Mill Creek Levee Maintenance

Story By Gina Baltrusch

The Mill Creek Flood Control Project was completed in 1942 and included levees along the improved Mill Creek channel to provide flood protection for the City of Walla Walla. Currently, the Mill Creek Project levees fail to comply with U.S. Army Corps of Engineers vegetation standards used to ensure reliability, resiliency and operability of levee, floodwall, and dam projects nationwide.
Non-compliant vegetation on levees blocks visibility for inspections, restricts access for maintenance, hinders flood fighting, and adds uncertainty to structural performance and reliability, which increases risk. The inability to inspect, maintain or flood fight could contribute to a breach or a delay in emergency response.
Since 1975, the Corps has completed several actions at the project related to reservoir and levee seepage. In the mid-1980s, vegetation was removed from inside the creek channel, but not the landward side of the levees. Wind storms in 2008 and 2012 uprooted trees, causing damage to the levee cross section which required repairs to ensure levee integrity.
The Corps regularly inspects its levees to monitor their overall condition, identify deficiencies, verify that needed maintenance is taking place and provide information about the levees on which the public relies. Inspection information also contributes to risk assessments and supports levee accreditation decisions for the National Flood Insurance Program administered by the Federal Emergency Management Agency (FEMA).
Corps levee vegetation standards require removal of woody vegetation from the levee crown, and to a distance of 15 feet from the levee toe on both sides of the levee or to the project right-of-way, whichever is closer. Roots growing into the cross section provide a path for water to flow through the levee, increasing the potential for seepage to occur, putting the integrity of the embankment at risk of failure.
“This is about public safety – Life safety is paramount for the Corps’ operations,” said Frank Wachob, a civil engineer in Walla Walla District’s Geotechnical Section. “With the trees on the levee, we wouldn’t be able to see if problems were developing during a flood event, much less take swift action to place sandbags or operate heavy equipment to construct reinforcements.”
Ensuring these levees meet required safety standards is a multiple-year effort. The following tasks need to occur: the maintenance zone cleared, stumps removed, levee structure repaired and grasses planted.
“We are considering the environment – The Corps has gone to great effort to minimize the amount of woody vegetation that needs to be removed,” Wachob explained. “We surveyed the levees and measured the maintenance zone from the base of the original design slope (called the toe) instead of the actual physical slope, which extends far beyond the design toe in many locations.”
Work began Oct. 8 – outside bird-nesting season – and a qualified biologist inspected the trees prior to removal.
“The Corps maintains about two-thirds of Mill Creek Project lands for habitat purposes (412 of 612 total acres). We estimate less than 6 acres of woody vegetation will need to be removed from the levee maintenance zone. Once the zone is cleared and levee repairs completed, grasses will be planted to improve the aesthetics and benefit insects. Vegetation outside the zone will be allowed to develop naturally,” Wachob added.

Photo illustration by Gina Baltrusch, U.S. Army Corps of Engineers.

Photo illustration by Gina Baltrusch, U.S. Army Corps of Engineers.

Photo illustration by Gina Baltrusch, U.S. Army Corps of Engineers.

Photo illustration by Gina Baltrusch, U.S. Army Corps of Engineers.

Interested community members tour the Mill Creek Levee maintenance proposal route following the Sept. 29 public meeting. Photo by Jennifer McFadden Allen

Interested community members tour the Mill Creek Levee maintenance proposal route following the Sept. 29 public meeting. Photo by Jennifer McFadden Allen

Interested community members tour the Mill Creek Levee maintenance proposal route following the Sept. 29 public meeting. Photo by Jennifer McFadden Allen

Interested community members tour the Mill Creek Levee maintenance proposal route following the Sept. 29 public meeting. Photo by Jennifer McFadden Allen

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