Variability of the hydraulic conductivity in the hyporheic zone in the light of laboratory research

The interaction between surface water and groundwater throughout the hyporheic zone determines the diurnal water level fluctuations in springs, streams and lakes. The previous studies have listed evapotranspiration, variability of hydraulic conductivity in the hyporheic zone, melting andfreeze-thawing processes, thermal expansion and water viscosity among the factors responsible for the water level diurnal fluctuations. The aim of the research was to investigate the variability of hydraulic conductivity in the hyporheic zone and its impact on the diurnal fluctuations of the water level. Experimental series were performed in laboratory conditions using seepage columns. Four types of sand sediments, with a hydraulic conductivity ranging from 6x10-6 to 5x10-4 m/s, were tested. The obtained values of hydraulic conductivity appeared to depend on the sand grain size, temperature, the direction of the hydraulic gradient, and the fluidization of the sediment (flowing sand). In all tests, higher hydraulic conductivity values were observed when water was set to flow in the direction opposite to gravity (flow up) rather than downward (flow down). In the sediments, which have not reached the state of fluidization, with an increase of the hydraulic gradient, values of hydraulic conductivity were two times higher during flow up than in the case of flow down. In the case of sediments being fluidized, the obtained values of hydraulic conductivity were roughly 3 times higher. The initiation of the fluidization process has been observed at the hydraulic gradient close to one. The results of laboratory experiments allow for a better understanding of the factors that have impact on diurnal fluctuations of the water level, especially when seepage of groundwater is present. Fluidization of the hyporheic zone after precipitation or thawing events may inhibit diurnal fluctuations

A mathematical model of the hyporheic zone fluidization

W strefie hyporeicznej dokonuje się wymiana wód powierzchniowych i podziemnych. W warunkach drenażu wód podziemnych, przy wzroście gradientu hydraulicznego, może dojść do fluidyzacji tej strefy. W artykule przedstawiono model matematyczny fluidyzacji strefy hyporeicznej, który wyprowadzono z bilansu sił związanych z filtracją wody przez strefę hyporeiczną. Model matematyczny umożliwia prognozowanie wystąpienia fluidyzacji w zależności od wzrostu położenia zwierciadła wód podziemnych. Przeprowadzono serię testów laboratoryjnych, które potwierdziły prognozę obliczoną z modelu. Przeanalizowano także konsekwencje hydrologiczne i hydrogeologiczne fluidyzacji strefy hyporeicznej takie jak: zanik dobowych fluktuacji zwierciadła wody w strefach źródliskowych oraz zmiana tempa opadania wody w rzece po wezbraniu.In the hyporheic zone, exchange of surface water and groundwater takes place. During the groundwater drainage and the increase in hydraulic gradient, this zone can be fluidized. The paper presents a mathematical model of hyporheic zone fluidization. It has been obtained from a balance of forces related to the water filtration through the hyporheic zone. The mathematical model makes it possible to predict the occurrence of fluidization depending on the increase in water table position. A series of laboratory tests were carried out, which confirmed the predictions according to the model. The hydrological and hydrogeological influences of hyporheic zone fluidization were also analyzed, namely: the decline of diurnal water level fluctuations in headwaters and the variable rate of decline of water level after rainfall flood