most types of applications. Once mechanically dewatered and processed, the dredged material could be delivered at a favourable cost compared to retail costs for local topsoil material and sand. Depending on the market conditions at the time, some or all of the cost difference could potentially be used to off-set the costs of producing the dredged material. However, HDR did point out that although sand is the most valuable component of the dredged sediments, it may require additional drying and processing at an off-site stockpile site to be competitive with local retail sand.
In June 2010, technical reports of the South Fork Rivanna Reservoir Dredging Feasibility Study were released for public consultation. The reports outlined the two-part dredging plan at a project cost of between US$34-40M.
RWSA re-iterated that the scope of HDR Engineering’s services was limited to the feasibility of dredging and did not include a review of the CWSP – which backed expansion of Ragged Mountain. Furthermore, the water authority believes that some citizens are inaccurately advocating that the least cost method for securing future water supplies is the dredging option. Recognising that this would lead to cost comparisons between the two methods, RWSA has prepared figures of cost analysis (see graphs).
By comparison, the reservoir expansion at Ragged Mountain will provide 6.5B litres of additional useable storage compared to 865M litres from the dredging plan. When the estimated costs of the entire two-part dredging plan are compared to Schnabel Engineering’s recent cost estimate for a new Ragged Mountain reservoir, the new reservoir is significantly less on a cost per unit of storage added.
HDR’s study included an evaluation of beneficial reuse and identified a potential revenue source from sand recovery within sediment at the upper end of the reservoir. While dredging this upper end would only restore 223M litres of water storage (3% of the volume added by a new reservoir at Ragged Mountain), potential revenues could recover a substantial amount of the cost for dredging this small section.
“There are at least two overarching questions that different citizens are asking from this study, depending on their advocacy,” Tom Frederick, executive director of RWSA said. “One question is ‘can dredging provide our future water supply needs cheaper?’ The second is ‘does the current market provide an opportunity for a small dredging project to benefit the South Fork reservoir as a community resource?’ We believe the answer to the first question is no and the second question is yes.”
The technical reports are now up for consultation. As IWP&DC went to press a public meeting was taking place to consider the proposals. RWSA is reaffirming that the plan to build a larger reservoir at Ragged Mountain should be sustained. However it also recommends a more thorough assessment of proposals to dredge the upper end of the South Fork reservoir, with sand recovery. It needs to be determined if this is a cost-effective option and a way forward.
To keep up-to-date with project progress log onto RWSA’s website www.rivanna.org