HR: 11:20h
AN: H42B-05    [Abstracts]
TI: Interaction of Hydraulic and Geotechnical Processes, and Sediment Transport in Controlling Channel Morphology in an Active Meander Bend
AU: * Simon, A
EM: asimon@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655 United States
AU: Kuhnle, R
EM: rkuhnle@ars.usda.gov
AF: USDA-ARS National Sedimentation Laboratory, P.O. Box 1157, Oxford, MS 38655 United States
AB: Evolution of meanders in incised alluvial channels is controlled by interactions between hydraulic forces acting on the bed and bank toe, and gravitational forces acting on in situ bank material. Vertical and lateral accretion of point bars lead to re-direction of flows in a downvalley direction that impinge on bank-toe surfaces causing undercutting, steepening and ultimate failure of the bank mass by gravity. The processes and forms inherent in incised meanders have been studied at an actively evolving meander bend on Goodwin Creek, Mississippi since 1996. Periodic surveys dating from 1977 to 1996 coupled with the dating of woody vegetation growing on the channel banks and bars were used to determine a migration rate of about 0.5 m/y since the mid-1960s. Up to 28 repetitive surveys were conducted at each of 10 monumented cross sections between 1996 and 2003 over an 82 m-long reach. Over the seven years of monitoring, 894 m3 of bank materials have been eroded from the reach while 436 m3 of sediment have been deposited on the bed and bar. Mean annual sediment concentrations have been essentially stable since the percent of cultivated land in the basin stabilized in the early 1990s. Migration of the outside bend by mass-wasting and toe removal of failed material has been matched by commensurate lateral accretion of point bars. Existing point bars grow vertically at a rate of about 5 cm/yr and now support establishing woody vegetation. Aggradation on the channel bed and lateral channel migration by bar accretion and opposite-bank retreat results in net erosion of all sediment but a net deposition of hydraulically-controlled sands and gravels. The erosion represents series of bank-failure episodes with subsequent removal of failed material by stormflow. A direct correlation between volumes of bar accretion and opposite-side bank erosion was developed, thus establishing a link between hydraulic and geotechnical processes. Peak-flow water-surface slopes increase with increasing discharge from 0.0025 to 0.0035 at 15 m3/s (81.5 m) and then decrease as stage and discharge increase further, causing a flattening of the shear stress-discharge relation at 35 N/m2. This change coincides with the elevation of the top of the newly formed berm where the establishment and proliferation of woody, riparian vegetation occurred once vertical accretion reduced the frequency of bar inundation. This marks the initial development of an inner channel and new floodplain below the level of the previous floodplain surface which now exists as a terrace.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1866 Soil moisture
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting