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Olsson Associates helps Maricopa County navigate variance request

Thursday, May 26, 2016

Alicia Krieger, Communications

Since Hurricane Katrina devastated the Gulf Coast in 2005, the U.S. Army Corps of Engineers (USACE) has been updating and refining their policy of allowable vegetation on and near levees and has started to enforce a “vegetation-free zone.” The current vegetation-free zonepolicy states that no woody vegetation is allowed within 15 feet of the levee prism on any levee owned, operated, or regulated by the U.S. Army Corps of Engineers unless a specific vegetation variance has been approved.

Working with the Flood Control District of Maricopa County and the Cities of Scottsdale and Tempe, Arizona, Olsson Associates is completing a vegetation variance request for the fourteen-mile-long Indian Bend Wash Levee System that is regulated by the U.S. Army Corps of Engineers.

The Indian Bend Wash project is a series of stormwater conveyance channels and conduits that were conceived and constructed in the 1970s and 1980s to control flash flooding from a 211-square-mile watershed that historically flooded homes and businesses in Scottsdale and Tempe. The project was developed as a multi-use facility, which contains hundreds of acres of recreational parks, trails, lakes, open space, and habitat restoration. The project protects more than $700 million worth of homes, businesses, and infrastructure from flooding.

“This is the first time that the Flood Control District of Maricopa County has submitted a vegetation request to the USACE,” said Dave Jensen, project manager. “We are proud to help the District navigate the process and how to submit the information to the USACE.”

Completing a vegetation variance request requires many components. This includes an inventory of all trees within the estimated vegetation-free zone, a wind blow-over review and analysis, a geotechnical assessment of the levee, a hydraulic analysis to estimate the scour potential of wind-thrown trees, and mitigation efforts to remove trees that might pose a threat to the levee.

Olsson Associates’ hydraulic and geotechnical engineers, landscape architects, and surveyors in Phoenix and Kansas City are working to show that most woody vegetation along Indian Bend Wash was designed as part of the Indian Bend Wash project and does not pose a threat to the stability of the levee. Olsson employees are also working to show the levee doesn’t hinder the operation, maintenance, inspection, or flood-fighting operations.

For those trees that may need to be removed, Olsson is developing a plan to remove the trees, reconstruct the levee segment, and plant additional vegetation outside of the vegetation-free zone.

“Olsson Associates will set the standard for future variance requests that will be required on additional District levees,” Dave said.

The vegetation variance request is expected to be complete in June.

 

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