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for larger waterways located lower in the drainage system buffer setbacks were measured from top-of- <br /> j .bank. <br /> Existing impervious surface areas within and adjacent to potential buffer management zones along each <br /> water quality protected waterway were digitally delineated, primarily utilizing 2004 aerial photos. <br /> Additional data input to the model included parcel ownership; non-tax lotted area (primarily public rights <br /> j of way); Waterside Setback Overlay Zone (west Eugene wetlands setbacks); and Water Resources <br /> Conservation Zone (Goal 5 setbacks). <br /> . <br /> Once the set up was complete, atest-run of the GIS model was conducted on two waterway corridors and <br /> their tributaries to ensure that the results were accurate and would address the Policy team's requests. <br /> Once QA/QC was completed, the model was run for all water quality protected waterways within the <br /> UGB. For each waterway corridor and tributary the model calculated waterway length, potential buffer <br /> areas by management zones, existing impervious surface areas by management zones, pervious areas by <br /> management zones, and percent impervious surface area by management zones. The model also provided <br /> <br /> ~ data regarding public vs. private land area affected by potential protection measures and the relationship <br /> (in lineal miles) between water quality waterways, west Eugene waterways, and Goal 5 waterways. <br /> Impervious Surface Cover Thresholds <br /> Using the Center for Watershed Protection's findings that impervious surface cover (ISC) is a reliable <br /> indicator of overall stream health, including water quality, and the framework it established for identifying <br /> varying levels of health, ISC thresholds were established for guiding the evaluation of waterways as to <br /> their existing level of impact, viability of water quality function and the feasibility of applying buffers. <br /> Table 1 lists the ISC thresholds by buffer zone that were used in the evaluation. <br /> Table 1. ISC Thresholds by Buffer Zone <br /> Waterway Corridor/Tributary <br /> Buffer Zone Percent Impervious Surface Cover <br /> • 0-25ft: up to 10% <br /> • 25-SOft: up to 25% <br /> • 50-75ft: up to 40% <br /> • Total: up to 25% <br /> Since the function of the "streamside zone" from 0-25 feet is to protect the physical integrity of the stream <br /> ecosystem and allowable uses in the streamside zone are very restricted (e.g. flood control, utility rights- <br /> of-way and footpaths per the Center for Watershed Protections guidance), the ISC threshold of 10% <br /> reflects the sensitive nature of this zone and its associated water quality functions. The "middle zone" <br /> from 25-50 feet functions to provide distance between upland development and the streamside zone, and <br /> some restricted uses would be allowed including some stormwater BMPs and bike paths. The ISC <br /> threshold of 25% in this zone is indicative of the fact that some impacts are allowed while still retaining <br /> the water quality functions of this middle zone. The "outer zone" from 50-75 feet functions to prevent <br /> encroachment and filters backyard runoff. Several uses would not be restricted in the outer zone including <br /> residential lawn, gaxden, compost, yard wastes, and most stormwater BMPs. The ISC threshold of 40% in <br /> 3/28/06 3 <br /> <br />