Larry Smith, Videra Drive HOA <br />12  <br /> <br />for estimating the amount of stormwater runoff to be treated, and is different for <br />volume based facilities and flow-through facilities as follows: <br />a. Facilities designed to store and treat a volume of stormwater shall <br />be sized using a water quality design storm of 1.4 inches of rainfall in <br />24-hours using Soil Conservation Service (SCS now the Natural Resources <br />Conservation Service) methodology. <br />b. Facilities designed to treat a rate of flow draining through them <br />shall be sized using a rainfall intensity of 0.12 inches per hour for facilities <br />off-line from the conveyance system, or 0.21 inches per hour for on-line <br />facilities, and using the rational equation.” <br />The stormwater plan presented does not address the loss of trees and the effect that will have on overall <br />stormwater flow to Videra Creek. Studies of forested areas in the northwest have shown that 15% to 50% <br />of water leaving a wetland site can be via transpiration, released to the atmosphere through trees. <br />Removal of trees here would substantially eliminate that path for stormwater which would instead add to <br />total flow and peak flow of the creek. <br /> <br />As shown in Table 3, the existing features provide at least 4.8 times the effectiveness as <br />compared to a new facility designed to meet the minimum standards set by Eugene code. <br />Existing wetlands may provide as much as 13.1 times more effective flow control when compared to a <br />typical new facility. This is consistent with behavior of outflow from these wetlands seen over the years. <br />These flows are very consistent, not increasing as much as one would expect during storm events. <br />Moderate flow continues for many weeks through dry periods due to the retention capacity upstream. <br />Page 62 of 87