TRANSIENT RESPONSE OF SAND BEDFORMS TO CHANGES IN FLOW

Field and lab studies indicate that bedform geometries lag changes in flow through floods, producing hysteretic relationships between bed morphology, roughness, and water discharge. Disequilibrium between bedform geometries and flow parameters complicates our ability to interpret stratigraphy for paleoenvironmental reconstruction. Here, we present results of flume experiments carried out at the Saint Anthony Falls Laboratory, University of Minnesota, in which we continuously tracked adjustment of sand bedform morphologies to abrupt changes in water discharge. We show how the timescale of bedform adjustment is driven by three primary factors: 1. directionality of adjustment, 2. preexisting bedform geometry, and 3. sediment flux. Directionality of adjustment (rising versus falling water discharge) determines whether bedforms grow quickly by irreversible merger (rising flows) or shrink slowly through secondary bedform cannibalization of relict larger bedforms (falling flows). Preexisting bedform geometry (height and length) determines the amount of bed deformation required for adjustment to new equilibrium, and sediment flux determines the rate at which this change is affected. These three factors all favor faster adjustment of bedforms to rising flows. We experimentally confirm this bedform adjustment hysteresis through a variety of increasing and decreasing discharge changes, across both sand ripple and dune regimes. Finally, we propose a simple conceptual model for estimating the adjustment timescale based on sediment flux and equilibrium bedform geometry.