Erosion and Sediment Control After Natural Disasters
No matter where one resides, sooner or later that area is bound to be affected by a natural disaster, including flood, fire, or landslide. And that disaster will undoubtedly create erosion damage that will require repair work.
Each type of natural disaster brings its own unique erosion control challenges.
In the aftermath of a fire when vegetation has burned, soil is very susceptible to erosion; and, because the fire season is followed by the rainy season in many areas, the danger is that the rains will wash large amounts of soil off of the slopes and into waterways.
If the waterways become clogged with sediment and debris, flooding can be a problem as well.
Flooding may occur as seasonal flooding that causes erosion-such as overtopping streambanks-or coastal flooding that results from hurricanes, with accompanying coastal damage and erosion.
Prolonged periods of rain result in landslides in some parts of the country. In some cases, clear-cutting of forests leaves large areas of bare soil on slopes, creating the right conditions for landslides.
Donald Gray, professor emeritus of civil and environmental engineering at the University of Michigan, emphasizes the importance of observing basic soil erosion control principles.
“These principles were formulated primarily with site development and earthwork grading in mind,” he says. “Nevertheless, most are also applicable to erosion and sediment control work that might follow a natural disaster.”
Gray says his “10 commandments” of erosion control are based on common sense, but are frequently violated in site erosion control work.
“Many erosion control measures and products have been introduced over the years; they are more effective when applied with conjunction with these principles,” he says.
They include:
- Fit the development or erosion control plan to the site. Avoid extensive grading and earthwork in erosion-prone areas.
- Install hydraulic conveyance facilities such as ditches, curbs, and down drains to handle increased runoff.
- Keep runoff velocities low by spreading flow out in wide swales and installing slope breaks or benches.
- Divert runoff away from steep slopes and denuded areas by constructing interceptor drains and berms.
- Save native site vegetation whenever possible, although this may not be an option after a fire.
- If vegetation must be removed, clear the site in small, workable increments. Limit the duration of exposure.
- Protect cleared areas with mulches and temporary, fast-growing herbaceous covers; this is particularly important after a fire.
- Construct sediment basins to prevent eroded soil or sediment from leaving the site.
- Most importantly, install erosion control measures as early as possible.
- Inspect and maintain control measures.
“Observance of these 10 basic principles should greatly minimize erosion losses,” notes Gray.
He says the most important lesson learned from previous natural disasters is to apply repair measures as early as possible.
“Vegetation takes awhile to get established,” he says. “There has to be a grace period before the rain starts when you hope you can get your grasses and herbaceous materials established. Another lesson is that you have to come in with a good plan-not just a haphazard application of different measures.”
Post-Fire Measures
Mike Harding, CPESC, of Great Circle International, frequently is called in as a consultant on how to prevent erosion following fires. He has pointed out that a fire/flood cycle speaks to how risks are taken when people build “in harm’s way.”
“You see a lot of things in the field that are preexisting conditions that are going to be exacerbated by a fire and flood,” he told Erosion Control magazine in 2005. “People need to be thinking about building retention structures or dirt drainage or diversion structures, because these areas are always going to burn at some point in the future. You need to have permanent control mechanisms in place instead of just worrying about going in after these fires and shooting a little mulch.”
With fires, the erosion control focus usually comes after the disaster. The primary consequence of fires is they burn off the vegetal cover and the vegetation-particularly low canopy vegetation, herbaceous vegetation, grass, and, to a lesser extent, bushes, says Gray.
“That’s very often followed by serious erosion problems in the year after the burn,” he says.
It is of prime importance to establish grass and vegetation cover on the burned-over areas right after a fire, Gray says. He cites the Oakland Hills, CA, fire of October 1991, when flames fanned by hot dry winds blowing from the Central Valley consumed 1,800 acres of the Oakland and Berkeley hills, resulting in 25 deaths and the destruction of 2,903 dwellings.
“They did extensive revegetation work in those hills to try to minimize the erosion the following year,” says Gray.
