Emergencies, Incidents, and Other Erosion Control Surprises

Nov. 1, 2000

In a perfect world there would be no such thing as an erosion and sediment control emergency at a construction site or other location, because the erosion control plan would provide the correct sequence of steps to be taken to protect against all threats—natural and otherwise—and would be followed properly. But in the real world there’s rarely such a thing as perfection, much less perfect weather, a perfect plan, or perfect implementation of a plan. Assumptions made in developing the plan might be faulty, the techniques and materials chosen to control the loss of soil might be inappropriate, time and budgets might limit the type and extent of any measures taken to prevent erosion, the plan might fall victim to shoddy workmanship, nature might simply overwhelm the best of human efforts, or any number of other likely and not-so-likely events might befall the project.

The bottom line: Just about any erosion control professional could face the need to revise or devise erosion and sediment control practices in a hurry.

Here’s a sampling of how other professionals responded to a variety of unexpected erosion and sediment control problems, all of which offer insights into the wide variety of events that can disrupt and even destroy erosion and sediment control plans and practices at a construction site or a natural area.

Preparing for Several Possible Failures

Sometimes exercising extra caution to prevent an emergency in the first place makes a lot of common and economic sense. In Redmond, WA, not only is sediment difficult to control, but to protect environmentally sensitive areas, water-quality standards are unusually high. What’s more, sites often drain to environmentally sensitive areas. Facing such a challenge, it doesn’t hurt to have a backup plan or two on hand.

Redmond officials heeded that advice and set the pace for other municipalities in the Pacific Northwest region of the United States in meeting new, stringent water-quality standards recently imposed to protect salmon, now an endangered species in that area. Such standards are very close to those required for drinking water.


Meeting these requirements is no easy task for developers and contractors. For one thing, soils at construction sites typically include colloidal-size clay particles – the really fine sediment that clouds stormwater runoff and can take weeks, even months, to settle out naturally. Turbidity readings of stormwater on Redmond construction sites of 5,000 to 10,000 nephelometric turbidity units (NTU) are common and may range as high as 28,000 NTU. To meet state and federal regulations, turbidity of stormwater discharged from a construction site into Class A water – such as a salmon-spawning stream – must be within 5 NTU of the turbidity level of that water, which is typically 50 NTU.

Many of the construction projects involve steep slopes and large, open excavations near salmon and trout streams. In addition, rain, which can be heavy during the winter, is likely throughout much of the spring and fall. As a result, developers and contractors working in Redmond are required to be prepared.

“Silt fence and other standard best management practices just won’t control the colloid-size particles from our glacial-till soils to meet those standards,” points out Guy Oliver, Redmond’s lead construction inspector. “So we require a basic erosion control plan, such as silt fence and sediment ponds with gravel cones, for each project. Then, depending on slope, type of soil, and proximity to receiving waters, we use a matrix to figure out how much additional protection we feel comfortable with in case the first one doesn’t work. Depending on the site, that could mean as many as four or more backup plans.”

Alternative Solutions

The first backup might be biofiltration. Stormwater collected by an onsite sediment basin might be piped 1,500 ft. into a level spreader that distributes it over a grassy field. If turbidity levels of water sampled at a certain point – say, 700 ft. downslope of the spreader – are still too high, then the contractor may be required to use ground infiltration or run the stormwater through a canister to reduce turbidity. Should the filters plug up with sediment or the ground infiltration prove unsatisfactory, then treatment of the water with a flocculant might be necessary. Another alternative might be a permit to discharge the stormwater into a sanitary sewer.

“Of course, another option would be to stop work during the wet season,” says Oliver. “The developer would have to weigh the costs of delaying construction against the costs of erosion and sediment control.”

For the past five years, Oliver has been working with several construction companies in testing different methods of removing fine colloidal soil particles from stormwater.

Electroflocculation. One removal method involves electroflocculation. Stormwater in a treatment cell is run through a series of electrified plates. This process changes the electrical charges of the soil particles, causing them to clump together until they become heavy enough to settle to the bottom of the cell.

