topic Re: Generate Flood Planning Layer in ArcGIS Spatial Analyst Questions

Generate Flood Planning Layer

JohnSpry1 — Tue, 31 Jul 2018 04:18:07 GMT

I have several flood depth rasters (floating point) from different flood studies and a dem (floating point).

The idea is to generate a flood planning layer (flood depth raster + 0.5m) using the flood depth rasters and a dem (floating point).

In raster calculator I did "Con("flood_depth_raster","dem")" which gave me the extent of the flood depth raster with the dem values.

Does anyone know a way to add 0.5m to this and fit to the dem?

Then i will create a flood planning level polygon from the raster to use in the gis in further analysis.

Thanks, John

Re: Generate Flood Planning Layer

NeilAyres — Tue, 31 Jul 2018 09:24:24 GMT

What you are saying here with this statement :

"Con("flood_depth_raster","dem")"

Is something like IF (flood_depth_raster is not null or empty, then return the dem raster, there is no else returned).

This would be more properly stated as something like :

Con(IsNull(flood_depth_raster), flood_depth_raster, dem)

I think you can just add the 0.5m to the output like :

Con(IsNull(flood_depth_raster), flood_depth_raster, dem + 0.5)

Re: Generate Flood Planning Layer

JohnSpry1 — Wed, 01 Aug 2018 01:49:39 GMT

Thanks Neil, apologies for not giving a better description of my overall problem.

Your answer is helpful and adds 0.5m to the vertical depth but I am also trying to stretch the flood_depth_raster horizontally to the dem so the extent expands as well.

Ultimately I will create an extent polygon from the new raster.

Any further ideas would be much appreciated.

Re: Generate Flood Planning Layer

DanPatterson_Retired — Wed, 01 Aug 2018 02:36:56 GMT

Xander Bakker‌ translated Bill Huber's original 2001 post which has been widely to simulate a flood.

See this link and Xander's python code

https://community.esri.com/thread/84057

Re: Generate Flood Planning Layer

JohnSpry1 — Wed, 01 Aug 2018 04:26:32 GMT

Thanks for the link Dan, I am looking at the script trying to figure out if it will work for my task.

Question:

In the python script under settings the line "nSourceElevation = 100 # The elevation corresponding to the source body" seems to use a static height?

What figure would could I use when the flood_depth_raster I have has a range of 0.6 - 50.2 AHD as you travel up the catchment?

Re: Generate Flood Planning Layer

DanPatterson_Retired — Wed, 01 Aug 2018 06:53:53 GMT

If memory serves... It is the elevation of the water surface at a location (say a lake or reservoir).

As you add a vertical increment increment of water to that location, it would obviously have a greater extent than at the start.

The cost distance portion effectively allows you to locate that new elevation in the surrounding terrain effectively.

Other portions of the algorithm deal with the boundary conditions etc.

Re: Generate Flood Planning Layer

XanderBakker — Wed, 01 Aug 2018 13:49:12 GMT

Perhaps if one would have a raster representing this flood depth raster graduating between the two elevation values. This would require changing the code and not validate against a single height value but against the flood depth raster. Not sure how complex that might be...

Re: Generate Flood Planning Layer

JohnSpry1 — Wed, 01 Aug 2018 23:59:46 GMT

Hi Xander, thanks for your thoughts

I ran the script and had a look at the outputs seem to simulate a bathtub type flood level rise. Not sure if or how the script could be changed to utilise the flood_depth_raster.

Now I'm thinking, take the polyline from the flood_depth_raster extent > densify it with vertices every 1m > create raster from polyline with cell values from dem > from this point I am looking at this link on focal statistics How To: Remove and replace no data values within a raster using statistical information from the surrounding data values as a way of expanding the flood_depth_raster with values from the dem that are within 0.5m.

Re: Generate Flood Planning Layer

DanPatterson_Retired — Thu, 02 Aug 2018 00:50:42 GMT

Yes it is the bathtub flood. As for flooding along a stream, ArcHydro seems to be the goto.

Although I do recollect some attempt to replicate a water level rise along the stream itself, (assuming uniformity along the length). I can't find a bookmark, but I recollect a protracted discussion before it dissipated without resolution if memory serves. Xander??