YAVAPAI COUNTY, ARIZONA Federal Emergency ... - FEMA Region 9

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The boundaries for the approximate study along Beaver Creek School Wash and Rimrock Creek in the were delineated using 2-foot contour mapping created from aerial photography flown at a scale of 1 inch = 100 feet provided by Cooper Aerial and collected in 1998. HEC-RAS and WSP modeling was done by Claycomb/Rockwell Associates, Inc., in 1999. The boundaries for the approximate study of 4.5 miles along Yarber Wash in the Town of Dewey-Humboldt were delineated using 2-ft contour mapping created from aerial photography flown and provided by Southwest Mapping Technologies, Inc., and collected in 2005. HEC-RAS modeling was done by Project Engineering Consultants, Ltd., in 2005. The boundaries for the approximate study along Oak Wash, Cherry Hills Wash, Rio Mesa Wash, Christina Draw, and Pipe Creek were delineated using 4-foot contour mapping created from aerial photography flown at a scale of 1” = 200 feet and provided by Analytical Surveys, Inc., and collected in 1992. HEC-RAS modeling was done by McLaughlin Kmetty Engineers, Ltd., in 1995. 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the 1-percent annual chance floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the 1-percent annual chance flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to one foot, provided that hazardous velocities are not produced. The floodways in this study - are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways presented in this study were computed for certain stream segments on the basis of equal-conveyance reduction from each side of the floodplain. Floodway widths were computed at cross sections. Between cross sections, the floodway boundaries were interpolated. The results of the floodway computations are tabulated for selected cross sections (see Table 9, "Floodway Data," shown in Volume 2). In cases where the floodway and 1-percent annual chance floodplain boundaries are either close together or collinear, only the floodway boundary is shown. In the case of shallow flooding in Yavapai County, the unpredictable flowpaths have made determination of a floodway meeting FEMA criteria impossible; therefore no floodway is presented in these areas (FEMA, 1999). The hydraulic modeling of Deception Wash, Bitter Creek, and Bitter Creek South Fork in the Town of Clarkdale indicated that floodway encroachments would not 55
e feasible in view of extremely high velocity zones (approximately 10 feet per second). This applied to all tributary stream reaches within the Town of Clarkdale. Tile floodways on these streams are coincident with the 1-percent annual chance floodplain boundary as shown on the FIRM (FEMA, 1982). In the City of Cottonwood, no floodway encroachments were computed for those tributary streams to the Verde River in which all or a major portion of the 1- percent annual chance floodwaters are contained within well-defined channels. Encroachments into these well-defined channels would result in extreme high velocity zones, as is indicated by existing flow velocities of approximately 14 feet per second. The floodways on these streams were made coincident with the 1- percent annual chance floodplain boundary. The tributary streams along which the floodway concept was seen as appropriate were Silver Springs Gulch and Railroad Wash (FEMA, 1981). In the Town of Prescott Valley, the 1-percent annual chance flood on North Navajo Drive Wash stays within the natural channel boundaries; therefore, no floodway encroachments were made for this wash (FEMA, 1982, revised 1990). The area between the floodway and 100-year floodplain boundaries is termed the floodway fringe. The floodway fringe encompasses the portion of the floodplain that could be completely obstructed without increasing the water-surface elevation of the 1-percent annual chance flood more than one foot at any point. Typical relationships between the floodway and the floodway fringe and their significance to floodplain development are shown in Figure 1. 56
Page 1 and 2:
YAVAPAI COUNTY, ARIZONA AND INCORPO
Page 3 and 4:
TABLE OF CONTENTS - Volume 1 Page 1
Page 5 and 6:
TABLE OF CONTENTS - Volume 3 - cont
Page 7 and 8:
TABLE OF CONTENTS - Volume 5 - cont
Page 9 and 10:
authority and acknowledgments for e
Page 11 and 12: In the March 29, 1983, revision, up
Page 13 and 14: The September 20, 1996, revision al
Page 15 and 16: update corporate limits, to change
Page 17 and 18: TABLE 2 - STREAMS STUDIED BY DETAIL
Page 19 and 20: TABLE 3 - LETTERS OF MAP CHANGE - c
Page 21 and 22: Precipitation in the study area is
Page 23 and 24: TABLE 4 - HISTORIC RECORDED DISCHAR
Page 25 and 26: The Verde Lakes Estates in the Town
Page 27 and 28: Problems created by large magnitude
Page 29 and 30: All flooding sources were analyzed
Page 31 and 32: approximately 1,600 feet downstream
Page 33 and 34: TABLE 5 - SUMMARY OF DISCHARGES FLO
Page 35 and 36: TABLE 5 - SUMMARY OF DISCHARGES - c
Page 47 and 48: Mapping, Inc. (Kenney Aerial Mappin
Page 49 and 50: 1985. It was determined from the si
Page 51 and 52: Values for the roughness coefficien
Page 53 and 54: TABLE 6 - MANNING'S "n" VALUES - co
Page 55 and 56: To minimize the impact of the levee
Page 57 and 58: ecommended as the approximate areas
Page 59 and 60: TABLE 8 - VERTICAL DATUM CONVERSION
Page 61: Approximate 1-percent annual chance
Page 65 and 66: average depths are between 1 and 3
Page 67 and 68: COMMUNITY NAME INITIAL IDENTIFICATI
Page 69 and 70: The only other study completed for
Page 71 and 72: Cooper Aerial Surveys, Aerial Photo
Page 73 and 74: Kenny Aerial Mapping, Topographic M
Page 75 and 76: U.S. Department of the Army, Corps
Page 77 and 78: Joes Hill, Arizona (1969); King Can
Page 79 and 80: Pipe Creek Railroad Wash Silver Spr
Page 81 and 82: Manning’s “n” values were ass
Page 83 and 84: FLOODING SOURCE FLOODWAY 1-PERCENT-
Page 85 and 86: FLOODING SOURCE FLOODWAY 1-PERCENT-
Page 87 and 88: TABLE 9 FLOODING SOURCE FLOODWAY 1-
Page 93: TABLE 9 FLOODING SOURCE FLOODWAY 1-
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YAVAPAI COUNTY, ARIZONA Federal Emergency ... - FEMA Region 9

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