Assessment of Riverbed Clogging in Reservoirs by Analysis of Periodic Oscillation of Reservoir Level and Groundwater Level

In the area of the the Krško alluvial field, the Brežice hydroelectric power plant (BHPP), with its surface water reservoir, was completed in 2017. The new BHPP reservoir dam is located approximately 7 km air distance downstream of the old Krško nuclear power plant (NEK) reservoir dam. The NEK dam was built in the 1970s. The primary purpose of the NEK reservoir is to provide fresh water for cooling the NEK nuclear reactor. To assess the impact of the newly built surface water reservoir on groundwater, we performed a series of data analyses prior to its construction. One part of the analysis relating to data from the monitoring facility of the NEK showed an interesting correspondence between the water level oscillation in the NEK reservoir and the groundwater oscillation in the nearby observation well. Based on measurements taken in 2000, we sought to estimate the clogging of the Sava riverbed sediments in the area of the old NEK surface water reservoir. To determine the permeability of the riverbed sediments, we applied geometry similar to that chosen by Hantush for his pumping test method. Using Fourier analysis, we determined the dominant frequencies from the hydrograph records of the NEK surface water reservoir and from the pressure probe in the nearby observation well. Based on the determination of the dominant frequency, we used the wave equation to compare the influence of different values of the hydraulic transmissivity of the clogged part of the NEK surface water reservoir on the transfer of its water oscillations to the groundwater in the observation well. For the hydraulic values of the non-clogged part of the aquifer (T, S), we assumed the values from the pumping experiments performed in the alluvial aquifer of Krško polje. We also assumed that the aquifer is homogeneous and isotropic, as Hantush had assumed in his method for the determination of semipervious river beds. The results obtained indicated the potential for estimation of the thickness of the clogging layer which, by analogy from applied geophysics, can be called the apparent thickness. This meant that the thickness could be determined on the basis of the default conceptual model rather than on real measurements. The presented method shows the potential for using the analysis of periodic oscillations in river reservoir level and nearby piezometers, as a method of monitoring riverbed clogging, in cases where periodical oscillations in reservoir level occur and observation wells are near enough to detect the oscillations.

Displacement Evolution of Reverse-Dip Rock Slope Considering the Change of the Reservoir Level

This paper investigates the toppling deformation characteristics of the displacement evolution in different portions for a reverse-dip rock slope, through a case study of Xiaodongcao slope in Chongqing city, China. Firstly, the elevation, slope angle, and aspect were obtained by the field survey, and then they were adopted in the partitioning process related to geological and geometrical conditions by the ArcGIS packages. Secondly, the spatiotemporal cloud map of the displacement was obtained by discrete monitoring data of surface displacement of the slope. Finally, the topping deformation was determined by superposing the cloud map of the displacement and the geometrical partition, considering the change of the reservoir level. The main findings are summarized as follows: (1) the horizontal displacement is close to the total one, meaning that the slope topping deformation is mainly in the horizontal orientation.(2) In the front and middle edges of the slope, the horizontal displacement is pronounced, which increases with the increase of the reservoir level and vice versa. The vertical displacement mainly occurs in the trailing of the slope, which increases when the reservoir level changes. (3) The area in relation to the strong superposed displacement increases with the variation of the reservoir level. The largest area of superposed displacement is distributed at medium gradient, low elevation and north aspect zones.