(28) Migration rate of combined-flow bedform: an analogue experiment
Numata, S., Sekiguchi, T. (Univ. Tsukuba), Yokokawa, M. (Osaka Inst. Tech.)
Contact information: Numata, S. (email@example.com)
A series of short-period combined-flow experiments was conducted focusing geometry and migration rate of bedform. A recirculating flume with a wave generator (12 m long, 0.2 m wide, and 0.4 m deep) was employed. Bedforms developed from a flat horizontal sand bed (5 m long, 0.2 m wide, and 0.05 m thick) and bed material was quartz sand with mean diameter of 0.4 mm. Combined flow with the following conditions was generated by combining waves and current, whose direction are opposite each other: the water depth was 0.2 m, wave period, T = 1.5 s, near-bottom oscillatory velocity uo ≤ 39.2 cm/s, and near-bottom unidirectional velocity, uu ≤ 41.2 cm/s. The development of bedforms was photographed at every 5 s. The experiments carried out 44 runs changing combination of uo, and uu. The analysis of the ripple size and the migration rate showed that:
(1) The migration rate decreased as the ripple height developed. Ripple height fluctuates even after ripples attained quasi equilibrium; thus normalization of migration rate with ripple height would be necessary for comparison of ripple migration rates under different hydraulic conditions. (2) The migration rate in the same ripple height increased with the increasing unidirectional velocity. (3) The migration rate in the same ripple height increased with the increasing oscillatory velocity under conditions of high uu. The migration rate in the same ripple height unchanged with the
increasing oscillatory velocity under conditions of low uu.