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Melodea

2d_SA and building obstructions

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Hi All,

 

I am modelling a small catchment that is densely urbanised and contains several large industrial buildings as well as residential areas.    I am using 2d_SA boundaries as my flow inputs and I am wondering what is the best way to model the buildings, as zshape blockouts or using the materials layer with very high mannings.  I am wanting to see that the water flows around the buildings - especially the industrial buildings.  I have also heard it mentioned that I could do cut out shapes from the 2d_SA polygons.

 

What do you think would work the best?

 

Thanks so much,

Melodea.

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Hi folks,

 

That is indeed a very interesting paper! When it comes to rainfall, things increase in complexity; what do you want to happen to the rain that lands on the buildings? Do you want it to run off rapidly, as if from a pitched roof? Or would gutters/drains be catching that water and rain onto the buildings should be removed? Or something else?! You probably don't want a light shower over a town to show all the buildings as flooded because water lands within the footprint, and depending upon your intensity and roughness within the building, depths may build up higher within the building than in the vicinity as water is unable to run-off the footprint.

Rather than simply applying high roughness, I would strongly recommend using a 'stubby buildings' approach, where the footprint is raised up to threshold level, thus ensuring that when there is only shallow flow adjacent to the building it does not cause the building to be flooded. My house stays dry even when there's a puddle outside!

A couple of possible approaches to throw into the mix then:

a ) Represent your buildings how you wish (I guess high roughness remains the standard), but as you suggest cut out the building footprints from the 2d_sa. On the assumption the drains are overwhelmed (the normal working assumption; people rarely include the impact of drainage in their models) then you'll need to increase the flow being added to the SA. That is, work out the volume of water that would be dropped across the area including the buildings, and then drop that volume on the area without the buildings.

b ) Estimate the capacity of the drainage system and and use a second 2d_SA with negative values to remove that flow from the building footprints. Thus for a light shower, or during the start of a more serious event the buildings would remain dry (unless water flowed in from the adjacent non-building areas), but if you have higher intensities such that drains are overwhelmed then you still see that in your model.

 

I actually prefer option A above, in combination with stubby buildings. You get all the rain landing in the area, it's consistent with common practice regarding drains and the building doesn't get wet inside unless adjacent water levels rise far enough (the roof doesn't just magically fail :) ).

 

I hope that makes enough sense. There's probably quite a big debate to be had here and in part it's going to depend upon what you're looking for from your model as to whether it makes any real difference anyway! I hope other people share their thoughts. :)

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Hi Guys and thanks so much for responding to my question.

 

Thanks Tachi, I have read the paper and have actually seen a tutorial on a similar topic with Bill that was very helpful.  My question is more specifically on how to do this when using a direct rainfall method such as 2d_SA boundaries.

 

PHA, I might try your method a) above and I am thinking instead of trying a graded Mannings based on depth.   i.e say 0.5 when the depth is less than say 100mm, and 0.1 when the depth is greater than say 300mm, with interpolation in between.    I might also try your stubby buildings approach as a second run.  It will be a good learning experience to do both methods and compare the results.

 

I'll let you know how it goes.

 

Any other suggestions will be warmly welcomed.

Thanks,

Melodea

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Hi Melodea,

 

There is a distinction between the direct rainfall (2d_rf) boundaries and source-area (2d_sa) inflows.  The former applies rainfall to each cell within the boundary, whilst the 2d_sa (by default applies) the specified flows to the lowest cells and then any wet cells within the polygon. When using the SA inflows there are a number of option you have to control how the flows are split between the cells within the 2d_sa polygon.  These SA specific commands are:

  • "SA Proportion to Depth == <on or off>" The default setting is ON.
  • "SA Minimum Depth == <minimum depth>"
  • "Read GIS Streams == " (see point 9 of the release notes)

When using the Read GIS SA command a number of options are available:

Read GIS SA == The default approach.

Read GIS SA ALL == The flows are split over the active cells within the polygon, this is similar to the direct rainfall approach.

Read GIS SA PITS == The directs the inflow only the 2D cells that are connected to the a 1D pit or "SX" boundary cell.

Read GIS SA RF == A rainfall hyetograph is specified rather than a flow.  This requires the additional attributes (catchment area, rain gauge factor, initial and continuing loss values) as outlined in Table 4.34 of the 2010 Manual.

 

For the SA boundary unless you are using the ALL option, for raised buildings (which remain dry) the SA boundary will not direct flow onto the building, only if the building becomes wet.  For the direct rainfall water will be placed on the buildings and high roughness at shallow depth will attenuate the flows from the buildings.  This may require a depth varying Manning's curve which has low roughness at shallow depths to allow the rain falling on the building to run off quickly (i.e. to represent the roof).  At high depths (when the building is inundated) a high n value can be used to prevent the building becoming a flowpath.

 

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With regard to the hydraulics of the industrial buildings, I would also consider the fact that they often have large entrance ways to allow egress for trucks and other large vehicles. This means that the "permeability" of the building sides is quite different compared with say a bricks-and-mortar residential building. You might want to consider using THIN z-lines to map the walls of the industrial buildings (leaving openings for doorways) rather than blocking out the entire building.

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