Keeping the Dirt Where It Belongs

March 1, 2010
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Consider the costly consequences at a 50-acre construction site for a commercial development in the Portland, OR metropolitan area this past fall, where attempts to protect bare slopes from the impact of winter rains and runoff was done the wrong way—using inappropriate and, in one situation, incorrectly installed best management practices. “This effort was a failure from top to bottom,” reports Fred Wright, a certified professional in erosion and sediment control (CPESC). He’s chief executive officer of Di-namic Resources LLC, an erosion and sediment control consulting and training service based out of Rockaway Beach, OR. Wright was called in to evaluate the damages after a storm had caused extensive erosion and the resulting runoff had polluted a stream with sediment.

The challenge in controlling erosion wasn’t so much the less-than-5% slopes as it was their length, which averaged more than 200 feet. The contractor treated the slopes with a hydraulically applied tackifier, made from guar, with no other additives to protect the soil from erosion. Although less expensive than other types of products for stabilizing slopes, this particular one lacked the performance needed to control erosion on slopes of that length. In addition, a single row of straw wattles was installed at the crown of each terrace to reduce the speed and force of runoff coming to it. Low-performance erosion control blankets were installed on the steeper parts of the slopes, where something stronger than guar was needed.

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Perforated pipes were also installed in ditches at the toe of each slope, then backfilled with rock. Contained runoff from these trenches was then diverted to a large open ditch that emptied into a sediment basin. However, in an attempt to economize, the rock check dams intended to slow and reduce the force of flowing water in this large swale had been improperly installed, Wright says. What’s more, the sediment basin wasn’t designed to handle the volume and velocity of runoff flowing into it.

A Costly Misunderstanding
“The BMPs were simply not designed to withstand the erosive forces generated on this site,” Wright says. “But the big failure was the lack of any meeting among all the key parties to discuss the erosion control challenges—both upstream and downstream—of this site before the design stage of this project.”

For example, he notes, the guar treatment, if applied correctly, acts like plastic sheeting, which sheds water off the slopes. “A 1-inch rain event, can generate about 27,000 gallons of water runoff from just one acre,” Wright says. “If improper BMPs are used in the design and are not installed per plan and detail drawings, the results can be catastrophic. Since the guar was applied to about 35 acres, there was a tremendous amount of runoff when several storms hit.”

As a result, this heavy runoff dug gullies into the slopes, blew out the straw wattles and terraces, scoured out the sediment basin, and dumped sediment into a stream.

A much more cost-effective approach to controlling erosion on the slopes over the winter would have been to apply an annual rye grass with a 2-inch-thick layer of straw uniformly spread over the bare soil until the grass established to hold the soil in place, Wright notes. “The contractor had already built the pads of the commercial buildings and thought it would have been too expensive to tear out the vegetation and repair the pads when work resumed in the spring,” he explains. “However, once the rye grass established, it would have controlled the runoff and only about 3 to 4 inches of soil would have had to be stripped to remove the vegetation. As it was, repairing all the damage cost much more than paying for and installing the correct BMPs.

“Overall, there was a lack of understanding by all the key participants in this project, including the engineers and municipal authorities who approved the erosion and sediment control plan, and a willingness to risk serious erosion to save some money,” he continues. “In my experience, those who skimp on erosion control or improperly design appropriate BMPs for the particular job almost always lose.”

This case represents only a few of the ways in which the wrong approach to controlling erosion and sediment on grading and excavation projects can have costly consequences. If not prevented or corrected, such missteps in protecting water quality from soil-disturbing activities can leave one vulnerable to financial penalties under the US Environmental Protection Agency’s National Pollutant Discharge Elimination System. According to this part of the federal Clean Water Act, anyone disturbing 1 or more acres of land for construction purposes must have an NPDES permit. It requires a storm water pollution prevention plan (SWPPP), which details how various BMPs—techniques, materials, or products to prevent or reduce the discharge of sediment into a stream, river, lake, or other body of water—will be used throughout the project to limit erosion onsite and curtail transport of sediment off the site.

Sediment from construction sites can cause problems even without ending up in a creek or a pond. “The most common public complaint concerning NPDES requirements is mud on streets next to a construction project,” says Jennifer Hildebrand, CPESC, CPSWQ (certified professional in storm water quality), national environmental compliance director for Weis Builders, headquartered in Minneapolis, MN. “You can minimize offsite tracking by routing trucks and other vehicles through a stabilized entrance/exit, such as a specified-size rock placed over a filter fabric or wheel-wash device that removes dirt and mud from tires and frames before vehicles leaves the site.”

