Abstract

A comprehensive series of wave-flume experiments were performed to examine the effects of settling of entrained sediment on ripple roundness, using three sizes of well-sorted quartz sand (D₅₀ = 0.32, 0.49, 0.73 mm). The observed ripple profiles were closely examined with reference to ripple roundness and ripple asymmetry. In order to quantify the effect of sediment settling, settling time factor Ω_s ((= η/(w₀T); η is ripple height, w₀ is settling velocity, and T is wave period) is newly introduced. It is shown that ripples developed under shoaling waves manifest the following geometrical features pertaining to ripples. (1) The measured performance of ripple roundness correlates systematically with Ω_s, irrespective of grain size. (2) The preferential deposition of entrained sediment on the ripple crest and the onshore-side slope of each ripple is accentuated under conditions of smaller Ω_s. Such deposition takes place under shoaling waves when the oscillatory flow direction is reversed from onshore to offshore, thereby allowing the entrained sediment to stall, to some extent, prior to being subjected to the subsequent peak offshore-going flow. Thus round-crested ripples ensue. (3) The way in which preferential deposition occurs under shoaling waves, is similar to the deposition process that occurs under combined flows on rippled beds (Yokokawa et al. 1995). (4) This reasoning led to a comparison of the ripple roundness and ripple symmetry in both flow regimes. Ripples formed under shoaling waves are in cases significantly round-crested with marked asymmetry. In contrast, equally round-crested ripples under combined flows (Yokokawa 1995) are commonly more symmetrical in profile. Thus, combined-flow ripples can be identified from the geological record using their degree of roundness and symmetry of their profile.