Flood Depth Grids

[Simon Jackson]

I am using 12D to get flood depth grids. I have a DTM tin and a Water level Tin. If I use the Tin Analysis---Intersect, it gives me a flood extent ( i.e. flat polygons) but not a flood DEPTH grid. How can I minus one grid from another (and remove negative values) - Which command would I choose to do this?

[Michael Henderson]

Simon

Try Survey - Conformance - Pavement Report. This is a routine designed to create a conformance report for a road surface. To use it you will need to cretae the grid of points yourself and then drape the dtm heights over these points. You could manipulate the settings to provide you with the depths that you need. On the pavement page set the data source to the model of points that you require depths for, set the control string to a dummy string that extends across the entire flood plane and set the design pavement tin to your water level tin. On the results model page, set your z values or your vertex text to conformances. You may also need to play around with the tolerances page.

Good Luck

[Lee Gregory]

One way to do it it to use the Grid option to create a regular gird of points using say Tins =>Utilities => Write DTM .

Then use the option Tins =>Utilities => Z diffs from tins to create the depth between the two tins as required.

[Simon Jackson]

Dont have the conformance Module license, but sounds interesting.

Ok. Bit confused. I have gone to Tins---Util----GRID DTM

This has created a points file of my Topography layer.

I then select Z Diff from Tins.

Model of Strings (bit confused about this layer - is this what is puts resulting difference into, I assume it has to be a points layer?) I put in the new points layer that I created above.

Original tin = DTM/Terrain Tin
New Tin = Water Level tin

However, the resulting file that comes out has no results - there is an elevation column but it is all zeros.

Help!

[Tom Lemon]

Hi Simon

You have used the correct option Tin => Util => Grid DTM to create the grid. When you create the grid, its best to place the grid points on an individual model, Model for points box in the grid DTM panel i.e Grid Points

The new grid points will have the z value of the tin that you specified in the grid DTM panel. Toggle on z values, tick-cross buttom at top of view.

To create Z-values from tin differences the panel should look like,

Data source - model
Model of strings - Grid Points (model you created above)
Original tin - Terrain tin
New tin - Water level tin
Model for tin differences - create new model i.e Grid Points Delta Z

To filter out the negative values this can be done under Utilities => A-G => change. Select the filter button (final on right) under data to convert. Select model tab and under name choose Grid Points Delta Z (model created above). Also select the Extents tab and fill in Minimum Z -999, Maximum Z 0. Click filter select.

At the buttom under target choose move to model and type in a new model name. The points with negative values will be transferred to this new model.

Hope that helps,

[Simon Jackson]

Thats worked perfectly, thanks for clearing that up for me.

Ive now got flood depth grids without negative values. However, there is some errors creeping in.

Ive worked out, that sometimes the water level extent extends further than the terrain coverage that I have. In these instances, after I do the Tin Difference, these areas have radically low values and affect that reach of the watercourse.

I will try and get some screen-shots into this thread tomorrow to better explain that, but was wondering if there was some sort of workaround to prevent this from happening.

Thanks again everyone for such prompt replies, ill get the hang of this 12D soon enough...

[Lee Gregory]

You could restrict the grid of points that are created the first time by using a polygon to restrict the points to only those areas that tin terrain exists.

Then the first Grid would only exist in the correct spots and so the Z-diff will only produce values at those correct points