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<br />5 <br /> <br /> <br />To meet the definition of an alluvial fan, the landform must meet the following criteria. <br /> <br />Composition. An alluvial fan is a sedimentary deposit of loose, unconsolidated or <br />weakly consolidated sediments (alluvium) which have been deposited during the Quaternary <br />period. <br /> <br />Shape. Alluvial fans are landforms that have the shape of a fan either partly or fully <br />extended, Modem and old flow paths on these fans radiate outward to the perimeter of the fan <br />terminus or toe. <br /> <br />Topographic Break, Alluvial fans are located at a topographic break such as the base of a <br />mountain, escarpment, or valley side. <br /> <br />Boundaries, Boundaries of an alluvial fan consist of toe and lateral boundaries. The toe <br />is usually defined by an intersecting stream at the downstream perimeter of a fan, Lateral <br />boundaries are the edges of deposited and reworked alluvial materials. <br /> <br />Step 2. Determine the Activity and Stability of the Alluvial Fan, Step 2 uses <br />geomorphic and weathering characteristics, and geologic age to determine whether the alluvial <br />fan is active or inactive, An active area is geologically very young (less than 10,000 years, <br />Holocene time) and unstable, while an inactive area is geologically older (greater than 10,000 <br />years, Pleistocene time) and stable. Activity can be analyzed by comparing old and recent <br />topographic maps and aerial photographs, land surveys, and documented reports of flooding. <br /> <br />Activity. The term "active" refers to that part of an alluvial fan where flooding, <br />deposition, and erosion are possible. Most alluvial fans have both active and inactive parts, and it <br />is important to identify both parts because this provides a map of where flooding can occur, as <br />well as where it probably will not occur. Types of characteristics used to determine activity <br />include drainage pattern, channel definition and incision, surface color, desert varnish, desert <br />pavement, and calcic horizon. Documented records of past flooding and debris flow on a fan <br />indicate an active alluvial fan. If flooding and deposition have occurred on part of a fan in the <br />past 1,000 years (during Holocene time), that part of the fan can be considered to be active. <br /> <br />Stability. Unstable areas have uncertain flow paths, This flow path uncertainty "is so <br />great that this uncertainty cannot be set aside in realistic assessments of flood risk or in the <br />reliable mitigation of the hazard II (NRC, 1996). Areas of flow path uncertainty and boundaries <br />for unstable areas are predictable using geomorphic evidence based on channel shape, surface <br />geology, soil development, and morphology. Stable areas include defined channels within <br />inactive alluvial fans. Flood boundaries for these stable areas are predictable using traditional <br />engineering modeling based on hydraulic factors of roughness, grade, and channel cross- <br />sectional geometry. <br /> <br />Geologic Age. Eight geomorphic and weathering characteristics, identified by <br />Christenson and Purcell (1985), are useful in separating alluvial fan deposits into three general <br />age categories. These characteristics include drainage pattern, depth of incision, fan surface <br />morphology, preservation of fan surface, desert pavement, desert varnish, B horizon, and calcic <br />