It’s somewhat ironic: When one’s trying to remove soil from an object, it often requires surfactants, manual labor, and copious amounts of water. Yet, when loose soil is struck by rain, it hitches a ride and quickly follows the runoff wherever it may go. Hence the importance of capturing soil and sediment before it reaches drains or natural streams.
Although sediment movement can occur at any time, it’s most likely during soil disturbances, such as construction. In many projects, utilities, sewers, and storm drains are installed first, so it’s possible to install sediment controls early, which will prevent sedimentation from that point on. With those facts in mind, manufacturers offer many solutions for capturing sediment. Some, such as wattles and filter socks, allow sediment to be contained where it is, such as on dirt roadways. Other methods capture sediment right before it enters drains. In many cases, a variety of methods are used to keep the soil on the site; when it comes to achieving cleaner water, there may be no such thing as “too many” BMPs.
Grabbing Sediment Before “Point of Entry”
Georgia is known for its red clay soil. So, when working on the site of the new Northside Hospital in Cherokee County, crews installed inlet covers as soon as possible to keep that soil from the stormwater. “We installed Silt-Saver sediment barriers around storm drain pipes,” explains Matt Cato, project manager for Canton, GA’s HydroSpec Erosion Control Management. “We’ve been using them for the past eight or so years; most of the time we use Silt-Savers. Of course, it depends on what the plan calls for. If it calls for a specific thing, we have to use that-but if we can, we like to use Silt-Savers.” HydroSpec purchases its Silt-Saver products from Macon, GA, distributor GrowGreen.
The Silt-Savers are installed early in the construction process, before the surrounding drainage basin is stabilized. “They work very well; they’re easy to install, simple to maintain, and much more durable than other methods approved by the state of Georgia’s Soil and Water Conservation Department,” Cato says. The product consists of two pieces: a plastic frame, and a fabric sock or bag that fits snugly over the frame. “The HDPE plastic frames are reusable. The filter fabric, which fits over the frame, is a mesh material, similar to that of a silt fence. The shape of the frame allows complete coverage of the inlet, unlike other methods that may allow unfiltered runoff to enter if overwhelmed. You can choose them in standard- and high-flow-rate material, depending upon your needs.”
Once the device is installed, it is backfilled with dirt and gravel to hold the fabric securely, ensuring sediment doesn’t get into the drainage system. “You have to buy a size to fit the pipe,” Cato notes. “The fabric is mostly one size fits all, like a sock; you don’t have to cut it, just fold it over and pull it over the frame. Frames and socks are available in round and square shapes, and you can buy a custom fit, if needed. The dirt and gravel keep it weighted down, and then you compact your backfill so nothing washes under the pipe.” Unlike some other drain protectors, the Silt-Saver frames can be reused. “We keep the frames in inventory, and reuse them; usually we only lose one if it’s damaged in removal or transport.”
The sturdy frames make it possible for the sediment traps to withstand the length of the construction project. “Depending upon the construction schedule, these sediment barriers might be onsite from six to 12 months. If there’s excess sediment in the fabric that you can’t broom out, or much wear and tear from wind, rain, UV degradation, and heavy equipment, you might have to replace the covering fabric before the job is completed. However, these things are strong, and it’s rare that we have to replace them.”
Although Silt-Savers are a preferred product, HydroSpec employs other “weapons” in its fight against sediment and erosion. “We also use polymer applications, silt fences, and wattles, as well as most erosion control and grassing items called for on construction drawings. Silt-Savers are employed during the grading and excavation phase of the pipe, when things are the dirtiest.” Cato concludes.
A Great Lake Trip Against Sediment
As sailors can attest, the vastness of the Great Lakes-a total of 94,250 square miles-often makes them as unpredictable as an ocean, with strong tides and occasional gales. Of course, the force of this water can also cause erosion at the lakes’ shores. Lake Michigan, the third largest lake and the only one completely within US borders, constantly nibbles away at Michigan, Indiana, and Wisconsin. Just north of Milwaukee, WI, a recreational park was losing a little bit too much shoreline.
