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46 ALLUVIAL FAN FLOODING because of the rearrangement of the channel system by remobilization of deposited debris, as described in the previous section. Also, the flood characteristics on adjacent or nearby fans resulting from the same storm commonly are different because of local differences in rainstorm intensity, sediment availability in the source areas, and the respective differences in recent flow path history. Although flooding accounts depict varied properties in time and space, they show certain distinctive properties when considered collectively. High flood flow velocities were reported for about half of the sites listed in Appendix A. Supercritical flow velocities for water floods are also suggested at other sites by the steep fan slopes and low hydraulic roughness associated with fine- textured surface sediments and simple channel geometries. Flash floods (short-lived floods with a short time to peak) are reported at eight sites, including accounts of translatory waves at the Chicago Creek, British Columbia; Horseshoe Park, Colorado; and Montrose, California, alluvial fans. At the Cottonwood Canyon, Magnesia Spring Canyon, and Montrose alluvial fans in California the reported combination of high flow velocities, flashy flow, unstable channels, and movement of flow paths produced an especially serious hazard that resulted in loss of life at two of the fans. Sheetflooding and distributary flow also are typical of alluvial fan flooding. Unstable channel boundaries are common, while stable channel beds or banks are less frequent, as suggested by accounts at only 5 of the sites studied. Flow path movement was reported for 8 sites, no movement for 13 sites, and either no mention of movement or absence of movement at the remaining 8 sites. A possible reason for the lack of flow path movement and disruption of the surface of some of the active fans is the short duration of the floods in concert with a limited supply of unconsolidated medium to coarse material from the drainage basin. Typical distributary channels appear to scour and fill; however, stable flow paths do occur during individual floods. The Carefree and Wild Burro alluvial fans in Arizona are examples of less active or inactive fans with some channel incision and a limited supply of medium-to-coarse sediment from the drainage basin. Most of the sediment delivered to the fans is carried by streamflow and is coarse sand and fine gravel. The basins are not very steep. For example, the basin of the Carefree fan is a pediment. On these fans the channels are slightly trenched below the surrounding fan surface and are lined with desert trees and shrubs. During recent major floods, there was no movement of the many distributary channels on either of these fans. Flood hazards on the least active fan surfaces, such as the Carefree (see Chapter 4) and Wild Burro alluvial fans, can be deceptively small because of the small size of distributary channels and the rather long distances from mountainous drainage basins. For example, shallow floodflow unexpectedly entered several small defined channels during a major flood on the Saddle Mountain, Arizona, alluvial fan (USACE, 1993). A subsequent field investigation revealed no new channel formation on the fan. Rather, small distributary channels that in places appeared to be simple topographic lows conveyed overflow from the larger distributary channels or carried runoff from high-intensity rainfall directly on the fan surface. Many less active alluvial fans in the arid southwestern United States have developed soils with small, slightly incised stable flow paths that convey shallow high-velocity floodflow during major floods. Flood control works such as levees and debris dams at several of the sites were partially or totally ineffective during major floods. For example, at Day and Deer creeks (Figure 2-2), Henderson Canyon (see Chapter 4), and Magnesia Spring Canyon alluvial fans in California, the flood control structures were overwhelmed, and floodwater followed original flow paths and fan ;\