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1 I I I I I I I I , I I OC1820 Table 7.2 Annual peak discharges used in incipient motion analyses Incipient Motion Site I Incipient Motion Sites 2-6 Qu = 364 cfs Q,OI = 457 cfs Q" = 541 cfs QI , = 675 cfs Q2 = 650 cfs Q, = 808 cfs Q, = 913 cfs Q,=1120cfs Q,O - 1080 cfs QIO - 1330 cfs Q" - 1290 cfs Q21 - IS 10 cfs Q,o - 1440 cfs Q,o - 1750 cfs Q71 - 1534 d's Q75 - 1 864 cfs QlOo - 1580 cfs QIOO - 1920 cfs 7.2.2. In-Channel Discharges The primary component of flow influencing bed material transport in cobble and gravel bedded streams, such as the Upper Arkansas, is the portion of flow contained within the channel banks. Overbank flows arc typically capable of only transporting sand and finer materials, except when flows concentrate in relic channels. Therefore, it is important to correctly identify the discharge amount contained within the bank boundaries once overtopping and flooding has occurred. Flows determined using the Log-Pearson III flood frequency analysis address the total discharge being conveyed through the river system; however, as stated, it is necessary to determine the discharge contained within the main channel banks. Flows influencing bed material entrainment and transport may be elevated above the banks, but do not include overbank flows. Therefore, comparisons of floodplain versus in-channel percentages of flow were used to ascertain those discharges conveyed within the channel that influence bed material transport. , I Two separate HEC-RAS hydraulic models were used to evaluate discharges confined to the area between the channel banks. The first model used was a reach-based model developed from cross-section data surveyed at each of the six 1M sites. The second model was the floodplain analysis model developed by URS. The reach-based model was used to determine reach average bankfull discharges and their respective recurrence intervals. Bankfull determination was achieved by executing the model for a wide range of discharges and then examining each cross-section independently to identify when the banks were overtopped. In general, the top of bank indicated in the cross-section survey was used as the reference point for bankfull. However, when the top of bank was not a good indicator of bankfull discharge, other indicators were used, including location, type, and condition of vegetation; and the highest elevation on interior point bars. . Once bankfull flows for each cross-section within a reach were identified, the cross-sections with the highest and lowest overbank discharges were eliminated and all remaining discharges were averaged. Calculations May 7. 1999 Fluvial Geomorphological Assessment Upper Arkonsas River Page 53