After the loose clay was removed, engineered soil, Filtrexx slope interrupters, compost blankets, and seed were installed.
“The original construction work on the park and 200-foot bluff was done by the Civilian Conservation Corps in the 1930s,” says Mike Marek, vice president and project manager with Milwaukee, WI’s Marek Landscaping. “It’s a recreational park and public beach for Shorewood, a village just north of Milwaukee. During July 2010, Shorewood received 4 inches of rain an hour, 7 inches in a 24-hour period, causing major flooding and damage throughout the village. The bluff had some minor erosion on it before this rain event, but after this storm, it needed work. The clay bluff was loose in several areas, and stiff to medium stiff in others. Below that, the 200-plus-foot-long beach is sandy to the water’s edge.”
Marek Landscaping was called in as a subcontractor to an earthwork contractor, which had stabilized the bluff by removing all loose soils from the site and excavating to clean, stiff clay. “With the exception of road construction materials, all slope stabilization materials were biological or biodegradable erosion control materials.” Marek says. “After clearing of loose soils, we placed a perforated drain tile, connected it to the road drainage system, and covered all with clear stone. Atop that, we placed a 4-inch layer of engineered soil, composed of half compost and half coarse sand. The compost was from a local supplier, who collects municipal wood and leaf and yard waste. Most of this material was placed on the site with pneumatic equipment. We then installed a 40-gram-per-square-meter biodegradable coir turf reinforcement mat, which was staked in with 8-inch wooden biodegradable stakes.”
The next step: installing slope interrupters from Grafton, OH’s Filtrexx International LLC. “We put in biodegradable, 8-inch Filtrexx FilterSoxx, secured with 2-by-2-inch hardwood stakes every 6 feet. The socks were filled with half compost and half filter media, a unique media for slope interrupters, which we typically want to show higher pass-through rates. We wanted to encourage some infiltration of water to the slope, to reduce runoff and to aid in plant growth. So we opted to restrict flow-through rate, compared to typical slope interrupter applications. The FilterSoxx were placed parallel to all the contours of the site, horizontally, every 8 vertical feet. We used laser levels to set them exactly on the contour.”
Two-inch compost blankets and seeding finished the installation. “We installed 42 species of native grasses and forbs native to Milwaukee county: tall grass prairie species, Indian grass, Canadian wild rye, switchgrass, big bluestem, and little bluestem, as well as native forbs. We sowed a little more than 25 pounds per acre-a heavy application, when you think of the actual seed count. We also used a multispecies annual cover crop. Except for the cover crop, which was installed during the hydromulching, all seed was hand cast. We weren’t relying on hydromulch for erosion control; we were for using it for maintaining seed moisture.
Gator Guard placement along roadsides helped keep
the road materials in place.
“The seed species list was selected not only for its stabilization performance, but also to help to support butterflies and insects,” Marek goes on. “This site is on the Great Lakes flyway, a major migratory bird route, and caterpillars are protein to fuel the migration. The butterflies and insects help pollinate the plants, feed the migration, and grow more plants, stabilizing, beautifying, and diversifying the ecology of the lakefront park. As for stabilization, these native plants are nearly drought-proof, and have hardy root systems that grow 10 to 15 feet into the soils, improving the soil’s strength, resilience, and infiltration rates, while bringing water balance to the site.”
Work on the park at Shorewood began in early summer 2011 and was completed by the end of September. “Several major storms during that period exposed further weaknesses in underlying soils, which hadn’t been observed in the soil borings. We cleaned them back to a greater extent than originally planned. Thus far, there have been no problems at the slope’s toe from Lake Michigan’s tide wash seiche.”
