Piedroba Dewaters Slurry, Returns Clean Water to Holmes Beach Canals


This project was a successful application of a total cleaning system (TCS), where the particular type of solids of the sediments permitted the dredger to avoid the costly construction of a dredged material maintenance area (DMMA), or the transportation and tipping fees for a municipal landfill. The surface waters were cleaned in tandem, and contributed to a higher quality benthic environment in the canals.

Maintenance dredging more than 11,000 miles of rivers and waterways in Florida normally involves the dual tasks of dredging and placement of the sediments. But these sensitive environments merit adding a third task: cleaning the water itself.

The City of Holmes Beach in Manatee County, Florida, had plans and permits developed by EBA Environmental of Bradenton, Florida to remove 6,890 cubic yards of sediments from eight of their 60-foot wide residential navigable canals. The contract was won by Piedroba Marine Construction (PMC) of Coral Gables, Florida. PMC commenced dredging by mid-May 2009, and self-included a third task: cleaning the returned water extracted from the dredged sediments.

Most of the material consisted of three percent gravel, 91 percent sand and only six percent silt and clay-sized particles. PMC ran its own sieve analysis/hydrometer tests on samples, and found that the material drained extremely well if aerated. Therefore, a solids recover)' system in lieu of land-fill disposal could provide an economic benefit to the city. PMC proposed that the city obtain permits from the Department of Environmental Protection (DEP) to permit the use of the clean sand recovered for re-nourishment of Holmes Beach and Ana Maria Island.

The Holmes Beach canals feature protruding docks, docked boats and piling, which favored the use of PMC's 15-foot-wide LWT 300SL dredge.

The dredge is capable of pumping a rate of 3000 gpm. but could be throttled back to as low as 1,200 gpm. (A lower rate could precipitate the solids in the slimy). The dredged slurry was directed to shore via a six-inch-diameter HDPE pipeline (versus an eight inch) to maintain a higher velocity, floated on 50-galIon sealed plastic containers.

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