A wide range of erosion problems of varying scale and complexity lend themselves to a common solution: channel armoring, a practice that has evolved from simply lining degraded channels with piles of stone or blankets of concrete to include a broad assortment of materials and techniques.
The city of Houston, TX, has a long history of devastating floods going back to the first few years after it was established during the early 1800s. The reason is simple, says Heather Saucier, a spokesperson for the Harris County Flood Control District (HCFCD), which encompasses Houston. She says the area is very flat and is laced with a network of meandering natural channels and very extensive flood plains that eventually drain to Galveston Bay. Receiving an annual average of 48 inches of rainfall, the shallow drainage depressions could fill quickly during severe storms and would frequently overflow to inundate nearby communities. Prior to flood prevention projects, the flat terrain, in many areas, provided the illusion of safety, only to reveal itself later, during heavy storms, to be an integral part of the extended floodplain causing havoc for businesses and residents located there.
To address these recurrent disasters, residents in the Houston area as early as the mid 1800s organized themselves to combat the flooding, culminating in the agency known today as the Harris County Flood Control District.The US Army Corps of Engineers (USACE) joined the effort to coordinate the large-scale engineering necessary to the task. The activities of these two agencies, working either in concert or independently, include construction and maintenance of detention reservoirs and conveyance structures, widening and deepening key natural channels, and land purchases to expand the right of way for stormwater by restricting development in floodplains. Together, these activities are designed to facilitate the safe delivery of storm flows from the settled areas in the county out to Galveston Bay while minimizing the risk to life and property.
Building A Better Bayou
The shallow floodplain depressions throughout the Houston area, known locally as bayous, offer a degree of natural defense against flooding by temporarily detaining inrushing stormflows. However, it was recognized as far back as the 1980s that increases in stormwater, delivered from impervious cover accompanying urban growth and development, would soon outstrip the capacity of the existing bayou flood control layout.During the 1980s, the USACE began drawing up plans for the renovation of Sims Bayou, representing one of the largest urban flood control projects in the United States to date. The plan would modify and enhance to the existing system to increase its capacity to detain and safely redirect the increases in stormwater projected with ongoing development in the watershed.
Mike DeMasi, USACE project engineer for the Sims Bayou project, says, “It started out with an old-school method. It was originally designed to be a concrete lined.”
But for residents, that idea did not paint a pretty picture. Community groups asked the USACE to consider a more environmentally sensitive approach that might enhance the ecological function and be aesthetically pleasing, while performing its flood control function.The Corps of Engineers took this community input under consideration. However, limitations on the width of the right of way that would be available to regulate the velocity and volume of potential stormflows constrained the options.
“We worked together and came up with the idea to design the bayou channel to be lined using articulated concrete block [ACB],” DeMasi says. “It came up to about the same flow as a concrete channel, although it pushed the right of way a bit.” The block would increase the flow over that of a dirt channel while at the same time resisting erosion of the streambed.
Because ACB comes in such a wide assortment of shapes and sizes, the USACE was able to try several different configurations of the material at various stages of the project, fine tuning their methods as they went.Neil Tollas, general manager for Armortech, a division of Contech Construction Products, a leading manufacturer of articulated concrete block for channel armoring, explains, “Most of our hard armor has 20% open space to relieve hydrostatic pressure and allow for vegetation.”
In general, he says, hard armoring requires very little maintenance and readily accommodates grasses and soft woods. According to Tollas, open-cell or closed-cell blocks, if properly installed, can provide strong protective armoring while allowing for revegetation of the streambanks and creek bed through the gaps in the cells. And that was exactly the effect the redesign endeavored to achieve.Construction started in 1990, and by sometime in 2012 the monumental project is expected to be completed.”We went through a learning process over a few years,” says DeMasi. “We tried mats, individual blocks. We used a half dozen variations on the product,” ultimately arriving at the decision to use individual interlocking blocks as the preferred material and hand placement as the preferred installation technique. The combination, according to DeMasi, facilitated installation with the least amount of disturbance to the soft soil and preserved the meticulous grading work performed to optimize the contour of the channel bed.
Accommodating construction techniques to the variations in soil structure along the channel also required some ingenuity. DeMasi says that in Sims Bayou and throughout much of Houston, after a big storm surface water levels can fluctuate rapidly. “Because of all the development, the water comes up so fast and goes down so fast it loads the banks up. If you have a sandy area, the water pushes the bank out. We have had foundation problems where we’ve had the ACB damaged because of underlying conditions. We’ve gone back and addressed those, and we think we’ve fixed that in the future contracts.”
