FLOOD02285 - Laserfiche WebLink

Home Browse Search

. 3 Indicators for Characterizing Alluvial Fans and Alluvial Fan Flooding Alluvial fans and alluvial fan floods show great variability in climate, fan history, rates and styles of tectonism, source area lithology, vegetation, and land use, For this reason, it is essential that any investigation of alluvial fan flooding include careful examination of the specific fan for which information is needed. The committee recognizes that the extent of site-specific examination will be constrained by factors such as the amount of time and money allotted to the project, the tools available to the investigator, and the investigator's experience. As discussed in lhis chapter, however, much information can be gleaned from topographic and soil maps, as well as aerial photographs. Nevertheless, it is essential to do at least one field inspection of the fan that involves walking across its surfaces and along its channels, In general, the more fieldwork done, the better the understanding of the processes of flooding on the fan of interest. According to the definition presented in Chapter I, for regulatory purposes alluvial fan flooding is a flood hazard that on active parts of alluvial fans has a I percent chance of occurrence, and it is identified by flow path uncertainty and deposition and erosion below the hydrographic apex. The criteria used to assess whether an area is, or is not, subject to alluvial fan flooding must determine whether the flooding occurs on an alluvial fan and whether it is characterized by deposition, erosion, and flow path uncertainty below a hydrographic apex. For these reasons, the process of determining whether or not alluvial fan flooding can occur at a given location, and of defining the spatial extent of the 100-year flood, are divided into three stages: I. Recognizing and characterizing alluvial fan landforms. 2. Defining the nature of the alluvial fan environment and identitying areas of active erosion, deposition, and flooding (as well as inactive areas). 3. Defining and characterizing areas on active parts of alluvial fans that are subject to a I percent chance of occurrence (the 100-year flood), the FEMA mandate. Progression through each of these stages results in a procedure that narrows the problem to smaller and smaller areas of uncertainty (Figure 3- I). In Stage I, the landform on which flooding occurs must be characterized. If the location of interest is an alluvial fan, then the user progresses to Stage 2, in which those parts of the alluvial fan that are active and inactive are identified. The term active means those locations where flooding, erosion, and deposition have 51