Emergency Erosion Control Healing With Old and New Methods
When it comes to emergencies involving water, for many small towns the worst nightmare is the 50- or 100-year storm that triggers flooding that can’t be contained with normal defenses. Typically, sandbags are a small town’s first line of defense against unexpected fast-rising water, but sandbags take time to fill and are awkward to transport, and it hardly ever happens that sand, bags, and high-rising water are in one place simultaneously. To combat these problems, H.T.C. Inc. in Milford, IA, has come up with a solution priced right for small communities and contractors that use sandbags to respond to flooding emergencies: a sandbag-filling attachment that fastens to the company’s Hydraulic Truck Conveyor, which in turn can be mounted on all standard dump boxes for easier delivery of sand and gravel without hand labor. The City of Milford has typically used the Hydraulic Truck Conveyor for such routine work as pothole patching, trench filling, and shouldering, but when flooding hit this small community, H.T.C. made the attachment available to National Guard units called in to help local civil-defense teams battle fast-moving river water. H.T.C.’s Valerie Watters reports that the guards and volunteers were able to fill sandbags in two minutes-a fraction of the time it usually takes for hand-shoveling wet sand-and deliver the filled bags more efficiently than is usually the case when lugging the heavy bags to places where they’re needed. The easy-to-use portable equipment made it possible to get the sandbags rapidly in place, which freed volunteers for other tasks.
While sandbags and earth dams have long been the first line of defense against rising water, a company in Carlotta, CA, is offering what it calls an environmentally safe, stable, temporary alternative. The Water Structures cofferdam is the brainchild of David Dooleage, who claims he got the idea while standing in a line filling sandbags. “I thought, ‘There’s got to be a better way,'” he recalls. It turns out there was: filling two balloon-like plastic sacks with water, then encasing them in a tube. “The water pressure inside the tube and the sheer weight of the water keep the larger tube from rolling around,” Dooleage explains. “All that sand represents is weight and mass, and the weight of water molecules is much easier to direct and place than sand molecules.” He claims that one Water Structures system can replace a thousand people working a sandbag line.
Another advantage is that Water Structures cofferdams are lightweight, easy to transport, and reusable. The company also cites the cofferdam’s low environmental impact. Without the major construction required with earthen dams and the leftover debris of sandbags and other temporary measures, areas recover more quickly. Onsite requirements include a portable pump and a local water supply to fill the tubes. Because the polyethylene liners and the geotextile outer tube are flexible, the dams conform to uneven surfaces and terrain, providing an effective seal between the dam and whatever surface it’s set up on.
In northern California, residents of a 725-unit condominium complex, who suffered $1 million in flood damage in one year, voted to invest $15,000 of precious homeowner association funds to rent a Water Structure. Their goal was to protect the complex from chronic flooding when water backed up from sloughs that run through the development into a pumping station and finally the American River. Frustrated when county officials ignored their demands to improve the drainage system, the condominium owners took matters into their own hands and fortified the complex with a Water Structures dam during the flooding season. The preventive emergency control worked, and at the end of the rainy season, the dam was packed up and removed with nothing left for the condominium owners to clean up.
Halfway around the world, an emergency of a different sort suggests that another advantage of Water Structures dams is how easy they are to install; no small consideration when water is cresting the levee or rising under the back door. The problem was a mechanical failure in a hydroelectric plant in KwaZulu-Natal, South Africa, where bolts on intake screens had came loose. The missing screens and bolts threatened four reversible turbines at one facility, and based on this failure, the hydroelectric company embarked on a program of inspecting screens at all of its stations. The problem was the time it would take to draw down the water reservoirs to levels required for screen inspection and repair.
Options included sandbags (2,900 of them and 12 hours of installation in addition to the problem that their plastic covers tend to slide) or a specially designed U-channel structure bolted to a trapezoidal concrete base with steel-plate gates slotted in. The latter option had even more negatives than the sandbags—the “temporary” dam would take three days to construct and more days to install, and even with a PVC liner, there was concern about getting an adequate seal given the uneven profile of the canal where the dam would be placed. Having exhausted his known options, the contractor decided to take a chance on a new American product—a 900-mm-high Water Structures cofferdam. Although the anticipated water depth was only 500 mm, the additional capacity was a hedge against water-level fluctuations during summer storm runoff. As an additional safety factor, Water Structures also provided a 1,225-mm backup dam.
