About the author: Adams is the project's lead storm water technical engineer for the Atkinson/CH2M Hill team. He can be reached at 425/405-1742, or by e-mail at [email protected]. Caley is senior storm water engineer for CH2M Hill's Bellevue office. He can be reached at 425/453-3567, or by e-mail at [email protected].
W. Reid Adams & Guy T. Caley
undefinedThe Washington State Department of Transportation (WSDOT) is in the process of completing its first major design-build (D-B) project. The $220 million Everett high-occupancy vehicle (HOV) project will widen approximately 6 miles of I-5 through Everett, Wash., one of the fastest growing communities in the greater Puget Sound area north of Seattle. The project will add a new HOV lane in the northbound and southbound directions, improve access ramps and traffic control and help fix one of the state’s most notorious bottlenecks.
An environmental benefit of the project is WSDOT’s commitment to environmental agencies and interest groups to completely retrofit the storm water drainage and collection system and provide storm water treatment for all the new and replaced pavement surfaces within the project limits (approximately 175 acres). Storm water management in general provided a significant challenge to the D-B team because WSDOT is committed to making sure each storm water treatment facility meets or exceeds WSDOT requirements, as well as local commitments made during the recent environmental assessment (EA) and finding of no significant impact process.
One of these challenges was to provide “enhanced” storm water treatment to facilitate the removal of toxic metals from storm water runoff. The D-B team proposed to accomplish this using several types of two-stage innovative treatment trains: pretreatment using hydrodynamic separators and post-treatment using enhanced-design biofiltration swales, mixed-bed media sand filters and constructed wetlands. Biofiltration swales and filters are specified primarily for small drainage areas and areas with limited space in the right-of-way. Storm water wetlands require a larger footprint, and in some cases, additional property was needed to construct these types of facilities.
Background
I-5 is a nationally recognized north-south transportation corridor that runs from the Mexican border to the Canadian border. In Washington state, it extends along the western portion of the state as the primary north-south transportation corridor, linking the major metropolitan areas of Portland, Ore., Seattle and Vancouver, British Colombia. Tacoma, to the south of Seattle, and Everett, to the north, are two of the largest metropolitan areas that surround the city of Seattle. A major goal of the state’s transportation plan is to have a continuous HOV corridor along this route, linking Tacoma to Seattle to Everett and improving mass transit and other transportation needs of the greater Puget Sound area.
The state legislature gave the project an accelerated timeline so that by 2008, project completion would coincide with local mass-transit improvements, the Boeing 787 Dreamliner Program (based in Everett) and the 2010 Vancouver Olympics. Recognizing that the project was to be a fast-track design, WSDOT decided the most efficient way to complete construction was to use the D-B process, which has been shown to save time by allowing the designers and contractors to start construction on early phases of work while designing later stages of work.
WSDOT selected the Atkinson Construction Co. and CH2M Hill joint-venture team through the competitive bid process, and work began in May 2005. The majority of construction is scheduled to be completed by mid-2008, more than three years sooner than originally anticipated.
Project benefits
Adding an HOV lane through the city of Everett creates a continuous HOV lane from Seattle to Everett and closes a gap in the I-5 corridor north of Seattle. Several other key benefits include:
- Provides long-term mobility by implementing flexible HOV requirements—adjusts the eligibility requirements from HOV 2 to HOV 3 or 4 as traffic conditions arise;
- Improves transit connections to and from the city of Everett’s multimodal transit station—provides a link for carpools, vanpools, local transit and regional express buses to and from I-5. The station also provides a connection with the regional transit authority (Sound Transit) express commuter trains and Amtrak intercity trains;
- Improves traffic flow—new/revised ramp configurations provide for smoother exiting and entering, reducing backup and decreasing congestion;
- Reduces noise pollution—provides sound barriers along portions of the interstate; and
- Reduces highway pollutants and erosion in nearby receiving waters and sensitive areas (Snohomish River, some tributary creeks and wetlands)—completely retrofits the I-5 storm water collection and conveyance system and provides storm water treatment for highway surface runoff.
Regulatory requirements
WSDOT and the Washington State Department of Ecology have jointly developed roadway redevelopment standards to ensure that projects meet the requirements of the Clean Water Act (CWA). The National Pollutant Discharge Elimination System (NPDES) permitting plan requires implementation of standards to carry out the intent of the CWA. WSDOT has a municipal NPDES Phase 1 permit that effectively requires it to mitigate impacts to water resources for its improvement projects. The redevelopment standards and lists of approved storm water treatment best management practices (BMPs) are incorporated into WSDOT’s 2004 Highway Runoff Manual (HRM). The design standards in the HRM are based on Ecology’s statewide storm water technical guidance. The I-5 HOV project follows the design standards in the 2004 HRM.
Storm water improvements
The existing highway system’s storm water collection and conveyance facilities were installed during the original highway construction and do not meet current roadway development standards. In the southern one-third section of the project, storm water flows via cross-culverts and shallow drains to small streams and channels that eventually reach the Snohomish River. The storm water that flows to these streams carries untreated highway pollutants to adjacent surface waters. In the northern two-thirds section of the project, storm water is collected in large storm drains flowing along I-5, and then is conveyed untreated to the Snohomish River in several separate WSDOT outfalls.
