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<br />Bosley Wash Drainage Master Plan Phase I: Watershed Floodplain Delineation
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<br />4.1.1 Watershed Delineation
<br />
<br />The Bosley Wash Basin is a fairly complex system and can be divided into two distinct regions,
<br />The upper basin, or the contributing area north of 1,70, is composed of steep, natural channels,
<br />In this portion of the basin, physical boundaries separate the watersheds, with subbasin
<br />boundaries drawn along ridgelines, as shown in Figure 2,
<br />
<br />In the lower basin, subbasin geometry is more a function of land use features rather than
<br />
<br />
<br />topographic characteristics, There is less topographic relief in this portion of the basin, and
<br />
<br />embankments associated with roads, canals, and other elements control the direction of flow,
<br />
<br />These features often cause the watershed boundaries to change with different flow events, For
<br />
<br />example, the watershed boundary of a 2-year event may be very different from that of a 100-year
<br />
<br />event due to overtopping of low-lying embankments or levees, These changes require the use of
<br />
<br />flow diversions in the model development. The approach used in this study was to identify the
<br />
<br />low flow threshold for nonnal delineation and then look at a revised delineation based on
<br />
<br />
<br />topography for large events, For example, the Stub Ditch was assumed to intercept flows up to
<br />
<br />
<br />its capacity but excess flow would overtop the ditch and flow down gradient. These features
<br />
<br />were integrated into the model in the routing elements, Table 1 presents a summary of the
<br />
<br />hydrologic characteristics of all watersheds within the basin,
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<br />Page 7
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