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SECTION 3 Flood Control Euaination 5. Average ground slope (dimensionless, ft/ft). 6. Subbasin width (feet). 7. Manning's roughness coefficient for impervious areas. 8. Manning's roughness coefficient for pervious areas. 9. Depression storage for impervious areas (inches of water over subbasin). 10. Depression storage for pervious areas (inches of water over subbasin). 11. Green -Ampt soil infiltration parameters: average capillary suction (inches), saturated hydraulic conductivity (inches /hour), and initial moisture deficit (volume air /volume voids). Table 3 -1 (provided at the back of this section) includes the major hydrologic information for each of the River Road Santa Clara subbasins. Specifically, the tables provide the information for parameters 1 - 5 listed above and the expected increase in impervious surface under future conditions. More detailed hydrologic information, including information described for parameters 1 — 11, can be found in Appendix B. The following subbasins were not included in the model for the reasons noted: • The A -1 Channel subbasins Al -000 and Al -005 were excluded from the model since they are located outside the City limits and the Urban Growth Boundary. • The Highway 99 major subbasin (including subbasins 99 -010 and 99 -020) were excluded since they drain to a roadside ditch along Highway 99N that is owned and maintained by the Oregon Department of Transportation. • Flat Creek subbasin FC -000 was excluded from the model since it is located outside the City limits and the Urban Growth Boundary. • Willamette Overflow subbasin WO -000 was excluded from the model since it is located outside the City limits and the Urban Growth Boundary. Hydrologic Data Associated with Drywell Drainage Areas After completing the subbasin delineations described above, a second step was conducted to delineate the portion of each subbasin where runoff is draining to drywells as opposed to the piped or surface conveyance system. Section 2.5.4 provides a description of the drywells in the River Road Santa Clara basin including 79 County wells, 72 City wells, and approximately 634 private drywells. At the time that this exercise was conducted, the GIS system for drywells was still under development and somewhat incomplete. Therefore, the delineation of drywell drainage areas included a subset (or approximately 759) of the total 785 drywells. For each subbasin that included some portion of area draining to drywells, the subbasin was subdivided into these two areas, and the hydrologic information described above (e.g., inlet node number, subbasin area, impervious percentage, etc.) was generated for both the subset of the subbasin draining to the drywells and the subset of the subbasin draining to the piped or surface conveyance system. The purpose of delineating these drywell areas individually was to simulate the effect of drywells in the hydrology portion of the XP -SWMM model and on the runoff calculations. In order to develop a model that would simulate the infiltration characteristics associated with drywells, the drywells were modeled as storage nodes that would store runoff generated up to the 5 -year, 25 -hour (3.6 inches) storm event. This was based upon the City's design criteria for 0:A25695978 Eugene RR -SC Final Basin P1an\Master P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 3-3