Fluctuation of hyporheic zone thickness due to inflow and outflow across the water-sediment interface
Determination of hyporheic zone thickness in streams is an important step for study of stream-aquifer interactions. Knowledge about hyporheic zone thickness is useful in stream restoration. However, because there is a lack of standard techniques for such study, evaluation of the hyporheic zone thickness for a given stream reach remains a challenge task for researchers. This paper presents Galerkin finite element flow and stream function models that can simulate the hyporheic zone thickness. The flow and stream function equations are solved for 2-D profile domains that can be across a stream or parallel to a stream. The numeral schemes for solving the flow and stream function equations and the treatment of boundary conditions are described. Hypothetical streams are used for simulation of the control of hyporheic zone thickness by the magnitude of inflow and outflow that occur at the stream-sediment interface. Groundwater flow velocity field is generated to examine the flow dynamics in hyporheic zones. The magnitude of groundwater flow velocity in hyporheic zone is greater than that of regional groundwater flow.
Depth‐dependent hydraulic conductivity distribution patterns of a streambed
Characterization of streambed hydraulic conductivity from the channel surface to a great depth below the channel surface can provide needed information for the determination of stream‐aquifer hydrologic connectedness, and it is also important to river restoration. However, knowledge on the streambed hydraulic conductivity for sediments 1 m below the channel surface is scarce. This study describes a method that was used to determine the distribution patterns of streambed hydraulic conductivity for sediments from channel surface to a depth of 15 m below. The method includes Geoprobe's direct‐push techniques and Permeameter tests. Direct‐push techniques were used to generate the electrical conductivity (EC) logs and to collect sequences of continuous sediment cores from river channels, as well as from the alluvial aquifer connected to the river. Permeameter tests on these sediment cores give the profiles of vertical hydraulic conductivity (Kv) of the channel sediments and the aquifer materials. This method was applied to produce Kv profiles for a streambed and an alluvial aquifer in the Platte River Valley of Nebraska, USA. Comparison and statistical analysis of the Kv profiles from the river channel and from the proximate alluvial aquifer indicates a special pattern of Kv in the channel sediments. This depth‐dependent pattern of Kv distribution for the channel sediments is considered to be produced by hyporheic processes. This Kv‐distribution pattern implied that the effect of hyporheic processes on streambed hydraulic conductivity can reach the sediments about 9 m below the channel surface. Copyright © 2010 John Wiley & Sons, Ltd.
Errors with Small Volume Elastic Seepage Meter Bags
The use of small volume elastic collection bags (condoms) has become popular in seepage meter studies in recent years, despite minimal field or laboratory validation of their use and, specifically, the impact of their elasticity on seepage measurements. A laboratory study was initiated after field results using small elastic collection bags produced seepage data that did not correlate with hydrometric data. The laboratory data demonstrate that condoms undergo significant mechanical relaxation during seepage measurement times typically observed in field settings. Unlike conventional nonelastic collection bags, which mechanically relax over several minutes, the condoms suffered from a slow mechanical relaxation or equilibration. Over nine hours, condoms gained 43 mL of water, ‐50% of maximum workable volume (between mechanical relaxation effect and elastic limit), under stagnant flow conditions. This long‐term equilibration invalidates simple subtraction of equilibration volumes from collection volumes as a correction technique. Previously published studies using flexible small‐volume elastic measurement bags (condoms) have not reported a mechanical relaxation effect. Overall, because the condom's small workable volume and inherent variability, we would not recommend any small‐volume elastic measurement bags for quantitative seepage measurements.