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Field and laboratory studies indicate that changes in riverbed morphology often lag changes in water discharge. This lagged response produces hysteresis in the relationship between water discharge and bed form geometry. To understand these phenomena, we performed flume experiments to observe the response of a sand bed to step increases and decreases in water discharge. For an abrupt rise in discharge, we observed that bed forms grew rapidly by collision and merger of bed forms migrating with different celerities. Growth rate slowed as bed forms approached equilibrium with the higher discharge regime. After an abrupt discharge drop, bed form decay occurred through formation of smaller secondary bed forms, in equilibrium with the lower discharge, which cannibalized the original, relict features. We present a simple model framework to quantitatively predict time scales of bed form adjustment to flow changes, based on equilibrium bed form heights, lengths, and celerities at low and high flows. For rising discharge, the model assumes that all bed form collisions result in irreversible merger, due to a dispersion of initial celerities. For falling discharge, we derive a diffusion model for the decay of relict high-stage features. Our models predict the form and time scale of experimental bed form adjustments. Additional experiments applying slow and fast triangular flood waves show that bed form hysteresis occurs only when the time scale of flow change is faster than the modeled (and measured) bed form adjustment time. We show that our predicted adjustment time scales can also be used to predict the occurrence of bed form hysteresis in natural floods. [ABSTRACT FROM AUTHOR]

For many gravel-bed rivers in New Zealand, there is a general channel degradation trend in the catchment, with sediments being transported down to lower reaches, where there is aggradation close to the coast. In some cases, the gravel bed material cannot be transported out to the coast, and there are natural deposition areas on alluvial plains. Repeat surveys of channel cross sections along the lower reaches allow volume balance estimates of the supply of bed material to these alluvial plain depositional zones to be determined for the inter-survey periods. Gravel-bed transport rates can be calculated from the bed material size and hydraulic parameters for given flood flows, using empirically derived formulae. If a sediment rating curve can be derived from transport rates for given flows and thresholds for the initiation of bed movement, then continuous flow records, generated at recorder sites, can be used to estimate bed material supply over the period of records. Comparing the volume balance estimates with those derived from the formula-based sediment rating curves indicates the applicability of different formulae, and allows some calibration or factoring of rating curve estimates. The results obtained from the application of volume balances and sediment rating curves are presented for a range of gravel-bed rivers in the North Island of New Zealand. The author carried out the sedimentation investigations or undertook detailed reviews of the supply estimation for the results presented. Some comment on these estimates of gravel supply in New Zealand rivers and the implications for extraction from rivers is included. [ABSTRACT FROM AUTHOR]

The transport of coarse sediment during floods often exhibits hysteresis patterns from changes in flow fields, channel geometry, bedforms, or sediment supply conditions. Flume experiments that simulate hydrographs tend to confirm that hysteresis is a consequence of the progressive organization of surface sediments in terms of grain protrusion, imbrication, orientation, and roughness. Hysteretic patterns are also highly dependent on the kind of sediment supply conditions, and the type of simulated hydrograph. A factor that has not been investigated extensively is the effect of the timing and sequencing of floods on bedload transport. Depending on its magnitude and duration, each flood leaves the channel bed in a different condition, which influences the bedload transport of the next event, representing the river bed's memory of past floods, which can determine future responses to natural disturbances. In this study, I investigated the effects of different sequences of events, i.e., the flood history, on sediment transport through a series of flume experiments that simulated three types of stepped and symmetrical hydrographs (ranging from short-duration/high-magnitude to long-duration/low-magnitude events) under sediment recirculation conditions. Hydrographs were simulated as a sequence of the same event, and with events in different sequences, in order to explore the effects of different antecedent conditions on sediment transport. The results show that a previous event decreases the rates of sediment transported by a certain hydrograph by around 40% if a high-magnitude event precedes another one and around 70% if a low-magnitude event precedes another event of similar magnitude. A low-magnitude event does not affect the rate of sediment transported by a subsequent high-magnitude flood, but a high-magnitude event reduces the sediment transported by a subsequent long-duration/low-magnitude event. [ABSTRACT FROM AUTHOR]