Generations of agricultural production on the eastern end of Henrico County, Va., left 182 acres of land in undesirable conditions. Common agricultural practices such as livestock grazing and tiling impacted the waterways, riparian buffers, and wetlands located on the site. These methods, along with the ongoing channelization and straightening of streams, severely impacted the quality of water and wildlife in areas of the county.
With pressure mounting from residential and commercial builders to develop a residential subdivision within the watershed, a stream mitigation bank was needed to offset the future impacts to these areas and provide a sustainable solution for what is to become an urban stream system.
Stream mitigation banks are used to compensate for adverse impacts to streams and associated riparian buffers. The U.S. Army Corp of Engineers, along with other state and federal agencies, approve stream mitigation banks to provide compensation for those impacts as established under Section 404 of the Clean Water Act. These agencies approve a specific number of stream credits available to be sold by the stream mitigation bank sponsor based on the restoration effort involved. Stream credits may be based on linear feet or on units determined by stream functional assessment.
Stream mitigation banks often require very little in the way of erosion and sediment control measures. The low-impact construction typically only requires silt fencing, a clean water pump around, and safety fence to limit construction access. These projects are, in themselves, stormwater management projects and therefore typical erosion and sediment control measures (i.e., ponds) are not needed for the projects.
Timmons Group was hired to design a $1.5 million sustainable development project to restore the streams within the James River watershed to their natural state – prior to agricultural disturbance – ultimately, to improve the quality of water and wildlife in the area. In addition, the project would provide Henrico County with compensatory mitigation options.
However, ensuring a sustainable outcome was no easy task. "Restoring a stream to its natural condition provides a set of unique challenges," said Michael Elander, P.E., a Timmons Group engineer and one of three project designers. "The biggest challenge with this project was its sheer size."
The site required hydrologic and hydraulic analysis – including but not limited to flow and velocity calculations at varying storm intervals – of more than 640 acres of drainage area and 10,000 linear feet of stream, consisting of 11 unique stream reaches and eight confluences. Furthermore, the natural streams provided their own set of hurdles. Unlike traditional stormwater design, which uses armored channels and oversized conveyances to avoid flooding, natural channel design seeks to match proposed channel parameters (such as width, depth, slope, cross-sectional area, and floodplain – 54 unique parameters in all) to a known stable reference condition to intentionally increase floodplain access. Frequent flooding is a desirable way to dissipate the power of the stream and reduce instability, thus providing a form of stormwater management by attenuating flows in the adjacent floodplains but still allow natural transport of sediment. However, each of the 11 stream reaches involved in Henrico County had unique design parameters; the stream changes size and shape constantly as it cuts through the land, causing increased complexity for implementing a natural channel design. "This can prove to be a daunting task without the proper tools," Elander said.
The complexity of this project required seamless collaboration and coordination among several stakeholders to achieve a design plan that was both feasible and economical. "As with any stream restoration project, the permitting process was challenging," said Rebecca Napier, P.E., the project manager for the Varina Stream Mitigation Bank. "Even though everyone agrees that these types of restoration projects are beneficial to our watersheds, much coordination is needed between federal, state, and local agencies."
In addition to collaborating with several government agencies, Timmons Group needed to demonstrate the benefits and expected impact of this project to adjacent landowners. In fact, Napier said that selling the project's vision was pivotal to the project's success.
Achieving seamless coordination and accurately representing the complexity of this project to all stakeholders – both internal project managers and external agencies reviewing the project – required dynamic software that could update on the fly, provide a clear and accurate workflow, and visually bring to life the project's goals. To achieve these objectives, Timmons Group leveraged the design, modeling, and visualization capabilities within the Autodesk Design Suite.
"Streams represent a complex geometry that is extremely difficult to represent by hand or with 2D tools," said Ken Hoen, senior designer at Timmons Group. "Autodesk's Design Suite gave us the tools to run with our ideas and make the streams appear exactly as we wanted."
Developing a conceptual design was one of the first stages in the project. Along with performing a standard topographic/existing conditions survey, a multidisciplinary team of environmental scientists, designers, and surveyors specializing in natural stream channel design collected a series of geomorphology data using the Rosgen Stream Classification system.
While survey and data collection were being executed in the field, the design team started its conceptual design coordination. Traditionally, the survey file would be brought back to the office, processed, and then handed off to the engineers for conceptual design plans. On this project, however, the design team was under a tight timeline to produce conceptual stream alignments, and it needed a quick and efficient way to convey the concept to others.
