Meeting the Velocity, Meeting the Volume
Channel-lining solutions can vary with the flow.
A group of property owners in the New Uri watershed, encompassing Florence, KY, began to complain to city officials about erosion from a 10-foot-high streambank that was taking a sharp turn in their neighborhood. The erosion was so significant that some property owners were losing their fences to cave-ins.
From a 2007 watershed study conducted by the engineering team of Viox & Viox, it was determined that a particular channel indeed demonstrated excessive erosion.
“We decided, in order to save the property owners’ property and stop the erosion, we needed to do a streambank stabilization project,” says Peter Glenn, project manager for the engineering and inspection division of the Florence Department of Public Works. A capital improvement project ensued, with work beginning in 2009 and the project finished in 2010.
|Photo: SINGHOFEN AND ASSOCIATES
|Photo: OGDEN DUNES WATER WORKS
Florence is one of a multitude of municipal entities that must decide how to best address the need to stabilize banks and prevent erosion—and head off further damage at the pass. Many channels serve to convey water or treat and infiltrate stormwater runoff.
For channels where flows have relatively slow and low volume, vegetation may suffice to slow the flow and prevent soil from washing away. Geosynthetic fabric may be used to line channels with greater flows and volume.
Some jobs call for hard-armor protection, such as lining the sides or bottom of the channel with concrete, although this doesn’t allow the water to infiltrate into the soil. The velocity of the water increases as it goes farther downstream, creating the potential for more erosion problems at the point where the concrete lining stops. In addition, sediment, nitrogen, phosphorus, and other pollutants from stormwater carried along with the flow cannot infiltrate into the soil or be mitigated by vegetation. In cases where hard-armor techniques are desired, articulated concrete blocks and gabions are often chosen.
In Florence, the city was addressing a hard “S” turn created when a development was laid out in the 1950s. To squeeze in a few extra lots, the developer created curves in the creek.
“Ever since then, the creek has been trying to go back to its normal alignment,” Glenn notes. “Those volumes of water can’t go through those hard turns without a lot of turbulence and erosion.”
Florence city officials met with property owners, obtained easements, and then considered options for making corrections.
“We looked at all kinds of different design ideas,” Glenn says. “Laying the embankments back to the point of the erosion would be difficult because we really didn’t have the space to do that with adjacent houses. We looked at pouring concrete walls, but aesthetically, it’s not too pleasing to have a concrete channel going through people’s yards.”
Rock gabion baskets were considered as an inexpensive alternative for channel lining, “but aesthetic-wise, that didn’t meet the city of Florence’s standards,” he says.
During its research, the city discovered an option from Redi-Rock, which manufactures precast concrete block for retaining walls. The city was able to choose various finishing options for the block.
Although Florence does not regulate aesthetics in such matters, Glenn says it is something he city takes into consideration.
“If we can do a project and make it aesthetically pleasing, we always try to take that extra step and find out if we can afford it, then we do that. The Redi-Rock product gave us that avenue,” he says. “The cost of a concrete wall and the cost of this concrete block system that Redi-Rock has are very similar, so it was very easy for the city of Florence not only to create that channelized rock wall but to do it at a cost that was equal to or maybe even less than conventional construction practices.”
Although the Redi-Rock product itself was not new, the Ledgestone rock face design Florence officials chose to match the limestone bedrock in the region was new.
“We were the first application of that particular rock face, so they had to make up a few blocks for us because they didn’t have any projects anywhere where they had used it,” says Glenn. “We went to the manufacturing facility to see if it was something we wanted to use, and it was.”
The walls were built to a height of 13.5 feet high, stacked like children’s building blocks using an excavator and a small crew, without the need for reinforcement. In total, 4,800 square feet were installed over two weeks’ time.
A reinforced concrete footer serves as the wall’s base, upon which two courses of 60-inch-deep Redi-Rock bottom blocks were installed, followed by 41-inch-deep Redi-Rock blocks for the remainder of the wall. One section featured a unique backfill design to compensate for the high water flow during storm events at the intersection of two streams.
