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I- 100-YEAR FLOOD PLA!N -I FLOODWAY FRINGE FLOODWAY FLOODWAY r RINGE STREAM CHANNEL FLOOD ELEVATION WHEN CONFINED WITHIN FLOODWAY AREA OF FLOOD PLAIN THAT COULD BE USED FOR DEVELOPMENT BY RAISING GROUND o ,.,1 FLOOD ELEVATION BEFORE ENCROACHMENT ON FLOOD PLAIN LINE AS IS THE FLOOD FU:'VATION BEFORE FNCROACHMENT LINE CD IS THE FOLDOD ELEVATION AFTER ENCROACHMENT .SURCHARGE IS NOT TO EXCEeO 1.0 FOOT (FIA RFOUIREMENT) OR LFSSFR AMOUNT IF SPECIFIED BY STATE Figure 3. Floodway Schematic It should be noted that the floodway for Cement Creek downstream of cross section 306 was determined using the flow confined to the creek only (980 cubic feet per second). A floodway through the shallow flooding area south of cross section 30G into the Animas River is not recommended because the area is developed and floodwater will be less than 1.0 foot in depth. It should also be noted that channel improvement is required downstream of the breakout point if the community chooses to encroach the creek at the breakout point. An encroachment will increase floodflow downstream frqm 980 to 1640 cubic feet per second. The channel improvement is to ensure that the encroachment will not cause the water-surface elevation of the lOO-year flood to use 1.0 foot more than the existing condition. 5.0 INSURANCE APPLICATION In order to establish actuarial insurance rates, the Federal Insurance Administration has developed a process to transform the data from the engineering study into flood insurance criteria. This process incluc1e~ the determination of reaches, Flood Hazard Factors, and flood insurance zone designations for each flooding source studied in detail affecting the Town of Silvcrton and the designated unincorporated areas of San Juan County. ]7