The Effect of Grain Size Distribution Modality on the Relationship Between Stress History and Entrainment Threshold

Many natural rivers contain sediment beds with a continuous grain size distribution. This surface distribution is a function of both the calibre and volume of sediment supplied to the channel and the time-integrated frequency and magnitude of discharge events that are capable of transporting surface material. However, it is increasingly recognised that bed surface composition and entrainment threshold are also dependent on the temporality of below-threshold antecedent flows (stress history). The experiments reported herein assess the influence of the sorting and modality of grain size distributions on the degree of stabilisation imparted by stress histories. Employing a bed slope of 1/200, an arbitrary benchmark for critical shear stress of entrainment was established for uniform sediment of D50 = 4.8mm using a quantitative visual technique for threshold determination (Yalin, 1972). Antecedent shear stress magnitude (stress history) was set at 50% of this critical shear stress in all runs. A series of flume experiments compared and contrasted uniform, unimodal (σg = 1.63) and bimodal (σg = 2.08) sediment beds of identical D50 (4.8mm). Beds were exposed to stress histories of 10, 20, 40 and 60 minutes duration. Changes to bed stability were analysed using a subsequent triangular ‘stability test' stepped in increments of flow depth with entrainment threshold determined using the Yalin Criterion. Increasing the antecedent flow duration is seen to significantly increase river bed stability. The influence of modality is discussed in detail in terms of compositional change, particle rearrangement and bed packing; data are specifically discussed with reference to previous work by Paphitis and Collins (2005) and Monteith and Pender (2005). The outcome of this research is pertinent to wider investigations in fluvial engineering; e.g. flume-based experimental technique, sediment transport models, the sensitivity of river morphology to climate change, and management of regulated rivers. Keywords: Graded, Sediment, Antecedent, Entrainment, Stability, Stress History