HELP 3 Landfill Modeling

[scottm8]

Ok where are my fellow landfill designers? I am experimenting with a somewhat non-conventional cover for a non-rcra site. The thought is to have the upper-most layer be 24" of gravel (Configuration 1).

It seems that as soon as I switch the top layer in HELP3 to a highly permeable material (such as gravel-texture 21, or sand-texture 1) the peak day volume collected on the cap membrane exceeds to the total peak day precipitation. How could a cap collection layer collect more water than falls on the site?

[Configuration 1]
layer 1 GRAVEL 24 HORIZONTAL
layer 2 DRAINAGE NET 1.97 HORIZONTAL
layer 3 GEOMEMBRANE 4.72 MEMBRANE
layer 4 BENTONITE 2.36 HORIZONTAL
layer 5 GEOMEMBRANE 2.36 MEMBRANE
layer 6 SILTY SAND 480 VERTICAL
layer 7 GRAVEL 24 HORIZONTAL
layer 8 GEOMEMBRANE 4.72 MEMBRANE
layer 9 BENTONITE 2.36 HORIZONTAL
layer 10 GEOMEMBRANE 2.36 MEMBRANE

Peak day precip. = 1.90"
Lateral drainage collected on layer 2 = 6.15"

Output is fine under Configuration 2, where the only difference is the replacement of the top gravel layer with a more traditional barrier/veg system.

[Configuration 2]
layer 1 ML - TEXTURE 8 6 VERTICAL
layer 2 SOIL BARRIER 18 BARRIER
layer 3 DRAINAGE NET 1.97 HORIZONTAL
layer 4 GEOMEMBRANE 4.72 MEMBRANE
layer 5 BENTONITE 2.36 HORIZONTAL
layer 6 GEOMEMBRANE 2.36 MEMBRANE
layer 7 SILTY SAND 480 VERTICAL
layer 8 GRAVEL 24 HORIZONTAL
layer 9 GEOMEMBRANE 4.72 MEMBRANE
layer 10 BENTONITE 2.36 HORIZONTAL
layer 11 GEOMEMBRANE 2.36 MEMBRANE

Peak day precip. = 1.90"
Lateral drainage collected on layer 2 = 0.86"

Output for configuration 1 attached. Thanks in advance for any responses!