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<br />For Tributaries 1 and 2, the lOa-year flood boundaries were deter- <br />mined by normal depth computations using field observations and <br />topographic maps enlarged to a scale of 1:2400, with a conLou[ <br />interval of 20 feet (Reference 11). <br /> <br />Approximate flood boundaries in some portions of the study drr'a <br />were taken from the Flood Hazard Boundary Map (Reference J2). <br /> <br />Flood boundar ies for the 100- and SOD-year floods are shown on <br />the Flood Boundary and Floodway Map (Exhibit 2). In cases where <br />the 100- and SOD-year flood boundaries are close together, only <br />the lOa-year flood boundary has been shown. Small areas within <br />the flood boundaries may lie above the flood elevations and, ther,,- <br />fore, not be subject to flooding; owing to limitations of the <br />map scale, such areas are not shown. <br /> <br />4.2 Floodways <br /> <br />Encroachment on flood plains, such as artif icial fill, reduces <br />the flood-carrying capacity, increases the flood heights of stl ~ ,1;'ll:~ <br />and increases flood hazards in areas beyond the encroachment its~l f. <br />One aspect of flood plain management involves balancing the economic <br />gain from flood plain development against the resulting increase <br />in flood hazard. For purposes of the National Flood Insurance <br />Program, the concept of a floodway is used as a tool to assist <br />local cormnuni ties in this aspect of flood plain management. Under <br />this concept, the area of the lOa-year flood is divided into a <br />floodway and a floodway fringe. The floodway is the channel of <br />a stream plus any adjacent flood plain areas that must be kept <br />free of encroachment in order that the lOa-year flood may be carrlcd <br />without substantial increases in flood heights. Minimum standards <br />of the Federal Emergency Management Agency limit such increases <br />in flood heights to 1.0 foot, provided that hazardous velocities <br />are not produced. The floodways in this report are presented <br />to local agencies as minimum standards that can be adopted or <br />that can be used as a basis for additional studies. <br /> <br />The floodways for Yampa River, Fortification Creek. Sand Gulch, <br />Sand Gulch Tributary, Brotherton Gulch, the upper 60 percent (ap- <br />proximately) of Cedar Mountain Gulch, the upper 40 percent (approxl- <br />mately) of Pine Ridge Gulch, Lay Creek, and Big Gulch were computed <br />on the basis of equal-conveyance reduction from each side of the <br />flood plain. No floadways were computed for the lower 40 percent <br />of Cedar Mountain Gulch and for the lower 60 percent of Pine Rid~c <br />Gulch due to lOa-year sheet flow diversions from Cedar Mountain <br />Gulch and pine Ridge Gulch, and due to the complex backwater conditio~ <br />between Pine Ridge and Cedar Mountain Gulches created by the triple <br />box culvert under State Highway 13 (County Road 7). However, <br />no encroachment is allowed within the stream channels. The complete <br />filling of these sheet flow areas will increase floodflows downstream, <br /> <br />16 <br />