2018-12-26_REVISION - M2008078 (28)

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Railroad. The railroad also experiences minor overtopping to the north and these overtopping flows return to Wolf Creek alongside an irrigation canal embankment. Lateral structures along the top of the railroad represent this overtopping. A Zone X shaded area represents the spills from Wolf Creek overland to the Railroad Split Flow channel, which was added in HEC-RAS as a separate reach to model the return of the flows to Wolf Creek. In Lamar, a split flow occurs along the north bank of Willow Creek in the vicinity of County Road Hh. This reflects overtopping of the left channel bank during the 1%- and 0.2%-annual-chance events. It should be noted that the grade of CR Hh is so low as to render any levee downstream of the road ineffective, as flows will spill to the north of the channel at the road and remain on the north side of the channel bank. To evaluate the spill potential, lateral weir structures were added to the Willow Creek HEC-RAS model at the higher of the natural ground elevation on the landward or riverward sides of the levee. The lateral weir structures on Willow Creek were optimized to determine the flows in Willow Creek Split Flow A. The modeling results indicate that a total of approximately 2924-cfs spill from Willow Creek to Split Flow A during the 1%-annual-chance event. These flows spill north toward the Atchison Topeka and Santa Fe Railroad, then continue northeast toward County Road 9. The canal just downstream of CR 9 was chosen as the limit of detailed study for this reach because that is also the limit of detailed study for Willow Creek. Approximately 865 cfs spill from Willow Creek Split Flow A over the railroad to the north and tie in to the approximate study for Willow Creek at CR 9. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the Flood Profiles (Exhibit 1) are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. 3.3 Vertical Datum All elevations are referenced to the North American Vertical Datum of 1988 (NAVD). All detailed studies included in this update were performed on NAVD (superseding all previous detailed study information), making vertical datum conversion unnecessary. The BFEs shown in the FIRM represent whole-foot rounded values. For example, a BFE of 1202.4 will appear as 1202 on the FIRM and 1202.6 will appear as 1203. For more information on NAVD88, see the publication entitled, Converting the National Flood Insurance Program to the North American Vertical Datum of 1988 (FEMA Publication FIA-20/June 1992), or contact the Vertical Network Branch, National Geodetic Survey, Coast and Geodetic Survey, National Oceanic and Atmospheric Administration, Rockville, Maryland 20910 (Internet address http://www.ngs.noaa.gov). 12