Fine-grained bed patch response to near-bankfull flows in a step-pool channel

Abstract

Fine-grained bed patches were monitored in a representative step-pool channel in the Arkansas Ouachita Mountains to assess their response to near-bankfull streamflow events. These patches are small, relatively well-sorted bed areas predominantly composed of gravel-size and smaller grains. They occupy 5.2 and 4.1 percent of the active and bankfull channel areas, respectively, over a 100-m study reach. During each of five simulated flow events with peak discharges ranging in size from 0.25 to 1.34 m³ /sec (1.0-to 1.6-yr return periods), 80 percent or more of the patches scoured or filled. Patch response frequency shows no difference between patch types, but does vary between hydraulic unit types. Patches show net aggradation in response to all events with fill volumes being two or more times greater than scour volumes. Patch response magnitude appears to vary somewhat between patch types, but not between hydraulic unit types. Both net volume and net depth change vary directly with peak discharge magnitude. The predominance of fill even at flows somewhat above bankfull stage suggests that bankfull events may not be sufficient for maintaining channel form in Ouachita step-pool channels.

Citation

Marion, Daniel A.; Weirich, Frank. 1999. Fine-grained bed patch response to near-bankfull flows in a step-pool channel. Proceedings, AWRA [American Water Resources Association] specialty conference: wildland hydrology; 1999 June 30-July 2; Bozeman, MT. Herndon, VA: American Water Resources Association: 93-100.