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Zahrab

pumping well in Clay/silt or sand layer in MODFLOW and FEMWATER

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Dear all

I am new with GMS. and have trouble modeling multiple pumping well to see the total drawdown.

I have three layer of clay, silt and sand. I wanna see how much water I should pump and from which layer to get certain drawdown. so I worked backward. giving discharge I checked the result drawdown.

I am interested to use wellpoint system in the silt layer (instead of deep well in sand layer)

now the question is, why in modflow I can assign any discharge to the well? I expected MODFLOW to give error saying because of low pearmiability this much discharge is not possible to be taken from clay/silt.

but that's not the case. any discharge I put in, the modele solve the problem and give me drawdown for it!

I used FEMWATER as well, for the same question, and it also solve for any discharge I define!!!

That looks wiered to me this can't recognize the layer can't provid a lot water?!!

any help is highly appreciated,

Zara

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Thanks for replyin.

I have a layer of clay with .0003 ft/min and silt with 0.003 ft/min. there is one layer sand underneath with 0.012 ft/min.

at 1000ft distance I have constant head at all direction. those are the only BC I have.

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as an example here I have about 40 well in two top layer with discharge of 12GPM/well which means about 480 GPM !!!

I know from silt layer it's not possible to get that much water, but my model adjust itself for every discharge I define and give me high drawdown instead.

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actually in my LPF package, all the layer are mentioned convertible.

I made my model getting help from Tutorial:"MODFLOW- Conceptual model approach".

Thank you guys for helping me. I wish I could post an image of the results here.

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layer 1 is clay, from +25ft to -19ft, k= 0003 ft/min

layer 2 is silt, from -19ft to -83ft, k= 0.003 ft/min.

layer 3 is sand from -83ft to -175ft, k= 0.012 ft/min.

at 1000ft distance from the job site (which is 100*100ft square area) I defined constant head =20ft all around in layer 3 ( means preimeter of 1100*1100 square )

my goal at first was to see how much dewatering and how many wells I need to make the first layer dry for my excavation area.

here I defined 43 MNW in layer 2 with desired discharge 12GPM/well !!! (which is impractical)

results shows layer 1 at the middle is dry, layer two has head of -40ft and layer 3 is almost +14ft.

I expect GMS tonot allow that much water running out of tsuch layer to my pumps.

in this case, it means I always need to know real world discharge from silt/clay/sand and then use model to predict the drawdown I can get for it. right? :(

Sorry for too much explanation

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Well, just briefly looking at this (without doing any calculations), your silt layer is of decent thickness and the conductivity isn't THAT low. I think one of the important things to consider is your boundary condition. It may not be far enough away. Right now the constant head continues to provide all the water needed to maintain the cone of depression. If you moved it further away, the cone would likely go down.

Something else to consider - I'm guessing that you are running this at steady-state. You might want to try a transient run to see how long things take to get to that cone of depression. Just additional information to have available.

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I tried decreasing k for silt to the value 0.0003

runing the model gives no error but it makes both tope layer dry and flow budget is almost zero!!!

results sounds too wierd so I think that counts as the model error when I set high discharge for low K .

I will try transient model for first parameter to see how it respond.

Thank you for helping me.

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Another thing you can try is instead of using wells and setting the pumping rate, you can use drains with really high conductance. Put the drain bottom elevation at the elevation that you want the draw down to be. Then you can look at the flow out of the drain boundary conditions.

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Thank you, I will try that as well. I am new in this modelling and face a lot of question for such an example.

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I stand by one of my earlier comments. Your constant heads are probably too close to the dewatering zone. Do a quick test by creating a grid that is double the size. You are going to get a different answer by doing that. You want to have little to no drawdown out at your model boundaries (unless you know you have something like a nearby river which could contribute water to your aquifer).

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Dear Sean,

I agree with you.

When boundary conditions are too close to the model area or better to the dewatering zone, I suggest to change constant head boudary condition (upstream) with a flow condition.

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