Landscape Architect & Specifier News

JAN 2018

LASN is a photographically oriented, professional journal featuring topics of concern and state-of-the-art projects designed or influenced by registered Landscape Architects.

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Page 53 of 133

54 Landscape Architect and Specifier News Modeling conducted by THA's team showed that in the case of Yreka Creek, the full width of the historic floodplain was not needed to reduce flooding and achieve channel stability. Excavating a new floodplain at least 175 feet wide and 8-10 feet below the surrounding grade (2-4 feet above the incised creek bottom) was determined to be what was needed to restore stream geomorphology, and surprisingly, would also contain up to a 100-year flood event. The floodplain width required could be achieved by excavating along one or both sides of the creek, depending on the disposition of creek side landowners and what types of structures or other improvements were in the way. The newly created floodway could then be developed as part of a greenway, complete with trails and native riparian revegetation. Surrounding businesses and private residents would benefit from reduced risk of flood damage and associated flood insurance costs, plus derive economic benefits from being located along a greenway. To date, this design solution has been implemented by THA along a half-mile of Yreka Creek and a quarter mile of its largest tributary, Greenhorn Creek. This involved the excavation of over 85,000 cubic yards of material. At one location, THA proposed using around 50,000 of those cubic yards as engineered fill on an adjacent undeveloped commercial property, saving the city $250,000 in hauling costs and saving the landowner a similar amount in constructing a building pad above the 100-year flood zone. This solution convinced that affected landowner to participate in the project, and Bottom: This is a recently completed bioswale and detention basin at Evergreen Elementary School. All of the school's runoff originally flowed directly onto the adjacent street prior to retrofitting the campus with a network of bioswales and basins. The same location after a snow shows the detention basin working, and use of the bioswale for snow storage, where it can slowly melt without causing dangerous ice conditions in the school parking lot. Right: The floodplain has been lowered, widened and an overflow channel lined with gravel added. This was prior to hydroseeding and constructing a paved trail under the trees along the creek.

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