For example, DeMasi says, to fix the problem “in areas where the soil composition was unsuitable material for a stable bank-if it was silty, or too sandy-we would over excavate that, and put a 2-foot cap over top of it with a better clay material to seal it, so that the bank wouldn’t load up as quickly,” providing a stronger foundation for the ACB.
The agency has completed 16 of the 19 miles of channel renovation planned for the project, comprising close to 10 million square feet of channel lining, over soils that vary from fat clay to sandy mix.”The big question is how big you make the channel, how to get the maximum capacity with as little maintenance as possible. We want to use the product that gives us the best detention and maintenance, because after construction we turn it over to our local sponsor, in this case Harris County Flood Control District,” DeMasi says.
Heather Saucier of the HCFCD says the community was very thankful that the USACE was able to offer open-cell ACB as an alternative to solid concrete lining. She says the effect achieved is so natural looking, “If you went out there today you would never guess that there is concrete block underneath the grass.”
The Sims Bayou project is not the only USACE project in the Houston area using ACB for channel protection. Barker Ditch provides protected side surface water runoff collection for the Barker Dam perimeter containment levee in the city, providing flood protection to the highly populated communities adjacent to the dam, and it faced some serious erosion issues.The combination of high water velocities in the channel bottom and the highly erodible nature of the native soil, for which local folklore had coined the term “sugar sand,” had been causing severe erosion and embankment failure for years, particularly in the area where the ditch empties into the dam outfall. Conditions there were beginning to put the integrity of the outfall control structure in jeopardy.
Sarah Xie-DeSoto, a geotechnical engineer with the US Army Corps of Engineers Galveston District, says the USACE designers assigned to develop a plan to restore the integrity of the ditch were encouraged to take an innovative approach. She says their final design included articulated block “on the sides, along the bottom, and on the lower side slopes, with a geosynthetic non-biodegradable permanent turf reinforcement mat on the upper slope.
“The ACB design allowed us to stabilize the bottom of the channel and side slopes and protect the toe of the dam, improving water quality and improving hydraulic capacity,” Xie-DeSoto says.
Although the facility is operated and maintained by the USACE, Xie-DeSoto says there are several parks connected to it, and the dam is often used for recreational purposes such as hiking and jogging trails. “The block allowed an aesthetically pleasing approach. We came back, vegetated it, and ended up with a green-looking channel, and it’s held up pretty well.”
She adds, “The performance is really good, and that’s why when we did the Clodine Ditch in 2008, we utilized similar methods.”
Rolling on the Water
Spring Creek in West Mobile, AL, had a reputation for washing out and regularly overtopping its banks. The heavily silted-in channel was too shallow to contain the volume of water rushing into it during heavy storms commonplace in the region. Overgrown and neglected, the ever-widening stream constantly scoured away land from abutting properties, carrying sediment-laden water to the Dog River watershed and ultimately to the sensitive waters of Mobile Bay, prompting the city of Mobile to take action.
Michael Tew, president of SJ&L Inc. in Mobile, AL, says his company had a modest amount of experience repairing streambanks when the Spring Creek renovation job came along. “We had done little areas, where somebody had a washout, and you’d go in and repair that, but we had never done anything that could compare to this; it was a half a mile of streambank.”
The job would call for excavating 30,000 yards of material to widen and deepen the channel to prevent flooding, and installation of 7,600 yards of gabion baskets to protect the streambank.
Tew says gabion walls were specified because they could provide an effective alternative to riprap. “The engineers that know about the systems and their performance chose the gabions because they are flexible, and monolithic-all of the units are interconnected before they are filled. And they are permeable. You can’t find that combination in any other system. Riprap is not monolithic because it consists of individual pieces. Concrete is not permeable, and not flexible; gabions offer to engineers a number of advantages that no other system can.”
He says one of the major advantages of gabions in a creek with fluctuating water levels it that it can “relieve any kind of hydrostatic pressure you’ve got building up against the wall. On a concrete ditch, that’s usually the thing that causes it to fail-when all the pressure builds up behind it and the concrete cracks.”
But with the porous construction of the gabion wall, Tew says, “you never have that hydrostatic pressure behind it.”
Gabion baskets themselves can come in a variety of shapes and sizes. For his Spring Creek bid Tew chose roll stock fabricated by Modular Gabion Systems (MGS), a division of C.E. Shepherd Co. George Ragazzo, a spokesperson for C.E. Shepherd Co., says “Roll stock is the quickest, the most economical, and best-looking construction method that any contractor can use to install a gabion wall.”
