Controlling the Flow
A city/county project solves detention maintenance issues, addressing flooding while meeting detention requirements for urban runoff and wetland and stormwater criteria
- By Rex Hansen, PE
- May 01, 2004
A nine-year, $5.29 million road improvement project in Washington County, Ore., is finally coming to a close in June 2004. With its completion, a community and wetland will begin to enjoy its benefits, including improved pedestrian safety and traffic flow, and protection from erosion and flooding for nearby Johnson Creek.
Paid for by a voter-approved property tax levy, this joint venture between the city of Beaverton and Washington County included widening 5,680 feet of a two-lane road and adding planted medians, new curbs, sidewalks and bike lanes; a new pedestrian bridge; and improved storm drains and treatment for stormwater runoff.
Flood Control Opportunity
Seeing an opportunity to address area flooding issues, meet detention requirements for urban runoff and satisfy wetland and stormwater criteria defined by the project's advisory committee, Rick Raetz, PE, principal engineer for the Washington County Department of Land Use and Transportation, designed two detention tanks, each with a small outlet orifice, to attenuate the smaller, more frequent storms that were down-cutting the nearby downstream channel bank.
"In many cases, for very little or no additional cost, we can realign our work in certain areas during reconstruction projects and alleviate some of the pre-existing problems," said Mark Boguslawski, PE, project engineer for the Beaverton Water Division, who acted as an advisor for this project. "In this case, we had some existing flooding issues in that particular part of the town and we were adding more impervious areas."
Each detention tank, installed underground, consists of a 48-inch pipe, several hundred feet long, that captures and stores the entire volume of each storm event and releases it at a pre-determined rate over time, typically equal to the rate of runoff from the pre-developed site.
By controlling the rate at which the volume of runoff leaves the detention tank, downstream channels are protected from scouring, flooding is reduced and a certain level of settling of particulates, such as sediment and organic debris that can transport pollutants or cause sedimentation of fish spawning beds, is provided.
To release the water at a specified rate, a structural orifice plate with an opening of a particular size is installed at the outlet. This opening size ranges from a 1/2 inch to 4 inches in diameter. For this project, a 1 1/2-inch diameter opening was selected.
Problems with Clogging
While the designed system addressed water quantity requirements and downstream protection for Johnson Creek, a known problem for detention facilities is clogging of the flow controlling orifice by trash, debris and organic matter. As the water flows out of the system, this trash and debris builds up against the flow controlling orifice, and eventually can stop flow out of the system completely. If this happens, the water level will rise until it reaches the bypass elevation.
As more water is added to the tank, the water continues to bypass, ultimately creating a situation where the rate that water enters and exits is the same and the detention tank is virtually off-line. The detention tank will continue to operate off-line until it is maintained, which usually occurs annually or biannually.
"If the orifice diameter is in the 2 1/2-inch range or bigger, we don't see these kinds of clogging problems. But, when we get into the smaller orifices, we see problems," Boguslawski said. "Given the nature of the roadways, with individuals especially throwing trash out at stop signs, the smaller orifices are very subject to clogging from things like trash and bark dust."
Typical solutions to this problem include installation of a trash rack, for larger diameter pipes, or a piece of screen over the pipe opening; placement of the outlet pipe well below the top of the wet storage pool; or installation of a down-turned elbow, often known as the snout.
With all of these options, it would seem that clogging could be prevented. However, only the piece of screen keeps trash and debris from getting through the pipe and into the downstream waterway. But even the screen has trouble; its small surface area becomes occluded quickly by debris that remains on the surface until maintenance.
Having used a snout in several other developer-built projects, the engineers were considering using it again for this site although, even with the snout installed, there would be a possibility that trash would leave the system. Adds Boguslawski, "Downstream of the down-turned elbow, we get some semi-floatable trash, such as cigarette butts and leaf debris that passed right on through. Also, there is a risk of the elbow becoming clogged by this trash."
For the city of Beaverton/Washington County project, the engineers acknowledged that clogging of the system was a real possibility. Through discussions about other city projects, the engineers heard about a new product that addresses the clogging issue, the Detention Management System manufactured by Stormwater Management Inc.
One Solution to Clogging
Available as a complete manhole unit with an integrated StormScreenÒ cartridge and built-in bypass, the system traps trash and debris larger than the screen-opening diameter (typically 2.4 millimeters) and keeps these materials away from the outlet orifice. And, unlike the other options for protecting the orifice pipe, the cartridge has a built-in surface-cleaning function.
As water enters the cartridge through the screen, all solids greater than the pore opening size are captured by the screen surface. Water continues to fill the cartridge until there is enough buoyant force to open the cartridge float valve, which allows the screened water to flow into the elevated discharge flume and out through the outlet pipe.
the Cartridge Works
This continues until the storm subsides and the water surface elevation drops to the elevation of the "scrubbing regulators" located on the cartridge hood. At this elevation, air is quickly drawn beneath the hood, causing high-energy turbulence between the inner surface of the hood and the screen assembly. This turbulence releases the trash, debris and heavier sediments on the screen surface and allows them to settle into the system sump.
