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<br />I MSite Reach Average Recurrence <br />(Geomorphic Bankfull Discharge In terval <br />Subreach) (cfs) (years) <br />1(2) 330 1.1 <br />2 (3) 575 1.3 <br />3 (3) 500 1.2 <br />4 (4) 1057 4.4 <br />5 (5) 515 1.2 <br />6 (7) 792 1.9 <br /> <br />I <br />! <br />1 <br />I <br />I <br /> <br />were then performed to identify the recurrence interval of the averaged bankfull <br />discharge (Table 7.3). <br /> <br />Table 7.3 1M Site average bankfull discharges and associated recurrence intervals <br /> <br />The second step used in determining the percentage of in-channel flows included <br />importing cross-section data from the reach-based models into the floodplain hydraulic <br />model. By doing so, bankfull discharges within the floodplain model match those of the <br />channel-based sites. The analysis provided channel and floodplain interactions <br />describing how much of the discharges were allocated between each of these features. A <br />decreasing percentage of the original discharge remained between the main channel <br />banks as flows increased above the bankfull level. This remaining percentage of water <br />was used to calculate hydraulic and sediment transport characteristics. <br /> <br />The in-channel discharges were then applied to the reach-based models for final <br />hydraulic analysis. Constraints were applied so that the percentages of flows, determined <br />using the floodplain model, were confined to the area within the channel banks, thus <br />representing hydraulics experienced within the channel (Table 7.4). <br /> <br />7.3. Hydraulic Analysis <br /> <br />The hydraulic analysis involved determination of main channel hydraulic conditions for <br />the range of annual peak discharges from the 1.1- to 1 OO-year events. Hydraulic analysis <br />was performed to attain an understanding of hydraulic characteristics of the river and to <br />generate parameters necessary for the incipient motion analysis. <br /> <br />7.3.1. Methods <br /> <br />Hydraulic analysis performed on each of the six 1M sites used HEC-RAS, Version 2.2, a <br />one-dimensional, standard step, backwater computation program (USACE, 1997). The <br />program allows for determination of water surface and energy grade elevations as well as <br />other hydraulic variables dependent upon channel geometry, slope, rouglmess and <br />discharge. <br /> <br />May 7, 1999 <br /> <br />Fluvial Geomorphological Assessment <br />Upper Arkansas River <br /> <br />Page 54 <br />