When U.S. Highway 98/State Route 30 was first built through Destin, Fla., the only thought given to storm water was how to get it off the road. At that time, Destin was little more than a roadside bait shop and a few fishing camps. Seventy-five years later, the highway is lined with high-rise condominiums facing what may be the most beautiful beach in America, and Destin is still "the world’s luckiest fishing village."
The local economy revolves around tourists who come to enjoy the pristine white beach and clear, emerald-green water. However, as the area became more popular, pollutants washed by storm water from the highway and bordering parking lots threatened the water quality of the harbor. All of the storm water falling along a 4-mile-long strip of the highway flows to three outfalls which empty into Destin Harbor. Two of the outfalls are 30-in.-diam. pipe, and the other is a 72-in.-diam. pipe. As a result, city officials asked Florida Department of Transportation (FDOT) for help.
Scot Golden, P.E., District 3 drainage engineer, said FDOT almost never does storm water treatment enhancement projects unless they are connected to new roadway work, so his department used a special enhancement fund to contract with Faller Davis & Associates, Tampa, Fla., to design a solution.
According to Nancy Faller, president of Faller Davis, land along the beach-front highway is far too scarce and expensive to allow storm water treatment ponds to be built. Therefore, she began a study to find the best alternative. Her investigation led her to several small-footprint storm water treatment structures which qualify as best management practices. However, only one of the technologies, continuous deflective separation (CDS), could handle the flow from a 72-in. pipeline, and it was the only technology that captured and retained all floatables which entered it.
Chris Landt, project engineer, CDS Technologies Inc., Winter Park, Fla., explained that CDS units can be designed to treat flow ranges from 1 to 300 cubic feet per second (cfs) and higher. Treatment flow is diverted from a storm sewer drain into a CDS unit where the non-mechanical screening technology uses the hydraulic energy of the water to screen and trap pollutants.
As water enters the separation chamber of a CDS unit, it begins a circular motion which is hydraulically designed to allow it to pass through a cylindrical stainless steel screen while the screen separates and retains pollutants. The circular flow is carefully balanced to assure that the tangential flow around the interior of the screen keeps the screen clean. Floatable debris, neutrally buoyant debris, sediment and other pollutants are collected in the center of the chamber. Heavier pollutants settle into a central sump where they can be removed by vacuum as needed (one to four times each year). After the water passes through the screen, it is returned to the storm water drain.
CDS units permanently remove virtually 100% of floatables from storm water. Once trapped in a CDS unit, debris cannot be flushed back into the downstream system. The units also remove 100% of all particles that are greater than one-half the size of the screen opening. Standard screens are available with openings from 4,700 microns (0.185 in.) to 2,400 microns (0.095 in.). Studies show the units remove 93% of all particles which are one-third the size of the screen opening and 53% of all particles one-fifth that size. A conventional oil baffle within the CDS units effectively controls oil and grease in storm water. Studies have shown that with the addition of sorbents, the capture efficiency of free oil and grease is approximately 80-90%.
Frank Synychak, senior drainage engineer, Faller Davis & Associates, was responsible for selecting the appropriate CDS units and designing how the structures would fit into the existing system. One drainage basin consisted of only 2.5 acres, so a unit which could treat 3 cfs was selected. A unit which can treat 11 cfs was selected for a 6-acre drainage basin. The largest basin consisted of 122 acres, so a CDS unit which can treat 30 cfs was used.
"No other technology could handle a 72-in. pipe, much less treat a flow that large," Synychak said. "My calculations show that five units from the next-best technology would have only been able to treat 5% of the total flow from the basin. One CDS unit treats it all."
All three of the units were precast concrete. Each installation design was different to meet site requirements. The smallest unit was the least complicated because the CDS unit could simply replace a short section of the existing drainage pipe. Installation of the next larger unit was more complicated because a building had been erected very close to the drainage pipe. It was feared that driving sheet piling so close to the building would be destructive. As a result, the unit was placed in the street, and the storm drain was diverted into it. The third and largest CDS unit also was installed in a street because that was where the 72-in.-diam. reinforced concrete pipe was located.
Fits right in
Charles L. Meister, P.E., city engineer, Destin, said the city has been very concerned about water quality in Destin harbor for a long time, so he was pleased when the treatment structures were in place. All maintenance and operation of the treatment units are the responsibility of the city.
"We don’t know yet just how often the units will have to be emptied," Meister said. "It will depend upon the amount of rainfall we get and how clean we keep our city streets. But, other than cleaning, there shouldn’t be any maintenance since the units have no moving parts."
Meister said he expects to see more municipalities go to units like this instead of retention ponds."When your real estate is expensive and scarce, you don’t have the luxury of buying land and building retention ponds," he said.