These two slope stabilization projects had the same goals: to protect the slopes from water-related erosion with engineering techniques and materials that made them look natural and remain maintenance-free for the long term. But the similarities end there.
One project restored a fragile stream and protects it from stormwater flow. The other protects a manmade pond carved into a ski hill from the slow rise and fall of the pond’s icecap.
The stream’s slopes are as shallow as possible. Inside the streambed, irregular features such as step pools, floodplain benches, and cross-vanes and J-hook vanes break up the slopes and slow or redirect the flow of stormwater.
Most of the materials are natural. The best management practices (BMPs) inside the streambed are all composed of rock, which allows water to infiltrate into the sides and the bottom.
On the banks, materials include coir fiber rolls, woven coir mattresses, and filter matting, which stabilize the soil and protect the native seeds, live stakes, shrubs, and trees that were planted. The materials will degrade within two or three years, as the vegetation matures and provides stabilization on its own.
The pond’s slopes are long, steep, and unbroken. The BMPs include three liners. The first two, a GCL (geosynthetic clay liner) and an HDPE liner, ensure that no water will leak out of the pond. The third, a polypropylene geotextile, protects the top liner from damage from the fill material above. Geoweb, a three-dimensional network of interconnected cells made of polyethylene, holds crushed rock in place on the slopes. All the BMPs are designed to last for many years.
Sullivan Branch Headwaters
Appearances can be deceptive.
The Sullivan Branch appears to be a pristine stream meandering through the coastal plain of Calvert County, MD. It joins larger streams that eventually empty into Chesapeake Bay, some 15 miles away.
No one would ever know that it had degraded for years as the area changed from rural to suburban and as the adjacent road, Solomons Island Road, which runs north to Baltimore, became Route 2/4, a busy four-lane divided highway.
“There was uncontrolled stormwater discharge from adjacent development and the roadway,” says Eileen Straughan, founder and owner of Straughan Environmental Inc. in Columbia, MD, which provides environmental planning, permitting, and low-impact site design services to government (federal, state, and municipal) and commercial clients.
“Stormwater flowed from the paved surfaces to a pipe, where it accelerated,” she says. “When it flowed to the stream channel it had such speed and power that it scoured the bed and banks, taking a lot of sediment downstream. The stormwater also contained all the pollutants you might expect to find coming from buildings, parking lots, and roadways.”
In the mid-2000s, the fortunes of Sullivan Branch changed. As mitigation for the impact of a further widening of the highway just north of Prince Frederick to six lanes, the Maryland State Highway Administration proposed to stop the erosion at the stream’s headwaters and restore the channel downstream.
In 2007, the engineering firm designing the highway, Wilson T. Ballard Co. of Baltimore, called in Straughan to develop design plans, construction specifications, and a cost estimate for the stream stabilization project.
Three streams form the headwaters of the main stem of Sullivan Branch, which flows for 10.4 miles through flat land and rolling hills until it joins East Branch Fishing Creek. The soil is sandy and highly erodible. Sixty-two percent of the 15.1-acre drainage area is impervious.
Straughan assessed the three tributaries as well as the main stem of Sullivan Branch and found erosion in all four streams, which was undermining existing outfalls and resulting in the loss of a large number of trees.
The upstream portion of Sullivan Branch was severely eroded and unstable, Straughan says. Although the downstream portion was moderately stable, sediment from upstream was impacting a wetland that provides habitat for rare, threatened, and endangered plant species. Some of the sediment was being transported as far as Chesapeake Bay.
The stormwater carried atmospheric deposits, especially nitrogen, and other pollutants from buildings, parking lots, and roads, including oil and grease as well as particulates from brake linings and paint from parking and roadway surfaces.
Past practices in land development, such as the way buildings, parking lots, and roadways were designed and constructed, led to the erosion, she says. The practice was generally to lop off the hilltops and fill in the valleys, creating level building pads and straight, safe roadways. This resulted in steep, unnatural, and often unstable slopes where development transitioned to the undisturbed landscape. In the case of Sullivan Branch, concentrated stormwater delivered to the top of the slope created dramatic channel erosion. Repairing the damage involved restoring not only the immediate area, but all the way downstream as well.
“Once you’re given an unnatural landform, it’s hard and expensive to mitigate the problems,” Straughan says. “The best strategy is to design development recognizing the stability of the natural undisturbed landforms of the region, and mimic those landforms while accommodating your development. This technique requires the designer to respect natural drainage divides and minimize mass grading that changes the natural drainage network. It also uses low-impact design strategies to manage stormwater from the buildings, parking, and roadways. These strategies capture stormwater from impervious surfaces, cleanse it with vegetation and filtration, and return it to the ground to replenish groundwater. The resulting groundwater enters the stream through springs and seeps all along the channel, rather than at concentrated outfalls at the top of slope.”
Straughan considered three alternatives to restore the stream. Grading back the existing banks would have required removing additional trees. Raising the channel invert to reconnect with the stream’s natural flood plain wasn’t feasible because of the highly erodible soil and the large amount of fill that would be needed.