A controlled-density fill was designed to reinforce the back of the wall so flow would go over the top. The wall was made impermeable to prevent erosion or scour.
To accommodate environmental permitting, the entire channel was constructed from the top of the stream banks, with contractor Dudley Construction Company cutting benches along the creek banks and Redi-Rock of Kentuckiana assembling the wall. Crews started at one end of the channel and backed their way out as they worked.
The rear of the storm channel is backfilled with crushed stone, allowing water to drain properly from behind the walls to prevent hydrostatic pressure buildup. An aluminum railing was constructed around the top perimeter to add an extra measure of safety for the residents.
Although the volume of the water was not affected by the project, the velocity was. The channel was streamlined as to facilitate the water’s movement so it would not end up surcharging an area. Additionally, the city took care not to disturb the channel bottom to keep it in its natural state.
Part of the project also entailed replacing the vegetation. “We spent tens of thousands of dollars planting trees and bushes,” Glenn notes.
Although the project is done, there are still issues downstream to be addressed through another capital improvement project at a city park in the same watershed. In that project, water quality will be addressed as well as quantity. Redi-Rock also will be used in that project to create a detention area, says Glenn.
“We’ve got more space on that property, so we can open the channel up quite a bit more and create a detention area where the water will actually pool and be detained,” he notes. “The longer we can detain it, the more opportunity there is for all of the particulates in the water to settle out as it drains through.”
Additionally, the detention pond will have a vegetated bottom to allow infiltration of the stormwater, thus reducing the quantity of water passing through it.
“That will enable people downstream from there to experience less flooding,” says Glenn. “We’ll create more of an elongated stormwater runoff area so if we get a flash flood type of event, they’ll see less of that and more of a steady stream of water for a longer period of time.”
Glenn says city officials are “extremely pleased” with the Redi-Rock product and the company’s ability to work with the city on providing the desired rock face while supplying everything in a timely matter.
“Dudley Construction did a fantastic job, and all of the homeowners were very happy,” he says.
Flooding has been an ongoing problem for residents of the Emory Court–Dupont Drive region of Tallahassee, FL.
The city in 2008 initiated a $6.2 million project that will include, among other measures, lining more than 1,500 feet of ditch with rock-filled gabion wire baskets from Maccaferri. The project is expected to be completed in June.
The city conducted an in-house stormwater study and brought in Singhofen & Associates, which specializes in stormwater retrofit projects.
“That area has been known to have a flooding problem for many, many years,” notes Robert Gaylord, a principal with the firm. “The city contacted us to help them develop a system that would work with the ditch they had there, and based on their modeling, they needed to make it wider in certain areas and make it function a little smoother. It was having a lot of erosion. It would have cave-ins from trees, which added to and compounded the flooding problem.”
Gaylord’s firm was given a minimum width to help reduce the flooding, but there were constraints because of yards, homes, buildings, and streets with utilities.
“Our space was limited. In the first part of the project, we evaluated a couple of different wall types. The gabion option was selected. We had a lot of success in Tallahassee with gabions and Reno mattresses,” he says.
Gabions were selected because they conform well to the channel and are similar to a gravity wall-type system, in that crews don’t have to do major excavation to put in tiebacks or reinforcement as they would for a mechanically stabilized earth wall, says Gaylord.
“You can basically can do a gravity wall system of up to about 15 to 18 feet,” he says. “Those work well. If the conditions change in the field, the gabions seem to flex pretty well with them. With concrete paving, if some erosion occurs, it’s going to crack and collapse. Tallahassee has had really good success with [gabions], so they were already in favor of it.”
In other areas of the project where there was no room to use gabions or tiebacks, the firm opted for sheet piling.
To install the gabions, Gaylord’s firm designed a U-shaped configuration, putting Reno mattresses at the bottom of the channel to the required width and extending the mattresses underneath the gabions, which were stacked to the required height.
“To check it, we’d cut sections at 5-foot intervals to see how it was fitting in there,” Gaylord says. “After that, it’s like building with Legos.”
