Problem with direction of Outflow

Hello All,
I am running a EFDC simulation model on Lake Ontario. It has an Inflow which is Niagara river and an Outflow which is St. Lawrence river.
With the simulation results it is evident that the flow of St. Lawrence river are coming into the lake instead of going out of the lake. The flow rate is defined as time series data in QSER.INP and also as in Outflow boundary condition.
I tried to change QSFACTOR in C24 card of EFDC.INP file, the model doesn’t run.
I manually changed the flow rate as negative flow rate in QSER.INP, the model doesn’t run.

Please advise me how can I correct the direction of outflow.



Do you have measured water surface elevation to use at the St. Lawrence River ? Usually when you use both flow boundaries i.e discharge on upstream and downstream boundary then your model either gains lots of water or loses lots of water so your model might become unstable. So, the easier way to do is to use water surface elevation on the downstream.

If you have only option to use i.e outflow you need to specify the discharge as negative since negative means the water moving out of the system.

I hope this helps.


Hello John,
So prompt in reply, that is great.
Well I do have water surface elevation data for both the rivers, but I was advised to use flow rate/discharge (which is actually hard to obtain!!!).
And as I mentioned yes I changed the flow data as negative flow in QSER.INP file but I am getting the error message like: ‘maximum iteration exceeded’. It runs for couple of time steps then gives the error message.

Can you advise me what is happening and where and how I should use surface elevation data?


You can try to reduce your time steps and then see if that works.


Reduced time step, still crashing. Noticed something, emailed you with screenshots.


1 Might because of your cell geometry,
2 what is your time step?

Hi All,
Thanks for your responses. Actually my problem was a bit complex. I assigned outflow in one grid cell and it was supposed to draw out a large amount of flow through that grid cell. The water elevations in nearby surrounding cells were shallow. So the solution to the problem was to make surrounding cells’ water elevation deeper, kind of creating a artificial canal so that the large amount of water can be drawn out through outflow grid cell.
Quite complicated, I got the solution from an EFDC Expert, not solved by me :wink:
However the model worked fine after that :slight_smile:


Hi Rumana,

Though we don’t know all the details of your model, generally the water depth must be sufficient to supply water to that cell – if you draw too much out of the cell in a single time step then instability can arise. One concern is that if you only deepened around that cell then your model may not be physically representative and may lead to mixing. Our recommended approach would be to create a deepened channel from the deeper part of the lake to the discharge cell.

Thank you very much for the response. I exactly did what you suggested and the model is working now.