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<br />drainage, that are on natural high ground, or that have
<br />been elevated to a point \vhere they are passable in all
<br />weather. As part of the rcviev\" process for new
<br />subdivisions in flood hazard areas, developers can be
<br />required to 511m\! hm-\' the subdivision streets relate to
<br />evacuation routes.
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<br />SPECIAL CONSIDERATIONS IN ALLUVIAL FANS
<br />Alluvial fans are triangular or fan-shaped landforms
<br />typically found at the base of canyons and mountain
<br />ravines. They are created over thousands of years as
<br />sediment is transported by flood\vaters and deposited
<br />onto the valley slope. These areas invite residential
<br />development with their excellent mountain and valley
<br />views and good local drainage.
<br />Unlike riverine floods, alluvial fan floods do not
<br />necessarily occur in the same place nor take the same
<br />flmv path from one event to the next. The hazards that
<br />occur on alluvial fans are also more '\-vide ranging and
<br />severe than those that occur during riverine floods.
<br />High-velocity vI.rater, erosion, and scouring to depths of
<br />several feet; deposition of sediment, debris, and mud to
<br />several feet; and flash flooding are typical during
<br />alluvial fan floods. Several of these hazards can occur
<br />during the same flood, and the next flood may exhibit
<br />completely different hazards.
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<br />Figure 4-4. Development on Alluvial Fans
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<br />The primary objective of flood protection on alluvial
<br />fans should be to minimize disruption to the natural
<br />flood processes. To that end, there are three general
<br />types of flood protection in these areas. The first are
<br />measures to protect the VI/hole fan; the second are
<br />measures to protect certain subdivisions or localized
<br />areas of the fan; and the third are to protect individual
<br />structures on the fan. Protection is accomplished
<br />through structural or nonstructural methods, or a
<br />combination thereof. Structural methods include dikes,
<br />channels, and debris basins that steer water and debris
<br />to undeveloped areas of the fan. Nonstructural methods
<br />include floodplain management ordinances or
<br />building code provisions requiring structures to be
<br />elevated and/or flood proofed, zoning and
<br />subdivision controls that limit allovl.'able
<br />development density, and programs to acquire land
<br />to keep it as open space.
<br />A debris flow basin placed at the apex of the alluvial
<br />fan to trap boulders and sediment and prevent it from
<br />depositing down the mountainside or damaging homes
<br />is a common means of reducing the potential for
<br />alluvial fan debris. At the site level, deflection walls and
<br />berms, debris fences, dikes, and channels can be built to
<br />route \vater and sediment around or through residential
<br />developments. Subdivision streets can be configured to
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<br />AlIuz))'al fill! floodillg can be very difficult to plan for or predict. At a minimultl, development 011
<br />fans should be limited to the It'ast floildpronc portfoll of the fan, as delermilled by Cflxif/eerillg
<br />studies. Subdivision streets can be cOHfigured to collvey wllter ami debris flows aUHlY from
<br />homes, but SiXllifical1t damage to streets is likely if used for this purpose.
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