Standard vegetative treatment after a fire includes seeding and hydromulching. Gray says this work is often supplemented by doing bioengineering with live fascines, which are sausage-like bundles of live-cut branches of wetland and streamside materials, usually willow or dogwood.
The bundles are placed into streambank trenches so that they grow perpendicular to the bank, providing protective vegetative over and a root structure to stabilize banks.
Another erosion control measure is straw rolls or wattles, placed across a slope in an attempt to slow down runoff velocity and enhance germination. Erosion control blankets also are used to help establish vegetation and provide temporary erosion control.
Aerial hydroseeding also can be employed, depending on the size of the damaged area. Hydroseeding can be also be executed from hydroseeding machines or truck-mounted tanks, says Gray.
“They are aimed and sprayed at the slope,” says Gray. “You can cover some pretty large areas with that.”
Advice From Those Who’ve Been Through It
When Laguna Beach, CA, got caught up in the devastating Orange County fires of 1993, public works director Steve May conducted an immediate aerial survey of the damage and hired a consultant.
When it comes to dealing with erosion control on a large scale post-fire, May points out, the situation requires a professional evaluation whose recommendations are applied. “You really can’t put out wholesale suggestions and recommendations that apply in all situations,” he adds.
After the 1993 fires, Laguna Beach hired laborers from the California Conservation Corps to implement professionals’ recommendations, which included sandbags and hay bales.
But using the hay bales became “a disaster in itself,” May notes, because they had not been fastened properly and ended up getting washed into storm drains and catch basins.
“Inappropriate measures can create more damage than no measures at all,” he says. “One example was the hay bales that were not adequately secured and ended up washing into the drain systems and creating their own problems.”
May says that in general, homeowners who live on steep terrain would do best post-fire to use Visqueen to keep water from hitting the soil. He suggests that homeowners who reside on relatively flat ground can effectively use wattles and sandbags to protect their property’s exposed soil post-fire.
However, he adds, “The idea is to never let the soil erode in the first place rather than trying to collect it once it gets put into motion.”
On November 15, 2008, fire hit the region of Anaheim, CA, prompting municipal officials to activate the city’s Emergency Operations Center to a Level 3, putting city services into logistical action.
“Our biggest task was evacuating folks from Anaheim Hills, the area where the fire was occurring,” notes Dan DeBassio, operations manager for the city’s public works department. “Public Works’ major involvement was supporting the police and fire departments in setting up detours and traffic control and getting people out of there as quickly as possible.”
Although eight houses had burned, damage was otherwise minimal that weekend because of several factors. The wind and rains were gentle—in fact, vegetation regrowth was observed soon afterwards. And because of foresight on the part of homeowners associations when some of the developments were built in the 1970s, there was little erosion damage.
DeBassio says the developments were built with defensible space—areas around the homes were landscaped and irrigated to prevent wild brush from cropping up near the homes. Drainage ditches diverted water from fire hoses and rain away from the area.
“We didn’t have a lot of erosion from the fire, and we had some rains soon after it, but the homeowners associations were proactive,” says DeBassio.
There was, however, an apartment building that was severely damaged. Its management company brought in workers after the fires and before the expected rains to set up erosion control such as sand bags, silt fences at the V-gutters, and straw wattles along the slopes.
DeBassio says the area’s slopes are intermediate and older, and therefore there wasn’t a lot of loose material. “We weren’t afraid of landslides; we were afraid of mudslides,” he says. “If we got the mudslides, how were we going to warn these people in the middle of the night?”
At a fire station close to the burn area, the public works department stored a few trucks equipped with barricades, cones, and signs in case of mudslides because of the potential for one of the major roads to get covered over in mud.
“We also had some heavy equipment so that, if need be, we’d shut down the road and start pushing the mud out of the way,” he recalls.
DeBassio says many people in the apartment complex feared mudslides and moved out, although the mudslides never materialized.
Sandbags had been placed around the apartment building. In fact, sandbags are available to the community throughout the rainy season and are provided to residents filled or unfilled—their choice—in advance of any potential rainstorm.
