DESCRIPTION
Proposed actions at the 190th Avenue Downstream Site include removal of existing levees and construction
of new levees farther back from the Puyallup River. Approximately 6,598 linear feet of existing levee
located along the right (north and east) bank of the Puyallup River between RM 21.3 and 22.5 will be
removed and a new armored levee (7,710 feet) would be set back to the north and east encompassing an area
of approximately 2,076,776 square feet (Figure E-19). Levee setback would reconnect the Puyallup River to
remnant riparian springs located at the Site.
Levee setbacks at Site 19 would maximize the area of floodplain inundation at high frequency events. The
estimated probable area of inundation at the Site is 991,727 square feet (23 acres) and the 2-year and 5-year
inundation areas are 1,380,091 square feet (32 acres) and 1,508,407 square feet (35 acres), respectively.
Increased storage volumes during low-frequency events are high (11,540 acre-feet for a 100-year event)
(Table E-19). Significant and beneficial increases in sediment transport are expected for low (25-year
events) and high frequency events (5-year events).
A significant portion of the Site is located within the 'severe' channel migration zone, as described and
mapped by GeoEngineers (2003). The total area of the Site is 2,076,776 square feet (47.7 acres), of which
1,963,655 square feet (45 acres and 95% of the Site) occupy lands delineated as having severe (high) channel
migration potential and 113,121 square feet (2.6 acres and 5% of the Site) occupy lands delineated as having
moderate channel migration potential (Table E-19).
Most of the 190th Avenue Downstream Site is covered in mature deciduous and coniferous trees (riparian
forest). There are a few open field areas with grasses and forbs. Levee setbacks would improve connectivity
to side channels, springs and low-lying riparian woodlands and create a complex channel structure with
multiple braided channels to maximize aquatic habitat diversity and use (Table E-19). Levee setbacks at Site
19 will reconnect the river to wetlands without excavation of a connecting channel. Levee setbacks will not
promote significant natural LWD recruitment and pool formation due to a lack of large diameter conifers and
wood recruitment.
Based on a HEC-RAS Model analysis of boundary shear stress for 5-year and 25-year recurrence intervals,
levee setbacks at the 190th Avenue Downstream Site appear to pose minimal risk of changes in downstream
deposition of bed material or downstream erosion.
