FLOOD00306 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I 11 07/16/99 Use and interpretation of diagnostic vegetation, just like the use and interpretation of desert pavement, varnish, or soil properties are generally specific to the individual fan in question. Within a geographic region, however, surface characteristics of alluvial fans may be correlated from one fan to another. Detailed topographic maps (i.e. 2-foot contour interval) are instrumental in identifying potential avulsion areas and in delineating the boundaries of areas subject to different flood, deposition and debris flow depths. Topographic maps also can be used to identify older alluvial surfaces within active zones that are not subject to flooding. Areas of question noted during the analysis of maps and aerial photographs should be closely examined during the field inspection. All flow paths should be walked to verify the active and inactive areas that have been delineated. Stage 2 is complete when the analyst has defmed and delineated all active and inactive areas of deposition, erosion, and unstable flow path flooding, as well as adjacent inactive fan areas. All inactive areas with stable flow path flooding and all active areas may be considered f1oodprone, but through Stage 2 the degree to which these areas are f1oodprone is not yet known. The delineated floodprone areas of Stage 2 should approximate the largest possihle extent of the lOO-year flood. 4D Types of Alluvial Fan Flooding Several types of flooding occur on alluvial fans. The most common ones are described in this section. 4D (1) Flooding Along Stable Channels A deeply entrenched channel or network of channels often is subject to inactive alluvial fan flooding. This type of flooding usually occurs within distributary flow systems that were fonned during climatic or tectonic conditions different than the present This flooding can occur at the head of the alluvial fan, but become unstable downstream. Conversely, unstable channels can become stable in the downstream direction because of head cutting into the toe due to changing hydraulic conditions downstream from the toe. Human intervention, directly by channel modification or indirectly by land-use change, can create stable channels. 4D (2) Sheetflow Some parts of alluvial fans are characterized by sheetflow, which is the flow of water as broad sheets that are completely unconfined by any channel boundaries. Sheetflow might occur where flow departs from a confined channel and no new channel is fonned. It might also occur where several shallow, distributary channels join together near the toe of a fan and the gradient of the fan is so low that the flows merge into a broad sheet. Because such sheetflows can carry high concentrations of sediment in shallow water and follow unpredictable flow paths, they are classified as active alluvial fan flooding. Sheetf\ow generally occurs on downslope parts of fans, where channel depths are low and the boundaries of channels become indiscernible. They are also more common at distal locations