“If done with the wrong type of plates, this electroflocculation is extremely toxic to aquatic life,” Oliver warns.

Filters. Another method being studied involves the use of polyacrylamides, which are commonly used as soil binders, with filters made by Clear Creek Systems Inc. in Bakersfield, CA.

“Polyacrylamides aren’t as effective in causing flocculation as cation polymers,” notes Joe Gannon, Clear Creek Systems’ vice president. “In one short-duration test at Redmond, however, polyacrylamides did cause soil particles to flocculate to a size big enough that they wouldn’t pass through filters. The treated water averaged about 20 NTU. We’ll do a longer study this winter to determine the long-term economics of this approach, including filter life.”

Batch Treatment. Oliver is also working with Klean Earth Environmental Company of Lynnwood, WA, in the evaluation of a mobile stormwater treatment unit for removing fine soil particles. This batch treatment process features a 6,000-gal. tank mounted on a 30-ft.-long trailer. Stormwater is piped into the tank, where it is mixed with a dry-powder flocculating agent. After the sediment precipitates out, the treated stormwater is released.

“Precipitation time averages about 30 minutes per batch,” says Bill Anderson, chief operating officer for the company. “That compares very well to precipitation times of eight to 24 hours with Catfloc in other treatment systems and to the one or two days to weeks for untreated stormwater in a sediment pond. This also gives us an effective treatment capacity of about 100 gallons per minute from each treatment unit.

“Our system can be used with smaller storage volumes required for underground storage tanks, leaving more surface area of the site available for development.”

When a Good Plan Turns Bad

Deviating from an approved erosion control plan can lead to an emergency situation. That happened last fall at a 5-ac. hillside subdivision in Nanaimo, BC. At the bottom of the project and across the road lay a 20-ac. wetland. The plan, approved in July 1999, called for using a drainage ditch with gravel check dams, perimeter swales, silt fence (along the perimeter and toe of the slope), and three sediment ponds to keep any eroding loam and glacial-till sediment on-site. The ponds were sized to handle runoff from a typical summer storm, usually a small rainfall event occurring in late August or early September. Permanent vegetation was to be seeded by September 30. This would allow one month of germination before the heavy fall rains were likely to begin. But because the earthmoving work took longer than expected, the site wasn’t hydroseeded until late October. Several days of heavy rains hit the site a week later with predictable results.

Sediment Control

“The sediment ponds weren’t big enough, and there weren’t enough of them to handle all of the runoff,” states Kevin Brydges, Nanaimo’s environmental coordinator.

The first emergency response was to double the size of two of the sediment ponds, enlarge another by 50% and increase the size of the main drainage ditch. The silt fence structures were replaced with a portable, porous berm system. The heavy rains continued.

Erosion Control

“The site wasn’t big enough to dig sediment ponds large enough to provide enough time for the volume of sediment we were dealing with to settle out,” recalls Brydges. “At that point, the best and most economical option was to cover the site for the winter.”

So 0.2-in. sheets of plastic, held down with staples, pegs, and sandbags, were installed over the entire site except for the sediment ponds. That action, combined with regular inspections and maintenance throughout the winter to ensure that the plastic sheets stayed in place, stopped the erosion and resulting sediment problems. The plastic sheets were removed in the spring. By that time, acting like a greenhouse, the material had promoted germination of what seed remained on the site.

Brydges estimates that these emergency measures cost the contractor at least $15,000.

Other Options

In addition to following the planned construction schedule, the contractor could have completed and permanently stabilized one section of the project with vegetation before proceeding to the next stage, Brydges points out. In the meantime, he notes, city officials are weighing the pros and cons of restricting construction to certain times of the year in areas known to have the potential for serious erosion problems.

When a Poor Plan Turns Worse

Three years ago, Nanaimo was the site of another erosion control emergency. Brydges notes that here, too, grading was done at the wrong time of year, and he adds that the original erosion and sediment control could have been improved.

