REACHES WITH ALTITUDE ERRORS in stream package

[Chang Liao]

I am getting an error in stream package using ModelMuse:

  REACHES WITH ALTITUDE ERRORS:

    LAY    ROW    COL    SEG  REACH      STR.ELEV.      CELL-BOT.

      1     10     66      1      1    186.5617065    188.0839996

      1     11     66      1      2    185.5171204    185.9889984

I used the bottom elevation of the layer 1 as the stream bed elevation.

So how did the STR.ELEV and CELL-BOT were calculated from the data?

[Richard B. Winston]

Have you tried coloring the grid with the stream elevation and looking
at the explanation for how the stream bed elevation was assigned in the
Grid or Mesh Values dialog box (Data|Show Grid or Mesh Values)?

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[Chang Liao]

Thanks for the reply.

I tried the coloring the grid, then it only supports the upstream or the downstream elevations, which are merelypoints.

I have assigned the Z-value of the stream shapefile using the layer 6 bottom elevation.

And in the segment streambed elevation dialog, I assigned it with layer 5 bottom elevation.

  REACHES WITH ALTITUDE ERRORS:

    LAY    ROW    COL    SEG  REACH      STR.ELEV.      CELL-BOT.

      6     24     77      1     15    158.0209961    168.6419983

  MODEL STOPPING DUE TO REACH ALTITUDE ERROR

The Data show dialog gives value for the cell and shows that the cell which has error has the correct elevation 168.64m.

However, the stream reach bed elevation should be 173.44m instead of 158.02m, which still confused me.

Interesting thing is that I moved that vertices a little and nothing goes wrong with the streambed elevation. So I think the reason might lies in the interpolation method for the elevation.

[Richard B. Winston]

The input to the SFR package requires elevations at the beginning and
end of a stream segment. MODFLOW interpolates between those values based
on the length of the segment in each cell.

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[Chang Liao]

After trying to resolve this problem for a while, I finally get some idea about how this problem occurs.

The requirement for the SFR/SFR2 package is that the streambed elevation MUST be higher than the bottom of the cell which contains the reach.

There are two ways the streambed could be prepared or calculated within SFR package.

It could be either directly inputted or calculated from the segment which contains it.

For direct input, we can set the individual streambed top elevation.

In some special case, the slope of one reach could be negative when upstream cell elevation is lower than downstream cell, and we have to change the slope value.

For the other way, it is very possible that the MODFLOW interpolates the elevation with errors if special case occurs and there is no control on it.

So I will stick to the first way.

[Francesca Moschini]

I get the same error, in the middle of a segment.

  REACHES WITH ALTITUDE ERRORS:

    LAY    ROW    COL    SEG  REACH      STR.ELEV.      CELL-BOT.

      1       103       43       176     15         958.5556641    998.0000000

      1       104       43       176     16         923.0745239    981.0000000

  MODEL STOPPING DUE TO REACH ALTITUDE ERROR

But the problem is that I get the error if I change my layers (or just the top layer)  into "convertible".

When my layers are confined I don't get any error.

I have 2 layers in total.

Any hints?

[Richard B. Winston]

1. Check the stream elevations at the beginning and end of a segment to
make sure they are appropriate.
2. Check the layer elevations along the course of the stream to make
sure that you don't have the stream flowing uphill somewhere.

[Francesca Moschini]

Thank you for your answer Richard!

I checked, my elevations are appropriate and I don't have uphill reaches anywhere. 
I think that the problem is due to the fact that my segment has a high/irregular slope, the upstream elevation is 1519 meters and the downstream elevation is 556. So the program calculates the streambed elevation equal to 958.55 and 923.07 m for the reaches 15 and 16, but the real elevations of those are 1404 and 1366 m.
So I changed my downstream elevation to 650 in order to "swindle" the problem. 

Though I think it's still curious the fact that the problem arise only when my top layer is convertible. When my layers are confined I dont have any error.

Thanks again

Francesca