|M.Sc Student||Goren Yonathan|
|Subject||Characterization of the Subsurface Flow Regime of Nahal|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Uri Shavit|
|Full Thesis text - in Hebrew|
The thesis summarizes a research project dedicated to the characterization of the hyporheic exchange flows (HEF) along two sites in Nahal Cziv, a mountain stream in Israel. This objective was achieved by a combination of field characterization of the subsurface, tracer experiments and numerical solutions of both water and solutes.
The stream subsurface is composed of different formations that were located using two shallow geophysical methods ERT and GPR. The combined subsurface image was used to sort the domain into boulders, intermediate and gravel-sediments formations. This mapping was used as an input for the 3D computational domain.
The numerical solution of the water Darcy flow was obtained by solving the differential mass balance equation assuming steady-state conditions. The solute concentration field was modeled by solving the ADE equation for transient conditions. Calibration of the hydraulic conductivity and the dispersivity was achieved by comparing the results of breakthrough curve field experiments with the concentration results obtained by the numerical model.
The average hydraulic conductivity and average dispersivity values of the gravel-sediments formations were relatively high when comparing with previous studies, indicating a potential high flow rates within the hyporheic zone of Nahal Cziv.
The values of the horizontal subsurface flow rate (QX), the hyporheic exchange flow rate (QHEF), the mean resident time distribution (mRTHEF) and the depth of the hyporheic zone (dHEF) were evaluated using a Lagrangian Model. The hyporheic exchange flows and their characterization was defined by following particles that were released from the bottom of the stream, penetrated into the hyporheic zone and then returned to the surface water. Calculated QX to a depth of 5m below the stream bottom varied between 0.005 and 0.038 m3 s-1, depending on the streambed topography. The average value of QHEF was 0.042 m3 s-1, which is 60% of the stream flow rate. It means that a large volume of the surface water interacts with the subsurface through the hyporheic zone.
A sensitivity analysis examined the role of three variables on properties of the hyporheic exchange flow: (1) the hydraulic conductivity ratio between the subsurface formations; (2) the effect of anisotropy; and (3) the water flux through the bottom boundary of the domain. It was found that changing the hydraulic conductivities ratio had no effect on the parameters that were examined. This result demonstrates that an accurate description of the complex subsurface structure is not required in order to estimate the flow and transport mechanisms within the hyporheic zone. Anisotropy, had a strong impact, whereas QHEF and dHEF decrease as anisotropy increases. Finally, the bottom flux was estimated and its influence on the hyporheic exchange flows was computed. Multiple discharge measurements at multiple cross sections along Nahal Cziv were obtained, showing that the stream generally loses water with a local increase in the first site. Nevertheless, in agreement with past publications, the sensitivity analysis shows that an increase of the bottom flow rate results in a decrease of dHEF and QHEF, regardless the bottom flux direction.