Transient response of sand bedforms to changes in flow

Raleigh L Martin
 PhD Student University of Pennsylvania, Department of Earth & Environmental Science
 

Abstract: 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.  This summer, I am conducting experiments in the SAFL Tilting Bed Flume to explore this bedform hysteresis.  In these experiments, repeat sonar scans are used to continuously track the response of sand bedform morphologies to abrupt changes in water discharge.   The timescale of bedform adjustment appears to be 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 effected.  These three factors all favor faster adjustment of bedforms to rising flows.  I will present preliminary results on bedform adjustment hysteresis for a variety of increasing and decreasing discharge changes.