BEDFORM MORPHOLOGY AND THEIR SEDIMENTARY STRUCTURES UNDER COMBINED FLOWS WITH 4- AND 6-SECOND OSCILLATION PERIODS

There is a wide range of unexplored stability conditions for combined-flow bedforms, especially with intermediate oscillatory periods, i.e., T = 2–8 s. The present experiment examined combined-flow bedforms for T = 4 and 6 s, and describes their morphology and sedimentary structure. Experiments were conducted using the Large Oscillatory Water-Sediment Tunnel in the Ven Te Chow Hydrosystems Laboratory, University of Illinois. The tunnel has a test section of 12.5 m long, 0.8 m wide and 1.2 m high, and half the height of the section was filled with uniform silica sand of 0.25 mm diameter. Bedforms were developed from an initial flatbed under the following combined-flow conditions: T = 4 and 6 s, oscillatory flow velocity Uo = 25 cm/s, and unidirectional flow velocity Uu = 0–40 cm/s. Both for T = 4 and 6 s, four distinctive types of bedform occurred in this order with increasing Uu value: (1) symmetrical small ripples with sharp and straight crests whose ripple wavelength L ≈ 20–30 cm, (2) discontinuous-crested asymmetric small ripples with L ≈15 cm, (3) round-crested asymmetric large ripples with L ≈ 20–30 cm, and (4) dune-like asymmetric large bedform with L ≈ 60 cm. For the cases with T = 4 s, the larger Uu value was needed for the formation of unidirectional-flow-affected bedforms than the cases with T = 6 s. Internal sedimentary structures observed in this experiment was almost the same with those developed under shorter oscillation periods. With smaller Uu, wave-formed-ripple-like structures such as offshooting structures were observed, while the characteristic structures such as truncated troughs and jointed structures at larger Uu. These characteristic structures should be crucial information for the reconstruction of paleo-hydraulic conditions from the rock records.