Calculating Conductance

[Stephanie]

I have a question about calculating stream-bed conductance. If you do
not want to model a layer of low permeability sediments at the bottom
of the river, but instead want to assume that there is a direct
connection between the bedrock and the river, what do you use for m
(thickness)? I have gone through the equation:

QRIV = KLW/M * (Hriv - hi,j,k)

If (Hriv - hi,j,k)/M is the gradient term, it seems to me that M
should be the distance from the river bottom to the center of the
cell. because this is the distance over which the head change takes
place. However, I have heard arguments that it should be 1 because
the gradient is vertical. Does anyone know what is the standard
practice? Thanks!

Stephanie

[Edward Banta]

Stephanie,

You may want to use a constant head rather than a river boundary where conductance is expected to be high.  (I think we have discussed this issue previously.)

If you do use river cells, you can interpret the conductance in more than one way.  

If the conductance of the riverbed itself is considered to be the limiting factor in determining leakage between the river and the ground-water cell, then in the formula Criv=KLW/M, M is the thickness of the riverbed and K is the hydraulic conductivity of the riverbed.

If, as in the case you describe, the conductivity of the riverbed is assumed to equal the conductivity of the aquifer material represented by the cell, then M (as you suggest) would be the distance from the river bottom to the center of the cell and K would be the aquifer hydraulic conductivity.  Generally, I would think it would be appropriate to use the vertical distance between the river bottom and the cell center as M, although some situations may call for using an alternative value for M.  M might equal 1 if the cell thickness is 2, but otherwise, I do not see the logic of assuming M equal to 1.

Ned

========================
Ned Banta
Hydrologist (GW Modeler)
U.S. Geological Survey
Denver Federal Center          
P.O. Box 25046, M.S. 415
Denver, CO 80225-0046
303-236-4882 ext. 223

From:	Stephanie <stephanie...@gmail.com>
To:	MODFLOW Users Group <mod...@googlegroups.com>
Date:	01/26/2010 09:26 PM
Subject:	[MODFLOW] Calculating Conductance
Sent by:	mod...@googlegroups.com

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[h2osteff]

Stephanie

I just wanted to echo what Ned said in that you can interpret the
conductance
many different ways in terms of how it links the river to the
aquifer. We have an
article in press that discusses this issue and its affects on
simulated
results:
doi:10.1016/j.advwatres.2010.01.008

Hope that helps.
-Steffen-