Beyond riprap: Designs on green erosion control

January 2013 » Features » PROJECT CASE STUDY
Stormwater pipe outlet channels highlight easily constructible, maintainable, and aesthetically pleasing erosion and sediment controls.
Benjamin S. Craddock, P.E.
Dry-stacked block vanes were designed with elevation drops that dissipated the force of the flowing water. A 10-foot portion of the swales directly downstream of the vanes was anchored with riprap to attenuate the impact of the water cascading over the vane.

For many designers, erosion and sediment (E&S) pollution control historically has been treated as an afterthought to the design process. With so many considerations driving a project–the client's vision and budget, the design budget, schedule, municipal approvals, and state and federal permitting – there often is little time, money, or energy left over for innovative E&S control design. Sometimes the designer will keep E&S controls in the forefront in creating the site layout, but more often than not, any thought regarding E&S design is left to the end, after the "real" design is complete. At this point, the E&S control designer is left trying to figure out how to "make it work" with the already-finalized site design.

Project
Stormwater pipe outlet erosion, Loganville, Pa.

Civil engineer

C.S. Davidson Inc.

Project summary
A combination of grass-lined swales and dry-stacked block vanes minimized structural controls and maintenance requirements for stormwater pipe outlet channels.

In trying to make it work, the E&S control designer consults the standard checklist and chooses the options that will function adequately, gain approval, and result in the least amount of design effort possible. Construction cost is a consideration, so the cheapest option always tops the list. As most E&S controls are temporary in nature, aesthetics are rarely a consideration; even the most hideous E&S control facility will only be visible for six to 12 months. Maintenance issues are also low on the list of priorities; after all, it will be the contractor's responsibility to maintain these controls. These are the temptations that confront each E&S control designer at some point in every project.

This way of thinking is too outdated to continue to have a place in our designs. Many state regulations are now explicit in assigning responsibility for implementing and sustaining functional E&S controls to not just the contractor, but also the owner and design engineer. In Pennsylvania, the design engineer is required to inspect and certify the controls were installed and function as designed, significantly elevating the importance of designing easily constructible and maintainable facilities. More stringent state and federal regulations regarding surface water quality are resulting in E&S controls being installed as permanent facilities, thereby increasing the desire to make them aesthetically pleasing for owners and the public. As development increases, the number of "easy" sites for developing continues to dwindle and the designer often is challenged by steep slopes, unsuitable soil, and limited site area, meaning that standard details may be too standard for many sites that designers encounter today.

Roadway improvements led to a high concentration of stormwater discharges downstream at greater velocities, causing severe erosion at the outlets from two recently installed pipe networks.

The project
C.S. Davidson Inc., an employee-owned civil engineering firm located in central Pennsylvania and northeastern New Jersey, went beyond the standard approach on a recent project. Serious erosion was occurring on property in Loganville, a rural borough in southern Pennsylvania. The area was a mix of residential and agricultural uses with lawn, pasture, and cultivated soil cover. Steep slopes contributed to soil and sediment being quickly washed away due to stormwater pipe discharges, cutting channels into the soil more than 4 feet deep.

In the mid 2000s, the Pennsylvania Department of Transportation launched a major road resurfacing, curbing, and stormwater improvements project to State Route 3001, the main artery through Loganville. As is sometimes the case, improvements in one area can lead to problems in a downstream area, and the higher concentration of stormwater flow, travelling at higher velocities, resulted in severe erosion appearing at the outlets from two recently installed pipe networks.

In addition to erosion issues, the formation of these channels also presented safety concerns and access problems for affected property owners. These eroded channels created sharp drop offs that made it impossible to navigate agricultural equipment, effectively prohibiting access to nearby pasture and cultivated fields. Residents were understandably concerned about property damage. Loganville Borough decided to act quickly to address these concerns and to comply with state and federal water quality regulations, including the Chesapeake Bay Initiative.

C.S. Davidson identified and pursued a Growing Greener II grant for constructing a solution to the problem. Growing Greener II is a Pennsylvania grant program that invests $625 million to clean up waterways; protect natural areas, open spaces, and farms; and shore up key programs to improve quality of life across the state. Loganville Borough received necessary grant funds and authorized C.S. Davidson to design a permanent solution.

