Applications and Innovations
| October 03, 2011 |
Fast, Dirty Water
Handling the stormwater/erosion dynamic
By Tina Grady Barbaccia
Sediment. That’s the problem.
Barry Fagan, Alabama Department of Transportation (ALDOT) environmental program engineer and a certified professional in erosion and sediment control, says that the “construction industry spends millions of dollars each year on construction stormwater management, yet sediment is among the top impairments of our nation’s waters with the construction industry being blamed for a good portion of it.”
Fagan says that BMP, the acronym for Best Management Practice, is often more likely to signify Best Manufactured Product because the desire for a quick, easy fix is replacing “actual thinking and application management.” In reality, a BMP is a physical, chemical, structural or managerial practice that prevents, reduces or treats contamination of water or that prevents or reduces soil erosion.
What’s more, he says, the amount of money spent on erosion control is not a reliable indicator of success. “Sediment control alone is ineffective,” points out Fagan. Other management tools and resources must be called into the fight, before the problem arises and after it has.
Having said that, Fagan points out that existing natural vegetation is the most effective BMP available. “It also happens to be one of the most economical,” he says.
In the Portland, Ore., erosion-control BMPs are required during all ground-disturbing activity until permanent site ground covers are in place. “Stormwater control is a vital element to prevent erosion,” according to Portland’s Bureau of Development Services. Stormwater control practices — including methods to convey, divert, treat and control stormwater flow rates and volumes — can be complex. Runoff volumes and rates also can be hard to predict.
Also, increasingly-tighter stormwater restrictions have made implementation of BMPs increasingly important. From an engineering perspective, it’s absolutely critical, explains John Kurdziel, P.E., director of technical services and market development for drainage pipe company ADS/Hancor Inc. When it comes to the removal of sediment from stormwater, there are several products available that are designed to handle sediments, he says.
And when it comes to sediment, there’s more than one way to attack it. “By diverting and conveying flowing water around, under and over a work area, the amount of sediment-carrying water that must be managed is reduced,” explains Fagan. “Water velocity also affects erosion and sediment control efforts. By increasing the velocity of water, its erosive energy, the mass of soil being transported and the size of the particle being carried are also increased.”
Removing chemicals and oil from the water is also vitally important and must also be considered a best practice, says Kurdziel. “Chemicals and oils can have a much bigger impact than just the sands and the sediment that gets pushed through the systems,” he says. “The intent is that the material coming through the unit will outlet to a stream or a natural waterway with the same type quality of rainwater that initially hit the ground. You need to mitigate it before it becomes a problem because you don’t want to have any chemicals or particulates going downstream that would not normally be there.”
Regulations are becoming more intense when it comes to water quality from construction sites. Scott Erickson, principal of Evolution Paving Resources and president of Salem, Ore.-based Quality Concrete, says “stormwater is a freight train coming your way. How you deal with it will depend on how much of the market you get.” By 2014, “the noose is going to be even tighter around pavement designers’ and developers’ necks,” Erickson points out.
The Georgia Department of Natural Resources has projected a 300- to 500-percent growth in pervious concrete in the next five years, Dale Fisher, executive director of the National Pervious Concrete Pavement Association, said during a press conference at CONEXPO-CON/AGG 2011 in Las Vegas. The agency is using pervious pavement for its Amicalola River Streamside Project in Georgia.
Low-impact pervious pavement allows water to pass directly through, which reduces the runoff from a site and allows stormwater to be cleaner and cooler. There have been recent advances in pervious pavement to further enhance its benefits. A pervious concrete infiltration fabric, PERC, has been recently introduced to the market. It’s made from recycled PET from soda bottles and has small pores to restrict dirt and particulates, which can contribute to clogging storm drains. Fisher says this type of technology will enhance performance for pervious concrete systems in paving installations. This type of fabric is engineered to maintain separation between the base and sub-base, which provides structural support to the pavement but allows stormwater to infiltrate freely into underlying soils, he says. “When used on an impermeable subbase, it mimics the microbial activity found in soils and contributes to improved water quality.”
The use of pervious pavements is creating dramatic paradigm changes for pavement designers. For years, pavement has been built like a bathtub — water runs onto the pavement and into a catch basin. Sidewalks, grass and everything else that’s associated with it was sloped so water would run into the pavement and then sheet flow to the low point of the basin. “There it disappears into a pipe or a swale and is out of everyone’s hair,” Erickson says.
The use of pervious pavement technology has been instrumental in keeping water onsite when it’s an appropriate stormwater solution for the application. “While the goal is to maintain as much water onsite as possible, we now want water that falls on landscaping to stay on landscaping instead of flowing onto the pavement,” Erickson says. “If we use curbs, we notch them so any overflow runs onto the landscaping.”
The City of Austin, Texas, is harnessing its problematic stormwater with pervious pavement to help control erosion through its Pease Park district. The city is building about 15 miles of trails through a section of the park to help corral the stormwater and prevent the trails eroding into the creek. “In general, the weather there is constant drought interrupted by occasional major floods,” says Erickson. “The city couldn’t even keep gravel on a trail next to a creek that goes by the park, so it decided to put in some pervious concrete trails.”
For the Five Pillars of Construction Stormwater Management and Three Tips on Designing a Retention/Detention System, go to the Better Roads digital edition at betterroads.com and click on “Experience our Full Digital Edition.”
Building new roads can overcome a number of mud and erosion problems. That’s exactly how one dairy farmer – Misty Meadow Dairy – in Tillamook, Ore., dealt with a constantly flooded fields. Tillamook is on the Oregon Coast, where there are multiple canyons, says Scott Erickson, principal of Evolution Paving Resources and president of Salem, Ore.-based Quality Concrete. At times, these canyons get up to 4 inches of rain in a short time, and flooding follows. “One farmer got so tired of maintaining his cow pastures that he put in 2.6 miles of concrete trails that were 11 feet wide so the cows could march in on it,” says Erickson, who was on the team that helped install the trails. “The farmer says it saved him more than $50,000 a year by not having to pay for someone to come in and fix the cattle’s feet with all the mud and rocks.” (For a video of pervious pavement being installed at the dairy farm, go to evolutionpaving.com/proudcts/slipform-pavers.htm.)