Fluid Transfer BC or Hydraulic Head BC?

Hi,

I have a bit of what I'm going to call an unusual situation (or maybe it's more usual and I've just never experienced it prior)... I have a model where my initial head is highest in the middle with general topography sloping to the west (left side of the model). The only reason I can figure to be causing this initial head condition is the fact that there is a relatively shallow ditch the runs through that area that doesn't drain adequately, there is typically ~60 cm of water (from spring run off) in it until mid summer until it dries out. I believe this ditch is influencing the very local groundwater conditions in the spring.


I am trying to figure out which boundary condition is best suited for this. Typically in rivers, etc... I would expect to use a cauchy boundary (in feflow a transfer boundary). however, I am newer to modelling and do not have a bunch of experience with this boundary condition. I understand that the difference for the model is that I have to calculate my in transfer rate (conductance layer) which is an easy enough calculation where:

phi=K/D

however, where I am getting a little hung up is the value of D in the model. is it the thickness of the model layer? say 2m (if that's the distance between slice 1 and 2 is 2m?) or is it some arbitrary made up value that I just pick? for this ditch I suspect there really isn't much of a conductance layer though it's mostly just stagnant evaporating water that is sitting on the same surrounding soils. Also I might add that the ditch itself is just a surface feature in the model i.e. I just have it following slice 1 as a dip in elevation.

Stratigraphy wise I know the area reasonably well and have a monitoring well located almost adjacent to the ditch the stratigraphy is as follows: low-intermediate plasticity clay overlying high plasticity clay.

What do you feel would be the best condition to use?

Hi phuntz, maybe we can work through some of this together since I am also dealing with 3rd type boundary conditions for a river at the moment and have been doing quite a bit of research into the fairly sparse Feflow documentation.
Hopefully DHI will come back and help us with some of this however.

-Considering your stream/ditch is ephemeral (dries up), and overlies clay, is it necessary to assign boundary conditions to it?  It seem it would not contribute significantly to recharge or discharge and mostly just handle runoff. Especially if the ditch was lined with some form of membrane or geotextile.

-Have you seen that the Fluid transfer BC has limiting conditions available that can be set to avoid to much (unrealistic) loss or gain into the ditch?  Modflow handles this automatically by not letting the head go below the stream bottom by default.  However in Feflow it seems we need to set this manually.  DHI, please correct me if I am wrong.

I will try and provide additional info if I come across anything new.  I know this is not much help and not a definitive answer, but maybe it will spark some thought to help us solve your problem.  I will also keep an eye out for definition of D for you, but suspect it has something to do with thickness since that is what D is frequently used for if I recall.

Cheers,
Drake

Hi Drake,

"Considering your stream/ditch is ephemeral (dries up), and overlies clay, is it necessary to assign boundary conditions to it?  It seem it would not contribute significantly to recharge or discharge and mostly just handle runoff. Especially if the ditch was lined with some form of membrane or geotextile."


The issue here is that the "clay" or alluvial clay as I called it geotechnically behaves as a CL/CI however, particle analysis indicates 50-60% silt and even up to 22% sand. Falling head tests have indicated K values of 7e-6 m/s to 2e-6 m/s, so it will seep and transmit water. also the ditch is just a ditch, no special geotechnical liner or anything in place just straight contact to the soils. Having said that I believe this seep is very local to the ditch and I just happen to have a monitoring well that picked it up...

"I will also keep an eye out for definition of D for you, but suspect it has something to do with thickness since that is what D is frequently used for if I recall."

D is thickness of the conductance layer. the equation is conductance=hydraulic conductivity/thickness.

Pardon, I misunderstood that you knew what D was and were trying to apply it to the model.

DHI may yet correct me, but I would expect that your D is the thickness of the river bed material.  This link is for ModFlow, but maybe it will be of some assistance conceptually:

https://www.waterloohydrogeologic.com/help/vmod-flex/index.html?vm_about_boundary_conditions____t.htm

Click on "River" in blue to get down to the pertinent section.

Out of curiosity, how have you done your node selection for the Cauchy/fluid-transfer boundary condition?  Did you select all the nodes on slice 1 (horizontal node selection) that represent your river bed, or did you select a vertical plane of nodes as is shown in the Feflow manual on page 113?

Also, just so DHI is aware that the question is not answered, phuntz still needs an answer to his/her question in the initial post

Hi,

the transfer rate is clearly defined in the FEFLOW documentation.
It is related to a virtual clogging layer in a river.

http://www.feflow.info/html/help72/feflow/09_Parameters/Boundary_Conditions/Flow/fluid_transfer_bc.html

The definition of the Transfer Rate can be found here:
http://www.feflow.info/html/help72/feflow/09_Parameters/Material_Properties/flow_parameters.html
in the section: Transfer Rate (Fluid)

Bests Christian

Thank you Christian.  There are numerous areas where similar information is provided online and in the manuals it is hard to know which one has the most current or best information.  I've come across the feflow.info page before but tend to go to other sources (pdf manuals, feflow integrated help) first since the page you linked for us tends to have some display issues on my monitor which oriented 90 degrees (portrait) for reading documents.

Phuntz:  regarding your grain size distribution, I would expect anything with that much clay to not transmit much water (just intuitively).  Is it possible the layer is layered/varved and transmitting the fluid horizontally along the bed of the ditch more like baseflow?  I guess that could end up seeping out the edges if the river depending on the base layers geometry.

Also, have you resolved your modeling issue yet or are you still waiting for answers?