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<br />Based on the very different behavior of Problem Area 6, it appears that this reach is
<br />likely degrading. This can not be stated with certainty at this point, but the high transport rate
<br />and cross-sectional shape suggest an incising reach, It is expected that this reach requires high
<br />maintenance (Resource Consultants and Engineers, rnc, 1994), The abnormally high transport
<br />capacity will likely cause deposition problems downstream as well, if this has not already
<br />occurred,
<br />
<br />The "Bristol Bridge" particle size distribution was used for this sub-reach for sediment
<br />transport calculations, The sediment yield for this problem area under existing conditions was
<br />63,800 tons/yr with a mean daily load of 175 tons/day,
<br />
<br />Problem Area 7, Hydraulic modeling of Problem Area 7 indicated that the capacity
<br />within the active channel banks is approximately 1,500 cfs, This is the value above which flow
<br />begins to significantly inundate the overbank areas adjacent to the river channel. Potentially
<br />damaging flows begin above approximately 3,000 cfs, though the flowpath that indicated for
<br />this area is quite wide with large undeveloped areas inundated, The 3,000 cfs operational
<br />discharge was modeled to indicate potential problem areas under current conditions. The flow
<br />area extents for this discharge were conriected to generate pseudo-floodplains, as described
<br />above, and are shown on Sheet 5,
<br />
<br />Examination of the profile indicated by the survey in this reach is relatively uniform,
<br />with a reach-length weighted average slope of 0,00122, and maximum and minimwn
<br />individual values of 0,00477 and 0,00017, respectively, The aerial photography shows
<br />relatively little encroachment (compared to other areas studied), with a reasonable buffer zone
<br />on either side of the active channel. This appears to correspond with a better behaved bed
<br />profile and illustrates the importance of the floodplain cross, section in stability, A profile plot
<br />of this area is shown in Plate 5,
<br />
<br />The "Bristol Bridge" particle size distribution was used for this sub-reach for sediment
<br />transport calculations, The sediment yield for this problem area under existing conditions was
<br />47,500 tons/yr with a mean daily load of 130 tons/day,
<br />
<br />Table 3-3 summarizes the sediment yields for the five problem areas in their current
<br />state, The large jwnp in yield from area 4 to area 5 is primarily due to the differing particle
<br />size distributions used. However, this is not the case for Problem Area 6, and represents the
<br />impact on channel morphology ofthe severe constriction from agricultural encroachment.
<br />
<br />Hydraulic and Sediment Summary
<br />
<br />The hydraulic and sediment analyses performed indicate several things, First, the
<br />conveyance capacity is less than the 3,000 cfs necessary for flood control releases in four of the
<br />five areas, Additionally, the erratic hydraulics likely cause seepage problems from localized
<br />high stages in other areas, Second, the channel form and profile are widely variable and these
<br />in turn cause a broad range of sediment transport potentials within the individual reaches, The
<br />profiles of some of the areas show marked disturbance. The "fits and starts" nature of the areas
<br />
<br />23
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