Applying a recharge BC on top of a 2D vertical (cross-sectional) model

Hi,

I want to build a numerical version of my 2D cross-sectional conceptual model for groundwater flow only (no transport at this time). FEFLOW 6.0 has an option to activate such type of projection, in Problem Settings. But I don't find appropriate Boundary Condition parameters to apply recharge on top of my model.

The problem is, I think, that the definition of the top surface of the model differs from 3D projections. Actually, I would need a way to apply a "linear" recharge on the upper linear boundary of my model (~ the XZ plane) ... rather than on the cross-sectional XY plane. Why is the "In/outflow on top" flow property removed when using a 2D vertical model? Wouldn't it be a simpler (and more coherent) way to apply recharge than imposing a 2nd type fluid-flux BC?

The main issue using such fluid-flux BC (e.g. a uniform downward recharge flux) is that it prevents the solver from inferring discharge zones along the top of the model. As a consequence, I think that using 2nd type BC for recharge is inappropriate for the upper limit of a flow model. But what can I use instead?

In short, I need your help to demystify recharge definition within a 2D-vertical model (please consider a simple case with steady-state uniform recharge).

Looking forward to reading from you!

Marc,
PhD student, Quebec, Canada.

Hi Marc,

For 2D cross sectional models (i.e. using vertical projection, thus gravity is pointing towards the negative y-axis), indeed the recharge is best defined by the 2nd type fluid-flux BC. Do a right-click on the fluid flux BC entry in the data panel and set the unit to mm/a for a more suitable unit. By "inferring discharge zones", I assume you refer to discharging groundwater via the ground surface e.g. at a valley bottom. I wouldn't put fluid flux BC's at those areas, rather estimate where in the model you expect such outflowing conditions at the surface and put seepage face BCs instead.

I normally model infiltration in 2d slice models using the well BC, assigning the equivalent infiltration in that particular area by means of injection wells assigned at the mesh nodes.

Hi,
Not sure if this helps for your problem, but I apply a constant flux boundary condition to the ground surface (top of cross-sectional model) and also assign a constraint (just click to add a constraint) that is equivalent to the ground surface elevation at that node. Then when the model reaches steady state, the top boundary conditions allow flux out of the model (discharge) at seepage areas where the water table intersects the ground surface, but recharge (flux in) along recharge zones.
Laurie

you can build a 3D model that is two elements wide, then you can use the in/outflow option.  Works quite well and is the only way to do a section for some types of flow problems.