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I have a bare earth DEM made from lidar data (A).

It works well and has passed all accuracy tests. It is used in storm surge models. The problem we are having is on underpasses. I need to allow water to flow through underpasses when one road crosses another. My thought is to:

1) Get all roads marked as bridges from OSM.

2) The select all the crossing roads that cross under them (intersect).

3) Then on the new selection of new road segments (2) convert these to raster.

4) Extract all the DEM (A) values under these road segments.

5) Select the minimum of these values under the new road segment (2).

6) Burn this minimum value into the DEM I have to cut a channel through the underpass.

Open to all suggestions, improvements, ideas on how to achieve this including entirely different solutions.

Other options are just get the intersect point and grow this out or maybe identify sinks and work back from this.

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2 Answers 2

This looks like a valid workflow for this task. However, depending on the level of accuracy that you work with, you will miss out on things like pipes under roads allowing for ditches and small creeks to flow past the road. These would probably require some field work to identify though.

You don't mention which software you use, but for ArcMap point (6) make sure to use Calculate cell statistics rather than Mosaic raster, since it doesn't do any unnecessary interpolation.

Best of luck, and please share your experiences here once you've tried it out!

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Thanks I was thinking Con function in ArcGIS (under the line) for step 6 using the snap raster as the original DEM and Zonal Minimum in ArcGIS for step 4. –  user two seven two three nine Apr 20 at 15:34
Thanks, my first task is underpasses for now. If I can get that operational then small creeks etc will come next. Maybe I could use the same method for creeks by getting blue lines and seeing where they intersect roads. –  user two seven two three nine Apr 20 at 15:36
up vote 4 down vote accepted

Ok here is the image

enter image description here

Blue are the bridges from OSM. The DEM is in grey-scale with buildings (I want buildings)

I selected the intersect roads that cross the bridges.

Buffered the selected roads by the approximate width of a road.

Ran 'Zonal Minimum' on the buffer.

Now you have the zonal minimum raster (red) with the low values.

The I just used CON to blend the two rasters (original and zonal min) so I have the original DEM with the new lower values just in the red area.

Seems to work great.

Now to do in batch.

The batch worked and the process appears to alter our flood model in the anticipated manner. Our new model outputs now align much better with the FEMA measured water marks in areas of underpasses.

I assumed the opening was total for bridges which I think is a safe assumption.

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