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HELP 3 Landfill Modeling

HELP 3 Landfill Modeling

HELP 3 Landfill Modeling

(OP)
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!

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