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Description: These terms are defined in Snohomish County Code:SCC 30.91F.430 "Floodway" means the regular channel of a river, stream, or other watercourse, plus the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than one foot.SCC 30.91D.100 "Density fringe area" means that portion of the special flood hazard area of the lower Snohomish and Stillaguamish rivers in which floodway areas cannot reasonably be established and in which development is regulated by maximum development density criteria.SCC 30.91F.440 "Floodway fringe" means that portion of a floodplain which is inundated by floodwaters but is not within a defined floodway. Floodway fringes serve as temporary storage areas for floodwaters.The density fringe area is unique to Snohomish County and to only a few other counties in the United States. The density fringe concept was developed by the county with FEMA in lieu of designating the entire lower valleys of the Snohomish and Stillaguamish basins as floodway. Developing a traditional floodway in these areas was complicated due to the multiple channels, split flows, tidal influence and historic flood control structures. The documentation for the initial density fringe study and mapping included the county’s interest in ease of maintaining and fostering the agricultural land use since proposed farm-related developments could follow a predesignated conveyance and obstruction amount without conducting a new hydrologic and hydraulic study for agricultural activities.The initial determination of the density fringe included an analysis based on flood water volumes over the entire density fringe area considering all of the existing lot coverage and obstructions present at that time, and identifying additional lot coverage and flow obstruction thresholds which would not result in a cumulative base flood level rise of more than one foot. These thresholds can be applied to new development without necessitating a full flood study:SCC 30.65.250 Density fringe area: maximum allowable density The land area occupied by any use or development permitted by this chapter that will displace floodwaters shall not exceed two percent of the land area of that portion of the lot located in the density fringe area.SCC 30.65.255 Density fringe area: maximum allowable obstruction The maximum width (sum of widths) of all new construction, substantial improvements or other development shall not exceed 15 percent of the length of a line drawn perpendicular to the known floodwater flow direction at the point where the development(s) is located. The length of said line shall not extend beyond the property boundary or the edge of the density fringe area, whichever is less.These density fringe thresholds for new development are often referred to collectively as “the 2 and 15” requirements.(Source: https://www.snohomishcountywa.gov/DocumentCenter/View/64614/Density-Fringe_Planning-Commission_staffreport_051019?bidId=)

Service Item Id: 88898a48c91546eaa2783ad3211bba64

Copyright Text: SCC 30.65.250, SCC 30.91D.100, PDS, FEMA

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