Instead of using ordinary preassembled individual gabion baskets, of which the largest standard size is generally about 12 feet in length, using roll stock gives the contractor the option of ordering gabion mesh in uncut rolls up to 100 meters long with no joints. “We bypass a phase of the conventional gabion fabrication,” says Ragazzo.
The contractor can shape the forms needed, cutting them from rolls onsite, while building the baskets at the point of installation.
Ragazzo says roll stock saves labor because there are no joints for up to 300 feet of a run. In contrast, he says, “If you have a 6-foot gabion, then every 6 feet you have a joint.”
To complete a row, the gabion baskets must be lashed together by hand at each junction and secured with quarter-inch wire. “You have to physically join the gabions together along these contact areas. That’s why gabions are known to be labor intensive.”
He adds, “When we entered the market, my primary objective was to reduce the labor intensity, and we managed to reduce it between 40 and 70%. When you eliminate 70% of the labor on a project, the project becomes a lot less expensive.”Tew says the MGS roll stock conferred an appreciable advantage in bidding for the Spring Creek stabilization project. Ragazzo describes the situation: Before roll stock became available, bids for constructing gabion walls were about $150 per cubic yard. By figuring the savings from roll stock into the price for Spring Creek, the contractor brought the bid down to $98 per cubic yard. “He saved about $52 per cubic yard, which is a lot of money,” Ragazzo says.
He also saved material. “When you join two gabions together the conventional way, you have double mesh panels where the cages contact each other. With roll stock, one can be eliminated, so you can save a lot of material. You just eliminate this joint.” As an added benefit, Ragazzo says, “A joint is always a weak point-you eliminate the weakness and you increase the structural strength of the system.”With the stone specified by the engineers ranging in size from 4 to 8 inches for the gabion wall and 3 to 6 inches for the gabion mattresses, Tew says, the logistics of getting the needed supplies into place by the truckload can be one of the big challenges.”
Building it is simple once you get going,” he notes. “You’re beside a creek, and you’re walking in a creek; you’ve just got to have ingress and egress, and that’s probably the toughest thing. You’ve got a road on this end, you’ve got a road on the other end, then you’ve got a half a mile in between without roads.”Tew says his management team came up with a 21st century solution for working on the points in between. “We had a good bit of grub and debris that we had cut down, and we mulched that debris and built a roadway out of it that we could run on. We kind of had a green job before there was such thing as green jobs.”
Tew directed his crews to begin work at the lower reaches of the stream and advance upstream, setting up a bypass pump in the creek bed to move the water around the work zone as it progressed through the creek. Divided into two teams, workers followed a “zigzag” pattern through the stream, alternating between excavation and gabion installation.
“The product worked very well,” Tew says. He notes that a plastic coating on MGS wire mesh provides resistance to damage from nicks from stones during installation, and there is virtually no waste. “No matter what you cut, or what you have, you’ve always got somewhere in a gabion where you can wire-tie it back in where you need it.”
Tew adds, “We garnered a good bit of knowledge on that job.”Workers placed a standard filter fabric underneath the gabion wall to prevent any soil migration from beneath the gabion. “We started out with a basket on the bottom that was maybe as much as 12 feet, then we raised up a basket in the region of 6 feet wide, and the final basket at the top was 4 feet wide so that it was cantilevered so there was something to support it,” says Tew.
For all its simplicity, Tew says, proper installation of the gabion does require some finesse. For example, Tew says dumping the stone into the gabion baskets by the bucket load is far from good form.
“To build a wall and build it right, you’ve got to have a good bit of hand labor placing the rocks-that’s not to say you do everything by hand, but a good bit of it is done by hand. We had a good crew put together to do that. To do it right, the more hand labor you’ve got, the better the finished product. You want them placed so the face of the basket is true and square, and it’s not bulging out, because you figure it’s only being held together with a piece of quarter inch galvanized wire.”
The Spring Creek project required approximately 2,630 linear feet of channel improvements and approximately 7,600 cubic yards of gabion wall. The result earned SJ&L Inc. a prestigious Build Alabama Award from the Alabama Associated General Contractors of America. An assessment conducted by John R. Chadwick III, Department of Earth Sciences, University of South Alabama, several years after completion concluded that the gabion wall had effectively eliminated erosion along the streambank. Furthermore, he reported that in spite of the extensive modifications to the channel, water quality “remains healthy and full of life,” harboring “bullfrogs, large mouth bass, blue gill brim, small minnow-like fish and water snakes.”
