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128th Street Comprehensive Setback Levee Feasibility Study
Study Location
The study area is east of State Route 162, west of McCutcheon Road East, south of 112th Street East and north of 144th Street East and along both the right and left banks of the Puyallup River between river miles 15.7 and 17.5, bisected by the 128th Street East bridge crossing (and north of the City of Orting limits).
This area is divided into four quadrants: NE, NW, SE, and SW to help assess the unique existing conditions, flood risk reduction opportunities and problems in each. A proposed levee setback in one quadrant may influence flood risk reduction in all three of the others. Therefore, all the quadrants are being evaluated concurrently.
The Puyallup River is Evolving
Over time, rivers erode rocks, dirt, and other sediments and deposit them downstream. This is one of several factors that can cause rivers to change course and flow into new areas over time.
The maps below show how the Puyallup River near the 128th Street East bridge crossing has changed its course over time.
The Height Above Water Surface Map (shown to the left below) shares more information about this. For example, the purple areas on the mapping indicate locations on the floodplains that are relatively lower than other areas, and the darker the purple color the lower the floodplain surface.
The patterns that are evident in the mapping show old river meander locations and other depressions that can provide increased flood storage and more frequent seasonal habitat benefits if re-connected to the river.
| The Height Above Water Surface Map (below) shows the difference in elevation between floodplain land surfaces and adjacent river water surfaces. | The Historic Channel Occupation Tract Map (below) shows how the dynamic Puyallup River has shifted its active channel over time. |
Historical Channel Occupation
The Historic Channel Occupation Tract Map (shown to the right above) reflects how the dynamic Puyallup River has shifted its active channel over time. Starting in 1880, the US General Land Office did a survey to start tracking river meanders. Incorporating this with the historical aerial photos taken between 1931 and 1998, we start to see a picture of how much the Puyallup river systems has changed in the last 140 years.
This Historical Channel Occupation Tract map is produced using a compilation of active historical Puyallup River channel occupation location areas changing over time. With this ever evolving channel, it’s important to understand the changes in history so we can help protect for the future.
Puyallup River (and 128th ST E Bridge Crossing) 1931 Historical Aerial Photograph
As the river moves and changes, flooding can become more pronounced during times of heavy rainfall. This flooding can damage property and public infrastructure, impact travel and evacuation routes, and cause a loss of fish habitat. Moving existing levees back from the river’s edge and reconnecting floodplains provide increased storage for flood waters. This additional capacity helps to reduce flood risks to the public, residents and property, create more reliable access to roads, reduce flooding of structures and risk of damage to the levees, fields and farmland, and create more fish habitat and spawning areas.
Existing Conditions
From our work to date, we have analyzed existing flood conditions in the study area, identified opportunities to reduce flood risk, and are starting to develop conceptual alternatives.
The video below shows a flood event passing through the study area. This animation was generated from a hydraulic model simulation of the November 2006 flood event on the Puyallup River. The animation shows the pattern, timing, and the spatial extent of flooding for existing conditions during a major flood. The peak discharge in the Puyallup River for the simulated flood event is 40,300 cubic feet per second.
What do we expect will happen if no changes are made?
There are several existing problems along the Puyallup River in the study area. If no changes are made to the current configuration of levees along this stretch of river, we can expect to experience problems in the future. These include:
- Flooding of the NE quadrant begins with backwater along Canyon Falls Creek.
- Homes may flood and become isolated during severe flood events.
- Levee and levee access road flooding impacts restrict the only access road for residents to or from the NE quadrant.
- Bridge abutments are at risk of scour erosion during flooding.
- Road flooding restricts the only access road for residents and emergency responders to or from the SE quadrant.
- Homes may flood and become isolated during severe flood events.
- Sediment deposited from floods may reduce the flood carrying capacity of the river channel.
- Erosion is widening the river near bridge crossings.
- Flooding may occur across segments of Highway 162.
- Continued and repeated levee damages due to flooding. The existing levee facility is at risk of damage from major flood events.