Wetlands are areas that are regularly or seasonally saturated by surface water or groundwater and are characterized by vegetation that is adapted for life in saturated soils.
The Valley is wet and flat with a high underground water table; water does not drain very well and the area floods easily.
Liquefaction susceptibility in the SR 167 corridor. Earthquakes Can Cause Unstable Soils to Liquefy. See
Larger Map.
There are 54 bridges on SR 167. See
Larger Map.
Water-soaked Terrain Increases Project Costs
The Green River Valley was historically a fjord called Lake Russell. Today it is criss-crossed by numerous streams, large areas of wetlands and floodplains, and it contains unstable soils, called liquefiable soils.
In addition to providing habitat for wildlife, wetlands help to retain flood waters during periods of rain, and they filter pollution out of the water. Flood plains temporarily store excess water when rivers, streams and lakes periodically overflow.
The study identified the potential of up to 105 wetlands and 27 wetland ditches within 300 feet of the highway. Affected wetlands will require mitigation by creating new wetlands or enhancing existing wetlands.
Earthquakes Can Cause Unstable Soils to Liquefy
Mount Rainier has had two lahars (massive mud flows) in the Valley that contributed to the soil’s liquefiable qualities. Liquefaction is when water-saturated, sandy soil loses strength during earthquake shaking. The land proved fertile for hops, peas, and lettuce that used to grow in the Valley, but creates challenges for building stable structures. When quakes occur, liquefiable soil can turn into a liquid that destabilizes bridges and overpasses causing them to crack or collapse.
The Washington State Department of Natural Resources produces maps that identify locations with liquefiable soils (shown in maps to the right). SR 167 is located in an area with a moderate to high susceptibility to liquefaction. Special measures will be required to stabilize bridges and structures when they are built and bring older structures up to modern earthquake standards by retrofitting or replacing them. These measures are expensive, increasing the estimated costs of improvements.
From Planning to Design: Each Project Will Need More Environmental Analysis
The environmental and geologic sensitivity of this area requires WSDOT to conduct rigorous analysis and meet heightened design standards when designing each project, particularly in the areas of:
- Wetlands, floodplains, and stormwater runoff
- Soil stability
- Fish-bearing culverts
- Protected fish and wildlife species
- Aquifer recharge areas
More information about environmental findings for SR 167 can be found in the corridor plan.