Stream-Aquifer Interaction in MODFLOW

[Ahmed Elwan]

Hi,

Reference is made to this discussion of MODFLOW users group

http://groups.google.com/group/modflow/browse_thread/thread/dc84035f22af0bf0

In this discussion, Vivec Gupta illustrated that if the GW is discharging to the stream, the river stage should be below the river bottom.

In my study area, I know that the GW is discharging to the streams, do I have to assign the river stage below the river bottom?

When I did so, the model didn't converge. 

Any suggestions will be invaluable.

 

--
Ahmed Elwan

[ashutosh singh]

Hi,

Be realistic. You cannot have river stage below river bottom.

If you use a river boundary condition.

You chose river bottom below the head in the cell next to the river cell, but the river stage is always above the river bottom.

In case you chose drainage boundary condition.

The streambed elevation is the river bottom elevation.

Regards

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[Bangshuai Han]

GW discharging to the streams does not mean that river stage is below the river bottom. It only means that the GW  head around the river is higher than the river stage. 

James Han

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[Ahmed Elwan]

Thanks Ashutosh. I was confused also about what Vivec Gupta had stated so I asked about that.

I'll give you an overview of my model.

I have an unconfined, steady state, one layer model. My study area is bounded on the east by no flow boundary (red) and on all other sides by rivers (blue) as you can see in the figure below. 

 

I'm using RIV, DRN, CHD and HOB packages. I applied drain conditions everywhere with drain elevation = DEM and drain conductance = (100 m2/d = 1.15E-3 m2/s). The river bed conductance is the same as the drain conductance. I am using river package along the rivers. As an initial conditions, I chose the river stage as (DEM-0.25) and the river bottom as (DEM - 1.5), as a start. I'm using one hydraulic conductivity value all over the study area which equals 17.2 m/d (2E-4 m/s). I used one point as a constant head value (= 40 m) at the uppermost NW of the study area. I am using GMG solver as it is effective for large grids. The HCLOSE is 0.001 m and RCLOSE is 0.001.I'm using the wetting option.

While running the model, I found that there is a problem in ModelMonitor, listing, that some cells are repeatedly converted from wet to dry and vice versa. I received a message stating that "GMG assembly error in subroutine GMG1AP". Anyhow, the model finished running but I found that all the right part became dry cells. and the error between observed and measured head is large. I tried to follow the instructions in these links

http://groups.google.com/group/modflow/browse_thread/thread/6a310f11f268b274/39392f0999f7d4e0 , http://water.usgs.gov/nrp/gwsoftware/modflow2000/MFDOC/index.html?frequently_asked_questions.htm for solving the problem of dry cells and convergence but it didn't work.

Another point, I suppose that "Leakage in" should be lower than "leakage out" as the groundwater is supposed to discharge into the streams. However, in the water budget, I found that "leakage in" is higher than "leakage out"!!!

Any idea, how can I solve this problem of model convergence, dry cells and to obtain leakage in lower than leakage out.

Thanks in advance.

[subrata halder]

Right answer from Han. --- On Wed, 13/7/11, Bangshuai Han <bangsh...@gmail.com> wrote: