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<br />Bosley Wash Drainage Master Plan Phase I: Watershed Floodplain Delineation
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<br />
<br />5,1 Survey Data
<br />
<br />Channel and overbank geometry was provided by the County and GJDD, A field survey was
<br />
<br />provided by the GJDD, which contained information on channels, culverts, and roadways,
<br />
<br />
<br />Aerial contour mapping was provided by the County for the lower portions of the basin,
<br />
<br />Topographic information for the upper basin was obtained from USGS quadrangle maps, This
<br />
<br />map information was assimilated into a single system, In order to combine these information
<br />
<br />sources, a single scale and datum were necessary, When delivered, the field survey used English
<br />
<br />units, while the GIS mapping used metric units, The scale and units of the GIS mapping were
<br />
<br />converted to match those of the field survey, In addition, the contours provided in the GIS
<br />
<br />
<br />mapping were approximately I ,0 met~r low, rehtive to the datum used in the field survey, The
<br />
<br />GIS mapping was adjusted up by 1,0 meter to tie into the field survey to make a continuous
<br />
<br />contour map, USGS elevations were unchanged, Cross-sections used in the model are depicted
<br />
<br />
<br />in Appendix 3 and survey data supplied by the sponsors is included in Appendix 4,
<br />
<br />5.2 Hydraulic Assumptions
<br />
<br />A number of hydraulic assumptions, some global and others localized, were used during the
<br />hydraulic modeling process,
<br />
<br />5,2.1 Roughness and Loss Coefficients
<br />
<br />Based on field observations and past experience, Manning's 'n' values were set for the hydraulic
<br />
<br />model on a reach by reach basis, although the uniformity of most of the channels created a
<br />
<br />consistent value of 0,035 for the main channel. According to Ooen Channel Hvdraulics (Chow,
<br />
<br />1959). this is a normal representation of an excavated earth channel, sluggish, with a stony
<br />
<br />bottom and weedy banks, The overbank consisted of floodplains of mature grass fields or
<br />
<br />scattered brush/heavy weeds and were represented by a roughness coefficient of 0,040.
<br />
<br />The model used general loss coefficients of 0,1 for contraction and 0,3 for expansion, Near
<br />culverts and other abrupt transitions, values of 0,3 and 0,5 were used.
<br />
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<br />Page J 9
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