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='~.::"""'-:~2 --:-;'51 ~ ::--UXlD .s"t-~~ ~c..EVA.'fIClN :::-TA"-;CI\. _':'701. 74 + :,C 5704 --;'6.0- 4C 570[, ~e .;.- 10 S-;ce- 7'?.... 5~ '571C E: +--'10 S"'12 83":- '30 ~:!4- e5+-20 S~ I;;' 671-20 1- SAMPLE PLOTTING TABLE (STEP 3) Critical Sub Critical Sub Cnllcal REGIMES OF FLOW , / ~ .f. " , 1,,-,' ~ LOCATING FLOOD CONTOURS (STEPS 4 AND 5) FINISHED FLOOD BOUNDARIES (STEP 6) information. Maintainin9 this continuity between studies will improve the workability of a floodplain management program, Jevelopers of land in the floodplain who use flood elevations established In a floodplain study as a guide must use that information accurately. The establishment of field survey datum continuity between the study results anel the on-the-ground conditions is a major factor to accomplish this, This can be done through: . Uniform Datum Standards --- The base levels, elevations. and mapping may be tied to a sinlJle local datum; it is preferrable however, that they be tied to the National Datum (MSL for verUcal datl1fT! and Slate Plane Coordinates for horizontal datum). By tying to the National Datum, uniformity is maintained within the region, the state, and the nation. . Benchmarks -- Permanent elevations can be established in the field through a program of monumentaUon in the field, This can be accom- plished easily by using Federaliy establish,.d benchmarks where avail- able. Where Federal benchmarks do not exist within a community, it is beneficial to construct a control marker network. By having control markers with known elevations, property owners can more easily and accurately determine their own flood hazard risks through conventional field surveying procedures. Floodways Two basic concepts for dividing the 1 OO-year floodplain into a ffoodway and a flood fringe are the Rise Concept and the Hazard ArHa Concept The Rise Concept, speGifies a maximum allowable rise (.5' or 1.0') in water ~;urface elevations due to encroachment into the floodplain while still allow- ing passage of the> 1 OO..year flood (provided hazardous velocities, which are those in excess of 3 feet per second (fps), are not produced). The Hazard Area Concept, specifies the area with a water depth of 18 inches or greater and a water velocity of 3 Ips or greater as the floodway which must be kept free of development. The Rise Concept is based on an "acceptable rise" criterion, This rise criterion assumes that encroachment occurs on the edges of the floodplain on both sides until the 1 OO-year water surtace IS increased by ,5 foot or 1.0 foot. It is suited to wide, flat-sloped, and wHII-dE,fined floodplains where con. veyance capacity must be preserved. By assuming encroachment into the floodplain from both sides, the floodway line may be calculated. Often thiS IS conservative, since development may not take place at every point. The rise floodway determination has some drawbacks. It is an expenSive process and it may allow for inconsistent development. FEMA's floadway ljuidelines require equal reduction in stream conveynacH capacity from both ,;ides of the floodplain. This equal reduction is an attempt to treat all land. owners on both sides of the stream equitably and give all an opportunity to develop some of this capacity by placement of fill. There is, however, some prOVision for variance from the requirement. Use of this variance provision can mean that some> property owners may have more developable land In the l'Ioodplain and some property owners may have less.