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County pioneers stream bank stabilization method.

IN SPRING 1990, engineers at the Westchester County Department of Public Works were looking at ways to reduce excessive streambank erosion in Tibbetts Brook Park in Yonkers, New York. Their primary concerns were structural stability, esthetics, and durability, since the park can attract thousands of visitors on any given hot summer weekend.

At the time, Creative Habitat Corporation had just introduced the innovative Fiber-Schine System for hydro-bioengineering with herbaceous plants. In cooperation with the company, coauthors Hugh Greechan and Paul Summerfield developed a streambank stabilization plan, which included Fiber-Schine engineering modules for toe stabilization at strategic points.

Hydro-bioengineering

Hydro-bioengineering, like traditional civil engineering, tries to repair stream corridors by imitating nature. Instead of relying on hard or rough surfaces, bioengineering draws on the cohesive strength of root systems and the flexibility of leaves and branches. Consideration of stream flow characteristics will determine the scope of the overall project.

The Fiber-Schine System is based on the observation that natural streams can have very secure and tight embankments. Under those balanced conditions native plants will establish themselves along the embankment and form a strip of so-called "riparian wetlands." These streamside wetlands are either composed of woody plants, such as the familiar dogwoods, willows, and alders, or made up of grass, such as herbaceous plants and wildflowers. In either case, plants growing at the streamside naturally protect the embankment with their extensive root systems.

Unfortunately, naturally secure embankments are the ideal world of undisturbed nature. Urban and suburban landscapes simply do not reserve enough space for stormwater detention, retention, or storage. Large portions of land that previously would have handled groundwater recharge and would have retarded stormwater runoff have been paved over or built upon. If its carrying capacity gets to be too small, a normal stream continually enlarges itself to convey the increased flow rates caused by urbanization. This results in stream erosion problems on the banks, on the bed, and around the bridges. Today's urban and suburban streams functionally resemble intermittent desert streams, carrying little or no water under normal conditions and "flash-flooding" when rain does fall. The technical and ecological consequences are difficult to assess and often very costly to anticipate and/or repair.

In Tibbetts Brook there was no doubt that shrubs and trees would have easily achieved the erosion control desired. So would have a number of other solutions. However, the stream flows through the central lawn area of this busy public park. The open grassy embankments significantly contribute to the park's landscaped environment. It was therefore important to maintain the general character of the grounds.

Bank and bed erosion had scoured out an ugly channel, with some portions of the embankment dangerously steep and abrupt while other sections had become fragmented and were sloughing into the stream at regular intervals.

To alleviate the problem, the embankments were graded back, gently sloping toward the stream, and then seeded. This method was believed to be sufficiently stable for expected storm events. The only weak link was the toe of the embankment. Here the Fiber-Schine System was applied in strategic locations where the engineers expected erosion through undercutting.

The engineered component that gave the system its name--the Fiber-Schine--is a 20-ft long, tightly stuffed bag of coconut fibers 1 ft in diameter. It serves as a planting substrate for native vegetation and controls erosion while the plants establish themselves over a period of years. In the long run the coconut fiber will bio-degrade. With proper plant selection this will not happen until a firm "natural" embankment has grown to stabilize the toe of the slope securely.

In addition to the installation method referred to above, we also simply placed the Fiber-Schines in front of an existing eroding embankment. This was the case where erosion was moderate, the site less accessible, and where regrading would have caused considerable damage to existing shrubbery and trees. The modules now provide a new "front line" of herbaceous plants as an added erosion control and water quality improvement measure.

Good Land Stewardship

The areas were planted with grasses, sedges, and wild flowers suitable for the individual locations. The resulting embankment looks like a natural ribbon of flowering meadow traversing the park. In spring there are plenty of yellow and blue flag irises, in summer the pink swamp milkweed and in fall red cardinal flowers and blue lobelia.

In May 1991 the general contractor had finished his grading on schedule to begin the installation of Fiber-Schines. In close cooperation with the manufacturer, a crew of three "unskilled" laborers installed the modules by hand. They were then secured by wooden stakes (4 ft by 2 in. by 2 in.) placed on 2 1/2-ft centers. Stream flow was so low that a shallow placement ditch had to be dug. Behind the Fiber-Schines the embankment was backfilled, secured with jute mesh, and seeded. On sections without regrading, the bags were simply placed as close to the existing embankment as possible and backfilled with a well-graded stone where necessary. The plants were inserted after all other aspects of the project had been completed.

Already during its first season, the newly planted embankment with its grasses and flowers was hard to distinguish from a naturally grown slope. In its second year the plantings delighted with flowers as early as May. The stream overtops its new embankment frequently, rising slowly up the gentle slope. Even though the water has enormous power, there is no new undercutting nor are there additional deposits downstream. The results have been so convincing, it would be hard to tell that just a year earlier this site had been eroding severely.

Mr. Hoeger is with Creative Habitat Corporation, White Plains, New York. Mr. Greechan and Mr. Summerfield are engineers with the Westchester County Department of Public Works, Yonkers, New York.
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Title Annotation:Westchester County
Author:Hoeger, Sven; Greechan, Hugh; Summerfield, Paul
Publication:Public Works
Date:Jul 1, 1993
Words:962
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