Lakes are not constants in a geologic sense, but are temporary, dynamic features of the landscape.  Over tens to many thousands of years, lakes change in size and depth due to various processes such as climate, erosion, rising or falling water table, and the accumulation of sediment.  This natural life cycle includes the process of eutrophication, which is the excessive addition of inorganic nutrients, organic matter, and silt to lakes, resulting in an increase in biological activity. 

Florida's lakes tend to be very shallow and surrounded by highly vegetated, nutrient-rich watersheds; thus, they naturally have shorter life spans than many deeper lakes in other regions.  A significant problem arises when the eutrophication process is accelerated due to human activities within the lake basin, such as:

• Changes in land use due to development
• Water level stabilization projects for flood control
• Discharges of industrial and wastewater effluent streams
• Increased urban runoff
• Greater use of pesticides and fertilizers
• Floodplain encroachment.

These factors have led to frequent algal blooms, fish kills, loss of submerged vegetation, deteriorated water quality, and reduced or degraded habitat.

Florida's Lake Restoration Programs

Major restoration programs, such as the Everglades Restoration, have been initiated throughout the state in more recent years to reverse the damage and protect Florida's natural systems.  These programs create many rewarding opportunities for engineers, scientists, and environmentalists to work together to balance the development of essential facilities for a growing population with the improvement and preservation of the environment for present and future generations. 

PB has supported state agencies undertaking the restoration and aquatic habitat enhancement of several of Florida's large lakes by providing study planning engineering, design, permitting, or construction administration services.  Some of the projects we have been involved in are listed in Table 1.

Engineering Considerations for Lake Restoration

Programs undertaken by several of Florida's state agencies (including the water management districts and the Florida Fish and Wildlife Conservation Commission) to restore many of Florida's lakes typically comprise three components:

• The physical removal via hydraulic dredging or drawdown and bottom scraping of the thick layer of organic sediment accumulated on the lake bottom.
• The storage and treating of this material. 
• The reuse of this material and the reclamation of the storage site.

Dredging.  Dredging design is based on approaches developed by the U.S. Army Corps of Engineers.  It involves the selection of appropriate equipment needed for the sediment removal process, including the dredge plant, which is usually on a boat or barge (Figure 1 on the following page), booster pumps, and a pipeline to convey the dredged slurry (mixture of solids and water) over some distance to a discharge location.

Storing and Treating.  Often the key factor to viability of a sediment removal project is finding a location to store and treat the material removed from a lake.  One solution is the construction and operation of a confined disposal facility (CDF) near the lake.  Surrounded by earthen embankments, design of a CDF involves significant geotechnical engineering considerations and must address many water quality, environmental, and permitting concerns. 

Once the dredged slurry is piped into a CDF, the solid sediment material is allowed to settle out and the supernatant water is released from the holding cell(s) via engineered water control structures.  We then typically convey the supernatant water to a polishing pond, where additional residence time allows for more settlement, thereby decreasing turbidity levels to meet water quality goals prior to final discharge.

Reuse and Reclamation.  Some final aspects of a dredging project-additional dewatering, beneficial reuse, and CDF site reclamation-present significant challenges to the project design team due to the physical and chemical properties of dredged material. 

Additional dewatering is needed because large amounts of water remain in the confined fine-grain material after the treatment described above, through which much of the water is released from the dredged slurry via CDF water control structures.  Additional dewatering approaches can include:

• Natural evaporation via air/sun drying
• Trenching
• Belt filter presses
• Solidification by admixture (e.g., lime, cement, polymers).

Natural evaporation is the least expensive option as long as sufficient storage area and time are available.  Digging trenches to increase dewatering at depth can also be quite effective.  Complex approaches like belt filter presses and admixtures are significantly more expensive but can overcome storage area limitations and provide faster results.

Dredged material that has been sufficiently dewatered can be reused beneficially for various purposes, such as:

• Construction fill
• Landfill cover
• Strip mine reclamation
• Agricultural soil amendment
• Horticultural industry needs.

Creation or enhancement of habitat can be achieved during reclamation.  Recreational use is another popular option for restoration of former dredged material storage sites.

Case Study:  Lake Panasoffkee Restoration

PB was hired by the Southwest Florida Water Management District (SWFWMD) to provide planning, permitting, design and construction management services for the Lake Panasoffkee Restoration Project.  Over the years, sediment build-up had destroyed fish spawning areas and promoted emergent vegetation to encroach the lake shorelines.  This build-up was primarily the result of a unique natural phenomenon in which dissolved calcium carbonate that entered the lake as upward discharge from the limestone aquifer below precipitated out due to the photosynthesis associated with lake vegetation.  The restoration plan was aimed at restoring fisheries habitat and historic shoreline conditions. 

Pilot Dredging and Preliminary Design.  The initial step in the plan was a pilot dredging project.  The pilot project provided information critical to the design, permitting, and construction of the full-scale project by confirming settling rates needed to size the CDF and demonstrating that the discharge water would meet state water quality standards.

During the preliminary design phase, data collection efforts included:

• Aquatic vegetation mapping
• Bathymetric and land survey
• Sediment coring and testing
• Water quality sampling and testing
• Sediment dewatering alternatives. 

The results from the analyses were used to further define the limits and depths of dredging and the sediment characteristics of the lake.  The bathymetric data were used to ensure dredging occurred deep enough to expose either "hard bottom" or "shell bottom," which would provide a stable surface on which fish could lay their eggs.  The average dredge depth was 2.5 feet.

Dredging Constraints.  A significant constraint during the dredging operation was that a minimum 60-percent areal coverage of submerged aquatic vegetation (SAV) be maintained in the open water for the duration of dredging.  This was a permit requirement to ensure that lake water quality was protected.  The initial dredging sequence was determined based on maintaining that SAV coverage.

Disposal Method.  The final design included hydraulic dredging of approximately 6.6 million m³ (8.6 million cubic yards) of soft inorganic calcium carbonate sediment and disposal of the dredged material in a 150-ha (450-acre) CDF located approximately 3 km (2 miles) from the lake (Figure 2).  Finding this disposal site was a key component in the project because the area in the vicinity of the lake is generally rural.  SWFWMD went to great efforts to find and purchase a parcel of land large enough for disposal of the dredged material.

The dredged sediment is in slurry form with about 90 percent water so it flows readily via pipeline.  A 700-horsepower centrifugal pump on the dredge barge sends the sediment slurry to the CDF via a 350-mm (14-inch)-diameter high-density polyethylene pipeline.  In addition, 500-horsepower booster pumps are used every 2135 m (7,000 feet) along the pipeline.

The CDF consists of three separate diked cells: two settling cells (seen in center of Figure 2) and a final polishing pond (bottom right of Figure 2).  Dredged material enters a settling cell for initial clarification and eventually makes its way to the polishing pond for additional improvement in water quality.  The clear water is then discharged into an adjacent natural wetland (bottom left of Figure 2). 

Project Status.  The dredging, which has commenced, is expected to take 3.5 years and be completed in December 2007.  The CDF will be reclaimed into pastureland after completion of the project.