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I I I I I I I I I I I I I I I I I I I VI. FLOOD HAZARDS The 100-year flood event is considered by the State and the UDFCD as the flood magnitude for which floodplains should be designated for regulatory purposes. The 1oo-year floodplain limits. and the 10- and 1oo-year water surface profiles are shown on the Flood Hazard Area Delineation maps and profiles, contained in the rear of this report. Beebe Draw and Left Bank Tributaries flood hazards are limited mainly to road crossing areas. Parts of two fannsteads and part of a garden nursery are within the 1oo-year floodplain limits. Other man-made structures within the 1oo-year floodplain ar~ irrigation structures and reservoirs and the Burlington Northern Railroad tracks. East 168th Avenue, East 160th Avenue, East 152nd Avenue (Bromley Lane) and Piccidilly Road would be overtopped by Beebe Draw during the 1oo-year flood. Interstate 76 will be overtopped by Redwing Creek, Meadowlark Run and Magpie Run during the 100-year flood. The frontage road at 1-76 will also be overtopped by Meadowlark Run and East 152nd Avenue will be overtopped by Magpie Run during the 100- year flood. The 100-year overtopping depths for Beebe Draw range from two feet at Piccadilly Road and East 152nd Avenue to four feet at East I60th Avenue and East 168th Avenue. The depth along with the overtopping velocity combine to create hazardous conditions for vehicles attempting to cross the flood waters. The majority of the floodplain area is either agricultural or pasture land. The average 1oo-year flood velocity is less than three feet per second. The channel velocity can be much higher at certain constrictions, irrigation structures or at the road crossings, but in general, the floodplain velocities are low. Average channel depth is about six feet except in areas upstream of road crossings where the depth can exceed ten feet In general, the overbank areas will be inundated by about two feet of water during the loo-year flood. The irrigation reservoirs, (Meek Lakes No.1 and 2, Bowles Reservoirs No. I and 2 and Foster Lake) are at or below the channel of the Beebe Seep Canal and are all inundated during major flood events. Additional flood hazards exist that are beyond the mapping limits for this study. The Speer Canal and West Burlington Canal will be overtopped by floods on Redwing Creek, Magpie Run and Meadowlark Run. The irrigation canals may divert flood flows outside of the watersheds to areas with lesser capacity where overtopping would occur. The Burlington Northern Railroad and 1-76 embankments divert flood flows parallel to the roadways. cross section. The 10- and loo-year water surface profile, channel thalweg profile and cross section locations are given on the flood profile sheets, and the loo-year floodplain and floodway limits are given on the flood boundary maps. The FHAD maps are found in the appendix. Sheet I through 20 contain the plan views and the profiles are given in Sheets 21 through 30. The floodways have been computed for the study reaches with allowable encroachment limited to that causing six inches increase in the 100-year water surface elevation. The floodway computations are based on passing the lOO-year flood with a proportionate degree of encroachment on both sides of the channel based on the conveyance of the overbank. The floodways depicted on the FHAD maps will result in a lOO-year water surface elevation increase of no more than six-inches, over the existing flood profile. Floodway widths, representing the limit of encroachment into the floodplain at each cross section used in the computation, are tabulated in Tables I through 4. The water surface elevations to be expected after encroachment are also tabulated. FlooQplain and Floodway Data The limits of future flooding are outlined on the FHAD maps and tabulated in Tables I through 4, floodplain and floodway reference data. The tables list the 100-year floodplain and floodway width and 100-year water surface elevation with and without the floodway at each 12