By using Autodesk Infrastructure Modeler, the team was able to compile data quickly from many different sources to generate a realistic 3D model of the existing conditions of the stream and surrounding areas. Through the use of custom styles, the portions of the stream channels that required improvement could be identified easily. The conceptual design could then be effectively conveyed internally for design coordination.
After the survey and geomorphology data had been compiled, the team used AutoCAD Civil3D as a building information modeling (BIM) tool to design the Varina Stream Mitigation Bank project. Using a combination of alignments, profiles, assemblies, cross-sections, corridors, feature lines, and surfaces enabled the team to create a final 3D model that not only was functional but also could be handed off easily to contractors for accurate bidding and construction.
The complex design of these natural stream systems required the models to be accurate within inches. If the final design was not constructed within a tight tolerance of the proposed design, the project as a whole had the potential to fail.
The parametric change engine in AutoCAD Civil3D instantly reflects changes made in one place throughout the entire model, so the team had tremendous confidence that the plans represented the complicated models accurately. Elander said the transition between design, plan production, and construction was virtually seamless using this software. "Civil 3D was a great tool for us to use with this project," he explained. "It really allowed the design team to model the tight corridors necessary in these complicated streams without worrying if the software could keep up."
One of the most difficult challenges to overcome during a design is when changes are required. In a 2D world, this hurdle can significantly increase the design timeline. Without 3D modeling capabilities, modifying an alignment of a stream reach or tweaking elevations would require a new project from scratch.
Fortunately, model-based workflows have made complete redesigns a thing of the past. Design teams can react to changes much faster and manage several design iterations. This was crucial for the Varina project because small elevation changes were constantly taking place in the stream. "It was extremely reassuring, as a designer, to be able to put as much complexity into the 3D model as I needed," Elander said.
Helping the client and reviewing agencies visualize the intended design was the next challenge. A set of construction plans – a 2D representation of the 3D model – can be cumbersome to understand. Believing that a 3D visualization would help illustrate its vision for the mitigated streams, the project team leveraged Autodesk 3ds Max to create a rendered picture of the intended design based on the 3D model created with AutoCAD Civil3D. "These rendered images open the communication doors between designer, client, landowners, and reviewing agencies," Napier said. "The team produced a video as well as several still images of the project to be utilized."
By using effective software tools, the Timmons Group was able to work more efficiently and produce a superior project design. This translated into scrapping the residential subdivision and restoring the Varina Stream Mitigation Bank to its natural state.
"Ultimately, it was the ability to conceptualize, model, and visualize the project before construction that resulted in happy clients and a distinct advantage to us in a very competitive marketplace," Elander said.
|Streams show signs of degradation at earliest stages of urban development|
The loss of sensitive species in streams begins to occur at the initial stages of urban development, according to a recent study by the U.S. Geological Survey (USGS). The study found that streams are more sensitive to development than previously understood.
Contaminants, habitat destruction, and increasing stream flow flashiness resulting from urban development can degrade stream ecosystems and cause degradation downstream with adverse effects on biological communities and on economically valuable resources, such as fisheries and tourism. For example, by the time urban development had approached 20 percent in watersheds in the New England area, the aquatic invertebrate community had undergone a change in species composition of about 25 percent.
Multiple streams in nine metropolitan areas – Atlanta; Birmingham, Ala.; Boston; Dallas; Denver; Milwaukee; Portland, Ore.; Raleigh, N.C.; and Salt Lake City – were sampled to assess the effects of urban development on stream ecosystems.
The study found that the effects of urbanization on the biological community vary geographically depending on the predominant land cover and the health of the community prior to urban development. In the study, the greatest loss of sensitive species occurred in Boston, Portland, Salt Lake City, Birmingham, Atlanta, and Raleigh metropolitan areas, where the predominant land cover was forested prior to urban development. The smallest loss of sensitive species occurred in Denver, Dallas, and Milwaukee metropolitan areas where land cover was primarily agriculture before urban development.
"The reason for this difference was not because biological communities in the Denver, Dallas, and Milwaukee areas are more resilient to stressors from urban development, but because the biological communities had already lost sensitive species to stressors from pre-urban agricultural land use activities," said Gerard McMahon, Ph.D., lead scientist on the study.
This article is based on the feature, "Restoring Varina's Streams" by Mike Aust, Timmons Group, that appeared in the November 2012 issue of Point of Beginning.
Mike Aust is a project engineer with Timmons Group (www.timmons.com).