The 10-ac. site sat on a 4-8% slope near a fish-bearing stream. Grading work involved major cuts and fills to level the area for a suitable playing field. Soils were a combination of clay and very silty material. A water table, very close to the surface, added to erosion concerns. “After an hour or two of steady rain, springs would begin to pop up all over,” recalls Brydges.

A ditch was created around the perimeter of the site to intercept any stormwater flowing onto the future playing field and minimize the amount of water on the site to be handled.

Early heavy rains hit in September while grading work was still underway. All the runoff from the site ran into one sediment pond, which overflowed, emptying into the fish-bearing stream.

Controlling Stormwater Runoff

“In addition to the rain, we had all this water from the high water table to deal with,” Brydges says. “We wanted to direct as much runoff as possible away from that one sediment pond and the creek and to stop the erosion.”

Three ditches were dug across the slope to intercept some of the water that was flowing to the undersized sediment pond. Three or four small sediment ponds (about 4.5 yd. long, 2 yd. wide, and 2 yd. deep) were dug within each of these cross-slope ditches. These three ditches emptied into a second large sediment pond, which was added. The cross-slope ditches also helped reduce the velocity and erosive force of the water flowing down the slope, Brydges adds.

Controlling Sediment

The original sediment pond was enlarged, and water from it was pumped onto a vegetated field and drained into an additional sediment pond. Water from this pond ran down a vegetated slope and emptied into the large sediment pond, which was collecting water from the three cross-ditches.

As a result, all stormwater from the site went out one exit point, across a large, heavily vegetated wooded area, and eventually found its way – free of suspended solids – to a wetland.

Controlling Erosion

Following this work, the site was seeded in October with fall rye and again the next spring with a mixture of fall rye and native grasses.

“Initially we cut the sediment load flowing off the site by half,” Brydges says. “By the end of the first year, we had reduced it by three-fourths. Today it’s covered with grasses and wildflowers, and no sediment is coming off the site.”

Controlling Erosion and Sediment After a Wildfire

For three weeks this past May, the Cree Fire burned more than 6,500 ac. of trees, rangeland, and rugged terrain in the Lincoln National Forest of south central New Mexico. When the flames died down, one of the priorities was to protect burned-over slopes in five major drainages from eroding when summer thunderstorms hit the area. Sediment washed from these slopes could threaten homes farther down the watershed, which lay directly in harm’s way at the mouth of the drainages.

“In some of the burned areas, soils are high in gypsum and very soluble in water,” says Greg Gray, range management specialist with the Smokey Bear Ranger District. “In others, really shallow silt and silt-loam soils also have an extremely high erosion potential.”

Gray helped direct erosion and sediment control measures in response to the fire. Work on the two-month project began immediately. Labor was provided by high school and college students, prison inmates, and the New Mexico Youth Conservation Corps.

Establishing Vegetation

Even before the fire was controlled, helicopters took to the air and began seeding the burned slopes with 40,000 lb. of a mixture of mountain brome, orchard grass, western wheatgrass, and yellow blossom sweet clover.

Protecting Slopes

About 300 ac. of the steepest slopes were treated with more than 10,000 straw wattles to control sediment. Measuring 25 ft. long and about 8 in. in diameter and weighing 35 lb. each, they were installed on contours. On slopes of 50% or more, they were spaced vertically and staked in place no more than 15-20 ft. apart. This spacing increased to more than 50 ft. apart on shallower slopes.

On 1,365 ac. of gentler slopes, trees were felled and placed on contours using the same vertical spacing as the straw wattles to slow runoff. The logs were laid as flat as possible, and soil was built up on their upslope side. In one severely burned drainage, both straw wattles and tree felling were used.

Protecting Property

About 20 trash racks were installed in the drainages to catch trash, which included tires and other debris, before it could wash into houses, onto roads, and through culverts. Made of railroad ties, logs 6-8 in. in diameter, and woven-wire fencing materials, they allowed water to pass through while pushing trash up on the rack.

