Estimate of river seepage by conditioning downward groundwater flow in the Toyohira River alluvial fan, Japan

Abstract

Estimate of river seepage by conditioning downward groundwater flow in the Toyohira River alluvial fan, JapanRiver seepage serves as the main source of groundwater in alluvial fans. However, reasonable estimates are rarely obtainable due to inaccuracy of vertical flows around the losing river. In the alluvial fan of the Toyohira River, Japan, to provide straightforward description of vertical hydraulic gradient, this study demonstrates to assign constant head boundary conditions on the shallowest aquitard instead of no flow conditions on the basement. Initial constant heads and layer-scale anisotropy are calibrated in terms of hydraulic heads. As a result, the simulated river seepage is in reasonable agreement with previous discharge measurements, with net water loss determined to be approximately 1 m 3 /s.Although the simulation fails water gain in the lower section, due to a lack of sporadic intercalations of fine deposits, the conditioned vertical hydraulic gradient modeling approach is found to be effective on a regional scale, especially when losing/gaining sections have not been identified in advance.

Vertical trend analysis of equivalent hydraulic conductivity in alluvial fan gravel deposits considering open void connectivity

“…In the Toyohira River alluvial fan, for example, the groundwater is mostly recharged by the precipitation from the surface and by the seepage from the riverbed (Mukai et al, 2008). When the decay exponent is very large, groundwater infiltration to the deep zones is inhibited and the groundwater flows mainly in horizontal directions within shallow high-permeability aquifers, as shown in the numerical flow simulation (Sakata et al, 2016). The vertical trend is probably more significant for the transport system into the deep zone, for example of infiltration of dense contaminants such as heavy oil in the developed area of the midfan.…”

Heat as a Tracer for Examining Depth‐Decaying Permeability in Gravel Deposits

“…The longitudinal and transverse thermal diffusivity values were 10 and 0.1. These thermal parameters were less sensitive to the heat transport simulation than K (Sakata ). The dependencies of the fluid density and viscosity on temperature were assigned from the literature (Voss ; Langevin et al ).…”

“…No flow and fluxes were also assumed for the bottom cells based on the assumption that the permeability of the bottom fine layer was negligible. The seasonal temperature profiles at DW‐1 commonly showed that the temperature gradient was only about 0.3 °C/100 m at the bottom depth (Sakata ). The initial conditions of the hydraulic head and temperature were estimated from maps drawn manually from observation data at the start date, June 10, 2010.…”

“…The realizations by SGS are significantly dependent on the variogram model. The variogram of R ( s ) in the fan was previously formulated by Sakata () who used 134 slug‐test results and performed variogram analysis using original data of R ( s ) and obtained a histogram that was approximately Gaussian at 95% significance level. Consequently, a spherical anisotropic model of distance h was obtained as: …”

“…In Sakata (), the sill b and nugget c were determined to be nearly identical in the horizontal and vertical directions ( b = 1.0 and c = 0.5). The range a differed in the x and z directions, a x = 1000 m in the lateral direction and a z = 20 m in the vertical.…”