Hazard output

[Mark]

Hello All

I realise that RAS mapper has a hazard (DxV) output which correlates a FEMA hazard classification.
But what I don’t get is how you can get a low hazard with a depth of say 5m (as long as velocity is around close to zero, since DxV < 0.2 is classified as low hazard)? That doesn’t make sense.

Do you know how to output hazard from RAS Mapper that accounts for depth more adequately – eg. D(V+0.5).

This is a semi duplicate post, however, the original post hasn’t received a response for over year.
http://hec-ras-help.1091112.n5.nabble.com/Creating-Hazard-Grids-td4198.html

[Scott Miller]

The HEC RAS User’s Manual describes a “depth * velocity” Mapper output, and a categorization for the values is not included in the description. Where does the “low hazard” classification for 5 meters depth come from?

FEMA wrote “flood severity thresholds are different depending on whether they are being considered related to the impact on humans, vehicles, or buildings.” Figure 31 in the linked FEMA document shows maximum thresholds for depth and velocity for each category shown.

[Mark]

Thanks Scott. The 5m “low hazard” comes from the next page (Table 1) in the document you referenced.
Replicating the hazard classification in Figure 31 or applying the Formula (D*(V+0.5)+Debris Factor) would be ideal – but I don’t know how to do that. In the other thread you responded about using basic logical conditions?

I understand the raster math approach. My problem is extracting a relevant velocity and depth grid that represents maximum hazard, since max hazard for a cell wouldn’t be specific to a single time step. I suppose you could export depth and velocity for each time step and make individual time step hazard grids and then merge max values. Problem is that it would be exceptionally labour intensive if you have hundreds of time steps.

Any advice?

[Scott Miller]

Yes. I see that Mapper can calculate the maximum of the product of velocity and depth, but it says it must use stored maps. So that would really be the product of the maximum velocity and maximum depth, which is not the same thing.

Max hazard would be specific to a single time step for a cell, but that won’t be the same time step as for other cells. So it’s a matter of calculating the hazard for each time step, then identifying the maximum for each cell. A lot of work, but maybe computers can do most of it.

I haven’t needed to automate HEC RAS, but it looks like this post might be a good start for what we’re looking at.

On the GIS end there would be enough individual raster calculator steps that you’d want to use model builder to easily repeat the process. It would not take much more added to model builder to iterate through all the model output. I think you could work out a scheme (file naming, or name trimming) to iterate through time steps or iterate through one set of results. Yes, it would be a lot of work just automating the process, but it can be done.

About logical conditions: in raster calculator the convert function (Con) is useful for testing one or more data sets and creating an new data set with one value for a true condition, or a different value for a false condition. This can be used for setting maximum thresholds and for converting continuous values to discreet values.

[cameron]

What would be easier would be to use a script to extract the velocity at max depth from the hdf file and then create a raster from that. You could then use either scripts or ArcMap to create the D*(V+0.5) raster.

[Anonymous]

One answer is to define dual hazard criteria:
Dynamic flood hazard – Depth * Velocity; and
Static flood hazard – Depth.

Define categories (e.g. high, medium and low) for both types of flood hazard. The flood hazard is defined by whichever is higher. For example, if the D*V criteria yields “low” and the D criteria yields “high,” the. The hazard is “high.”

This approach is commonly used in Europe, most notably by the Swiss for which it is established by National Law. Doing a Google Search will yield references for specific adopted criteria. FLO-2D for example, has built-in functions implementing the the Swiss dual-criteria for both flooding and debris flows.

RAS Mapper doesn’t have this capabilty and therefore it needs to be implemented using if-statement GIS operation on Max Depth and Max D*V raster datasets.

Hope this helps.

[Mark]

Thanks to Scott, Cameron and Mike for your replies.

One final check with regards to DxV – Scott, you mentioned that RAS uses stored maps (max depth and max velocity). When adding a DxV grid to results, I have no ‘stored maps’ other than default (D, V, WSE) which aren’t necessarily set to max. So when calculating DxV, how do I know that its using max values independent of time step?
The manual only says “the maximum value is the maximum of depth times velocity” based on the user mapping interval.

It would be great for some inbuilt results calculator to create grids like hazard based on interrogation of all time steps. Until then I will go with what you have recommended – batch raster calculations for depth and velocity outputs for all time steps (when I need detailed), and a dual hazard criteria using DxV and D when not as much detail is necessary (with a caveat of course).

[Scott Miller]

I gave D*V in mapper a try. Good news! It turns out that “Must Use Stored Maps”, when you add a new map in the “Manage Results Maps” window, refers to the output, not the input. The only available output option gives you the raster you need. When I generated the map, it took longer than usual to calculate, which makes sense if Mapper is checking the maximum D*V of each output interval. I compared the result with the product of the maximum velocity and maximum depth for several 2D mesh cells, and the latter was consistently higher, sometimes by 20% to 40%. So, Mapper does not simply calculate max*max. Documentation has room for improvement. Thanks to the developers, though, for including a smart function.

That UK equation might be suggested to the HEC. I understand there is a lot of functionality planned for Mapper that just isn’t implemented yet.

[Mark]

Hi Scott.

Awesome work. This knowledge really takes off what would have been a load of effort to get realistic D*V max values. As I don’t have spatial analyst, I went with QGIS and classified my D, V and D*V grids between 1 and 5 according to the FEMA document’s hazard graph, and then combined them preserving max values in each to derive the composite hazard as Mike mentioned. So I think I’m good.
For my needs the FEMA approach is fine which benefits me as I don’t need to use the D(V+0.5) approach from the UK.

Thanks again for checking that RAS uses the correct approach to D*V.

[Author]

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