About 20 sediment-retention structures, made with steel or wood posts and supporting 2-ft.-high woven-wire fencing material, were installed along drainage channels. Some were backed with filter cloth; others filled quickly with pine needles and trapped sediment without any filter cloth.

In some drainages, several earthen-dam ponds, once used to provide water for livestock, were cleaned out and used to slow water flow. To keep drainages flowing, road culverts were cleaned or enlarged.

Concrete traffic barriers and sandbags were used at the mouths of some drainages to divert runoff flows away from houses and other structures. In addition, heavy equipment was used to create new drainage channels. At one site, a channel dug across the lawn of an expensive home was protected with a turf reinforcement mat to provide a green look when it became fully vegetated. Riprap was used to reinforce the banks of channels that passed by homes and to protect culverts.

Effectiveness of the Response

An unusual and light June rain, which fell immediately after seeding, produced good germination. However, before the grass could grow much and before the wattles were installed and the trees felled, several heavy rains hampered work.

The straw wattles proved very effective. In fact, on some slopes where they were used, there weren’t enough trees to fell.

“Even though ash and sediment filled in behind them, they were effective in slowing stormwater runoff,” Gray reports. “Now grass is becoming well established to continue protecting these slopes from erosion.”

Some wattles and felled trees failed following thunderstorms, but regular maintenance has kept the trash racks and culverts clean and performing properly.

Gray credits close cooperation among the US Forest Service and other federal, state, and local agencies, such as USDA’s Natural Resources Conservation Service, the New Mexico Department of Forestry, and the Lincoln County Highway Department.

Gray reports that stormwater runoff following the fire has had little effect on the nearby fishery and that few of the erosion and sediment control measures have failed. He says he wouldn’t do anything differently. “So far the practices have worked really well.”

Learning From a Rare Storm

Lessons learned from the aftermath of an emergency situation can help reduce the impacts of future events and the cost and intensity of any response. Consider what happened this past July in Eagan, MN, a fast-growing suburb in the Minneapolis – St. Paul area. A freak storm parked above the city for more than a day, dumping as much as 10-12 in. of rain on some areas. Much of that fell within two and a half hours. Local officials consider this a once-in-2,000-years event.

Normally erosion and sediment control measures at construction sites in this area are designed for periodic rainfalls of about 1 in. or less, states Jay Michels with the Minnesota Pollution Control Agency.

Runoff from the storm blew out temporary sediment basins, flooded basements, dumped large amounts of sediment into lakes and wetlands, and took out retaining walls, roadbeds, and utility infrastructure.

Among the lessons learned:

Well-defended sediment ponds with properly placed emergency overflows pay off. “One of the biggest lessons we learned is that when building a temporary sediment pond, you don’t just plop dirt on a berm and expect it to hold,” Michels says. “Compacting the soil helps dramatically. Put the emergency overflow where it will discharge away from a wetland or other area you are trying to protect and use riprap to defend the overflow from erosion.”

Keep the construction site as green as possible. “Areas where water flowed through vegetation held up much better than exposed sites,” Michels says. “In several cases where water was channeled through vegetation, the areas would have eroded very deeply had the vegetation been scraped away. The contractor left it, at our urging, and it worked very well.”

Some silt fences work better than others. “Most silt fences were worthless in a storm of this magnitude,” Michels says. “The fences built with silt installation equipment, however, stood up fairly well.” These fences consist of a 36-in. monofilament silt fence fabric with 5-ft. steel posts spaced at 4 ft. on center for channelized flow and 6 ft. on center for perimeter applications.

Correcting for Incorrect Information

Several years ago, faulty soil survey information resulted in an incorrectly sized sediment basin at a residential subdivision construction project in Shellharbour, New South Wales, Australia. Morse McVey & Associates Pty. Ltd. in Picton, NSW, developed the original erosion and sediment plan for the project. Based on a soil survey, the firm designed a sediment pond to handle runoff from what was assumed to be coarse soils that were not easily dispersible. The sediment basin failed after the first storm, a relatively small event.

