Boundary condition for a underground drainage

Suppose we have a high-permeable unit (A) sitting on a low-permeable unit (B). Landslide is happening on Unit A, possibly due to the rainfall recharge cannot penetrate Unit B and accumulate at the bottom of A.

We are design several horizontal drainage to help with the dewatering during rainfall events. see the picture attached.

Please give me some advice on how to set up the boundary conditions on the horizontal drainages.

Thanks~~

I've been involved in a problem very similar to yours, except that the top of Unit A was a 1-direction slope.
We simulated the horizontal drains by assigning 3rd type (Cauchy) BCs with levels set using the actual drain elevation at every node.
This way, we got the drained flow rate and the water levels in Unit A as output under different rain events.
Hope this helps,

Giovanni Formentin