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SECTION 3 Flood Control Euaination — The open waterway profile of the A -1 Channel between Bushnell Lane East and Irving Road (node 72730 to node 72797) on Figure 3 -7 was unknown, and therefore the system was field verified and surveyed. — Conditions of the western tributary of the A -1 Channel from node 72102 to 71215, shown on Figure 3 -5, were unclear, and therefore the system was field verified and surveyed. 3.2 Model Validation Process As described previously, an initial model calibration was applied City -wide during the development of the 2002 basin plans. However, a separate model validation was conducted specific to the River Road Santa Clara basin because of the unique conditions of the basin associated with fairly high permeability soils and the use of drywells to handle some of the drainage. In addition, photos from a large storm event were available from within the basin for use in validating the model. Flow monitoring data were not available for a calibration process; therefore, a model validation process was conducted based on photos and observed freeboard elevations provided by the City. The information was provided from the Willamette Overflow subbasin in the Willamette Overflow Waterway at Lone Oak Way (node 74406) for three days of rainfall in 2005: December 28 30 and 31 Validation of the model was based on comparisons between model - simulated water surface elevations at node 74406 (converted to freeboard elevations) and the freeboard observed during the rainfall event(s), as provided below. To start, the base hydrologic /hydraulic model that was used for the model validation process assumed that the drywells in the Willamette Overflow Basin were functioning as they were designed to infiltrate all of the runoff from the 5 -year, 24 hour design storm (model scenario 2 from subsection 3.1.1). Another assumption in the base model was that the impervious percentages were equal to the mapped impervious percentages areas as opposed to using effective impervious percentage areas. The comparison between simulated and observed freeboard elevations revealed that the model - simulated freeboard was less than the observed freeboard (i.e. the model was conservative as it was simulating higher water surface elevations and hence, lower freeboards). Model parameters were adjusted in an attempt to reduce the differences between model - simulated and observed freeboard. These adjustments to the model input parameters were applied to the entire Willamette Overflow subbasin model, and the model was run for the period from December 27, 2005 to January 3, 2006 using real rainfall data. A summary of the daily rainfall depths that were used in the model validation is provided in Table 3 -3 below. Several additional model runs were conducted to evaluate the model's sensitivity to changes in input parameters. The results of these sensitivity analyses indicated that the impervious percentage area was the most sensitive model input parameter. 0:\25695978 Eugene RR -SC Final Basin P1an\Nlaster P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 3-6