Some might think the work was over-engineered; Marek disagrees. “One big advantage of this “˜two belts and suspenders’ approach: yes, the additional redundant layers added costs to the erosion control solution, but when we compare our seed bed prep costs, topsoil import costs, spreading limitations, and the weight of standard topsoil when saturated, it makes sense. We’re eliminating having to put seedbed prep equipment on a dangerously steep slope by using engineered soils and compost. Also, the rapid establishment of cover crop and native seed eliminated the need for erosion repairs after construction.”
During the project, Marek saw evidence that the plan was working. “Even before the hydromulching, the site experienced a 2-inch-per-hour rainstorm, and with the compost blanket, there was zero sediment wash-on a 2:1 slope! The work held up miraculously.” He’s also quite satisfied with the Filtrexx products. “We’ve been using them for the past eight years, so we have experience with most of the Filtrexx product line. Their biodegradability was a big point in our choice, along with their years of proven analytical data. Filtrexx holds back sediment and doesn’t fall down. We had the right flow-through rate for the site because of their rigorous testing, and their design process came up with specific sock materials and weaves for our specific application.”
The steeper the slope, the closer wattles were placed.
Slow Water, and Divert Its Flow-Sediment Contained
Water travels downhill, so any site not tabletop-flat is an erosion control challenge from the outset. Idaho’s private, 130-acre The North Ridge at Terrace Lakes Subdivision, part of a 1,200-acre overall development, needed road realignment, a large fill, construction of new roadways, and installation of new waterline and hydrants. Such improvements were designed to entice investors to purchase building lots for private cabins and getaways. Stubbing utilities to each secluded building lot made each lot more inviting for purchasers. Nampa, ID’s Syman Co. was hired for this project to prepare a stormwater pollution prevention plan (SWPPP), implement all necessary BMPs, and perform weekly inspections as directed by the owner. The BMPs included silt fence, straw wattle, Gator Guard wattles, straw bale check dams, slope drains, flow diversions, rock check dams, and settling ponds to collect runoff from the new and existing roadways.
“Work began in July, 2011 and continued through 2012,” says Syman project manager Brad Simpson. “The initial work was the road realignment and a large culvert installation with a equally large fill to accommodate the new realignment. Once the road was realigned, Gator Guard was used along the edge of the roadway to divert water away from the slopes and to slope drains or burrow ditches that contained either rock or straw wattle check dams. Gator Guard wattle was also installed diagonally across the roadways at 20- to 30-foot intervals, to prevent the road from riling or rutting. Gator Guard was used because it’s designed for traffic areas; it will pop back up. Water doesn’t pass through; it instead diverts the water in the direction we wanted it to go. Roughly 2,500 linear feet was installed, and it’s still in place and functional.
Placing Gator Guard at intervals slowed the water flow, reducing erosion and sediment washing off the surface of the narrow mountain road.
“The new dirt roads contained decomposed granite, and as we were going into winter snow and rainy season, we diverted water off the roadway to drainage areas,” Simpson goes on. “Placing Gator Guard at intervals slowed the water flow, reducing erosion and sediment washing off the surface of the narrow mountain road. The road was on a gradual slope; however, when it got steeper, we shortened the space between the wattles.”If it’s needed, maintenance on the foam-filled, lightweight Gator Guard wattle is simple, he notes. “We’ve had great success using Gator Guard wattle on this project, due to its toughness-that’s why we’ve used the product on multiple projects over the six years we’ve been in business. We’re often able to reuse the wattles as long as they are still in good condition. And they can be repaired; if a wattle is torn, you can use a silicone adhesive to repair it. If it’s been damaged beyond repair, it can be removed and reinstalled relatively quickly and easily. If we have to dispose of the Gator Guard wattle, we use an onsite dumpster and notify the disposal company to make sure they have no special requirements for handling. Several of our clients use Gator Guard wattles because they can be reused, and they’re not biodegradable, so wattles hold up for extended periods of time without breaking down and falling apart.