The process
The typical approach to a stormwater channel with this type of erosion would be to design an adequately sized swale based on the expected design flows, consult a standard nomograph for rock lining, determine the largest rock size that would satisfactorily prevent erosion, and provide a standard detail for construction. However, C.S. Davidson saw this as an opportunity to design the entire project with erosion control and stormwater best management practices in mind from the beginning. To maximize water quality filtration and groundwater recharge opportunities while respecting the residential feel of the neighborhood, the designer wanted to keep as much of the drainage area in an open grass condition as possible. Since borough residents would be using and maintaining the constructed facility, ease of maintenance was an important consideration in the design process. Keeping in mind that this E&S control facility would be a permanent feature located in a residential area, the designer also wanted to avoid the industrial appearance that a dry streambed filled with large rock would provide.

Stormwater channels in the most severely eroded areas were realigned to allow most of the construction to occur over more stable virgin soil, reducing the likelihood of future problems while avoiding unnecessary structural controls.
Flatter channel sections upstream and downstream of the vanes allowed stabilization with grass vegetation.

C.S. Davidson consulted with the York County Conservation District to receive input on various possibilities the designer was considering. Eschewing a one-size-fits-all mentality, the firm decided to apply numerous methods of erosion control to turn the vision of the Borough, the residents, and the Conservation District into reality.

The design
Stabilizing the upper reaches of both channels proved difficult because of the steep slope of the topography surrounding both properties and the high velocity of the stormwater that was concentrated at the pipe outlets. Placing new soil directly back into the existing channels could have been problematic since it was determined that the silty soil type would not provide a solid base. These fill materials could become unstable if further structural controls were not placed to mitigate the erosive condition. Realignment was performed for the most severely eroded areas to allow the bulk of construction to occur over virgin soil, reducing the likelihood of future problems while avoiding unnecessary structural controls.

For these steep areas, a combination of grass-lined swales and dry-stacked rock vanes were installed to provide a channel with stepped elevations at the vanes. The vanes were designed with elevation drops that dissipated the force of the flowing water, as well as provided for flatter channel sections upstream and downstream of the vanes that could be stabilized with mere grass vegetation. The rock vanes incorporated a 36-inch-wide by 24-inch-high weir that was designed to convey the torrent of stormwater that was released during low-frequency storm events. Elevation drops ranged from 18 inches to more than 5 feet from the weir elevation to the swale bottom below. A 10-foot portion of the swales directly downstream of the rock vanes was anchored with R-5 riprap to attenuate the impact of the water cascading over the vane; much less rock than if the entire portion of swale had been rock-lined.

The rock vanes were constructed of a locally produced green block call KSTONE. This block is produced by Kinsley Materials of unused concrete left over from other projects, resulting in the economical and environmentally beneficial reuse of a material that would otherwise be discarded. The block design was much more constructible than a typical rock-lined swale, according to contractor, Tim Smith of Fitz & Smith. "The KSTONE worked much better than stone. It saved us trips to the quarry and helped installation time."

Upon initial inspection of the nature of the high stormwater flows, soil conditions, and steep slopes at this site, a standard, off-the-shelf remedy would likely have resulted in a fully structural facility, leaving no place for native vegetation. Instead, storm pipe was installed in select areas to allow access to all areas for agricultural and maintenance vehicles, along with 550 feet of grass-lined channel. Several typical E&S control methods such as temporary erosion control blanket and riprap aprons at pipe outlets were used as well. These features were proposed, not because they were the easiest options, but because they were the most appropriate for the intended application.

The result was not just a fully functioning erosion control and soil stabilization facility, but an eco-friendly, sustainable drainage system that was also safe and maintainable. The stepped swale provides the added benefit of being visually pleasing.

The use of unconventional methods allowed this project not only to fulfill the vision of each stakeholder, but also to produce a project that came in under the original budget. All construction expenses were fully covered by the procured Growing Greener II grant.

By going beyond the usual and through cooperation with the Borough, the residents, and the York County Conservation District, the design team came up with an innovative and attractive solution. A problem was not just solved, but it was solved in both a fiscally and environmentally responsible manner.

Benjamin S. Craddock, P.E., is a project manager and Derek J. Rinaldo is a project designer at C.S. Davidson Inc., an employee-owned civil engineering firm in central Pennsylvania and northeastern New Jersey.

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