A Wide Range of Practices
A number of BMPs, each with their individual performance limits, have been developed over the years to control erosion and the resulting sediment on construction sites. These include:

  • Silt fence, bio-filter bags, wattles, filter berms, and inlet protection for controlling sediment
  • Gravel construction entrances/exits, check dams, and surface roughening for controlling runoff
  • Buffer strips, temporary/permanent vegetation, ground cover, mulches, and plastic sheeting for preventing erosion

In some areas of the country, alternatives to these and other common BMPs have proved effective. Wright notes three that are popular in and around the Portland, OR, area.

One of them is the sidewalk sub-grade gravel barrier, which is used frequently in residential developments. The location of a future sidewalk on the lot side of the curb line is excavated to the designed depth and width before construction of the house begins. Then, the rock sub-base is installed. “The rock acts as a filter to trap sediment and slow down runoff from the bare surface of the lot before it gets into the street and down a storm drain. Also, unlike silt fence or other vertical sediment control devices, vehicles can drive over this BMP without damaging it.”

Straw can be an effective temporary ground cover. It protects bare soil from the erosive impact of raindrops and helps slow runoff velocity, allowing time for more of it to soak into the ground. A uniform 2-inch-thick cover of straw is the standard practice for short-term protection of exposed soil in his area, Wright notes. “Once the straw becomes wet, it compresses down to about 1-inch thick and protects the soil from erosion by both wind and water. Also, a number of contractors use wood chips produced by grinding up trees cleared from the construction site to control erosion,” he says. “This form of recycling also eliminates the costs of hauling and dumping the woody debris in a landfill. Usually, the wood chips are spread about 4 to 6 inches thick.”

In some cases an impervious cover, such as plastic sheeting, can also serve as a temporary BMP. “We see quite a bit of it being specified to prevent erosion of stockpiles of soils on construction sites and to protect steep slopes with a high risk of failure if they become saturated with stormwater runoff,” Wright says. “It has to be used in conjunction with some kind of flow dissipater at the bottom of the slope, such as a filter berm or swale that will divert the runoff into an approved storm water discharge system.”

Avoiding Missteps
Here are some tips to help you make the most of BMPs to protect water quality during your grading and excavating activities.

Know what you’re doing—Controlling erosion and sediment effectively requires a certain level of expertise. In fact, regulatory authorities require contractors to have a qualified person on staff to see that BMPs have been installed properly according to manufacturer or project specifications. In some states, contractors must receive a minimum amount of training to stay current on the latest erosion and sediment control requirements and practices. Others, such as California, have recently adopted construction permits requiring that certified professionals be used. Often, this training is provided by construction-industry associations as well as various government agencies and private consulting firms. An excellent source of training is the International Erosion Control Association, www.ieca.org, which offers a variety of educational sessions at its annual conference, and through regional chapters. Erosion Control magazine www.erosioncontrol.com, the official IECA journal, offers in-depth articles about techniques, products, and materials for preventing, minimizing, and controlling erosion and sediment on construction sites and other areas.

To help keep their construction sites in compliance with NPDES requirements, some grading and excavating contractors rely on the services of erosion control subcontractors who inspect and advise clients on the proper use and maintenance of BMPs. Jeanne Duarte, CPESC, CISEC (certified inspector of sediment and erosion control) is one of those. She’s president of Storm Water Resources in Valencia, CA. In addition to designing the SWPPP for a project, she and her staff also advise contractors on proper installation and maintenance of BMPs and how to solve any compliance problems.

“Often we’re hired to manage implementation of the SWPPP for the contractor,” she explains. “We inspect BMPs and write up the results of our inspections on a regular basis. If we find any deficiencies, we’ll give the site superintendent anywhere from 24 hours to seven days to correct any problems, depending on the type of BMP involved.”

She encourages her clients to take a team approach to controlling erosion and sediment. “It’s important to get the project designer and the guys in the field, the equipment operators, and foremen onboard so that they understand the reasons for the BMPs and what must be done to keep the project in compliance with the NPDES requirements,” she says.

Follow the plan—Steve Gucciardi, CPESC, is the senior erosion control coordinator, and Jay Wilson, CPESC, is the erosion control trainer coordinator with the city of Charlotte, NC. Much of their work involves inspecting erosion and sediment control practices at construction sites.

“The Number 1 problem we see is the contractor not following the approved SWPPP,” Gucciardi says. “Usually, it’s not that they’re trying to save money. Instead, they install BMPs like they always have in the past without realizing that the correct installation methods may have changed over time as ordinances and standards are revised. Procedures that were correct five or 10 years ago for installing, say, an inlet structure, may not be the correct ones today. But, if you don’t refer to the inlet detail on the plan, you may not know this and you’ll install it the wrong way—the way you used to do it. Until that error is corrected we wouldn’t issue a grading permit.”