As private homeowners and businesses took their own measures—including hiring erosion control specialists—the public works department made sure the storm drains were clean and put sandbags around the drainage inlets in advance of the rain. The department also assisted at the EOC by distributing erosion control information at the same site where those affected by the fire were getting donations of food and clothing and assistance with building permits, utilities, and other services.
“That was the most useful way for us to get information to the people who needed it,” he says.
Anaheim’s public works department educates its residents post-fire through informational brochures from other government entities and erosion control contractors.
DeBassio says one of the best guides to post-fire erosion control was produced by Orange County and is used in Anaheim, as well as other municipalities, in educational efforts. The guide is available here.
Another educational guide used is from the Natural Resource Conservation Service Southern California Watershed Recovery Program on how homeowners can prevent soil erosion on their property. It is available here.
Understanding Landslides
In the case of flooding and landslides, preventative erosion control measures can be used to minimize damage that can occur in areas prone to such disasters.
After landslides, the problem is a “very disturbed condition that is vulnerable to a whole host of erosion processes,” says Gray. “Landslides disturb the slope, weaken the soil, and make it vulnerable to advanced erosion, such as gullying.
“In this case, you have to look at potential schemes for stopping gullies from forming,” says Gray, adding that the goal is to prevent them from recurring.
One example of gully control is that of check dams, he says. These are temporary or permanent barriers that slow water velocity and help prevent erosion.
“You’ve got to be careful how you install check dams; otherwise, you can get washout around them unless they’re well inset into the channel,” he says.
Live gully repair is an effective soil bioengineering technique, he adds. The technique involves the use of live cuttings and compacted soil to hold the repair.
“You can still go with the usual erosion control schemes like mulching and vegetative seeding, erosion control blankets, and so on, but you have to look beyond just those to techniques capable of dealing with more channelized erosion,” says Gray.
The California Geological Survey (CGS) has produced a number of maps since the 1960s that illustrate landslide features and mark potential slope-stability problem areas.
The CGS classifies landslide types either as rock or as soil, which can be debris (coarse fragments) or earth (fine fragments). The categories include:
Rockslide– A landslide involving bedrock in which the rock that moves remains largely intact for a portion of the movement. Rockslides can range in size from small and thin to very large and thick and are subject to a wide range of triggering mechanisms. Rockslides commonly occur on relatively steep slopes, with gradients ranging from 35% to as steep as 70%.
Earth flow– A type of soil-flow landslide in which the majority of the soil materials are fine grained (silt and clay) and cohesive. The material strength is low through much of the slide mass, and movement occurs on many discontinuous shear surfaces throughout the landslide mass. Earth flows commonly occur on moderately steep slopes, with gradients from 10% to as steep as 30%, although steeper slopes may be found in headscarp and toe areas. Prolonged rainfall typically initiates earth flows.
Debris slide– A slide of coarse-grained soil, most common in unconsolidated sandy or gravelly units, but also common in residual soils that form from in-place weathering of relatively hard rock. The overall strength of the debris-slide mass generally is higher than that of earth flows because of granular constituents, but there may be a very-low-strength zone at the soils’ base or within weathered bedrock. Debris slides commonly occur on slopes as steep as 60% or 70%. A single heavy rainstorm or series of storms may be sufficient to trigger debris slides.
Debris flow– A soil flow in which the majority of the materials are coarse grained (fine sand to boulder-sized particles) and noncohesive. Debris flows are most often triggered by intense rainfall following a period of less intense precipitation, or by rapid snowmelt. High pressures in pore water cause the soil and weathered rock to rapidly lose strength and flow downslope. Debris flows can move at rates ranging from meters per hour to meters per second and travel relatively long distances, creating a significant life and property threat.
Rock fall– A landslide in which a mass of rock detaches from a steep slope by sliding, spreading, or toppling and descends mainly through the air by falling, bouncing, or rolling. Intense rain, earthquakes, or freeze-thaw wedging may trigger this type of movement. Rock falls occur on steep slopes of hard, fractured rock.
