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CE-QUAL-W2 bathymetry widths calculation method

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I have some concerns about bathymetry widths calculation. The bathymetry widths calculated by WMS are up to 50% less than the real measurements.

The projection seems to be correct and when I use the ruler I get correct values. The TIN resolution is fine and good.

Now I am suspicions of the following issues:

1- The storage calculation from TIN is not correct and consequently the layers widths.

2- The TIN map resolution is not fine enough.

3- The projection is not correct or the ruler tool is not correct or the ruler shows the values in a different projection system (i.e The ruler shows the length at meters while the widths are calculated in a different unit).

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The first thing that comes to mind is that your X, Y, and Z values might be in different units. For example, are your Z values in meters and your X and Y values in feet? Make sure this is not the case. If this is not the case, you can always check the calculations in WMS by looking at the storage capacity curve for each segment. WMS computes the width of each layer in your W2 model by using the following equation for each layer of your model:

Layer Width = change in volume / length * change in elevation

So if you have a 10-foot change in elevation for each layer, you could determine the change in volume between two elevations from the storage capacity curve. WMS gets the length from the line you manually draw when you define the segment orientation/numbering in setting up your model. Use the equation above to manually compute the layer width. The width of the layer is shown in your segment attributes and it should match your computed value. Let me know if I can help with anything else.


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Hello Chris

Thank you for the reply. The TIN is made from x y z points in (Longitude , Latitude, Elevation in meters) e.g. (-106.5 , 51.6, 555)

At first I used the (Geographic (Latitude/ Longitude)) projection but the distance calculated between two points were not correct. So I reproject the maps to UTM.

Projection : UTM

Zone: 13(108w-102w)Northern hemisphere

Datum: NAD83

Planar Units: METERS

I checked the distance between two points that I know the distance in reality (51 4' 45"/106 37' 10" and 51 6' 17"/106 39' 0.5") . The distance should be around 3500 meters which is correct in UTM projection . (In the Geographic projection it was 0.0394 m).

So I have everything in meters and when I am using the ruler I have correct values for distance. The segment Length is also correct but the layer width is not. I also checked the calculated storage capacity which is not correct and consequently the layer width.


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The problem is the TIN resolution

Solution for this problem is to make the WMS to use a finer resolution for calculating the storage:

These are the results for one of the segments number after I increased the TIN resolution.

This table have widths for first 11 layers. The 3000*1500 is the resolution in my new TIN and 300*150 is the one that I and the WMS team used. You can see about 20% difference between the values is almost equal as the amount I was concerned about.

3000*1500 300*150 difference 1863.5 1506.2 19% 1800.0 1461.8 19% 1727.2 1425.8 17% 1650.8 1395.9 15% 1566.9 1385.8 12% 1496.1 1190.1 20% 1423.9 1166.2 18% 1366.0 1151.6 16% 1322.9 1071.9 19% 1280.5 1049.6 18% 1242.0 1028.5 17%

What I did is:

1- to convert the TIN to a DEM ( here you can define an interpolation ratio). I adjusted the new DEM resolution to 3000*1500 (it was 300*150).

2- Convert back the new DEM to TIN.

And from this TIN I used the rest of the work. The calculation time will increase significantly but the results are more convincing.

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That's a good way to solve the problem with incorrect layer widths. Sometimes your TIN triangles might cross segment boundaries. If this happens, your computed layer width would be incorrect. Make sure your triangles are mostly contained in each segment to compute the correct layer widths. Thanks for your post.


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