Morse McVey did a very quick soil analysis and discovered that the provided soil information was wrong. Within 10 days, that sediment basin was replaced with a much larger one sized to retain runoff from the very fine, highly dispersible soils that were actually on the site.

Responding to Natural and Human Challenges

Surprises on another Morse McVey project were not solved nearly as easily nor as quickly. That project involved a 9-ac. residential subdivision in Allambie Heights, a suburb of Sydney. The site is near Manly Dam, a popular recreational area upslope from bushland that is home to threatened and endangered species of plants and wildlife.

Erosion and sediment control planning for the project began in 1996. However, strong opposition to the project by environmental protection groups, including four court cases that cost the developer more than $1 million in legal expenses, delayed the start of construction until March 1999.

In the meantime, the project led to the development and testing of erosion and sediment control practices that are now part of new standards for managing urban stormwater in New South Wales. In fact, one version of the project’s erosion and sediment control plan is used as a model for subdivisions throughout the state. The court hearings also produced a landmark court decision affirming the value of sound stormwater management planning in protecting environmentally sensitive areas.

Once construction began, more legal challenges and onsite protests, unusually wet weather, and other surprises extended construction time of the high-profile project from nine months, as originally planned, to 17 months. “Throughout all of this, we had to ensure the utmost in erosion and sediment control to protect the environment,” states Rick Morse.

Among the unexpected developments that affected those plans:

Insufficiently Accurate Map Contours. Because of heavy vegetation, surveying contours accurately was difficult. As a result, some of the catch drains, which were built before land-clearing operations began, were placed outside of actual drainage flows.

“Once we cleared the land, we realized the contours were wrong,” Morse explains. “So we had to move those catch drains and install an extra-small sediment basin to ensure that water would flow in the right direction.”

Unauthorized Wheel Wash. Unknown to Morse, the site superintendent built a stabilized vehicle access/exit point and wheel-wash facility using a simpler, more economical design that did not conform to the approved soil and water management plan.

When Morse discovered this, he allowed the superintendent two weeks to show that this facility would work as well as the original design. It did, and Morse approved this alternate design and changed the soil and water management plan accordingly.

Disrupted Work Schedule. The project called for constructing two bridges across an intermittent creek. To minimize any sediment problems, the work was scheduled to take place between May and December 1999, normally a time of relatively small storms. The original soil and water management plan prohibited access to the creek from January to mid-May. However, La Niña and legal actions combined to delay much of the site development work from the start in April 1999 through January 2000.

Rather than shut down the project for five months before beginning the bridge work, the soil and water management plan was changed to allow work on or near the creek between January 1 and May 15, provided certain conditions were met:

  • Maximum soil cover had to be maintained during possible erosion events (C-factors in the Universal Soil Loss Equation would be kept below 0.05).
  • All soil cover had to remain stable under concentrated water flow, where appropriate.
  • Construction activities could not pollute the creek, directly or indirectly, with sediment. Among other things, this meant the contractor had to have materials, such as erosion control blankets and pegs, readily available in case they had to stabilize banks of the creek.

The long spell of wet weather altered other features of the soil and water management plan too. “We had to change phasing of the whole site because of the wet soils,” says Morse. “Also, we had to change the design of a water-quality control pond at the bottom of the site to accommodate heavier runoff flows.”

Reviewing the Results

Looking back, Morse would have eliminated the original requirement that prohibited working in or near the creek during the normally wet time of year. “We have to be more careful in considering what the words used with such restrictions really mean for a project,” he notes.

Morse reports that the general erosion and sediment control practices represented a little less than 8% of the total site development costs. Still, despite the surprises and changes, the money was well spent, he adds.

“The project’s soil and water management plan was reviewed probably more thoroughly than any other plan in New South Wales,” he remarks. “Inspection and monitoring programs show it has worked as predicted and will continue to improve quality of the receiving waters. And it has helped advance the erosion and sediment control industry.”

About the Author

Greg Northcutt

Greg Northcutt writes frequently on construction and business issues.