Jump to content
GMS, SMS, and WMS User Forum
matevz

Specified head

Recommended Posts

Hi,

can anyone help me with this :

I have a small model with three layers. In the edge of the model I have assigned specified head values (in such maner that the groundwater flows in the right direction). I assigned the specified head value to around 251.15 but when it calculates i get a value for GW head on the altitude around 268? Why, i dont know!

I have also assigned the conductivity values for each layer. I dont get any errors with Check model application.

Does anyone has an idea of what this COULD be?

Thanks!

matevz

Edited by matevz

Share this post


Link to post
Share on other sites

Do you have recharge entering the system? The specified head is only for those cells that you assigned it to. If you have more water in the system upgradient of your specified head cells and it can't get out, your groundwater elevation will be higher (this will also happen as you lower your conductivity values.

Share this post


Link to post
Share on other sites

Hi, Thanks for help.

The model work fine i i have only one layer if i have three there is a problem. But the reason why I need three layer is because

i have to model diaphragm wall in this model. The lenght/depth of the wall is not the same and is not fully penetrating the model. As it is not

possible to "tell" GMS to which depth the HFB is reaching i have made 3 layers. So that now i can design in the model

the HFB, designating in the coverage properties to which layer it reaches.

But my layer borders are horizontal.

Thanks.m

Share this post


Link to post
Share on other sites

Without seeing your model, this is difficult to answer. However, I'm going to make a couple of assumptions: 1) The model without a barrier works fine for one layer (as you stated); 2) You split the model into 3 layers and, even without the barrier, the water elevation is higher than before (if you have the barrier in, that could explain the high water level, but I'm assuming this problem is without the barrier).

My guess is that your vertical anisotropy is the problem. If you have anything other than 1, your result will likely be different than the single layer model. This is because the water is more likely to flow horizontally than vertically. If your vertical conductivity is not the same as the horizontal conductivity, then your three layers don't act like the single layer that you previously had.

Hope that is the issue....

Share this post


Link to post
Share on other sites

Hi,

attached is the file of the model - 3.gpr is the three layered mode which doesent work at all anymore...I get some strange messages.

I didnt even manage to put in the horizontal flow barrier yet, because i cant seem to get it to work.

My goal (as usual) is to compute how moch water would have to be taken out of the (later designed) open excavation if I lower the water table for

3 meters. DO you think that GHP (general head..) package is the right package to use here? If so, what/how to compute the conductance for this GHP?

Thanks a million for the help. Seriously, if you was here i'd buy you a beer ;)!

Thanks, matevz

Edited by matevz

Share this post


Link to post
Share on other sites

I just downloaded the file and checked it quickly, but have to run off to a meeting right now. Two things - as I noted, the vertical anisotropy is currently 3 and it should be 1 for all layers if you want to allow the water to move vertically as much as it does horizontally. Second, some of your changing/constant head boundary condition elevations are now below the bottom of the top cell (as you split your original single layer into thinner layers, now your top one may be dry in some areas....unless you lower the bottom elevation of that top layer). I'll look at it a bit more when I get back, but wanted to give you a couple of things to work with.

Okay - I'm back. Change your top layer bottom elevation to 250 and your vertical anisotropy to 1 across the whole grid. That will correct your errors and should give you the solution you were looking for.

Regarding your excavation question, there are a few different ways to do it. You mentioned one. I would probably go with drains at the elevation you want. You still have to put in a conductance - assuming that you are keeping the excavation dry, make it a higher conductance (doesn't matter too much if you are pumping it dry, but you could try a couple of different values for sensitivity). Use the flow budget to determine the amount of water taken out by the drains.

Edited by Sean Czarniecki

Share this post


Link to post
Share on other sites

Hi,

thanks a lot for that reply. It helped a lot.

But i still have few question about conductance :).

The amount of water pumped out of the excavation is very different if i input 0.001 or 0.000001 for conductance? The difference is more than double.

Do you have any idea to how i could estimate this.

thanks

Share this post


Link to post
Share on other sites

Others may be able to answer this in a better way (I would have to look things up to give you a better explanation), but let me put it this way. Your conductance is related to what kind of resistance there is to the water getting to where it wants to go. For example, river bed material with a lot of fines will put up a lot more resistance to water going to and from the river. In the case of an excavation, there probably isn't too much resistance, so a very high value is appropriate.

I would guess that your conductance of 0.000001 may not have dropped the water level to the elevation you wanted (possibly too much resistance).

Share this post


Link to post
Share on other sites

Hi, thank you for your help.

You were guessing right, the conductance of 0.00001 results in actualy no lowering of water table.

I have another questioon. Is it possible to design and model a system of drainage pipes in GMS (like it shows in the picture)?

Edited by matevz

Share this post


Link to post
Share on other sites

Hi,

ok, i guessed that right. What u get out of this is the amount of water that has to be taken out with this drainage system, as i understand? Buit that meaning that the pipes are completely hollow? I dont get this conductance estimation...i must be really dumb!

thank man!

m

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...