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<br />.
<br />
<br />second profile set represents the computed water surface elevations and the mapped water surface
<br />elevations for the 25-, 50-, and the SOD-year (4%-, 2%-, and the O,2%-chance) floods, The third
<br />profile set represents the computed water surface elevations and the energy grade line elevations for
<br />the 10- and the 100-year (10%- and the PYa-chance) floods, FEMA's RASPLOT and a CAD
<br />package were used to produce the water surface profiles, Appendix D provides plots of the water
<br />surface profiles,
<br />
<br />Flood Plain Maps
<br />
<br />The flood plain maps were prepared to FEMA's standards, As previously stated, FEMA
<br />guidelines stipulate that in a supercritical flow regime, the critical depth elevation is to be mapped,
<br />
<br />Two sets of flood plain maps were produced at a scale of 1 inch equals 200 feet, Exhibit 2
<br />includes the 10-, 25-, 50-, 100-, and the 500-year (10%-, 4%-, 2%-, 1%-, and the O,2%-chance) flood
<br />plain maps showing cross sections and 5-foot index contours overlain onto digital orthophotos, This
<br />will allow easy and quick identification of the extent ofthe flood plain as well as structures that are at
<br />risk. Exhibit 3 includes the 10-,25-, 50-, 100-, and the SOD-year (10%-,4%,2%-, 1%-, and the
<br />O,2%-chance) flood plain maps showing base flood elevations (BFEs) and cross sections overlain
<br />onto a base map with 2-foot contour intervals, BFEs represent lOa-year (1 %-chance) flood
<br />elevations and are shown by wavy lines drawn normal (perpendicular) to the direction of the flow of
<br />floodwater. BFEs are located where the slope of the water surface profile changes, unless close
<br />spacing between the BFEs and/or structures prohibits this interval.
<br />
<br />. Cross section 2010 does not appear on Exhibit 2 or Exhibit 3, This cross section is upstream
<br />of the analysis reach and was only utilized to establish the boundary conditions for the HEC-RAS
<br />model.
<br />
<br />RESULTS AND RECOMMENDATIONS
<br />
<br />The hydraulic analysis of this reach primarily revealed a subcritical flow profile that is fairly
<br />well contained between the bluffs to the east and west of the river. Upstream of this reach, from
<br />Vallecito Reservoir to the northern border of the reservation, super critical flow is the dominant
<br />regime, This flow from upstream is likely to be sediment laden, which could produce significant
<br />sedimentation along this reach. In addition, transitions at bridges could cause scour around the bridge
<br />abutments, Sedimentation and scour analyses were not investigated in this flood plain study, Again,
<br />it should be noted that the prevalence of many historic channels along this reach indicates the channel
<br />alignment is continuously changing as a result of erosion and sedimentation, This could result in
<br />future changes to the flood plain boundary
<br />
<br />A hydraulic jump is a rapid, unsteady transition over a short distance from a shallow to a
<br />deeper depth, The hydraulic analysis did indicate the potential for a number of hydraulic jumps
<br />primarily located immediately downstream from the bridges. Additional cross sections were added
<br />near these bridges to determine approximately where the jump occurs.
<br />
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