The company went with the third alternative, which combined traditional drainage structures and stormwater management BMPs with stream restoration techniques and native riparian plantings.
Straughan Environmental oversaw the construction, which was carried out by general contractor Pessoa Construction Co. of Fairmount Heights, MD. Work began in January 2009 and was completed by the end of February.
“There was snow,” Straughan says, “but the idea was to get it done before the fish started to lay their eggs. The stream closure period starts March 15 in this area.”
At the top of the slope, runoff from the roadway enters an underground stormwater structure, where it slows, allowing some sediment to settle out. Cleaner water flows through two concrete storm drainpipes to the bottom of the slope. These pipes were placed in the eroded channels and the channels were backfilled. Stormwater empties into a large constructed forebay at the head of Sullivan Branch, where additional sediment settles.
Straughan used the Rosgen natural channel design approach, named for hydrologist David L. Rosgen of Colorado, to design Sullivan Branch. The principle is to use a natural, stable stream as a model for restoring the impacted one.
A stable stream was located in the same watershed, which shares the same land features and climate as Sullivan Branch, and its characteristics examined, such as its plan form, its riffle pool sequence, the depth of pools and riffles, and bed composition data, which provides insight into the way the stream transports water and sediment. The design of Sullivan Branch was based on these characteristics.
“Natural channel design can be very successful if it’s applied correctly,” Straughan says, “although sometimes it can be very hard to find a stable stream in the same watershed in an urban setting to use as a reliable reference.”
Before beginning work on Sullivan Branch, Pessoa used a 6-inch pump to divert the stream flow into a 550-foot flexible pipe around the construction area. Crews worked from downstream to upstream.
They kept most of the restored channel within the existing channel to preserve as many trees as possible. Still, they ran into a conflict implementing the design.
“One property owner wanted to avoid losing one more tree on his land,” Straughan says. “In order to preserve that one tree, we moved the channel, and it created a small problem. It affected stability in one of the constructed riffles, causing flow to split to either side of the riffle, rather than be uniform across the riffle.”
Pessoa graded the banks and reduced the slopes to between 2:1 and 3:1 wherever possible. Crews constructed a series of rock step pools along the steeper slopes to transition the channel to the existing streambed elevation, and multistage floodplain benches to allow water to spread out during flood events and to reduce shear and velocity. These benches will contain runoff from at least a two-year storm.
The existing materials in the streambed were too small to resist shear, so crews added larger channel bed materials. Inside the streambed, they built rock cross vanes to direct the force of the water to the center of the channel, and rock J-hook vanes to redirect high-velocity flows away from the outside curves of the banks, as well as riffle pools on the inside curves to allow water to run fast.
Pessoa added a small plunge pool to one of the tributaries to transition it to a gently sloping channel before it joins Sullivan Branch. Crews also filled and revegetated eroded gullies.
The crews installed the coir fiber rolls on the outsides of the curves and woven coir mattresses along the new floodplain benches to stabilize the banks and protect the seeds and the silky dogwood and black willow live stakes they planted. These are all native species and will help stabilize the stream as well as take up nutrients before they infiltrate into the soil. The coir will degrade in two to three years.
Pessoa also used a filter matting that allows quick growth of the native species that were planted as container plants. These include red chokeberry, American beech, American sycamore, river birch, southern arrowwood, and sweet pepperbush.
A third party began monitoring the site in the spring of 2009 and will continue through 2104.
“They indicate that the design is working,” Straughan says. “The upper reaches are looking really good. There was a decision to do nothing with one lower tributary, though, and it’s contributing some sediment to the downstream reach of the stabilization. I think this is a strategy we’re going to have to do time and time again to restore streams affected by past development. We’re getting better and better at it.”
Canyons Ski Resort
At Canyons Ski Resort in Park City, UT, skiers on the Orange Bubble Express chairlift can look down and see the pond where much of their snow comes from.
“We chose that site because it was one of the flattest places on the ridgeline,” says Ryan Bradley, a design engineer with NV5 (Nolte Vertical 5), who designed the pond. “We made adjustments to the slope to make sure we got the most out of the topography that we could. The goal was to hit as close to 20 million gallons of water as possible.”
Maximizing the area of the pond entailed constructing 2.5:1 slopes that are 30 feet long. Finding a material that would protect such long, steep slopes was just the beginning for Bradley. The material also had to resist scour from ice as the pond level rose and fell, allow wildlife that had climbed down to drink the water to climb back out, and be attractive, because the pond is not only visible to skiers but also a destination for off-season visitors to the resort.
The Geoweb slope protection system from Presto Geosystems, in Appleton, WI, satisfied all four requirements.
“Because of the steepness of the slopes, we worried about any protection being dragged down,” Bradley says. The Geoweb material has a tendon system that holds it securely from the top, so there’s no need for anchors that would penetrate the impermeable liners underneath. In addition, the tan color, which Bradley requested, blends in with the shades of the mountains.
Crews installed three liners under the Geoweb system: a geosynthetic clay liner, an HDPE liner, and a nonwoven geotextile.
