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<br />similar to that of the associated watershed and vary from <br />virtually no vegetation on fans in parts of the southw~st to <br />grass and trees on fans in Boise. <br /> <br />Location, Density, and Geometry Qi Developments <br /> <br />Many types of developments have been constructed on fans. <br />ranging from low density ranchlands to high density <br />subdivisions. streets commonly run across the fan slope and <br />homesites are created by a series of progressively lower <br />terroces as one moves downslope. streets oriented along the <br />maximum slope are not common in most subdivisions. <br />Development usually begins near the toe of the fan and moves <br />progre~sively upslope in concentric rings or in somewhat <br />detached blocks of houses. Such development has in the past <br />been largely uncontrolled. figure 3.4 shows the Rancho <br />Mirabe fan in 1979 and illustrates the typical development <br />pattern leading, ultimately, to complete occupation of the <br />fan. <br /> <br />3.3 Types and Distribution Qi Fans <br /> <br />Textbook ond journal discussions of alluvial fan <br />characteristics indicate that fans are a common geological <br />feature in the semi-arid regions of the western United <br />States (Ritter, 1973; Schumm, 1977; Bull, 1968). In many <br />areas the fan formation is considered to offer the most <br />desirable building sites, particularly for residential <br />development. These two facts suggest that the overall <br />potentiol for flood damages on fan areas will increase with <br />time as development pressure grows in the western United <br />States. The purpose of this section is to assess the extent <br />to which flooding problems on fans and related alluvial <br />formations (i.e, aprons, washes) are likely to increase. <br /> <br />32 <br /> <br />