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# River Conductance

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Dear all

i am working on a model for a study area where the area is surrounded by a river from one side

and when i start using GMS (MODFLOW MODULE) i defined the river and it request that i add a value of conductance of the river ...... how can i calculate this value and what does it represent

my second question is .....is it better to simulate the river as specified head boundary instead of river .... and wt is teh difference between the two cases?

thanks for ur help

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Quickly (you should really read the MODFLOW module write up on the river module) - The difference between specified head boundary and river is that conductance term. It allows you to limit the transfer of water to/from the river cell. For example, if the bottom of your river is sand and gravel, there will be good connection (conductance) to the aquifer. If, however, the river bottom is made up of tighter material, the connection may not be as good. You could make the conductance very low and have minimal interaction between the river and the aquifer.

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Dear Sir

i searched the interned for the calculation of conductance

and i got the site http://www.xmswiki.com/xms/GMS:Conductance where i used the equation mentioned in it

my case is a river bed width = 50 m, water depth in the river = 3 m, and the aquifer depth is 90 m, hydraulic conductivity = 25.5 m/day

what i calculated for the conductance based from whet i understood is (25.5*50)/90 = 14.166 (m2//day)/m

the value is too small, and i would like if u check it with me and tell me if something is wrong with my understand to the equation

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If you read the information again, you will see that it is the conductivity and thickness of the river bed materials, not the aquifer. If you said that your river bed is a little siltier than your aquifer, it would have a lower conductivity.....and the thickness is often assumed, but there is no given number to use (0.5 m?, 1 m?). In general, since I often don't know the info, I often start with a high conductance assumption and then adjust if I know the connection isn't that good or see during calibration that there isn't a good connection.

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what i understood is the river bed thickness represents the thickness of aquifer below the bed ..... which is what i used in the calculation

and i used the same value of conductivity as the aquifer

thanks for ur notes ........ i will start from it

as i know that the main source of water in the aquifer comes from that river, and very small amount is from the rain, i will start with a big value of conductance

thanks for ur time

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