The following questions and answers were drafted based on comments and questions received from member of the public between February 15 and March 16, 2023.
Public Works and Environmental Services Alert:
The following questions and answers were drafted based on comments and questions received from member of the public between February 15 and March 16, 2023.
No. In communications with staff from the Virginia Department of Environmental Quality they stated that they are not aware of any requirements within their permitting programs that will require Fairfax County to dredge the lake.
Yes, staff believe that the County would be assigned an additional waste load reduction requirement for the portion of the Accotink Creek TMDL downstream of the lake; but they do not believe the amount would be significant in comparison to current requirements. All restoration work done in the watershed would receive credit towards that load allocation. The County has done and is already planning a substantial amount of restoration in the Accotink Creek watershed.
Accotink Creek does discharge a lot of sediment due to the excessive storm flows from its highly developed drainage area eroding stream banks. That sediment is harming water quality, but the primary impact is to the stream channels themselves and the aquatic animals that live in them and terrestrial animals that depend on them. The Accotink Creek Total Maximum Daily Load (TMDL) was assigned to Fairfax County based on:
The Chesapeake Bay Expert Panel, US Geological Survey and George Mason University have provided a tremendous amount of data that reveals what is likely to happen to the sediment before it gets to the Potomac River, and what is actually happening when Accotink Creek reaches tidewater:
Dredging Lake Accotink is inefficient compared with previous county dredging operations, becomes increasingly expensive over time, and is ineffective in addressing the root cause of Accotink Creek stream degradation.
Previous lake dredging operations in Fairfax County included low-cost disposal, were much smaller in scale and/or were able to be accomplished using much lower cost dry dredging methods (see comparative cost table below). In addition, other lakes managed by Fairfax County, for example, Huntsman, Mercer, Royal and Woodglen, are sized to handle the flows and sediment loads within their watersheds. It was thirty-eight years between the time Huntsman Lake was first filled and when it had to be dredged, and even then it was nowhere near full. Dredging of these other lakes was able to be accomplished by draining the lake, drying the sediment onsite, and trucking the material to a county or other local low-cost site for disposal.
Lake Accotink has a massive watershed and would need to be dredged again every five years at significantly increasing costs in order to keep it from filling in. All dredging of Lake Accotink would require hydraulic dredging, is large in scale, and sediments would need to be pumped through a pipeline to a separate processing site to be dried and trucked off-site for disposal at a commercial landfill.
| Dredge | Year Dredged | Quantity in Cubic Yards | Cost Estimate | Cost Per Cubic Yard |
|---|---|---|---|---|
| Lake Accotink Base Dredge | Future | 500,000 | $95,000,000 | $190 |
| Lake Accotink Maintenance Dredge 1 | Future | 150,000 | $46,500,000 | $310 |
| Lake Accotink Maintenance Dredge 2 | Future | 150,000 | $59,400,000 | $396 |
| Lake Accotink Maintenance Dredge 3 | Future | 150,000 | $96,800,000 | $645 |
| Lake Accotink Maintenance Dredge 4 | Future | 150,000 | $123,500,000 | $823 |
| Prior Lake Dredgings | Actual Cost of Prior Dredge Projects | |||
| Lake Accotink | 2008 | 180,000 | $9,975,000 | $55 |
| Lake Barton | 2010 | 32,500 | $2,115,000 | $65 |
| Huntsman Lake | 2014 | 43,000 | $3,500,000 | $81 |
| Woodglen Lake | 2015 | 40,000 | $3,150,000 | $79 |
| Royal Lake | 2017 | 80,000 | $6,300,000 | $79 |
| Lake Barton | 2022 | 19,100 | $1,996,000 | $105 |
Accotink Creek captures most of the sediment generated in its stream channels before it reaches the Potomac River and the Chesapeake Bay. As stated above, the US Geological Survey found that the Accotink Creek system is balanced, capturing about as much sediment as is produced. Dredging Lake Accotink does not provide significant benefit for reducing impacts to the Chesapeake Bay.
The Accotink Creek TMDL:
We think that the average annual sediment load to the lake at this time is about 46,000 cubic yards, and the lake is capturing about 19,000 cubic yards of that. The rate of incoming sediment likely increased through the 1980s as the watershed was built out. It also varies based on storm events. Tropical Storm Lee in 2011 was the largest storm on record for the Accotink Creek watershed, and, anecdotally, it delivered a very large sediment load to the lake.
