Lake Dredging

Lake Dredging

Chair:

Bob Charles, bobandmel04@hotmail.com

Vice Chair:

Glenn Weigle, llagweigle@gmail.com

Background on Lake Linganore Sediment & Dredging

All lakes, whether natural or engineered like Lake Linganore, fill in with erosion sediment over time. Sedimentation rates increase with land development due to increased run-off / reduced infiltration leading to increased surface water flow rates and stream bank erosion. As sediment builds up on the bottom of a lake, it displaces lake water and the volume of water the lake can hold diminishes.

Lakes like Lake Linganore, which are used for recreation and as a drinking water reservoir, require periodic sediment removal to preserve the intended uses of the lake. Sediment removal frequency varies considerably but normally more often than every 45 years (the number of years sediment will have been allowed to accumulate in Lake Linganore).

Occasionally, sediment deposition areas (“forebays”) are engineered and constructed at the stream lake entrance to protect the lake and to facilitate sediment routine sediment removal. Lake Linganore is rumored to have had forebays but no historical plans or photographs have been found and if the forebays existed they are lost.

The USGS estimates that the volume of sediment deposited in Lake Linganore averages more than 8,000 cubic yards per year, equivalent to about 11 feet of soil placed on a football field annually. Approximately 5 million gallons of water storage capacity are lost to sediment each year. The USGS recently found that the sediment deposits have resulted in the upper portion of Lake Linganore (above Boyers Mill Bridge) losing 65% of its water storage capacity (View the USGS Scientific Investigations Report 2013-5082) and the lower portion of Lake Linganore losing 17% of its water storage capacity (View the USGS Scientific Investigations Report 2010-5174). Sediment accumulations approaching 17 feet in certain portions of upper Lake Linganore (see “Figure 8” excerpt from USGS study below) have posed problems for boating and other community recreators and have likely contributed to water quality degradation throughout the lake. The sedimentation pattern indicates that that the bulk of the sediments entering and settling in the lake are coming from outside the community (upstream).

Dredging Plan Concept

The current dredging plan is to remove 8 feet of sediment from only the upper portion of Lake Linganore. Accordingly, it is estimated that approximately 253,000 cubic yards of sediment will need to be dredged from the upper lake to accomplish this objective. This volume is equivalent to the volume of sediment deposited in the lake over a 31-year period.

A number of on- and off-site alternatives have been identified and explored for re-use of the dredged sediments by Maryland Environmental Service (MES) under a $0.6M, reduced hourly rate, sole-source design contract with Frederick County. MES’ work began in January 2015 when MES was contracted by Frederick County to perform an analysis of spoils re-use / disposal alternatives, preliminary dredging design / planning and permitting. Once the preferred sediment disposal alternative is known, MES is under contract to prepare a preliminary, 30% design suitable to initiate the permitting process. As of February 2016, MES’ top alternative for dredged sediment placement is Frederick County’s Reich’s Ford Road Sanitary Landfill. The dredged sediment would be stockpiled at the landfill to be used as needed for daily clean soil cover without taking up landfill capacity for waste.

Provided it can be completed cost effectively, the dredging plan would include the construction of forebays to better protect the lake and facilitate future sediment removal efforts. Depending on final costs of the dredging / sediment disposal & potential costs for building the forebays, they may or may not be feasible.

Rough Costs

The total cost to implement the dredging plan has been preliminarily estimated to be about $15M. Costs will become more accurate as the design / plans progress. Precise costs will not be known until after the final design is completed and dredging contract has been executed.

In December 2014, a cost sharing Agreement was reached between LLA, Frederick County and the City of Frederick (View the Agreement). Since The City of Frederick uses Lake Linganore as a reservoir for a significant portion of its water supply, it agreed to pay for 50% of the cost (~$7.5M). Frederick County counts on the reservoir as a backup water supply and, therefore, agreed to pay for 25% of the cost (~$3.75M). LLA membership will be responsible for the 25% balance (~$3.75M).

Schedule

Final design and permitting is not anticipated to be complete until 2018. Once underway, the dredging can be expected to take up to 2 years (i.e., completed by 2021), depending in part on the number of independently operating crews to optimize efficiency.