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<br />V. FLOOD HAZARD AREA DELINEATION
<br />
<br />A.
<br />
<br />GENERAL
<br />
<br />The Flood Hazard Area Delineation (FHAD) identifies those areas, structures, and building$, which are
<br />expected to be inundated during a 1 OO-year flood event. Certain areas within the 1 OO-yearfloodplains must be
<br />kept free of obstructions in order for the 1 OO-year flood to be carried without significantly increasing the energy
<br />grade line, This area, identified as f1oodway, is defined and determined as being the limit of encroachment into
<br />the 1 OO-year floodplains where the resulting energy gradeline profile is not increased more than a specified limit
<br />(i.e" 0.5-feet, 1,O-feet), The 1,O-foot rise f100dway is used by the Federal Emergency Management Agency and
<br />, the 0.5-foot rise f100dway is the f100dway adopted for floodplain regulation purposes by the local entities,
<br />
<br />B. FLOODPLAIN/FLOODWAY
<br />
<br />The 1 OO-year floodplain limits and water surface profiles are shown on the FHAD drawings, sheets 1
<br />through 34. Also shown on these drawings are the 10. and 1 OO-year water surface profiles, cross-sections of
<br />existing crossing structures, and typical cross-sections of the channel for each sheet.
<br />
<br />Presented in Table V-1 is the pertinent f1oodplain/floodway information for this study, This table includes
<br />the following items:
<br />
<br />. Description, labels, and stations of all cross-sections
<br />. Thalweg elevations
<br />. 10-, 50-, 100., and 500-year peak runoff rates and water surface elevations
<br />. 1 OO.year flow velocities, top widths, and cross-sectional areas
<br />. 100-year, 0,5- and 1,0- foot rise f100dway data
<br />. Floodway Distance Left of centerline, Floodway Distance right of centerline, and total f100dway widths
<br />
<br />The 10-, 50-, 100-, and 500.yearflood profile elevations were determined using the U,S. Anny Corps of
<br />Engineers, HEC-RAS, water surface profile computer program (Reference 8). The 0.5-foot and 1,O-foot rise
<br />f100dways were determined using encroachment Method 4 and Method 1 of the HEC-RAS program. This
<br />method as applied herein sets left and right encroachment targets based upon equal conveyance reductions
<br />
<br />and adjusts them until the target level of 0,5- or 1,O-foot rise in the 1 OO-year energy grade line elevation was
<br />achieved.
<br />
<br />There are several locations within the study limits where application of the regulatory f100dway would
<br />result in only a few feet of allowable encroachment due to hydraulic constraints. In these areas, the f100dway
<br />was set equal to the floodplain (Le, no encroachment), The f100dway left and right widths are measured from
<br />the stationing line, which is not always the low point of the channel.
<br />
<br />The flood contours shown on the drawings are shown such that interpolation between contours should
<br />result in a 100-year water surface profile which is within 0.5 feet:f:: of the computed 100-year water surface
<br />profile,
<br />
<br />C, COMPARISON WITH CURRENT EFFECTIVE FLOOD INSURANCE STUDY,
<br />
<br />The existing floodplain infonnation for Kinney Creek and Fonder Draw shown on the effective
<br />Flood Insurance Rate Map (Reference 6) was prepared based on the FEMA's approximate hydrologic and
<br />hydraulic study method, Therefore, no detailed hydrologic and hydraulic analysis backup data for the study
<br />area was available, The limits of the Fonder Draw current effective approximate floodplain as shown on FEMA
<br />Flood Insurance Rate Maps closely match the limits of the new Fonder Draw detailed floodplain. The new
<br />Kinney Creek detailed floodplain extends approximately 5600 feet further upstream than the current effective
<br />approximate floodplain, The new Kinney Creek floodplain now covers the tributaries to Kinney Creek that were
<br />not included in the current effective approximate floodplain, The current effective approximate floodplain is very'
<br />similar in size to the new detailed floodplain over the entire study area, The most significant difference between
<br />the two floodplains occurs on Fonder Draw immediately downstream of Bingham Lake. The new floodplain
<br />shows flow passing through the dam's spillway and re-entering the stream approximately 1650 feet downstream
<br />of the spillway, The current effective approximate floodplain only showed flow passing through the dam's piped
<br />outlet works, not through the emergency spillway,
<br />
<br />V-I
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