The average snowfall in Park City is 365 inches, and the resort depends on supplementary water to make snow, although the amount varies from year to year. There are other reservoirs on the resort site, but this new pond ensures that there will be a sufficient supply, Bradley says.
To make snow, water drains through a screen that filters out fish and debris into a 20-inch subgrade pipe. The pipe leads to multiple pump systems that feed up to four snowmaking stations on the mountain. At the pump stations, the water is chilled and pressurized to up to 800 psi to atomize it. Now snow, it is sent to Techno Alpin snow guns around the mountain. Each gun produces a mountain of perfect powder, he says. A snowcat piles the snow in strategic locations and spreads it out in high-traffic areas on the lower mountain. There’s enough water in the pond to cover approximately 60 acres with snow 1 foot deep.
“We can typically drain the pond almost completely dry during the winter,” Bradley says. “We leave about a million gallons for fire suppression.” The lake fills through the same subgrade pipe as it empties. Both operations take about 10 days. “We will drain and refill the pond multiple times during one season,” he says.
An aeration system is being installed that will super-chill the water before it reaches the pump stations. Aeration also will keep the water moving in the pond so it doesn’t freeze.
NV5 designed and managed the construction of the pond. The company was formed when Nolte Associates Inc. and Vertical 5 Inc. combined in 2010. Its headquarters are in Hollywood, FL.
“We do pretty much everything for the resort,” Bradley says, “all the utilities, roads, site development, and even the new golf course.”
NV5 had considered creating the pond with an earthen dam, but the resulting high-hazard dam classification would have added extra cost to the project. The company’s investigation showed that the dam would have been able to handle a breach on one of the two sides, but not on the other, which was above a populated area.
“That’s why we had to go so deep, to the subgrade of the existing topography,” Bradley says.
Construction began in August 2009 and continued seven days a week, 10 to 12 hours a day, until the project was completed. “It took about four months,” he says. “Snow was our time constraint.”
Crews excavated to 30 feet below ground level, creating slopes that were both steep and long. Although they expected to hit a large amount of rock, they didn’t hit any, Bradley notes. “We were pleasantly surprised by that.”
They were able to use standard equipment for the excavation and simply smooth the sides with a roller. They used the excavated soil to improve the nearby ski trails.
The geotechnical investigation the previous spring had found groundwater in the bottom third of the proposed pond, Bradley says. NV5 wanted to ensure that there would be no uplift once the pond was lined, so crews installed a number of drainage systems, which serve as a leak-detection system in the summer.
The first of the three liners installed was Bentomat DN, from CETCO Lining Technologies in Arlington Heights, IL. This is a reinforced needle-punched GCL (geosynthetic clay liner) with a layer of sodium bentonite between two geomembranes. According to the CETCO Mountain States technical rep Joe Kaul from Kaul Corp. in Golden, CO, it has both high internal shear strength and excellent interface friction on both sides of the GCL and is ideal for steep slopes.
The Bentomat liner provides a layer of defense in case the 60-mil textured HDPE primary liner above it begins to leak and also provides puncture protection during construction.
“Pumping water up a mountain can be expensive,” Bradley says. “We’re being very careful not to lose any of it to infiltration.”
Crews also installed a Mirafi nonwoven polypropylene geotextile from TenCate to protect the HDPE liner from damage from the fill material above. This geotextile is specifically designed for subsurface drainage and soil separation.
Some resorts don’t cover the liners in their snowmaking ponds, Bradley says, but leaving them uncovered would have left the liners in this pond very prone to scour as the ice cap rises and falls. In addition, because the pond is out in the open and liners are very slippery, they’re also a potential hazard to animals and people.
The Geoweb system is made of polyethylene. It can be installed with anchors, but because NV5 didn’t want to puncture the liners, the tendon system was used. Crews dug a 3-foot trench around the circumference of the pond and lay PVC pipe in the bottom of it. With tendons threaded through the Geoweb sections at the design intervals, they then attached the tendons to the pipe and expanded the sections down the slopes of the pond. They buried the pipe, extended the liner system over the trench, and covered it with soil.
The Geoweb cells were filled with crushed rock that was mined from the site. To fill them, a tractor drove around the top and the bottom of the pond with the rock on a conveyor belt, also known as a Telebelt. A remote-control system directed and adjusted the flow of the rock into the cells, keeping all heavy equipment off the liner system.
“The project went really well,” Bradley says. “The whole thing really came together. Everyone’s very happy with it.” Not one drop of water has leaked, and the only real maintenance that needs to be done is feeding the fish. The aerator will keep the algae in the pond in check.
The pond has become an amenity for the resort, not just a reservoir for snowmaking built off to the side and hidden by fences, he says.
Once the vegetation is planted, one side of the pond will have native vegetation, such as aspens and native grasses that blend in with the environment. The other will have a more manicured look, with two docks and an outfitters’ cabin where visitors can rent fishing poles and paddle boats or perhaps host a mountaintop wedding. The pond already stocked with tiger trout. A hiking path circles it.
“This looks like a natural lake,” Bradley says. “It will become a destination that will be used year round.”