No. The sediment analysis for the Lake Accotink completed in 2021 revealed that with one exception, all of the 200 samples taken were within the federal limits for toxicity. There is no indication that having sediment accumulate would pose any risk to the public.
Bathymetric surveys of Lake Accotink sediment were conducted in the study in 2016 to assess lake management alternatives and the study in 2021 as part of the lake dredging project. The assessment in 2016 concluded that Fairfax County would need to remove about 350,000 cubic yards of material to attain an average 8 foot depth of the lake. The study completed in late 2021 determined that the lake had gained about 50,000 cubic yards of sediment since 2016. So the total with this estimation to attain an average 8 ft depth is about 400,000 cubic yards of material. However, excavating sediment out of Lake Accotink is like trying to dig a hole at the beach while the waves are coming in: as you dig the waves wash more material into the hole. Since the amount of sediment arriving at Lake Accotink is fairly consistent over time at about 46,000 cubic yards per year, staff estimate that in addition to the 400,000 cubic yards of sediment to reach the 8 ft depth based on current sediment depths, we would need to excavate an additional 100,000 cubic yards of material out of the lake over the three years of the base dredge in order to compensate for all of the sediment filling in the lake as it is dredged in order to meet the desired 8 ft average depth.
Doing smaller dredges would not provide any real benefits because so much sediment is arriving at the lake that gains would be quickly erased. In addition, water quality in the lake is impacted by suspended sediment while dredging is underway; so more frequent dredging regardless of the amounts removed would diminish water quality for longer periods of time. If we tried reducing the amount of material dredged to keep the volume of trucks down, we would likely end up dredging continuously with either very few short breaks between dredging events or no breaks. The cost per cubic yard of material would likely not go down with slower dredging events, and would continue to go up over time with inflation.
A dredge of 500,000 cubic yards of sediment would fill a football field to a height of 281 feet. A big tree in our region is about 90 feet tall. So the base dredge for Lake Accotink would fill a football field to a height of about three times taller than the largest trees in our region. A sediment pile that high would also be taller than a twenty-five story building.
Fairfax County considered the potential for employing a dry dredge at Lake Accotink as has been done at every other major dredging operation done by the county. It was determined that the Accotink Creek watershed is too large and the stream too flashy to employ a dry dredging approach. In a dry dredge, sediments need to be kept dry for may weeks in order to use equipment to remove them. During a rain event, Accotink Creek would quickly overwhelm any efforts to keep storm flows out of areas where sediment was drying. Storm flows would also prevent equipment from maintaining a channel to keep water out of drying areas. In addition to the major extensions in the dredging timeframe, trucks would have to enter and exit through Queensbury Avenue.
When the lake management alternatives were evaluated they included the idea of installing a lake forebay to remove sediment to reduce the need to dredge the lake. It was determined that a sediment forebay could not be constructed that is large enough to reduce the amount of sediment captured by the lake enough to reduce the frequency of lake dredging.
Turbidity curtains are used during construction to contain sediment suspended by land disturbing activities. Turbidity curtains could be used by dredging contractors if it would reduce impact; but dredging impacts on the scale of those necessary for Lake Accotink would likely not be appreciably reduced by their use.
The lake sediment is a mixture of silt, clay, sand, gravel and organic debris (leaves, sticks, branches, etc.). It is not usable as a compacted fill material that you can build anything out of because it does not have the right mixture of different materials.
Clean dredged material can be spread on a site that needs material, but we could not identify any site in the region which could take our material other than quarries that have stopped excavating rock and are now permitted to receive material that meets the cleanliness standards of their permit – which the sediment from Lake Accotink does.
Another problem is that the quantities for the base dredge are very large, as are the subsequent maintenance dredges every five years. County staff and consultants did talk to one of the largest regional materials processors, Luck Ecosystems to determine if there was the potential to reuse all or some of the sediment in Lake Accotink. In that conversation, Luck determined that our dredge volumes exceeded their capacity to use mechanical separation to extract any useful portion of the Lake Accotink sediment for reuse.
