Channel armoring can cover a wide range of applications.
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.
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| Photo: HARRIS COUNTY FLOOD CONTROL DISTRICT Harris County’s Clodine Ditch before work began |
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.
By David C. Richardson
http://www.erosioncontrol.com/november-december-2011/method-every-season.aspx
