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elevations were computed using the slope area method. The Spring Creek hydraulic analysis was conducted by Anderson Consulting Engineers, Inc., in 2003 (Reference 72). Water surface profiles were generated using the USACE HEC-RAS model. A combination of field survey data and ortha-photo based topographic mapping (dated April 1999 and prepared at a 2-foot contour interval) was used as the base information for defining cross section geometry and delineating floodplains. From Timberline Road to just downstream of Welch Street, topography in the form of I-foot contours was generated based on field survey data. Field survey data was used to define cross-sections at bridges and the geometry of the low flow channel at nearly all cross sections defined in the HEC-RAS model. In addition, the field surveys included irrigation ditch embankments and other features needing more definition than was available on the topographic mapping. For the main Spring Creek channel, Manning's "n" values generally range from 0.030 to 0.070, with heavily vegetated areas exhibiting values up to 0.090. Overbank Manning's "n" values generally range from 0.060 to 0.090, with isolated areas as smooth as 0.035 or as rough as 0.100. A detailed splitflow path east of Shields Street (Shields Street Divided Flow Path) was identified and analyzed by Anderson Consulting Engineers in 2005 (Reference 71). This path consists of several subreaches through the Hill Pond and Windtrail neighborhoods and along Shire Court. Water surface profiles were generated using the USACE HEC-RAS version 3.0.1. Manning's "n" values range from 0.035 to 0.050 for the channels and from 0.100 to 0.150 for the overbank areas. Many shallow flooding areas were identified. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the Flood Insurance Rate Map (Exhibit 2). The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the Flood Profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. 3.3 Vertical Datum All AS reports and FIRMs are referenced to a specific vertical datum. The vertical datum provides a starting point against which flood, ground, and structure elevations can be referenced and compared. Until recently, the standard vertical datum in use for newly created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of 1929 (NGVD29). With the finalization of the North American Vertical Datum of 1988 (NA VD88), many FIS reports and FIRMs are being prepared using the NA VD88 as the referenced vertical datum. 34