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I am using ArcMap 10.7.1.

I am trying to delineate multiple urban watersheds in GIS for the City of Denver (sometimes called sewersheds) using Spatial Analyst. I am looking at several drainage points along the South Platte River and trying to determine their catchment areas to understand what land uses/areas are draining to each drainage point. These points include surface water gulches and underground outfalls.

Up to this point, I have a relatively high resolution DEM and effectively 'burned' the storm sewer mains into the DEM to account for city drainage and get reasonable drainage paths for the underground outfalls. I was getting some weird looking watersheds for the surface water gulches as their flow accumulation paths weren't showing up. As a result I burned in the surface gulches and the South Platte River as well.

Half of the sites are on the left (western) side of the Platte and the other half are on the right (eastern) side of the Platte. The watersheds on the left side of the Platte look good as drainage goes from left to right into the Platte River. However, the watersheds on the right side don't work, as I've noticed that the flow direction is going the wrong way on GIS. Theoretically, it should be going right to left (east to west) draining into the Platte River; however, the flow direction raster GIS computed has the flow direction going left to right (west to east) which is wrong. It doesn't make sense as the elevation is increasing along the flow paths west to east, so GIS is saying that flow is going uphill. Technically the burned gulches and storm mains are at the same elevation, so maybe this is the problem? I've filled the DEM too. This thread had the same issue but I wasn't able to pull any solutions from it: Flow direction generated for river is incorrect

I'm not sure what to do now.

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  • Remove dem cells under river and do fill, direction, accumulation. Streams should flow towards river unless river runs on a ridge. When confirmed, check where you messed up with burning etc.
    – FelixIP
    May 13, 2021 at 0:10
  • @FelixIP Thanks for the reply - I'll try that. Should I delete the DEM cells for all streams (river and surface gulches) or just the river?
    – pilofg
    May 13, 2021 at 3:20
  • Just a river, , no gulches
    – FelixIP
    May 13, 2021 at 3:39
  • @FelixIP That didn't seem to do the trick. Flow is still going opposite of what it should.
    – pilofg
    May 13, 2021 at 16:12

1 Answer 1

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Sounds like you are using either a LiDAR dataset or a drone-based structure from motion surface. Check your surface model metadata or LiDAR report. Are your surface model z values are referencing the WGS84 vertical datum?

If so, use vDATUM to convert the surface to NAVD88 and try the hydrography again.

See this article here on how water can flow uphill.

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