Origin of Laminae Deposited by the Movement of Ripples along a Streambed: A Laboratory Study

The hydrodynamic sorting processes responsible for the deposition of laminae and cross-laminae in a laboratory stream with ripple bed forms were analyzed quantitatively. The results of the analysis indicate: (a) sorting of the entrained sediment mix into relatively fine and coarse laminae is related primarily to the sorting processes operating within the heavy fluid layer (contact load) flowing along the streambed, and (b) finer and coarser laminae are composed of grains derived respectively from the upper and lower parts of the heavy fluid layer. The finer laminae are formed by the settlement of suspended-load particles in the trough areas, whereas the coarser cross-laminae in the streambed are composed mainly of bed-load particles that were deposited on the ripple foresets. The experimental procedure involved the sampling of the suspended sediment at various levels above the streambed to determine the decrease of median grain size (D₅₀) as a function of height (y) above the streambed. An estimate was also made of the size composition (D₅₀) of the bed load. The D₅₀ values for both the bed load and the suspended load were plotted logarithmically against d-y/y values, where the symbol d signifies the depth of the stream; interpolation of this plot for the region near the streambed gave an inferred size gradation for the density-stratified fluid of the heavy fluid layer. By adopting a simple model for the flow pattern and sediment movement near the streambed and using a particle trajectory method previously described, it was possible to correlate the size composition of the trough material with the distribution of grain sizes in the heavy fluid layer. The tentative methodology described here requires testing over a wider range of flow conditions and for different types of sediment mixes.