ALLUVIAL FAN FLOODING by Committee on Alluvial Fan Flooding

Author:Committee on Alluvial Fan Flooding
Language: eng
Format: epub
Tags: Earth Sciences : Earthquakes, Floods and Natural Disasters. Earth Sciences : Geology and Landforms
Publisher: NATIONAL ACADEMY PRESS
Published: 1996-10-08T00:00:00+00:00

Because alluvial fan flooding is associated with high rates of erosion, sediment transport, and deposition, it is common for such flows to shift position as sediment is dropped and forms obstructions to the flow. In some events, previous channels are completely blocked by deposits, and a new channel is formed. This process is known as avulsion and can be identified from aerial photos or field mapping by the presence of topographic lows (abandoned channels), the upstream parts of which filled with sediment. Areas of potential channel avulsion sometimes can be identified from construction of longitudinal and cross-fan profiles, because avulsion is likely to occur in places where sedimentation has raised the channel floor surface to a level that is nearly as high as the surrounding surface of the alluvial fan.

In addition, human modification of alluvial fan surfaces and urban development on alluvial fans have resulted in cases where human-made obstructions themselves have been the cause of alluvial fan flooding. For example, construction of culverts to divert water from one part of a fan to another sometimes results in rapid sedimentation downstream from the mouth of the culvert. The result can be that alluvial fan flooding then occurs in an area that might not have been mapped as susceptible to this type of flooding before human alteration of the landscape. Special attention is needed to identify areas where engineered works might aggravate or cause alluvial fan flooding during the time period designated as active by the investigator.

Specific steps that should be followed before undertaking any final delineation of alluvial fan flooding hazards include detailed office and field reviews of historical information and the evaluation of the present landform. Initial office procedures include the review of topographic maps and aerial photographs to determine the location and the morphology of the landform to determine whether it is a true alluvial fan. Other data that should be gathered early include historical maps and old photographs to document channel changes, changes in channel morphology, and the areas of the fan that may be classified as either active or inactive. Soil and geologic mappings should be examined to confirm the relative geologic age of fan deposits. Climatologic data and appropriate hydrologic analyses will be needed to determine the magnitude and frequency of flooding to be addressed. Aerial photographs and geologic information of the catchment area will provide indications of the amount of sediment and debris that can be delivered to the fan.

Field investigations by a trained observer should include gathering information on elevation differences across the fan and in a transverse direction if detailed topographic maps are not available. Vegetation types, soil characteristics, and the presence of desert varnish should be added to the office maps to confirm the active or inactive portions of the fan. Observations and measurements of channel conditions should be made to determine areas of possible avulsion. Detailed inspection of diffluences or abandoned channels should indicate the most likely flow paths. The results of the initial office and field investigations should provide sufficient information to direct the final analysis.

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