ASSESSMENT OF RIVER-FLOODPLAIN INTERACTIONS
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ABSTRACT
GROUND WATER FLOW VELOCITY IN THE BANK OF THE COLUMBIA RIVER, HANFORD,WASHINGTON
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ABSTRACT
A suite of In Situ Permeable Flow Sensors was deployed in the bank of the Columbia River at the 100H Area on the Hanford Site to characterize the hydrologic regime within the banks of the river. The three dimensional flow velocity was recorded on an hourly basis from mid May to mid July, 1994 and for one week in September. The first data collection interval coincided with the seasonal high water level in the river while the second interval reflected conditions during relatively low seasonal river stage. Two flow sensors located approximately 50 feet from the river recorded flow directions which correlated very well with river stage, both on seasonal and diurnal time scales. During time intervals characterized by falling river stage, the flow sensors recorded flow toward the river while flow away from the river was recorded during times of rising river stage. The flow sensor near the river in the Hanford Formation recorded a component of flow oriented vertically downward, probably reflecting the details of the hydrostratigraphy in close proximity to the probe. The flow sensor near the river in the Ringold Formation recorded an upward component of flow which dominated the horizontal components most of the time. The upward flow in the Ringold probably reflects regional groundwater flow into the river. The magnitudes of the flow velocities recorded by the flow sensors were lower than expected, probably as a result of drilling induced disturbance of the hydraulic properties of the sediments around the probes. The probes were installed with resonant sonic drilling which may have compacted the sediments immediately surrounding the probes, thereby reducing the hydraulic conductivity adjacent to the probes and diverting the groundwater flow away from the sensors.
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The interaction between the Brazos River and the adjacent floodplain aquifer was studied for 200 days in 1995 at a ground-water research site near College Station, Texas. Two In Situ Permeable Flow Sensors (ISPFS) and a grid of well nests were used to correlate river stage to the magnitude and direction of ground-water flow at depths of 13.7 m and 18.3 m in the unconfined alluvial aquifer. Linear relationships between ground-water flow and river stage were determined at each depth. The floodplain aquifer responded differently to changes in river stage at the 13.7 m and 18.3 m depths. The horizontal velocity, parallel and perpendicular to the river, decreased with increasing river stage and increased with decreasing river stage, at both depths. However, the rates of change varied between the two depths. This caused the magnitude and direction of ground-water flow to be different at the two depths. The upward vertical velocity increased with increasing river stage at the 13.7 m depth and decreased with increasing river stage at the 18.3 m depth. At the 13.7 m depth, vertical ground-water flow gradually changed from upward to downward flow with long term river stage decline. Downward ground-water flow was not observed at the 18.3 m depth. Assessment of river-aquifer interactions indicates that a direct and measurable relationship exists between river stage and ground-water flow components at the site. The magnitude and direction of groundwater flow in the alluvial floodplain aquifer may be predicted if river stage is known.