Tim Ezzy

Sometimes I use "Recreate" in the post-processor. I then select "Rehash F.E. Problem" and it automatically prompts you with the message "Rehash a finite element problem from the underlying database using the set time = XXXXdays  as initial condition for the simulation run", whereby xxxx days = finishing time of previous run. Save it as a new problem with a unique identifier. Additionally, the starting heads appear to be the same as the finishing heads on the previous runs.

Gday All

I would greatly appreciate any feedback/encouragement on my methods for a mine seepage investigation I am conducting. cheers!!

[b]Background Info[/b]:  I am investigating a tailings pond seepage 3D transient model. This pond is using a perimeter interception well scheme to capture seepage and recycle back to the pond. The underling aquifer setting is mainly a fine to coarse-grained sand aquifer up to 60m thick and 200km2. I am utilizing up to 100 4th type BC's to represent the wells in the sand aquifer and am using minimum head constraints equivalent to 4m above base of screen interval. I am using a 2nd type flux boundary to represent the pond ( to avoid 1st type BC in close proximity to high number of wells) with flux volumes into the model calculated off the mine water balance and divided into the area of the pond's surface area. I have set a max head constraint on this flux boundary to represent the tailings pond maximum elevation, so that groundwater cannot mound above the pond elevation. Pond is initially about 40m above phreatic surface and thus will drain into the aquifer and develop a "mound". The pond has a massive open pit situated 1km to the side and I have run my first simulation representing this open pit as a 1st type CHB using a head that is equivalent to the base of the aquifer. This boundary has aggressively dewatered the sand aquifer and appears to be preventing mounding from occurring beneath the tailings pond flux zone.

[b]Questions[/b]:
1) Are 4th type wells the best boundaries to represent wells in this case? I read a post by Eladd about using a 2nd type boundary (Max = 0) and very high Kz.
2) Is there a better way to simulate the seepage from the pond so that the head elevation and water flux into the model are honoured?
3) Is the open pit dewatering best represented as a series of 4th type wells simulating a typical drawdown zone? Or can I modify the constraints on a 3rd type BC to represent a "drain" type scenario (similar to Dwain edington post on March 08, 2007)?

Any feedback would be sweet!

Gday Elad,

I was wondering how you faired with these changes. Specifically, what constitutes a "low" residual saturation depth? I am experiencing similar water budget issues with unconstrained setting and have applied a depth of 0.1m for residual depth. Is this an adequate number (in the ballpark) or should I be up-scaling  this value significantly? 

cheers mate
tim

Thanks guys!

It appears that I have three distinct aquifers here. The two bottom sand aquifers are confined, however, the degree of communication between those layers is currently unkown as I do not know if the thick clay layers pinch out in any areas. We have recently installed bores and piezometers into all layers and will be test pumping and sampling soon to gather more information on the interconnection (or lack of) of the sand horizons.

Assuming that these two basal sand layers were fully saturated, then a fixed layer in the clays above and below them may be a potential solution. I am currently running PEST (SVDA in native format) for both the fixed and unspecified options to compare the results. The perched aquifer in the upper sand unit is limited in extent and ceratinly appears to dry up on occasion. I do not want slice movement to connect the water tables in the upper sand and the middle sand units and convert the middle sand aquifer into an "unconfined aquifer".

To follow this on. Hypothetically, if I wanted to depressurise the basal confined sand (which has a fixed clay layer above and below) with a gallery of pumping wells, would this impact on the stability of the model considering the aquifer may not remain fully saturated at some point in the future?

cheers
tim

Gday Feflowers

I am fairly new to Feflow with a background in MODFLOW, and I need some advice, please.

I am creating a flow and transport model in FEFLOW for a glacial Quaternary aquifer system. I have excellent drillhole coverage (over 200 holes in a small area) and have been able to produce a fairly accurate geological model which generally consists of sand and till sequences that are individually 10-20m thick, so that the whole sequence is between 50 and 70m. The sequence is sand, clay, sand, clay, basal sand, basal clay. 

I have defined the problem as unconfined (phreatic) aquifers. When i select the clay layers as "fixed" layers, I get a good convergence for steady state, however, when I set the clay layers as "unspecified", the convergence becomes a serious problem, and the calibration is not good. Can someone give me some advice on when to set fixed versus unspecified for saturated and thick clay sequences in unconsolidated aquifers... the help option in the program gives a brief explanation but does not give specific examples of when to utilise each.

Thankyou

Tim