Eugene Planning Commission <br />July 9, 2019 <br />Page 5 <br /> <br />Stormwater Concerns <br />Much of the Neighbors’ testimony concerns impacts from stormwater management of the <br />proposed development. <br />As an initial matter, LUBA did notremand for review of stormwater issues. Stormwater <br />was addressed during the initial proceedings and stormwater management is now a resolved <br />issue. Any new stormwater issues raised by Neighbors, have been waived. The Planning <br />Commission should reject stormwater arguments as outside the scope of the remand. <br />That said, in an abundance of caution, Applicants have had the engineer responsible for <br />designing the stormwater system, Nathan Patterson,P.E.,review and respond to the <br />GeoSciences, Inc. allegations. That technical memorandum is attached as Attachment 3hereto. <br />Mr. Patterson explains that the project’s stormwater management analysis and proposed <br />facilities are designed using an accepted stormwater methodology (the Santa Barbara Unit <br />Hydrograph hydraulic modeling method) and accepted technical solutions to managing <br />stormwater flow. In plain language, that methodology captures stormwater from a larger area <br />(the developed area), containsit, and releases the water at a rate that equals the pre-development <br />peak flow rate equivalent for the width of the spreadersas measured for a 10-year storm. The <br />TechnicalMemorandum explains: <br />“By collecting runoff from a larger area at the top of the hill into underground <br />storage and pipes, there will be less runoff soaking into some of those areas Mr. <br />Schleider seems so concerned with at the top and eastern facing slopes. Those <br />other locations will receive equivalent peak runoff rates which should avoid any <br />detrimental issues as the review suggests.” <br />To reiterate that main point, the Technical Memorandum states: <br />“To put it in more simple terms, during a 10-yearstorm event on the undeveloped <br />site, the same peak flow rate would drain across the undeveloped 38’ wide <br />section. Again, we are matching pre-developed rates across the same 38’ wide <br />section, despite collecting runoff from a larger area.” <br />In even plainer terms, GeoScience, Inc’s and Neighbors’ claims of significant amounts of <br />water being dumped into theoff-site“creek” (which does not appear on any City waterway map) <br />by the proposal during periods of creek flooding are wrong. The rate of flow from the site does <br />not change. <br />The Technical Memorandum analyzes where the GeoScience, Inc. analysis goes wrong <br />and leads towhat the memorandum describes as a gross mischaracterizationof the stormwater <br />analysis.First, Geosciences uses an unprofessional measurement of flow and, second, attempts <br />to calculate runoff volumes by calculatingabsorption/transmission of water into the soil using <br />unorthodox and unprofessional techniques. The problem with the GeoScience, Inc. analysis is <br />5 <br />