Dredged material from other projects in Fairfax County has not been sold. Because of the small quantities of most of the previous dredge projects, the county has been able to use the material to reinforce the cap on the Interstate 95 landfill. That landfill is not in need of additional material, and the quantities involved in the Lake Accotink dredge are far in excess of those done previously.
County staff and consultants have spent several years attempting to locate a willing buyer or some entity willing to accept the processed sediment for low or no cost. Unfortunately, no such entity exists in our region.
County staff and consultants explored the potential to use rail transport and agree that if it were feasible it would solve a significant number of issues with traffic conflicts, air pollution, noise, cost, etc.
What we discovered was:
The concept for lake dredging that was promoted in 2019 envisioned using the Dominion Virginia powerline easement in Wakefield Park to process the dredge spoils. That concept did not include any analysis of whether the powerline easements could be used to process the dredge spoils. Analysis in the last two years, to include coordination with Dominion Virginia, has determined that the restrictions under the powerlines in Wakefield Park in terms of flooding potential, powerline height and structure distance restrictions, and the need to access the powerlines for emergency and routine maintenance, has determined that the Dominion Virginia powerline easements in Wakefield Park are not sufficient to conduct dewatering operations of the dredged material. The most suitable site, from the perspectives of being publicly owned and not requiring access through a residential neighborhood, is the Wakefield Park Maintenance Area which includes about seven acres of forest, of which about four acres is forested wetlands.
When Lake Accotink was drained for dam maintenance about twelve years ago it was pretty unsightly. The difference then was the drop in the water level exposed the sediment, and the lake bed remained bare mud while the maintenance was underway and until the lake filled again.
If the dredging is not done, the lake would remain at full pool, so there should be water visible. The anticipation is that it would remain open water for the foreseeable future with a gradual filling in starting at the edges. The establishment of a wetland would require initial funding to ensure that the wetland is both an ecological feature, as well as a community amenity. Eventual establishment of emergent wetland vegetation is anticipated to start at the edges and at the upper end where the creek flows in especially around the old island.
Funds would need to be set aside for managing the wetlands. This management may include both guiding how wetlands develop and managing them for long term ecosystem health and wildlife habitat. We do not currently have a cost estimate for wetlands management.
There would be no increased flooding impact if the lake filled in with sediment because Lake Accotink always sits at full capacity (“full pool”). It was not designed as a stormwater facility, so it contains very little detention capacity, and that capacity would not change if the lake filled in. No future water/sediment surface elevations can exceed the top of the existing spillway. As a result, there would be no increased flooding impact from the lake filling in with sediment.
The estimate of $30 million for dredging Lake Accotink presented to the Board of Supervisors in 2019 did not have the benefit of intensive analysis, included some assumptions that are no longer considered valid and could not have anticipated increases in sediment volume or market prices for construction. The primary reasons the cost went up are:
In spring 2021 staff began to see the costs going up dramatically. They went back to the Board of Supervisors and requested permission to seek an additional $30 million in loan approval from the Virginia Clean Water Revolving Loan Fund – raising the construction cost to $60.5 million. This cost was shared with project stakeholders. However, as analysis refined both the project constraints and needs and prices were adjusted for inflation, staff understood by summer 2022 that the cost to dredge would be over $90 million.
The Fairfax County Department of Public Works and Environmental Services was tasked with assessing the implementation of both a base dredge and a maintenance dredging program for Lake Accotink. As part of the request for approval to seek additional state loan funds for the base dredging program in June 2022, members of the Board of Supervisors requested information on the maintenance dredging program. A 5% inflation rate is currently used in developing estimates for all county projects from fire stations to school renovations.
There was no bond. Fairfax County applied and was approved for $60.5 million in state loans. The state loans require repayment with interest over 20 years.
Fairfax County has not drawn on the approved loans.
The county has secured a $60 million loan guarantee from the Virginia Clean Water Revolving Loan Fund, which would have to be paid back. There are no state or federal grants available to support maintenance or dredging of existing facilities.
This is a very good idea, but unfortunately Virginia law does not allow us to impose a special tax district in this manner. It’s also worth noting that the residential neighborhoods immediately surrounding the lake were built before modern stormwater controls were required and are significant contributors to the runoff to our streams which produce the sediment entering the lake.