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<br />Table 6.4 Relationships between bankfull discharge, effective flow (4% duration), and <br />channel morphology <br /> <br />I <br />I <br />I <br />I <br />,I <br />I <br />I <br />I <br /> <br />order of 15 days per year on average. In a stable, appropriately sized channel, the <br />effective discharge (Qeff) is equal to the bankfull discharge (Qbf). These values are <br />compared for Subreaches 2-6 in Table 6.4. The results indicate that based on the entire <br />period of record, all subreaches with the exception of Subreach 4 are undersized with <br />respect to the channel forming flow (Qeff>Qbf), and hence are be prone to enlargement, <br />If the flow conditions prior to the implementation of the Mt Elbert Conduit are <br />disregarded, however, the channel is generally capable of conveying the channel forming <br />flow within its banks (Qeff-Qbf). <br /> <br />Sub- Qbf Qeff Qeff Width % Bankfull Equilibrium <br />reach (1998) (pre-1982) (1990-1993) increase Split Topwidth Topwidth* <br /> 1939-1997 Flow - (ft) <br />2 330 400 400 22% 10 58 50 <br />3 550 900 550 37% 32 87 63 <br />4 1057 900 550 15% 89 104 89 <br />5 515 900 550 9% 18 109 62 <br />6 nla 900 550 15% 95 nla nla <br />7 792 900 550 14% 18 95 77 <br /> <br />*Hey and Thorne. 1986: Wbf=2.73Qbf1l,5 <br /> <br />Although the results depicted in Table 6.4 suggest that the channel is generally capable of <br />conveying the estimated effective discharge, it is critical that the width:depth ratio ' <br />(channel shape) is appropriate, such that the flow energy/sediment transport conditions <br />are in balance with the sediment load. The application of hydraulic geometry <br />relationships that have been developed for similar streams (Hey and Thome, 1986) <br />indicate that the channel is generally overwidened with respect to the current hydrology <br />(Table 6.4; Figure 6.7). This excess width in Subreaches 3-7 reflects the historic increase <br />(onset of augmentations) followed by reduction (bypassing via Mt. Elbert Conduit) in <br />channel forming flows. The channel historically enlarged in response to flow <br />augmentations, and has yet to narrow sufficiently to equilibrate to existing available <br />flows. The overwidening has been exacerbated by the degraded condition of the riparian <br />corridor both due to grazing and tailings impacts; livestock have also increased channel <br />width by driving gravitational collapse of undercut banks. <br /> <br />May 7. 1999 <br /> <br />Fluvial Geomorphological Assessment <br />Upper Arkansas River <br /> <br />Page 44 <br />