WSDOT takes extraoridinary steps to protect water quality: SR 522 Bothell project provides an example
Crews on the SR 522 project worked hard to prevent muddy water from getting into the Sammamish Slough. |
In Washington’s rainy climate, WSDOT and the highway construction crews who build projects for us work hard to prevent rains from washing soil into streams and rivers.
When construction of transportation facilities disturb soils, crews prevent erosion at these sites by spreading straw, planting grass, building ponds and using other erosion-control measures. Construction areas that have a high potential for erosion due to the project area’s size, steepness, soil type or proximity to waterways are inspected by WSDOT weekly.
Acres of soil were disturbed during construction of the new access to the UW-Bothell and Cascadia Community College campus. |
The
SR 522 – UW Bothell/Cascadia Community College Campus Access project provides many examples of
WSDOT’s efforts to protect water quality as crews construct a new south entrance to the University of Washington – Bothell and Cascadia Community College campus from SR 522 and I-405 in King County.
top What are the environmental concerns for the SR 522 project?
- Sammamish Slough and North Creek waterways are next to the project site
- Several acres of soil are being disturbed that are prone to erosion
- Steep slopes along the project site present a high risk for soil erosion
How is WSDOT protecting the environment on this job?
- Plastic sheeting is used to protect slopes and reduce erosion
- Slopes are seeded with grasses to hold bare soil in place
- Wattles, which are rolled bundles of straw, are secured on slopes to slow water down and filter it as it passes through
- Coir logs, which function like a large wattle, are placed on steep slopes to prevent sliding due to saturated soils. These logs (made of coconut fiber) will be left in place to naturally decay as the landscape matures
- Wheel wash systems are used to prevent trucks from tracking soil onto the roadway pavement
- Soils are amended with compost so that they absorb more water and reduce runoff
- Sediment ponds are built to collect roadway runoff and allow pollutants to settle out of the water
- Pumping stations have been constructed in low areas to collect and deliver water through pipelines to one of two Chitosan sand filter systems on the project site to remove fine sediments
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What is a Chitosan sand filter system and how does it work?
Two Chitosan sand filter systems were used to treat stormwater from the SR 522 construction site. Each of these units can treat up to 1,000 gallons of water per hour. |
A Chitosan (pronounced kye-toe-san) sand filter system is a device used to filter dirt from water on the construction site. Each of the two Chitosan systems being used is capable of pumping up to 1,000 gallons of treated water per hour. This is how it works:
- Water from the site is pumped to a series of tanks that allow time for solids to settle
- This water then passes through sand which contains the Chitosan chemical, a product made from crushed crab shells
- The chemical acts as a binding agent and causes small particles of dirt in the water to bond to the sand
- The treated water is then discharged to the Sammamish Slough
What was unique about this system’s use on the SR 522 Bothell project?
WSDOT designed a dispersal system for the treated water that would not disturb the layer of silt on the bottom of Sammamish Slough. |
In this situation, we were releasing treated water to the Sammamish Slough. The problem is that the slow-moving Sammamish Slough has a heavy layer of silt on the bottom and to avoid disturbing it, WSDOT needed to find a way to dispense the runoff without creating muddy water in the Slough.
Transportation Engineer Pat Mallahan designed a dispersal system that would allow the discharge water to be released by broadcasting it at the surface of the slough using fabric bags to control the direction of flow.
“We tinkered around with some ideas and came up with a design that sends the water through plastic pipes and filter bags that float on the surface,” Mallahan explained. “Even during low slough water conditions, the dispersal system has worked well without reducing amount of water we can release or disturbing the silt.”
top How is WSDOT’s erosion control performance on a statewide basis?
Plastic sheets protect the slopes from erosion along SR 522. |
Every highway construction project requires an erosion control plan as part of the permitting process. WSDOT creates the plans, which are put in place by its contractors. In addition to weekly inspections, each fall we inspect construction sites to document:
- How thoroughly these plans are implemented
- How prepared of the project is to prevent erosion
- How effective the plans are at preventing erosion
- Identify areas for improvement
Results from 2008 annual site inspections demonstrate that performance is at an all time high, with all measures above 80 percent compliance and no decrease in performance from previous years. Details on this annual report can be found in Environmental Stewardship section of the December 31, 2008 Gray Notebook.
Need more information?
SR 522 – UW-Bothell/Cascadia College Campus Access Project:
Janice Fahning P.E., Project Engineer, 425-225-8799, Fahninj@wsdot.wa.gov
Jeri Schurman, Chief Inspector, 425-225-8792, Schurmj@wsdot.wa.gov
Pat Mallahan, Chief Inspector, 425-225-8773, Mallahp@wsdot.wa.gov
Construction site erosion and sediment control program:
Dick Gersib, Environmental Services Office, 360-570-6637
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