It took only an hour for a crew of four, using the requisite two portable pumps, to install the two dams, and despite the uneven concrete surface, the Water Structures dam sealed effectively. One slight glitch marred the otherwise successful experiment: A high wind came up and pulled the inner tubes of the 900-mm Water Structure off the parapet ledge of the canal. The inner tubes began to deflate, and the Water Structure deformed, losing its effective seal. It was later determined that the water depth had risen 150 mm over the 900-mm dam’s design depth, but with the 1,255-mm backup in place, the repairs were completed. The two units were deflated and pulled out for use at the other installations, and the contractor estimated that as a result of the innovative dams, work was completed three days ahead of what was already an ambitious schedule.
The South African experience points out, however, the importance of securing a good seal between the dam and the surface on which it’s placed. In Clear Lake, CA, where Water Structures were used to protect homes from record-breaking El Niño floods, the problem was solved by directing seepage under the structure to a small electrical pump, which discharged the water over the top of the dam.
Retaining Louisiana’s Gulf Coast
Years of erosion from natural wave action, boat wakes, and hurricanes and other types of stormwater might not seem nearly as urgent as towns flooded in freak storms, but Army Corps of Engineers (ACE) Project Director Jack Fredine calls what’s happening along Louisiana’s gulf coast a national emergency. The state is losing an acre of coastal land every 24 minutes, and if the current rate of erosion isn’t slowed within the next 40 years, some 800,000 ac. of coastal wetlands will be lost. In some places, the Louisiana shoreline will recede 33 mi. inland from its present location. Currently the state contains 40% of the United States’ coastal wetlands and accounts for 80% of the nation’s coastal wetlands loss. Crucial for wildlife, the wetlands also protect mainland residents and infrastructure and are the nurturing ground for 25-40% of the nation’s seafood catch.
“This is not a Louisiana problem, it’s a national problem,” informs Fredine.
Although the coastal areas of Louisiana are naturally exposed to what Chuck Villarrubia, program supervisor for the Louisiana Department of Natural Resources (DNR), calls “the constant subsidence of land mass” (sinking), human interference has disrupted the natural mechanisms by which the area has traditionally replenished itself. Levees built to control the Mississippi River’s annual flooding, which reached dramatic proportions in the disastrous flood of 1927 when the river still flowed through New Orleans, inhibit the laying down of river sediment that historically counteracted the land’s subsidence. Furthermore, the channelization of the Mississippi at its delta to accommodate shipping has added to the problem. Although ACE has lead the way in these kind of nature-altering projects over the last 50-60 years, Fredine says he’s now happy to be involved in efforts to remediate some of the unforeseen effects of previous projects.
As a first effort in slowing the coastal erosion it helped foster, ACE has developed two freshwater diversion projects to help protect the ecology of the coastal wetlands and control “the continuing emergency” of Louisiana’s dwindling coast. The levying of the Mississippi River for flood control has not only blocked the river’s historic spring overflows, but it also impedes the flow of marsh-supporting fresh water, nutrients, and sediment to coastal areas. The first of two diversion projects, the Caernarvon project was completed in February 1991 at a cost of $26.1 million. Using only gravity feed, fresh water is diverted southward from the river through five 15-ft.2 gated culverts and can be dispensed through outflow channels at a rate as high as 8,000 ft.3/sec. Prior to the diversion, this area lost some 1,000 ac. annually. The Davis Pond diversion structure, which is now being built, will be capable of diverting up to 10,000 cu ft/sec. of water, a rate that in one day’s time would cover 20,000 ac.-ft. Fredine estimates the gross area of Davis Pond’s influence at about 707,000 ac. or about 1,200 square miles, roughly the size of Rhode Island.
