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6 horizon. (Since these characteristics are based on arid desert environments, desert varnish, desert pavement, and calcic horizon may not apply to non-desert and coastal environments.) The degree of development of each of these characteristics is indicative of the relative age of the alluvial fan. That is, geologically young (less than 10,000 to 15,000 years old) fans show none to weak development, while intermediate (10,000 to 700,000 years old) and old (greater than 500,000 years old) fans have progressively stronger development of these characteristics, Step 3. Apply Methods to Define the Flooding Classification on the Alluvial Fan. Step 3 is the utilization of appropriate methods to predict the flooding hazard on alluvial fans; and, to classify alluvial fans based on the nature of flooding potential with consideration of existing and any future development in the areas. A detailed theoretical analysis of alluvial fan flooding is not the intent of this study. However, a general discussion of methods that can be used to define the flood hazards is appropriate. Conceptually, there are three categories of alluvial fan flooding: (1) clear water flows that can be analyzed using traditional hydraulic methods, (2) hyper- concentrated sediment flows that can be analyzed by sediment transport theory, and (3) debris flows that can be analyzed by various empirical methods such as the bulking factor, the Bingham models, and other methods. The choice of methods is directly related to the type of fan to be evaluated. Inactive Alluvial Fans. For inactive fans where flow paths and flood boundaries are stable and predictable, traditional engineering models are mainly used to compute water-surface profiles and define the 100-year flood. An inactive fan with flooding is typically considered as riverine flooding. Traditional Models. Two engineering models that can be used to define the flood hazard on inactive, stable fans are HEC-HMS and HEC-RAS, HEC-HMS is a hydrologic modeling system capable of simulating rainfall/runoff modeling, The model can be used to predict flood flow data at the fan apex to determine the 10-, 50-, 100- and 500-year peak flows if streamflow records are not available. HEC-RAS is a hydraulic modeling system which uses peak flow data and channel cross sections to delineate stable distributary channels of a river system. The model can handle a full network of channels, a dentritic (tree-like) system, or a single river reach. However, the disadvantage of this model is that the assumptions of channel stability, friction, flow distributions and losses, and flow-path locations may be severely limited. Backwater models by USGS or NRCS can also be used for inactive alluvial fans. A hydrodynamic model such as FLO-2D can also be used for some areas on inactive fans. Active Alluvial Fans. For active fan areas where flow paths are unstable and unpredictable, the flood hazard is assessed using geomorphic evidence and possibly supplemented with engineering modeling, Since the frequency and extent of flooding on active fans cannot be determined exactly, the status of existing or planned development in the area is also a major factor for consideration in Step 3 for active fans. Geomorphic Evidence. Using geomorphic evidence is preferred for active alluvial fan areas where there are changes in channel conditions during flooding. This method uses channel shape, surface geology, soil development, and geomorphology to define the flood hazard on an alluvial fan. The types and sources of information are the same as used in Steps I and 2; that is,