Stormwater Policy Team 04.04.06 - Laserfiche WebLink

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the outer zone is consistent with the allowance of more unrestrictive uses in this zone, while still retaining important water quality functions. Evaluation Results This section, along with portions of Sections IV and V of this memo, includes a summary of the parcel/corridor impact evaluation results. Amore detailed data table including imperviousness by buffer management zone for each waterway corridor and tributary can be viewed at: \\Cesrv200\public\Shared\StormwaterManagementTeam\WQProtectedWaterways\SWMT 3-15- 06\Attach B ResultsDataTablexls. Exceedences of the impervious surface cover (ISC) thresholds in the three n management zones are indicated by shaded cells in the data table. A total of 60.5 miles of water quality waterways were below the threshold of impervious surface area for all three buffer zones (i.e. where imperviousness within each management zone was lower than the threshold ISC). This gives an indication of the degree to which the water quality waterways currently possess water quality function at a level deemed appropriate for water quality function within the potential i regulated area (67.5% or 60.5 miles/89.6 miles). A total of 29.1 miles of water quality waterways exceeded one or more of the ISC thresholds. One of the factors observed for waterways that scored consistently over the ISC threshold across all management zones was the presence of a waterside street parallel for most of the waterway's length. Examples of this situation include: • Highway 99 Channel (35% - 66%ISC, tota148%)) • North Beltline Floodway (42% - 52%ISC, total 26%)) • Roosevelt Channel (26% - 43%ISC, tota131%) • West Beltline Floodways (10% - 57%ISC, tota128%) • Debrick Slough/Delta Ponds -Delta Highway (4% - 39% ISC, total 11%) • Upper Amazon Creek, bounded on either side by East and West Amazon Drive (1.4% - 82% ISC, total 18%) Another unique situation was the Amazon Creek Main Stem between Bailey Hill Road and the concrete- . lined channel where existing high ISC is due to waterside development and the bike path. Although scoring relatively high in terms of exceedences of impervious surface area thresholds, reaches within the waterway corridor contained long and large chunks of pervious area, and contained a bike path portion which contributed to the high numbers but is a use that would be allowed within the buffer area under proposed water quality protections (i.e. the Center for Watershed Protection acknowledges provision of limited uses in certain zones including bike paths). Two other conditions were found that were unique to certain locations: incorrect classification of a private roadside ditch as a "waterway" (e.g. Highway 99 roadside channel functions as a roadside ditch) and very short tributaries (<SOOft in length). It is the Stormwater Management Team's recommendation that Highway 99, as with other roadside ditches, be removed from protect status. Waterways less than 500 feet in length were also removed from protect status. Figure 1 shows the number of acres of potentially regulated land by buffer management zone and the percent impervious surface cover by buffer management zone for all water quality protected waterways, excluding the Highway 99 roadside channel and its private roadside ditch tributaries. This provides a sense of the level of impact (as defined by ISC).. The overall ISC for all management zones combined is 3/28/06 4