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paul_ollett

zsh grid editing

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Hi there,

I've been using the zsh feature a lot to burn surveyed channel cross sections into a DEM generated by laser points. When using the zsh feature, the polygon (extent of DEM edit) and polylines (breaklines) are currently kept in the same table. Would it be possible to separate these, so that in the tgc file you would need to write something like:

Read MI Z Shape == mi\2d_zsh_polygon.mif | mi\2d_zsh_TINlines.mif | mi\2d_zsh_TINpoints.mif

The reasons are:

When I edit the polylines, I keep selecting that darn polygon in the same layer. Typically now I keep these separate, then re-combine when finished editing;

Flexibility in using different sets of polylines (eg, pre/post development);

Compatibility with ArcGIS which does not allow polygons and polylines (if I'm correct) in the same layer.

Regards,

Paul.

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Hi Paul,

Just out of curiosity, how wide is your channel in relation to your grid size, and is there a certain width to cell size ratio where you would start using this approach instead of a deactiated 1D channel? Also, are the 2D solution produce comparable results to a 1D deactivated channel?

Cheers

Richard

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Hi Richard,

Most of my modelling work is for urban drainage networks where the cell size is 2m. Typically our open channels are sufficiently wider than this so that the grid adequately defines the channel shape (once the channel is burnt in). If the channel is narrow (<=4m) and incised I’ll typically model this in 1D. (not sure what you mean by deactivated 1D channel)

In 2D schemes the cell’s hydraulic radius = flow depth. A 2D scheme does not model side wall friction, so in the situation where the channel is narrow and incised, 2D won’t include the full affect of side wall loses. In most cases our urban catchments are very flat, and the hydraulic grade is dominated by instream structures (eg, road crossings).

The second thing with 2D is that for the solution to be reliable, the cell width should be >= flow depth (due to assumptions in how the Shallow Water Flow equations are derived). This is why I don’t use a 1m grid for urban modelling, as we often experience flows > 1m deep. On the other hand, using a 5m grid does not do as good a job (from my experience) at picking up gutter flow and other urban features. I’ve generally found 2 to 3 m grids to be best for urban modelling.

When modelling a 100yr ARI flood, where the drainage network is mostly drowned, the 2D modelling I’ve done produces good results. However, for smaller return periods (eg, 2-yr), I’d would more rigorously check results for 2D open channels.

Cheers,

Paul.

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Thanks Paul, sorry what I meant was a 1D channel with the corresponding 2d cells deactivated.

Cheers

Richard

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