An Organic Edge
When it comes to controlling erosion, Daniel Schaaf doesn’t believe in drawing a distinction between “hard armoring” and “soft armoring.” His company manufactures a product called ShoreSox that he says provides a highly effective means to restore vegetation to damaged, denuded shorelines using organic materials.
According to Schaaf, natural vegetation that integrates biological functions and ecological function with healthy root systems can be just as resilient against erosion as any wall of riprap or concrete.
Rick Reimer says residents with waterfront properties in the Thousand Lakes Region of Minnesota have been watching with increasing dismay as their shoreline frontages, deprived of a healthy vegetated cover, have been devoured by erosion. Reimer, who directs Kandiyohi Soil and Water Conservation District, says the windblown waves have claimed several tons of soil from the water’s edge-reshaping the gentle sloping shores into hazardous, nearly vertical precipices plunging sometimes as much as 4 feet.
Reimer says a number of factors, including agricultural practices and land development practices, over the years have contributed to the declining stability of the riverbanks and lakefronts in Kandiyohi County, MN. However, he says that lately landowners have begun removing their livestock from areas near sensitive streambanks and taking other measures to reduce the environmental damage to the waterways. Although he applauds these efforts, he doesn’t believe that these measures alone will be enough to stop, much less reverse, the erosion process already under way.
Equally concerned that the risk of bank failure due to erosion extends along local streams, Reimer says 150 feet on the shoreline of the North Fork of the Crow River and a 400-foot section of the Middle Fork of the Crow River are in immediate peril of collapsing into the waterway. In one instance, he notes, fencing that was erected several feet from the water’s edge now hangs precariously over the river; in other instances, relentless erosion has begun to expose the roots of 100-year-old oaks once well ensconced in healthy riparian woodland soil.
Although the loss of soil and land is troubling to the local community, Reimer says the consequences of continued erosion will doubtless show their effects downstream in the form of deteriorating water quality. Currently, according to Reimer, soil losses from the area average 20 tons per year, carrying sediments and nutrients such as phosphorus toward the receiving waters of the Mississippi and contributing to the ongoing degradation of this vital national resource.
Reimer says he is eagerly awaiting the opportunity to repair streambanks and shorelines in Kandiyohi County, but he wants to avoid using riprap or concrete. Instead he plans to use ShoreSox, and he says several lakefront landowners are waiting in line to try it out.Schaaf says the system has received good reviews for its performance in a number of scenarios.Mike O’Connell of Shoreline Services Co., who has experience using ShoreSox on lakefront properties in Florida, says the traditional solution for backyard erosion along the lakes in the upscale areas near Ormond Beach has for a long time consisted of building a wall of large stones called coquina rocks at the water’s edge. A very attractive light-colored stone, coquina rocks in earlier times “were what the Spanish used to build their forts,” he says.
But, he says, when the rocks are used for erosion control, they don’t quite work out, as the waves eventually wash the soil from between the rocks. With the loss of this foundation, “within 10 to 15 years they end up in the lake.” O’Connell says ShoreSox provide a viable alternative and help him solve three problems.
First, financially they are much more economical. He says protecting a bank with coquina rocks now runs in the range of $125 per linear foot, while ShoreSox can be installed for $50 per foot.
Second, unlike the rock, which degrades to a muddled gray brown after a few years, ShoreSox’s appearance can improve with age as it recruits native plants and integrates with the natural shoreline.
And third, O’Connell says the product helps him get around a big logistical issue. In contrast to working with stones 1 in foot diameter or larger, which require a utility vehicle to haul up to the bank, with ShoreSox he is able to carry in all of his material by hand, avoiding injury to some of the “very nice lawns” his customers maintain in the upscale lakefront communities.
Schaaf says he conceived ShoreSox to provide an erosion control alternative that uses “American-grown agricultural organics.” He says that in addition to eliminating the expense involved in long-distance freight for typical erosion control media, by using locally harvested organics, ShoreSox can also provide a new source of income for farmers. According to Schaaf, cornstalks that had previously been plowed under are now being harvested and processed to create the primary medium for ShoreSox erosion control systems-a feature Reimer, who hails from the farm belt, finds attractive. He says this will be his first attempt to use a living organic alternative to shore up lakefronts soils, and he expects to begin his first installation this fall.
A device bioengineered specifically for shoreline stabilization, ShoreSox “becomes part of the earth,” Schaaf says. “And when part of the shore bank gets properly planted with the proper native species-the proper aquatics-that once used to be there, those are the things that filter out the nitrates, the phosphates, the pollutants; that’s what nature does better than any of us. We’ve simply created the kind of a system that allows nature to heal itself.”
