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Raster mapping of Drain Elevation Not Good


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I have created a number of drains and getting 160 errors were the mapping of the drain elevation is bad. The raster value is the (surface elevation - 1m) [highlighted in lilac] is 13.1m.

Cell elevation (based on surface elevation raster) is 14.18 m, bottom is 12.0. The mapped drain elevation is 11.3. WTH? I've deleted and recreated and mapped MODFLOW 3 times, no change.

image.thumb.png.107fce3d6fd3808cf9db5e6c0829d990.png

 

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How are the drains mapped?  If they are from arcs, the elevations will not necessarily follow the variation in topography, especially if topography is irregular and node spacing wide relative to the grid. If as a polygon, for the same reason, elevations may not match if topography is irregular and the grid and raster resolution are not similar. Solutions are :

  • Use the automatic layer range allocation with "Auto-assign BC to one cell", so they can go into the layer below if below the top layer (if that is conceptually acceptable)

  • If using arcs, redistribute vertices more finely along the arc, to a similar spacing to the grid, convert them to nodes so it can have more variation over distance.

  • Simplest and most robust option is to use a Seepage Face rather than Drain, so that the drain elevation in the grid is set relative to the upper cell top elevation. In your case set the “elevation offset” set to -1m.

     

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Thanks for the response.  The drains are mapped via polygons and the drain elevation raster pixels are ~253m x 253m (derived from smoothed 1arcsec DEM) and the FD grid cells are 250m x 250m. Where the drains are applied are relatively flat, so I was hoping for better agreement.  The seepage face option sounds promising - much easier to implement and less data dependence.

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  • 3 months later...

Hi,

I have the same issue to define the bottom elevation of drains (the TIN and Raster aren’t good to define the drain elevation). When I map the drain coverage to MODFLOW, drain elevations always place below the cell bottom elevation. I have a set of drains defined with arcs in layer 1 (top layer) and with polygons in layer 3.

I am using the MODFLOW USG and unstructured grid (UGrid).  

I tried the 'Seepage Face' instead of 'Drain'. However, I am getting this error: “The seepage face type requires a stacked grid. It requires a UGrid with horizontally identical layer (IVSD=-1).”

Does anyone know how to solve this issue?

 

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