To provide storm water treatment for the existing and proposed highway surfaces in the project area, storm water will be conveyed to centralized storm water treatment facilities and discharged to the Snohomish River through existing outfalls. This design presented a challenge to the D-B team because the space on which to construct wetlands was limited without the purchase of expensive properties. Also, constructing several dedicated facilities to treat all the storm water would have required constructing new storm drains parallel to I-5 that flow with the longitudinal gradient of the road surfaces to direct drainage to the facility(s).
The Atkinson/CH2M Hill D-B team proposed to modify WSDOT’s conceptual plans by separating the conveyance/treatment schemes into two separate sections, each with a different design concept. Along the northern section of the project area, the proposed conveyance/treatment concept is to use as much of the existing storm drain system as possible. New outfalls were not proposed because they would require additional permits by Ecology, significantly extending the project schedule. Because surrounding land in the northern section of the project was mostly developed, purchasing additional property would have added significant costs, with limited available properties adjacent to I-5.
The D-B team proposed to optimize BMPs described in the HRM, such as using mixed-media sand filters, smaller storm water treatment wetlands and enhanced biofiltration swales. Where required, pretreatment for the filters and wetlands will be accomplished using manufactured (and Ecology-approved) hydrodynamic separators. The D-B team proposed three mixed media sand filters and one small constructed wetland with hydrodynamic separators for pretreatment for the I-5 mainline in the north section, and five enhanced bioswales were selected for ramps and other smaller drainage areas where space is limited.
Storm water wetland & city park
The most unique and largest facility will treat storm water from almost 2.5 miles of I-5 in the project’s south section. The concept of a constructed wetland to treat highway runoff prior to discharge to the Snohomish River was approved by the city and regulatory agencies. This constructed wetland is located almost 1 mile from I-5 on a former industrial site that WSDOT had purchased from the city. The site is adjacent to the Snohomish River and a city park with walking trails. Unlike most WSDOT storm water treatment facilities, this facility will be accessible by pedestrians via the city park and riverfront trail system.
To convey storm water to this facility, a new 36-in. storm drain trunkline will be installed parallel to I-5 to collect storm water from pavement and some median areas. The trunkline will convey flows away from the interstate, down a steep hillside through a residential neighborhood, to a dedicated storm water facility adjacent to the Snohomish River. A unique pedestrian bridge and aqueduct will be constructed to carry the storm water over a railroad mainline and into the wetland park facility through a series of waterfalls and subsurface pretreatment devices. The aqueduct will also be constructed to allow pedestrians access to the wetland trails below. During storm events, visitors can view the water flowing over constructed waterfalls into wetland pools and observe the treatment processes involved. The wetland cells will be constructed with berms surrounded by walking trails with interpretive signs so visitors can observe how and why the storm water is treated.
Design challenges included numerous existing wetlands that can not be disturbed by construction. To minimize these impacts, flows will be conveyed over the wetlands through a flume supporting a walking trail on top. The flume is designed to provide a narrow passage of treated flows through the existing sensitive areas while allowing continuity of existing drainage patterns beneath.
Best management practices
WSDOT has committed not only to provide storm water treatment for all new and replaced highway surfaces within the project area, but to provide an “enhanced” level of treatment to remove some toxic metals, such as lead, copper and zinc, which have been shown to be present in highway runoff. Removing some of these compounds will be a significant environmental benefit to human health and the environment.
Although the list of approved enhanced BMPs is small, having only recently been tested for removing heavy metals, WSDOT has a suite of BMPs available for use in their HRM, along with design guidelines and details for preparing plans and specifications. The D-B team selected the best-suited BMPs from the HRM to use at each location. For example, mixed media sand filters and hydrodynamic separator vaults treat larger tributary areas, while enhanced biofiltration swales were selected for smaller pavement areas. The swales will be constructed next to ramps, replacing typical ditches. Both the filters and swales use a combination of sand, compost, perlite and other natural soil amendments to filter heavy metals.
Other storm water improvements
This project provides other significant benefits as a result of upgrading I-5’s storm drain collection system. While building new conveyance systems, the collection structures will be upgraded to include some larger opening inlets/catch basins with sumps, spaced closely together to allow more storm water to flow into the pipes, which will reduce on-street flooding and provide some peripheral treatment via the sumps.
In the south section, I-5 runoff will be diverted from the existing hillside drainages to the new storm drain trunk. Several of these small streams have historical erosion problems; by removing the highway flows, erosive peak flowrates will be decreased. A portion of I-5 constructed along a hillside is experiencing subsurface conditions such as groundwater seepages and slides. Highway runoff along this area, as well as some underdrains that collect groundwater, will be rerouted to new storm drains and thus to the storm water treatment wetland, improving stability in the area.
Trenchless technology will be used to rehabilitate existing storm water pipes that convey offsite flows through the project area and downstream, and for existing pipes that will be used as part of the proposed highway conveyance system. WSDOT directed that these pipes be assessed for condition and then relined or replaced so the system could achieve at least a 50-year life expectancy. Trenchless installation methods will be used in key areas to avoid lane closures and speed up the construction process.