Flooding
Erosion control after floods typically entails more than just hydroseeding, says Gray. “You have to look at other systems like erosion control blankets and Visqueen,” he says.
It was April Fool’s Day when severe weather began to sweep across Florida’s Panhandle region, but the consequence was no joke. The excessive storms that hit north Florida during the first week of April dumped more than 20 inches of rain in some areas over a few days, causing severe flooding in area rivers that exceeded recorded historic levels. The Withlacoochee River had crested as much as 2 to 3 feet above previous recordings.
“All of the water gauges were underwater, and survey crews were sent out to do the water elevation recordings,” notes Bill Steves, a transportation infrastructure program manager for the Sarasota, FL, office of Reynolds, Smith and Hills, an architectural, engineering and environmental firm. The firm provides general engineering consulting services for Madison County, FL, a small, rural county of 19,000 residents that was among the hardest hit by the flooding.
Steves says the Withlacoochee River and the Alapha River both feed into Florida’s Suwannee River in the Suwannee River watershed in north Florida. The rivers flow from Georgia, which also was affected by the weather event.
Ironically, the region is not an area that is considered to be flood prone, notes Steves.
“People within most of the areas didn’t even have to have flood insurance for new homes,” he says. “It wasn’t even thought about. This was one of those freaks of nature where it just started dumping and it kept dumping.”
Not only were residents not required to purchase flood insurance, but erosion was not considered a potential problem either, says Steves. The area is generally managed by the Suwannee River Water Management District and the US Forestry Service; when the water exceeds the riverbanks, it generally meanders into flat woods under those agencies’ control, and no houses are generally affected.
This time, however, more than 8 feet of water had gushed over one of the county roads, which the county had been pumping out for more than a month. The water also flooded over bridges. Madison County has nearly 400 miles of county-maintained dirt roadways.
“When those roads went underwater, we lost a lot of the lime rock roadbeds that were not paved,” says Steves. “A lot of them were either millings, lime rock, or maintained dirt roadways. A lot of the top portions-the stabilized portions-were washed out into the woods.”
Some residents provided short-term erosion control on their own with sandbags supplied by the state and sand supplied by the county. Steves says most of the county’s efforts focused on clearing downed trees and getting the roads back into order so people could travel in and out of their homes.
In addition to the land issues, there were property and human concerns as well. One couple’s newly built house had water up over the tops of the doors. Most of the homes in the area are built on stilts. Some 140 homes were affected in some fashion; 40 were wiped out.
Educating the public during a natural disaster is key to saving lives.
“Our Emergency Operations Center has a reverse-911 emergency notification system, and they kept on it very well for being a small county,” says Steves. “The local chamber of commerce assisted. Through anybody who had any electronic master list, we were able to email people and put out alerts.”
One life was lost during the severe storm. Steves explains that an older man who lived with his dog in a stilt home was intent on riding out the flooding in his home, despite pleas from people for him to seek safe haven. When the man finally decided to leave, the water was cresting, and he lost his footing and got washed toward a fence which temporarily halted him, but he was unable to recover. He was found dead nearly 2 miles down the river.
Steves relays the story of a county commissioner who, after the storm, was fishing with his son at a location where the Suwannee River flood levels had started to subside. He looked up at a tree that caught his eye and noted a refrigerator lodged in it, 30 feet above him.
As soon as Florida Governor Charlie Crist declared a state of emergency, Florida agencies stepped in to assess the damage. The State Emergency Response Team enlisted the American Red Cross to assist affected residents.
“They immediately started taking inventories of the damage to our roadways,” says Steves. “We were given estimates of what it was going to take to fix everything. They were telling us how to keep a record of it, so once it was declared a national disaster, we could get the Federal Emergency Management Agency [FEMA] in and start getting reimbursed for everything we had to do.”
Ten days after the governor declared a state disaster, he sent a letter to President Barack Obama asking him to declare a major disaster for several of the north Florida counties. Toward the end of April, more than $1.5 million in federal aid was provided to the area.
