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Alan Davidson

Post Processing in ArcGIS

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Hi all, I have a quick query regarding processing grid files using ArcGIS. We use XP-SWMM here, and can export an Asc results file which can then be imported to ArcGIS. For a flood envelope depth map which I have created in ArcGIS, the majority of it looks fine, however, some areas of lower depths around the edges of the extents have some wet areas missing. Approx 95% of the results match to the results shown in XP-SWMM. Why is this? I am using a 20m cell, and know that ArcGIS is using a 10m grid. Also, the grid network is orientated NW to SE, while the ArcGIS orientation is N to S. Any ideas?

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Alan,

This is a common problem we have encountered when results from models are mapped to finer scale DEM's (eg LiDAR/ALS etc). waterRIDE has a number of tools for automatically stretching water surfaces to ensure that the true flood extent is represented when results are mapped from one framework to another (such as to a finer scale DEM in your case).

Generally, stretching by 1 cell around the model "wet edge" is sufficient to overcome such issues.

Regards,

Cameron

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Hi Alan

This is caused by the different resolutions (and in your case different orientations as well) between the TUFLOW grid and ArcGIS grid. As Cameron mentions, you need to extend or buffer the flooded water surface. If using a GIS, this can be done by using gridding options such as IDW that have options to buffer the 3D surface (ie. extend it outwards). The ground DEM is then subtracted from the 3D water surface model to give a 3D depth model. The 3D depth model can then be contoured or shaded for positive depths only (any negative depths will represent those fringe areas where the water surface has extended into dry land). I'm not sure how to do this using Spatial Analyst, but I'm sure it would be possible!

Cheers

Bill

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Hello,

Does anyone know how to do this with SAGA GIS, Quantam GIS or, at a stretch Grass GIS (which I don't really know how to use very well)?

If so could you point me in the right direction? I'm mainly interested in how to do the buffer successfully, I (think I) know how to do the rest.

Cheers

Brett

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No, sorry (I'm not familiar with those packages).

We are, however, planning to build this buffering feature into TUFLOW_to_GIS, in which case, any GIS or 3D modelling software that imports ESRI ASCII grids will be able to make use of this.

Cheers

Bill

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Great to hear. Is this likely to happen soon?

Also, is this same buffering technique the only way to fill in results in the empty 1D area of the model results? I've looked around the forum and can't find anything related so the work around is either really obvious or nobody bothers. Al and I are producing flood depth figures for a very large model and the empty areas where the 1D channel is will probably draw some questions.

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We have come across this issue when trying to combine 1D and 2D results, not just for TUFLOW but for other models such as MIKE FLOOD. An approach which gives good results is to map/overlay the 1D results on top of the 2D results (you may need to stretch/buffer 1 cell of the 2D results if there is a 'gap').

Alternatively, you could try mapping to a common framework (eg finer scale DEM). Map the 2D first then the 1D (depending on your network setup you may wish to map the 1D before the 2D).

Buffering/stretching across the 1D domain can lead to unwanted 'wandering' around bends etc.

Cameron

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Thanks. But I have to admit that I had no idea that a 1D flood map could be produced. How is that done?

The only way I can think of would be to export all the 1D nodes with xyz (z being the maximum water level) and using the river polygons from the inactive area as an extent to produce a grid. Or is there some easier way?

Any suggestions would be greatly appreciated.

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TUFLOW results contain a triangulated version of the 1D network in the *.2dm file. You can separate the 1D and 2D portions of you wish, or combine them by mapping/overlaying.

The attached figure shows a 2D grid (triangulated) in red and the 1D network in blue, which has (in this case) been mapped to the 2D framework. The thematic colours are water level.

post-2725-1285651714_thumb.jpg

post-2725-1285651597_thumb.jpg

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@ btwycross,

To create the 1D mapping in TUFLOW you will need to specify 1D water level lines (1d_WLL), for more info see section 4.11 of the TUFLOW manual (or section 5.3.6 of the tutorial model).

WLLs are lines of equal water level, hence they should be perpendicular to the direction of flow. TUFLOW automatically triangulates between the WLLs to produce a surface of the results.

Hope this helps.

Phillip

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