Fredine is quick to point out that the goal is to import fresh water, not sediment. “I can’t in all honesty say I’m building any land,” he says. “The idea is to reduce the salinity levels in the marshes and introduce nutrients to fertilize the wetlands. A certain amount of the muddy water brings sediment along with it, but that is a very small part of the picture.” The vegetation the two diversion projects are designed to nurture comprises floating root mats, 2-3 ft. thick, that rise and fall with the tide. The fresh water promotes a more lush vegetative growth, which in turn provides more shelter for the juvenile fisheries, and detritus that build up at the mat bottoms. “It’s all organic,” states Fredine, “but it helps in the formation of land—or at least in reducing rapid land loss—by increasing the vegetated area that acts as a subtle buffer against wave action. Moving fresh water south reverses current trends in which increasing flows of incoming salt water are killing existing freshwater vegetation faster than new saltwater plants can become established.”
Villarrubia reports that aerial photography indicates an increased presence of floating vegetation and what he thinks is also an increase in biomass. “We’re seeing some increase in sediment in some areas, especially the ponding area that the fresh water flows immediately into,” he says, “so I think we’re getting sedimentation in some areas, along with the increased nutrients. If the marsh grows fast enough, it will keep up, which is what used to happen here before we levied everything.” Villarrubia says he hopes to get a better idea of how far south the freshwater effect extends so the various agencies involved can manage the diversion and decide on whatever supplemental action is needed (adding sediments or nutrients) as the flow moves farther south.
Researchers at Louisiana State University and the University of Southwest Louisiana are currently documenting preliminary conclusions from aerial photographs taken of the area influenced by the freshwater flow. Figures from DNR monitoring from 1992 to 1994 show a sevenfold increased in freshwater marsh plants while salt marsh vegetation has decreased by more than half. Study results also indicate a net increase in marshland of 406 ac. within the sampled area, which originally contained 2,289 ac. This translates to a 5.9% increase in marsh each year.
ACE is also involved in an additional project aimed at reclaiming Louisiana’s barrier islands as a hedge against mainland erosion. “The barrier islands are a natural process,” says Kenneth Bahlinger, landscape architect with the DNR Coastal Restoration Division in Baton Rouge. “They’re the remnants of old Mississippi River deltas that are now gradually washing away. The islands are very valuable because they protect us from storm surge and hurricanes. Between the barrier islands and the nearest cities, there are about 20 to 40 miles of marsh. We figure that every mile of marsh decreases storm surge about a foot. We pumped sediment from a nearby lake, building containment dikes and pumping about a million and half cubic yards of sediment per island. We finished in the summer of 1998 and installed sand fencing on two of the islands. This year we planted vegetation. Our philosophy is, if we’re going to pump sand, let’s try to contain it. We’re not going to stop the islands from totally eroding away, but we’re going to slow down the process.”
Bahlinger selected the three grasses used to vegetate the islands for their growth rate and ability to hold off erosion. “The smooth cord grass [Spartina alterniflora, 34,623 plants] is ideal and the most common marsh grass out there,” Bahlinger explains. “It loves being inundated with the sea at high tide and being exposed at low tide. Of the other two—marsh hay cord grass [Spartina patens, 80,729 plants] is a very diverse plant; it grows anywhere from just above the marsh platform, where you find the smooth cord grass, up to the top of the dunes. It can tolerate the drought as well. The bitter panicum [Panicum amarum, 24,343 plants] is not as common out there, but it loves the dry weather, and it’s a pretty vigorous plant. We planted them in rows and spurs to maximize wind control. You’d be surprised at the size of the sand pile that accumulates around just one of these small plants.”
Both the freshwater diversion projects and the barrier island restoration utilized amphibious excavation equipment manufactured by Wilco March Buggies and Draglines Inc. in Marrero, LA. “The marsh buggies are almost the lifeblood of building levees out in the marshes,” says Fredine. “They’re basically a barge with wide tank tracks—so wide they won’t sink into the marsh—with a backhoe mounted to deck. It’s one of the few practical ways a piece of construction equipment can move around here without sinking out of sight in the marshes.” On the barrier island project, the buggies were used to haul equipment and pipe. Fredine points out that the marsh buggies are undergoing the same type of metamorphosis as ACE. Previously instrumental in laying oil pipelines through the coastal areas—cutting the channels that have increased saltwater infiltration in the freshwater marshes—the buggies are now being used to help reverse the situation.