Each cartridge can treat a peak flow up to 0.5 cubic feet per second (225 gallons per minute) and can maintain this peak flow even when the cartridge surface is 85 percent occluded and only 15 percent of the screen surface is available.
Also offered without the manhole, the system's screen assembly can be easily installed in pre-existing detention systems, including underground detention vaults and tanks and aboveground detention basins or wet ponds.
The engineers decided that this system offered the reassurance they needed. It was important that the reconstruction project's detention tanks would be effective, and the screen system needed to be retrofitted, without the manhole, into the installed detention tanks.
"The incredible amount of surface area built into the StormScreen's design and the small screen openings make it greatly resistant to clogging," said Boguslawski. "Without this option a Styrofoam cup would be able to plug the small orifice; but with the Stormwater Management solution, there's much more surface area still available for water to pass through."
A Detention Management System was installed in each of the detention tanks in February 2004. Due to site construction, the system was installed in two phases; the discharge flume was attached first, and the cartridge was added later in the month. The flume was attached to the wall of the tank's concrete control structure on the upstream side of the outlet pipe. When it came time to install the cartridge, it was set on the pre-threaded opening of the flume and screwed into place. Combined installation took approximately thirty minutes per tank.
Jim Brink, project engineer for the city of Beaverton and liaison between the city and Washington County for this project, was on-site for the installation of the cartridges in February. "This system is quite handy," Brink said. "Clogging is the leading reason detention structures don't work; and even though our systems are on a routine maintenance schedule, clogging usually doesn't get discovered until we perform regular maintenance."
Maintenance of the Detention Management System without a manhole involves annual inspection of the cartridge screen surface and cartridge replacement, if necessary. If a complete system is installed, removal of sediment from the manhole floor is required. Maintenance should be done annually, although site conditions and pollutant loading will ultimately determine the maintenance frequency and whether cartridge replacement is required. Typically, inspection and maintenance can be done along with the detention tank's standard maintenance.
Performance of the system installations at this site will be assessed during the next routine maintenance check. At that time the engineers will also assess how well their attempts to address the smaller, more frequent storms have been in correcting the pre-existing flooding conditions. The engineers are optimistic.
"We would like to use this screen cartridge system again in areas where we're using small orifices, such as where we have extended dry detention," said Boguslawski. "The Detention Management System is now another tool in our tool box to use for areas where there is a high likelihood of having semi-floatable debris clog detention pipes."
Sidebar: How the Cartridge Works
The passive, siphonic StormScreen cartridge provides removal of trash and debris and some total suspended solids (TSS) from stormwater runoff by means of direct screening. The cartridge consists of a stainless steel screen with a pore opening of 2.4 millimeters (2400 microns), which ensures the capture of all solids of greater size, and a hood with integrated scrubbing regulators that facilitate the cartridge's surface-cleaning function. The cartridges can be installed into small, prefabricated catch basins or incorporated into large cast-in-place facilities that treat hundreds of cubic feet per second. Each cartridge is designed to treat up to a peak flow rate of 0.5 cubic feet per second (225 gallons per minute).
Creating the Siphon
When water first enters a screen cartridge, the float valve in the cartridge is in a closed (downward) position. As water fills the cartridges, polluted influent is drawn into the screen cartridge through the pore openings in the screen surface. All solids greater than the screen's pore opening size are captured by the screen surface. As stormwater passes through the screen surface, it begins to fill the cartridge center tube. The air in the cartridge is displaced by the water and purged from beneath the filter hood through the one-way check valves located in the cap. Once the center tube is filled with water (approximately 18 inches deep), there is enough buoyant force to open the float valve and allow the treated water to flow into the elevated discharge flume that supports the cartridge.
As the screened water drains, a vacuum is created. This causes the check valves to close, initiating a siphon that draws polluted water throughout the full surface area (7.5 square feet) of the cartridge. Thus the entire cartridge is used to screen water throughout the duration of the storm, regardless of the water surface elevation in the vault.
Cleaning the Screen Surface
This continues until the water surface elevation drops to the elevation of the scrubbing regulators. At this point, the siphon begins to break and quickly draws air beneath the hood through the scrubbing regulators, causing high-energy turbulence between the inner surface of the hood and the screen assembly. This turbulence agitates the surface of the screen, releasing accumulated debris on the screen surface, flushing it from beneath the hood, and allowing it to settle on the surface below. This surface-cleaning mechanism restores the permeability of the screen surface and enhances the overall performance and longevity of the system.
This article originally appeared in the 05/01/2004 issue of Environmental Protection.