The U-shaped configuration was chosen because of the soil type. “Typically what I run into in Tallahassee as with other places in Florida is clay soils,” says Gaylord. “A lot of times in Tallahassee, the ditches will stand up near vertical anyway, but when we line things, sliding becomes a factor. [The lining] tends to slide and it does tip—it’s a little different than other places. The U-shaped configuration helps protect the toe of the gabions and prevent the sliding factor. We ended up putting in a backfill behind the gabions to help relieve the hydrostatic pressure and to move the clay away a little bit.”
One of the biggest challenges of the projects was designing maintenance ramps, Gaylord notes. “We had to make sure the trucks had access, so we used a combination of gabions and sheet piles with some articulated block so they could drive on it,” he says. “We had to make sure the trucks could get in, and they had to back out because there was nowhere to turn them around. They basically had only one point of entry and one point of exit in this ditch.”
Gaylord says one lesson learned for future applications is to lay out the sheet pile for the structural engineer. With gabions, he notes, “Usually rock size is the biggest issue, and overfilling is where we have the biggest problem. Once you get that done, it’s a go.”
|Working in limited space, crews lined 1,500 feet of channel with gabions and Reno mattresses.
|Photo: SINGHOFEN AND ASSOCIATES
Protecting Bridge Pilings
Tidal current flows have been eroding the sand around the pilings of four bridges in Hillsborough County, FL. Inland Construction and Equipment of Panama City is using Cable Concrete, an articulated concrete block mattress from International Erosion Control Systems (IECS), to protect them.
“We’re placing aggregate material first for a leveling course, then placing the 4-inch-thick erosion control mats with geotextiles under them,” explains Sam Stone of Inland Construction. “Then they’re all bolted together in a standard situation.”
Some 60,000 square feet of the Cable Concrete is being used in the project.
“It was a government-specified product, and I chose IECS because they were competitively priced and worked with me on the size of mats that I wanted,” Stone says.
There project presented considerable challenges, he says. “We’re placing articulated block mats underneath 5 to 20 feet of water underneath a bridge that’s only 5 feet off the water. It’s not easy work by any means.” However, Inland Construction developed a method for placing the mats underneath the bridge.
“Basically, we have a barge that has a hole in the middle of it and a wench that wenches up and down underneath the barge. I place mats on the bottom of the bay with the crane, take my barge over the top of the mats, hook on to them, and raise them up to the bottom of the barge. The barge then goes underneath the bridge and lowers the mat down in place,” Stone says.
The project is expected to be completed in April.
Flooding at Ogden Dunes
Ogden Dunes, IN, is a picturesque town to the west of Indiana Dunes National Lakeshore. Also bordering the town is an area water manager James Kopp describes as a “long lake, more like a swamp, about a mile long and 3 to 4 feet deep with a varying width.”
At a narrow end is Ogden Dunes, where three years ago there was so much rain that 18 inches of water covered the road for four days. The town used sand dikes as a temporary structure to control the water.
“The Indiana Department of Environmental Management [IDEM] didn’t like that, so we had to come back and do something else,“ notes Kopp. ShoreSox—a biodegradable berm filled with organic material such as cornstalks—were chosen to deal with the problem.
“We put the ShoreSox along the edge and dug the edge out a little bit to give us a better water’s edge or barrier and put the socks in,” says Kopp, adding that the town also planted vegetation in the ShoreSox.
The installation process entailed using a backhoe to dig down 20 inches and put the ShoreSox along the banks. “We built it up with the material we took out to make it straight and give it some water depth on the other side,” says Kopp.
The cornstalk material in the ShoreSox helps absorb water and prevents it from channeling, allowing it to puddle and sink into the sand.
“After it rains, everything’s gone in about 20 minutes,” Kopp says.
In addition to the ShoreSox, the town planted wildflowers in the area, funded by a $10,000 grant.
The ShoreSox have held up well, notes Kopp. “We had to get something that IDEM would accept,” he says. “They had experience with this. It seemed like the best solution for our application.”
