Cheng Cheng, advised by Xun-Hong Chen, will defend his dissertation at 8 a.m., Tuesday, Jan. 24, in 901 Hardin Hall. His topic is "Understanding of the Hydrologic Connections Between Wide-channel and Adjacent Aquifers Using Numerical and Field Techniques." All are invited.
Abstract
Streambed sedimentary structure plays a vital role in controlling the interactions between surface water and groundwater. Previous researchers usually used a lumped parameter, streambed conductance, to analyze stream-aquifer interactions and assumed that a low-permeability clogging layer occurs at the channel surface. However, a number of recent studies noted that hyporheic processes can affect streambed permeability and declogging processes may exist. Accordingly, the assumption of a low-permeability clogging layer at the channel surface can lead to underestimations of the real-world stream-aquifer interactions. Streambed vertical hydraulic conductivity (Kv) and thickness (M) are two of the most important parameters of streambed conductance, and thus a detailed characterization of streambed Kv and M is crucial to understand and quantify the exchange between the near-channel and in-channel water.
In this dissertation, the variations of streambed Kv values relative to the depth are investigated for three rivers (the Big Blue River, the Little Blue River, and the Platte River) in Nebraska. The Kv values in the top 2-m of the streambed sediments are generally larger than 1 m/d. In addition, streambed Kv values decrease below the channel surface, which may be a result of hyporheic process and indicate that stream-aquifer interactions are more intense near the channel surface. Consequently, this study confirms the absence of a low-permeability clogging layer at the channel surface. Furthermore, the estimated values of M may vary largely in space. From these results, the constant head boundary is proposed to be an alternative solution in the simulation of stream-aquifer interactions. This approach is applied in a regional groundwater flow model to evaluate the impact of groundwater irrigation on the streamflow in the lower reach of the Platte River. Additionally, the model provides an accurate estimation of the streambed leakance of the Platte River using numerical and field techniques, which is beneficial in assessing stream-aquifer interactions when an analytical solution is employed. Furthermore, the statistical distribution of streambed Kv values along a 300-km segment of the Platte River is also examined. We found they are normally distributed at nearly each of the 18 test sites in the Platte River.