FEMA representatives had traveled to the Panhandle and estimated it would cost up to $5 million to pump the water down. Madison County officials elected to use their own 6-inch irrigation pumps, while renting a few more at a total cost of $20,000, Steves says. He estimated it would take at least a month to pump all of the water down.
“The FEMA folks couldn’t even get mobilized for a month, so we’ll have it pumped down before they get there,” Steves said at the time of the flooding. “It wasn’t worth it to us to let them spend the money.”
Most of the affected area is agricultural land. Representatives of state agencies expressed concern that chemical and other agricultural runoff would affect water quality.
“We got an emergency permit from the Suwannee River Water Management District to allow us to pump,” says Steves. “It took a route down into a ditch system and spread out into a sheet flow on some of the lands controlled by the Suwannee River Water Management District. All of the people from the Florida Department of Environmental Protection and environmental folks thought that by going through open ditches and out into flat woods, it would purify itself enough not to contaminate the river to any degree,” Steves explains.
Government officials also were concerned about how the Panhandle region would handle rain that may be dumped on the area from the beginning of hurricane season, which began on June 1.
“If we get any kind of rains that are even somewhat less than what happened in this past month, the grounds are saturated and there is not a lot of storage in the ground anymore, so it could escalate to something just as bad quite quickly,” says Steves.
After stabilizing the situation, Madison County officials intended to sit down for a “lessons learned” meeting, says Steves.
“If our county had thousands of people, a large-scale emergency operation would have probably been very beneficial,” he points out. “But right now what’s going to help us the most is to get reimbursed for what we did to get roadways opened and get rid of debris.”
With an eye to the future, Steves says Madison County will ensure it will be ready for the next potential flooding event by being prepared with pumps.
“We’re going to make sure all of our pumps are set up, and we’re going to develop procedures so we know we can go to get assistance like pump rentals,” he says. “Our biggest problem wasn’t getting the pumps-we had water management districts all over the state offering us pumps. Our problem was getting the lines that carried the water out. We had to borrow farmers’ irrigation pipe to get all of it pumped away, because every pump we were using in the area had to have 4,500 feet of pipe to get [the water] over the hill.”
Once the pumping is complete, post-flood erosion problems are revealed.
“We try to stabilize all of our roadways to keep things within the roadways so it’s not rushing out into private property or environmentally sensitive property,” says Steves. “Most of our roadways are controlled by ditches on the side and side swales with ditch blocks. All of those we upgrade are definitely stabilized when we’re able to pave them, so everything is pretty much controlled.”
Solid stabilization and grassing on the sides of the roads ensures that the county will generally not have any problems, Steves says.
“We follow the same guidelines as the Department of Transportation does for ditches with higher slopes-we pave the ditches,” he says. “We make sure that those that are less than 2% are vegetated well.”
Steves says Madison County “does not like to do erosion control so much as prevention. We try to do what we can by stabilizing banks and paving as many roadways as we can each year, but with a limited budget for a small county, we pave maybe only 2 or 3 miles a year.”
Steves says that from his perspective, the biggest lesson learned from this storm was that a local government entity must be prepared to help itself.
“When you start getting larger government in, there’s a lot of duplication,” he says. “The officials were trying the best they could. They would bring someone in for a couple of days, and then we’d have somebody new come in and they were asking the same questions over again.”
It’s important to be prepared to with good record-keeping that will satisfy FEMA or any other government agency responsible for assisting for reimbursements while getting back to normal, adds Steves.
“It really is record-keeping nightmare. If you come up with a good computerized system to record everything and have it documented well, that’s about the best thing you could do.”
Another option for municipal entities is the Emergency Watershed Protection Program of the Natural Resources Conservation Service. The program purchases floodplain easements on non-federal agricultural floodplains that have been impaired within the previous 12 months or have a history of repeated flooding. The program is intended to refurbish existing areas or strengthen them to withstand increasing runoff.
