May 16, 2016 04:32

Managing Stormwater

managing stormwater
In municipalities across the country, the infrastructure for handling runoff from rain events is failing miserably. Many of these systems were built at the turn of the last century, and cannot handle the amount of stormwater coming from impervious surfaces. Their failures lead to street flooding or overflow into the sewerage system. Worse still, this problem will get harder to fix the longer it is ignored, because the systems will continue to deteriorate and populations will continue to increase.

As professionals in this area, we are in a unique position to help. We can lend our know-how, experience and vision to the monumental task of addressing the country’s stormwater management needs. With the situation as pressing as it is, you might be inclined to wonder, where is the most immediate need? As with everything in stormwater, the answer depends on the circumstances.

Sometimes you’ll find yourself in an emergency situation. The immediate needs of the municipality will dictate which solution you have to implement. Colorado Springs, Colorado, saw massive flows in Rockrimmon Creek after heavy rain and snowmelt in September, 2015. In a channel below a residential development, high velocity water was causing scouring and landslides on the channel’s sides.

“It was scary to see how much soil had been falling off the banks, going all the way up to the houses,” said Alex Keenan, sales representative for Contech Engineered Solutions in West Chester, Ohio. “When I arrived onsite, I could almost see the foundation of one of the houses, because of a slide that had occurred right there.”

City engineers determined that, because of the nature of the situation, an immediate end-all solution was needed. They decided to convert the open channel to an underground culvert system, designed to handle the volumes of water currently flowing through.

“So instead of bringing in more fill and trying to refill and regrade, or install hard armor, something that would’ve taken more time, the idea was to basically just channel the creek and those high-velocity, highflow storm events through a large-diameter pipe,” Keenan said.

“That’s where we came in.”

City engineers specified an eight-foot-diameter pipe to convert the most damaged portion of the channel to a culvert system. Contech supplied 1,400 linear feet of a high density polyethylene (HDPE) pipe product for the project. The company recommended steel welding between the piping sections to make the structure watertight.

Wildcat Construction Company, Inc., in Colorado Springs, Colorado, was awarded the contract. To speed installation, because time was of the essence, some crewmembers were welding pieces of pipe together while others were down the line putting in more sections. This turned out to be prudent, because the site had heavy storm events all during the installation. As a result, stormwater actually flowed through the pipe while it was still under construction.

Paul Sook, vice president of Wildcat and the Colorado division manager, described how his team handled the project. “We went in with the city and created a solution to limit the continual erosion of this channel,” he said. “We regraded the area, installed the drainage pipe, and eliminated the problem of future erosion.”

The design decision in the Rockrimmon Creek Rehabilitation Project was the result of a truly extenuating circumstance. At least one property owner’s deck had already slid into the channel by the time the project got underway. It’s possible that the heavy erosion at Rockrimmon Creek—like any number of other similar situations going on elsewhere in the country—could have been mitigated by increasing the amount of stormwater best management practices (BMPs) in place upstream.

Their swift and needed efforts eliminated the area’s erosion problem, but the changes they brought did have environmental ripples. Bugs and bunnies alike used to live in that creek, but the pipe that replaced it makes for a poor habitat. In far too many American cities, the modus operandi for stormwater management is emergency mitigation when it should be prevention.

Consider the temporary lakes and rivers which can form quickly along urban streets. Grates and inlets become clogged with leaves and trash, leading to sewer systems that are easily overwhelmed. Even moderate rains can fill entire lanes with water. Motorists everywhere have accepted that they must occasionally cope with flooded roads.

As undesirable as both flooding and erosion are, neither is the biggest problem with current stormwater practices. The water in question flows over asphalt streets and picks up petrochemical and heavy metal deposits—‘the nasties,’ as some professionals refer to them colloquially.

This water has run off the tops of buildings and concrete sidewalks, rife with animal waste and bacteria.

It’s flowed off of people’s lawns, coated with residue from fertilizer, herbicide and pesticide application. Left unfiltered, all of that gunk will find its way into our wetlands, streams, rivers, lakes and oceans, causing untold harm to fish, wildlife, the environment and ultimately, us.

Stormwater used to leach into the soil slowly and naturally. With time and progress, urban development covered large swathes of land with hard, impervious surfaces. This created runoff, for which we dug channels and installed conduits to pipe it out of sight and out of mind. All the while, little thought went into where all that water was going and what it was carrying with it.

Now, with water infrastructure in the national spotlight, we are thinking about it. It’s no secret that we need to handle stormwater runoff in a different way. Currently, the main path for improvement is Low Impact Development (LID). The idea is simple: instead of sending stormwater away, we keep it in the neighborhood, filter its impurities and let it replenish the watershed.

LID principles can be applied to both new and existing development, from low-density to ultra-high density urban settings. So where do we go with applying LID to stormwater? We need to start by asking more questions. Can we filter away all of the materials found in stormwater that are hazardous to our groundwater and soil? Can we find a way to keep the precious resource around, instead of sending it away and downstream? Fortunately, the answer to those questions is yes.

