By Hydrosimulatics INC  

Canyon Seepage

 A sandstone aquifer connects a reservoir with a river in a canyon as shown below. Impermeable shale lies above and below the aquifer, and along the river and reservoir where the aquifer is not connected. A portion of the aquifer near the river is unconfined. Proper management of the reservoir requires knowing the leakage (rate of flow), Q, from the reservoir to the canyon.

1.      Using an analytical approach, determine the length of the unconfined portion of the aquifer, Lun. Then compute Q. Assume 2D horizontal flow.               

  (Hint: Treat the overall problem as two connected problems: the upstream section of the aquifer is confined, and the downstream section is unconfined. Note that flow is conserved throughout the system.)

2.  Verify your findings using a simple numerical model created with MAGNET. Use the numerical model to determine the sensitivity of calculated Q and Lun to hydraulic conductivity of sandstone.


                               Figure 1: Plan view (left) and cross-section view (right) of the site of interest.


   Additional information/instructions:

  •   Average depth of the river, driv = 5m; 
  •   Average depth of the reservoir, dres = 150m; 
  •   Thickness of the aquifer, Baq = 50m; 
  •   Length between the river and reservoir, L = 1 km; 
  •   Hydraulic conductivity of the aquifer, K = 10 m/d (best estimate).
  •   Use the ‘Distance Display’ capabilities when editing geometric features and/or the ‘Ruler’ tool to properly represent distances between model features.