Failure to follow the plan can also result from poor or no communication among the key project participants, Gucciardi adds. He recalls one instance where he met with the developer and general contractor in a preconstruction meeting for a commercial development. During that meeting they discussed the sequence of grading activities and the need to install the approved relocation of a storm drainage system prior to the clearing for any erosion and sedimentation control measures. The grading contractor did not read the construction sequence and started to clear for the installation of a sediment basin that was undersized to handle the drainage area carried by the existing storm-drainage system.

During the construction of the sediment basin it rained and destroyed the basin and filled in a stream with sediment. In an effort to prevent any more stormwater from entering his construction site, the grading contractor packed the outfall of the existing storm system with dirt. The next time it rained, water backed up and flooded a major roadway. It also caused flooding in an apartment complex, resulting in the submersion of parked vehicles.

“The grading contractor received a notice of violation, and a fine and was sued by an insurance company, ”Gucciardi says. “That was the contractor’s last job.”

Use the proper practice properly—Regardless of the type of BMP, to be effective it must be installed in the right place, for the right purpose at the right time. “Many contractors tend to blame the BMP when it fails,” Wright says. “But usually any failure is due to improper selection and installation of the practice or product. It’s the responsibility of the project designer, the installer and the local municipality or other authority to know if a particular BMP is appropriate for a given site and purpose.”

In fact, Duarte’s Storm Water Resources inspectors frequently observe the use of an inappropriate erosion or sediment control practice or misapplication of an appropriate BMP.

“These errors result more from the contractor not knowing when and how to use a particular BMP than an unwillingness to install them in the first place,” Duarte says. “Most contractors are focused on construction and may not be on top of what needs to be done to install and take care of BMPs. But, once we explain how and what they need to do they respond well.”

Among the common mistakes Gucciardi and Wilson see in the installation of BMPs on grading and excavation projects:

  • Construction entrance/exit—Featuring a specified size of rock placed over a geotextile, this sediment control measure is designed to shake sediment loose from tires and frames as vehicles pass over. However, Wilson notes, contractors don’t always make it long enough—at least 100 feet—to be
    effective.
  • Silt fence—Failing to trench in the bottom of the fabric to the required depth and lack of compaction (North Carolina has a standard detail for achieving the desired degree of mechanical compaction). This can allow stormwater runoff to dislodge the fabric, leaving it useless for trapping sediment on the up-slope side. Spacing posts farther apart than required and/or installing posts on the downslope side of the fabric instead of the uphill side weakens the ability of fence to withstand runoff forces. “Putting silt fence at the top of a slope, where there is no runoff, wastes money,” Wilson says. “And, installing it to run up and down a slope converts the fence from its designed purpose of containing sediment to an unintended use as a diversion to concentrate the flow and increase its erosive force.”
  • Grade-control structure—A series of check dams built of rocks, wattles, or other suitable materials limits erosion of diversion ditches by slowing the flow and, thus, the force of stormwater runoff. “Maintaining the proper flow is critical for controlling down-cutting of the ditch,” Wilson says. However, he’s seen some contractors build diversion ditches with such flaws as sharp bends that increase erosion of the outer bank of the bend and even with an uphill gradient that causes stormwater to back up and flow over the ditch banks, increasing rill erosion of the slope. Also, he’s found that some contractors extend the spacing between check dams beyond what is specified in an attempt to cut costs. This runs the risk of allowing the velocity of the stormwater flow to exceed the desired limit.
  • Storm drain inlet protection—This is one of the cases, he notes, where contractors tend to install the device to keep sediment out of storm drains as they have in the past, rather than consulting the approved set of plans for the latest procedure detail.
  • Drop outlet structure—In trying to dissipate the erosive force of water below the discharge point of a storm drain pipe, contractors don’t always use the specified size of riprap with the required geotextile beneath it as additional erosion protection. “When then happens, water moving through the rocks will eventually erode the soil and form a channel,” Gucciardi says. “As it enlarges, the channel starts flushing out the rock. If not repaired, the resulting erosion could threaten buried utilities.”
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Cover Your Maintenance Needs
Sooner or later, just about every type of BMP requires some sort of maintenance to continue working properly. However, that’s not always done. Duarte reports that one of the biggest problems she and her crews encounter is lack of maintenance of BMPs—such as not removing sediment collected by a silt fence or storm-drain inlet protection device, not replacing a broken fiber roll or nor repairing a damaged check dam

“A critical factor in the success of any grading and excavation project is the hidden cost of maintaining BMPs, which will always be there,” adds Wright. “You have to prepare for the inevitable and budget enough in your project bid to cover this maintenance. Otherwise, it will be like tossing money out the window, because it’s not a matter of if the BMPs will need maintenance but when.”