To paraphrase Rodney Dangerfield, stormwater don’t get no respect. It’s just rain mixed with the stuff from our yards and a little trash from the street that runs down a storm drain. Of course, it’s not a bad thing, so long as it’s treated before it enters water we might drink or swim in.
The truth of the matter — in Tampa Bay and most cities across the U.S. — is that stormwater is the primary way most pollutants enter our rivers, lakes and estuaries. The “stuff” it picks up includes fertilizers that cause harmful algal blooms, pesticides and herbicides used in our landscapes, dog poop laden with bacteria, and various petroleum products carrying heavy metals and toxic contaminants.
In newer neighborhoods, stormwater is held in retention ponds where most contaminants can settle to the bottom. In older neighborhoods, though, stormwater simply rushes into the nearest creek or river with all the pollution it’s picked up along the way.
And fixing the problem isn’t as simple as it sounds. It takes a lot of land – or a lot of imagination and innovation – to clean stormwater. Two new projects underway in the Hillsborough River watershed highlight the challenges facing water managers as they grapple with strict limits on the amount of pollution entering Tampa Bay.
The Robles Park project, for instance, actually started at Glenwood Avenue when the city of Tampa asked Southwest Florida Water Management District’s SWIM (Surface Water Improvement & Management) program for assistance in building baffle boxes into stormwater pipes that drain directly to the Hillsborough River. A feasibility study looked at the amount of water, combined with the elevations and the amount of land available, and showed that the project wasn’t going to work.
Backing up – literally moving up the watershed – the SWIM team and city staff looked at available land and the newest technology, then designed a series of smaller baffle boxes to treat stormwater draining into Robles Pond. While Robles is further away from the river, the pond is part of a watershed that drains near Stewart Elementary School (see Bay Soundings, Winter 2011.)
“Robles Pond is a natural basin that drains about 170 acres of very dense neighborhood with a lot of rooftops and a lot of solid surface where rain can’t sink into the ground,” said Nancy Norton, professional engineer with SWIM.
The first step of the project, a three-way partnership between the city, SWIM and the Florida Department of Transportation, will be dredging the pond to make it deeper and remove the sediment that’s already been deposited. Five of the seven drain pipes (including three draining from nearby Interstate 275) will be fitted with the latest technology in baffle boxes that capture nitrogen and phosphorus as well as sediment.
Baffle boxes work much like retention ponds in areas where less space is available. They slow stormwater down so sediment has a chance to settle out. The new boxes, developed by SunTree Technologies in Cocoa, include three chambers to capture the sediment – where hydrocarbons and other toxic substances are most likely to be found. The latest innovation includes a filtration screen set above the baffle box to catch leaf and lawn litter, an important source of nitrogen and phosphorus in stormwater, that wasn’t captured in previous versions of baffle boxes.
A second stormwater retrofit at River Tower Park in Sulphur Springs includes a two-acre pond 16 to 18 feet deep that will capture stormwater from densely populated neighborhoods before it enters the Hillsborough River. Like the Robles Park project, it’s a partnership between the city, SWIM and FDOT where the actual project changed dramatically from the original plan.
“The city came to us for help with a shoreline restoration, then FDOT got involved because of the interstate,” explains Stephanie Powers, environmental scientist with SWIM. “Working together, we’ve designed a project that will restore 1,200 linear feet of riverfront and treat the stormwater from a 350-acre drainage basin.”
The pond, designed with weirs that allow water to overflow into the river once pollutants have settled out, will be dredged on a regular basis to remove contaminants. “There were some questions about how large the pond needed to be to capture the stormwater, but luckily the park is large enough to accommodate it,” Powers said. “Over time, as the plantings grow in, it will provide an aesthetic benefit as well as stormwater treatment.”