In first picture, you can see an image of a lake(Bellandur lake, bengaluru, india). In second picture, you can see the flow accumulation data for the same area I made(after "fill"ing the sinks). As you can see, the water accumulates rightward of the lake where there are houses and city.

Why is this happening?

Instead of flowing rightward, shouldn't the water flow inward the lake?

enter image description here enter image description here

  • What were the precise steps (tools and parameter values) that you performed to get these results? – PolyGeo Sep 24 '16 at 19:43
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    Make your lake a sink (which is what lake is) by assigning NODATA to it and repeat your exercise starting with Fill – FelixIP Sep 25 '16 at 20:41

To understand why, have a look at your flow direction raster. In a flat filled area such as this, the flow direction is limited to the flat filled surface regardless of whatever topography may have been present there before filling. There are alternatives to this that either apply a downstream trend to the filled surface (helps a bit) or better yet, you could decrement (breach) the DEM instead of filling to preserve the inherent topography of the area.

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  • How do I breach the DEM then? – Aakash Bikram Rana Sep 29 '16 at 9:39
  • I've played with a couple different solutions: 1. SAGA-GIS has a "Sink Removal" module which can accept a "deepen drainage routes" parameter. It's 64-bit and is available as a GUI or command line app or Python toolbox for integration in ArcGIS. 2. WhiteBox is a GUI based option with a "Breach Depressions" module. WhiteBox gives the user the opportunity to easily view all code used in a particular tool. Keep in mind that while filling sinks has only one derivative solution regardless of methodology, breaching depressions can (and will) produce quite varied results depending on the method used. – Jae Sep 29 '16 at 10:41
  • @AakashBikramRana there are also "hybrid" solutions that fill some sinks and breach others in an attempt to minimize either # cells modified or change in Z values. uoguelph.ca/~hydrogeo/pubs/Lindsay-HP-preprint.pdf – Jae Sep 29 '16 at 10:54
  • What is this hybrid solution? How do i do it? – Aakash Bikram Rana Sep 29 '16 at 10:56
  • I would recommend downloading WhiteBox and having a look in the "DEM Preprocessing" section of the "Hydrological Tools" toolset. – Jae Sep 29 '16 at 11:04

As to why is this happening, look at the actual values of the DEM in the area it is filling up. As the previous answer stated, if the land there is fairly flat and not far above the surface of the lake, it will flood. If it doesn't meet those two points, then there is probably an error in the way you worked it.

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