The state of Indiana prefers soft-armor solutions, adds Kopp. “It didn’t make sense to put in a hard wall,” he says. “This is a pond, so it would have been expensive to put in a wood wall, which does not maintain itself once it gets wet, and putting in piling didn’t make a lot of sense. We didn’t have the money to do it, either. This was an inexpensive, functional way of addressing the problem.”
Time is one of the best indicators of the success of a channel-lining project. A project in Houston, TX, involving Southwest Erosion Control’s Tri-Lock articulated concrete block systems has held its ground for 10 years.
|Photo: SINGHOFEN AND ASSOCIATES
The project remedied years of flooding.
|Photo: OGDEN DUNES WATER WORKS
Temporary sand dikes controlled the initial flooding.
The Test of Time
The work took place near a number of condominiums overhanging Spring Branch in northwest Houston area near Interstate 10. A 3:1 slope went down about 60 feet to the water.
The Harris County Flood Control District approved the installation of the 4015 Tri-Lock block, which is 6 inches thick. Each block covers 1.54 square feet and weighs approximately 70 pounds.
“We graded the area and laid a filter fabric, Mirafi 700X, and then we hand-laid the blocks in place,” says William Simmons, CEO of Southwest Erosion Control. It took a crew of seven to place about 2,200 feet of blocks a day for a total of 65,000 feet.
In the upstream area, workers rolled the block down to about 3 feet below grade and covered it with dirt, just as in the downstream area. The installation transitioned into some rock riprap the city of Houston had used under a bridge abutment.
“On the west side, where the condominiums were, we just went up under the condominiums to a concrete retaining wall and butted into that and then grouted that seam, about 12 inches wide, to connect the blocks to the concrete abutment,” says Simmons. “We backfilled with top oil, sealed it, and got vegetation growing on it.”
The area was about 50% developed at the time the project took place; since then, it’s become nearly 90% developed, Simmons notes.
“The volume and the flow have increased since the water is coming off parking lots, roofs, and streets,” says Simmons. “It flows a lot faster and there’s less ground to absorb the water, so we’re getting a lot more water in there now.”
The 6-inch block can hold up to 20 feet per second. When the project was installed, it was designed for 14 feet anticipating a 100-year-event.
Stormwater discharges into Spring Branch. “As opposed to concrete piles, this allows water to go back into the ground and allows you to revegetate, and it’s flexible,” says Simmons. “It doesn’t crack like concrete would, and it’s also cheaper than poured-in-place concrete. Another advantages is the aesthetics—you want it to blend in more with vegetation, and it doesn’t look so commercial.”
Regulations for vegetation are normally site-dependent, Simmons says. “If it’s back in a channel where nobody is going to see it, it’s not so much of an issue,” he says. “When it’s facing a highway or some high-dollar homes or a development where they really want it to look nice, it is.”
The Tri-Lock articulated concrete block is often chosen for the “dollar factor,” Simmons says. “It’s more economical than mattresses or poured-in-place concrete, plus the fact that you can revegetate,” he says.
If the site needs to be disturbed after the blocks are placed, such as to install new sewer line, he says, “You can take the blocks up, do your work, put the fabric back down and re-lay the blocks. Access is an advantage if you have to get back in there for something.”
Simmons says the only drawback of the block system is that it cannot serve as a retaining wall.
“The slope has to be stable, and we generally like to stay on a 2:1 slope, although we have laid steeper than that,” he says. “The steeper the grade, the more difficult it is to lay the blocks.”
|Photo: OGDEN DUNES WATER WORKS
On a stable slope, the blocks can be placed by hand in about 3 feet of water without having to use cofferdams, Simmons says.
“Deeper than that, we would make them up on mats and swing a crane to lay them in the water. Then we’d get out of the water and hand-place them to hold up the slope,” he adds.
The Houston project has held up so well that the Houston Flood Control District used the blocks again for a bayou-lining project, Simmons notes.
Carol Brzozowski specializes in topics related to stormwater and technology.