Bad Sentiments for Too Much Sediment
While the challenge in Louisiana is not enough sediment, two contractors in rain-soaked western Washington State recently discovered that too much sediment in the wrong place is not a good thing at all. In December 1998, EMCON Inc. in Bothell, WA, responded to a cry for help from a contractor building a 10-ac. apartment complex. The state Department of Ecology had shut down the project for releasing dirty water into a local creek, which drained directly to a larger creek that supports a salmon spawning ground. According to John Macpherson, EMCON senior water-quality program manager, the criteria for discharge for the project was no greater than 5 nephelometric turbidity units (NTUs) above the background turbidity of the receiving stream, which in this case routinely had a turbidity of less than 5 NTUs. By the time regulators caught up with him, the contractor was on occasion discharging water as high as 1,000 NTUs off the site.
“With permitting help from the Department of Ecology,” explains Macpherson, “we were able to set up a 12,000-gallon treatment system on-site for cleaning up the water prior to discharge. The procedure called for the contractor to fill the tank on an as-needed basis during rainstorms, then to call us to treat the water. The treatment consisted of recirculating the water in the tank at the same time that we meted in slightly less than 2 gallons of an approved water-treatment agent. When they began concrete work on the site, we had to add acid to reduce the pH back to neutral before performing the chemical treatment (occasionally the pH would reach 11).” Macpherson stresses that his crew carefully documented the quality of the water before and after treatment and monitored the receiving creek to ensure there were no negative effects.
After treatment, the water was allowed to gravity-settle for two hours and was then discharged at a controlled flow rate to a county stormwater retention basin adjacent to the site. According to Macpherson, after treatment the turbidity of the water averaged about 3 NTUs, which made it possible for the contractor to continue work through the worst of the rainy season, stay on schedule, and protect the salmon.
“The owner of the construction company subsequently received a VIP tour of our facility,” says Macpherson. He admits that he wouldn’t have been anywhere close to being on schedule if it weren’t for the EMCON mobile treatment capability. “In this era of ‘profit-at-any-cost’ corporate mentality, it is wonderful to be able to offer a valuable service at a competitive cost.”
Just a year later, Macpherson had another chance to produce what he calls a “win-win outcome.” “I received a call from a developer/contractor on a Thursday afternoon. He had been red-tagged—ordered to stop all construction—by the county land-use inspector just hours before. Stormwater runoff from the 15-acre condominium site was discharging into a small stream, increasing the turbidity from 5 NTUs to several thousand NTUs—considerably over the maximum standard of 5 NTUs above receiving-water turbidity. Not only was this site shut down, but if they couldn’t stop the water quality violation, they were prime candidates for a fine and/or a lawsuit.”
Macpherson also notes that the county listed several other areas where the contractor fell short of the legal requirements, including the fact that he had no erosion and sediment control plans, no receiving-water monitoring plan, and no spill-prevention plan or equipment. Macpherson visited the site on Friday afternoon to get a feel for the nature of the problems and then worked through the weekend to produce the required plans, which included what he calls “an innovative design for managing all of the stormwater on-site.” The project was reopened the following week using EMCON-installed temporary water-holding/ settling tanks and a specially designed 100-gal./min. stormwater infiltration system. These and other changes were demonstrated to the county inspector.
“Although the contractor continued to resist covering disturbed soils and implementing other recommended BMPs,” says Macpherson, “our plan was comprehensive and robust enough that, for the next five months of wet weather, there was not a single incident of surface-water discharge violation. EMCON is now working on two other projects for this contractor, and its water treatment systems will be installed prior to breaking ground.”
And from Louisiana comes something in the “there’s-never-anything-new-on-the-planet” category. For nine years the DNR has been stockpiling Christmas trees in freshwater marshes to slow down wave action. The idea originated in Holland, and so far some 780,000 post-holiday trees have been donated to the project. “First we build a wood-brush fence,” says DNR’s Bahlinger, “any number of feet long, about 5 feet wide, and 4 feet high. Then we fill the enclosures with clean, discarded trees and tie them down to keep them from washing away. The installations slow the wave action, reducing wave energies, but let the water through. Sediment drops out behind them. The tree-filled fences create a great habitat for fish, crabs, and shrimp.”
Bahlinger says the Christmas tree fences work best at water depths of less than 2 ft. “We go out every few years and replace some of the wood in the fences, then we just keep adding trees on top of them. The sap in the evergreens acts like a preservative.”
