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I I I I I I I I I I I I I I I I 1 I I OC1831 discharge levels. Incipient motion results indicate several different types of sediment transport responses to the range of hydrologic and hydraulic conditions characteristic of the system. Table 7.9 Site averaged incipient motion characterization Q Reach Average Particle Diameter (io) (crs) RI R2 R3 R4 R5 R6 QLI 2.55 2.00 1.82 1.44 1.30 1.57 Q" 3.26 2.\4 2.24 \.76 \.57 1.82 Q2 3.61 2.34 2.75 \.89 1.71 1.98 Q, 4.26 2.81 3.13 2.28 2.03 2.18 QIO 4.60 2.97 3.30 2.28 2.10 2.32 Q2l 4.96 3.09 3.47 2.55 2.30 2.51 Q,O 5.\7 3.\4 3.57 2.7\ 2.44 2.66 Q" 5.30 3.21 3.67 2.82 2.52 2.75 QIOO 5.36 3.25 3.69 2.90 2.57 2.80 The bed material sizes mobilized at 1M Site 1 are much larger than at other 1M sites for discharges of the same return interval. 1M Sites 2 and 3 transport the next highest sediment sizes, and downstream, 1M Sites 4 through 6 are less able to transport sediment, as indicated by the smaller particle sizes transported at each peak discharge. Site 5 (Geomorphic Subreach 5, downstream of the Highway 24 bridge) transports the smallest sediment sizes for each return interval evaluated. The bed material sizes transported within 1M Site 1 and 1M Site 5 range from 2.5-5.3 inches and 1.3 -2.5 inches, respectively. Relatively large sediment particles are mobilized through 1M Site 1 due to steeper channel and water surface slopes, as well as partial confinement against a high terrace surface. The channel slope through 1M Site 1 is approximately 0.9% compared to an average slope between 0.6% and 0.7% for other sites. Floodplain access is restricted on the left bank where the river parallels a terrace, restricting flood access primarily to the right bank. The lowest transport capacities occur within 1M Site 3 and 1M Site 5, due to relatively low shear stress which result from extensive floodplain access and shallow water surface slopes, 1M Site 5 (Geomorphic Subreach 5) mobilizes the smallest particle diameter for the entire range of flows, due to low banks, a relatively shallow water surface slope of 0.6%, and a wide channel top width that all contribute to lower shear stress. Transport characteristics and channel geometry, specifically split flows, are similar in both 1M Site 4 and 1M Site 6. Results indicate that split flows transport smaller sized materials, a result of both lower energy slope and the lack of depth of water providing added shear stress on the bed. May 7. J 999 Fluvial Geomorphological Assessment Upper Arkansas River Page 68