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<br /> <br /> <br />How Are Alluvial Fans Identlllad? <br /> <br />The gentle, imperceptible slopes and extensive size of many <br />fans can sometimes make their identfficatlon in the field <br />dffficun. As a result, many homeowners do not realize that the <br />panoramic views of the valley below are due to their elevated <br />location on an alluvial fan. The initial identffication of the <br />presence and extent of fans w~hin the community Is therefore <br /> <br /> <br />-,:., ':;;"_._.._.':.O'--'._.....i <br />:~ ~ ...!*""'~";"'$""B!'V'''''- ......1:."" "" ''''';;;: <br /> <br />Figure 4. <br />Thl. U.5. GeoIoglcsl SutvBY topographic quadrangle map has been .~ <br />hanced to show the concentric contour pattern typica' of .lIuvlal flms. <br />Depicted her. ar. two alluvial f.ns In Wenatchee, Washington. <br /> <br />best accomplished through the use of topographic maps or <br />aerial photos. Topographic maps (such as those produced by <br />the U.S. Geological Survey) reveal concentric semicircular <br />contours that bow downsiope, as shown on the figure above. <br />The use of soil survey maps (such as those developed by the <br />Soil Conservation Service) can also indicate the presence of <br />alluvial (water-transported) soils which often comprise fan <br />formations. In confirming the location and extent of alluvial <br />fans, the final detailed interpretation of maps, air photos and <br />field data should be made by a professional geologist, hydrolo- <br />gist, and/or engineer. <br /> <br />What Are The Flood Hazards Associated With Alluvial <br />Fans? <br /> <br />Alluvial fan floods do not exhib~ the more predictable behavior <br />and well-defined boundaries normally found in most riverine <br />floods. The behavior and path of flood water in any individual <br />flood event as ~ proceeds from the apex to the toe is a direct <br />resun of the flood processes previously illustrated. These <br />processes vary as a function of the flow's sediment content <br />and velocity, the fan's slope, soil and vegetative cover, and <br />types and amount of fan development. <br /> <br />Alluvial fan flows are subject to lateral migration and sudden <br />relocation during the course of a flood, and may not even follow <br />the same path in subsequent floods; in any flood event, <br />however, a part of the fan will always be subject to floed <br />hazards. Thus, ~ is generally not appropriate to utilize the <br />location of past flow paths in the prediction of future hazards. <br />The full range of hazards that may be encountered on fans <br />include: <br /> <br />high-veloc~y flow (as high as 15.30 feet per second), <br />producing signfficant hydrodynamic forces (pressure <br />against buildings caused by the movement of flowing <br />water) <br /> <br />erosion/scour (to depths of several feet) <br /> <br />deposition of sediment and debris (depths of 15-20 feet <br />have been observed) <br /> <br />debris f10wsflmpact forces <br /> <br />mudflows <br /> <br />inundation, producing hydrostatiClbuoyant forces (pres- <br />sure against buildings caused by standing water) <br /> <br />flash flooding (little, if any, warning times) <br /> <br />An often-overlooked "hazard" is the tendency to underesti- <br />mate both the potential and sever~y of alluvial fan flood events. <br />The infrequent rainfall, gently-sloping terrain, and often long <br />time spans between successive floods contribute to a sense of <br />complacency regarding the existence of possible flood haz- <br />ards. Though the intense rainstorms which produce fan floods <br />occur randomly, they nevertheless can develop very rapidly at <br />any time, and can recur with any frequency. <br /> <br />3 <br />