Those goals can be accomplished by using the correct BMPs. You’ve probably employed some of them already. They include bioretention cells, curb and gutter elimination, grassed swales, and inlet protection devices. There are more—many more—but that’s saved for another article. The most pertinent ones for this discussion are permeable pavers and underground chamber systems.

Permeable pavers are a very appealing solution, because unlike a detention pond or an open swale, they don’t take up additional space. Rather, they give a secondary use to developed spaces. Areas that are already meeting the needs of parking, driving or foot traffic can serve to promote infiltration.

This isn’t the only infiltration solution, of course, but other methods have their drawbacks. Ponds and swales take up valuable land, pose a safety hazard (should someone fall in), and can also be breeding grounds for mosquitoes and other vermin. Permeable paving fits into an urban environment and still retains the power to reduce the amount of runoff from a site.

An impermeable surface would just let the water run off someplace where it can’t be managed so easily—say, into an overwhelmed creek, just like Rockrimmon. “What you have happening in cases like Rockrimmon Creek is the tremendous expense of pipe installation, not to mention the loss of wildlife habitat,” said Bill Handlos, P.E., president of Presto Geosystems in Appleton, Wisconsin. “One way to mitigate these situations is by using vegetated and unvegetated porous pavement systems upstream.”

In a vegetated system, trays are placed over a bed of a mixture of topsoil and open graded base course (with a void ratio of 30 to 35 percent), then a geotextile layer can be added as an optional addition below the base. The units are then filled with a topsoil and seed. A vegetated system can help to stabilize an area, transfer water into the ground and maintain a green landscape while giving the area utility.

They are commonly used in overflow parking areas, as well as fire lanes and secondary access roads on the lawns behind or around apartment buildings.

Because a high volume of traffic would cause the soil to compress and eventually kill the grass, they aren’t recommended for areas that will see frequent daily use, but they’re ideal for weekly or biweekly use. Under those conditions, a vegetated system can keep an area green and pervious, even if it still needs to be driven on occasionally.

“If you’re going to use the porous paving on a daily basis, you’re better off with unvegetated stabilized aggregate, pervious concrete or porous asphalt,” Handlos said. Unvegetated systems are designed to hold aggregate in place under heavy use. The difference is the infill: topsoil for vegetated, versus open-graded base course for unvegetated systems.

In this sense, permeable paving combines the best of both worlds. The stability of well-graded aggregate meets the perviousness of poorly-graded aggregate to form a surface that can both bear loads and reduce stormwater runoff onsite. Additionally, the water is filtered as it flows through a paver system. Studies have shown that when permeable pavements are used, runoff volume is reduced by as much as 60 percent.

In some circumstances, you’ll want a solution other than a permeable pavement system. Say you need an underground solution that allows the water to transfer into the ground. At the same time, it also has to be able to wend the water into a municipal inlet, at a restricted flow rate, In a case like this, you can make use of a stormwater chamber system.

“What you want is a detention system, storing water onsite for a temporary amount of time, discharging it at a specified rate that you know you can control,” said James Maier, an engineer at Triton Stormwater Solutions in Brighton, Michigan. “In a chamber system, it’ll go into an outlet structure at the same time that it infiltrates into the ground.”

Chamber systems manage stormwater underground, freeing up space on the surface for landscapes, development, or other purposes. These systems consist of lightweight, ridged plastic halfpipes. You line up rows of these chambers over a bed of gravel, and water flows into the system through an inlet pipe. Whatever doesn’t transfer into the ground can exit at a controlled flow rate through another pipe that leads to the municipal system.

With chamber systems, the main way to mitigate clogging is by using a containment row, also known as a main header row. Maier explains how it works. “In our system, it’s basically a row that has a solid sediment floor in it so that when the water goes in there, it slowly rises and cannot soak into the ground,” he said. “That way, all the particulates will settle down to the bottom of the chamber as the water rises. That water overflows into the storage rows.” The pipe connecting each containment row to the next is at a higher elevation than the inlet pipe, so that as the particles settle, the water flowing into the storage chambers stays relatively clean.

In the final analysis, no matter what the products out there are capable of, we aren’t going to solve the puzzle of stormwater management overnight. Nevertheless, Low Impact Development provides a framework in which to approach this problem with efficiency, environmental protection and everyone’s safety in mind.

As our population grows and the spread of suburban sprawl continues, the need to retrofit our infrastructure looms even larger. We will continue to learn more and amend our BMPs so we can move with confidence toward the next level of stormwater management.

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In many ways, we are fortunate that, in our chosen profession, we are able to help people when certain disasters occur: the tornadoes in Missouri, Oklahoma, Louisiana and Georgia, the flooding in Louisiana, the snows in the northeastern part of the country, the rain in California, and the snow in Colorado....

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