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Hi,

i have a proble with computing the conductance of a polygon. I have designed the polygon wtih general head, which represent the elevation of water table in an open excavation. I know that conductance is coputed (in GMS) as C=k/t, where k= hydraulic conductivity and t= thicknes of material. My question is what is the thicknes exactly. In my model, the general head falls somewhere in 4th layer, so is the thicknes :

1- thicknes of layer

2- thickes of material above general head

3- thickens of material below general head

4- complete thicknes of the model ?

matevz

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• 1 month later...

The conductivity for a polygon is entered in a conductance per unit area, and derived from this equation:

C=(k/t) *lw

Where t represents the thickness of the material in the direction of flow, and lw represents the cross-sectional area perpendicular to the flow direction. So the t is the thickness in the direction of flow.

Hope that helps,

Benjamin Daniel

XMS Tech Support

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• 4 weeks later...

Hi,

But what i still dont get is:

I am modeling dewatering of an construction pit. The project is like this. I know the k koeficitent, i know where and how water flows in that are. I have an area in which a have to lower the ground water table for certain depth (let say 3m). And the goal of my model is to calculate how much water has to be taken out of that pit in order to lower the water table for designated height. For that i was using a GHP (general head). Around the area i have designed a diaphragm wall (using HFB package).

I have designed the polygon which represents the area in which the water table (WT) has to be lowered.

What I dont get is what is in this case the t in equation for computation of Conductance (in GHP using polygons to represent it in conceptual model) ?

Hope you can uderstand what i mean.

Thank you again.

p.s.: Is the GHP package the right package to use in such case at all?

m

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• 3 weeks later...

The thickness in the Conductance equation refers to the "skin" of material between the water and the aquifer. For example, it may represent the thin layer of alluvial clay between the river and the main sand aquifer.

If you are modelling a dewatering situation, you may be better off using a drain polygon. If you use a general head boundary, it will supply water to the aquifer if the level in the aquifer drops below the specified head in the GHB polygon, whereas a drain can only take water out.

What I dont get is what is in this case the t in equation for computation of Conductance (in GHP using polygons to represent it in conceptual model) ?

Hope you can uderstand what i mean.

Thank you again.

p.s.: Is the GHP package the right package to use in such case at all?

m

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