Literature Review & Summary Report Design Basis Memorandum St. Johns River Water Management District Contract # SK940AA
BCI Project No. 19-15089 September 2007 Page 1
Sediment removal from lakes and other waterbodies has proven to be an effective method for removing contaminants and limiting the internal recycling of nutrients (Cook et al). There are many methods for dredging sediments, and in each case, the disposal of the dredge material presents the greatest challenge in project design. The disposal of dredge materials is often complicated by the lack of land around a waterbody that is available for disposal. The use of rapid dewatering technologies can mitigate the lack of available land for disposal and facilitate lake dredging projects primarily in urban areas. Rapid dewatering is defined as any process that accelerates the separation of solid and water fractions in dredge slurries, and the use of rapid dewatering techniques often reduces the amount of land needed for disposal.
In Florida, most lakes have experienced some level of development around the shoreline. The value of lakefront property is often significantly greater than that in the remaining watershed. Many lakes in need of restoration have been degraded as the result of extensive development in the watershed and associated stormwater runoff and habitat loss. Vacant land may be non-existent or zoning may be such that sitting a dredging disposal area in the vicinity of the lake may not be practical. As a result, the location of final disposal sites for dredging projects in urban lakes may be many miles from the waterbody and the area available for dewatering and temporary storage may be limited to a few acres or less.
The design of lake dredging projects is driven by the physical characteristics of the dredged sediment and the availability of suitable disposal areas. Consideration must also be given to the type of dredging most practical for a specific application. The two primary dredging types are mechanical and hydraulic. Mechanical dredges use a clamshell, shovel, backhoe, or other such device that scoops material from the lake bottom. This may be a land based or barge mounted crane. Barges may be used to move the material from the dredging site to a disposal site, or to trucks for haulage to a suitable site. Since the nature of mechanical dredging is such that lake bottom material is removed with the other sediments, the mixed material (slurry) is often mud-like and thick enough to transport and store without further dewatering (Figure 1).
Hydraulic dredges use water and a pumping system to remove and transport sediments. Dredged material moves through a pipeline, often thousands of feet, assisted by a booster pump (or series of pumps) if necessary when the distance warrants. Due to the very dilute nature of the hydraulically dredged sediment discharge, subsequent mechanical or chemical dewatering and thickening is often necessary.
In cases where land is readily available, sediment disposal areas can be designed that will provide sufficient sediment retention/detention time to allow gravity settling and drying without the use of mechanical or chemical dewatering technologies (BCI, 1989). In many cases, dredge slurries must be rapidly dewatered to facilitate storage in relatively small areas or where the sediment must be transported to a distant disposal site. Rapid dewatering of hydraulically dredged lake sediments has been tested and successfully used in Florida and Ohio by BCI and others.