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Limitations erosion parameter?

Started by Tom, November 24, 2014, 02:02:10 AM

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Tom

When I tried out various values for the erosion parameter, I noticed that it did not differ whether I had it set on 2, 10 or 50 meter of lateral erosion per year.  I wondered how this was possible, because in theory I had expected a significant difference. Next step was decreasing the time step, so I could see better what happened. Result; for erosion rates that were large enough, each time step one cell transformed from tidal flat to estuarine water. After doing some more runs, I found out that the erosion length per time step is limited to cell size and the time step. If the cell size is for instance 5m, a maximum of 5m per time step is eroded. I know this is rather much and probably most erosion rates are (much) lower. However, when the box with ' Pause with Examination Tools' was unchecked in order to skip many clicks, not even one transformation from tidal flat to estuarine water was visible in the end map. The last cell in the offshore direction is Open Ocean, so maximum fetch is assumed if I'm right. Is this only happening to my simulation or is it a known problem?

I have another question about the condition of erosion to only happen at the land-water boundary. This boundary is defined as the boundary in the NWI category between open ocean/estuarine water - land type. In my simulation, I use a transect with a tidal flat next to the open ocean. During the tides, the water reaches up to the marshes, but there is no erosion. I assume that the boundary is fixed and doesn't account for the tidal range? I think it would be nice if cells with an elevation that lie within the MTL+/-1 HTU range also experience erosion.

Thanks in advance,
Tom

Jonathan S. Clough

November 24, 2014, 09:30:33 AM #1 Last Edit: November 24, 2014, 09:32:11 AM by Jonathan S. Clough
Hi Tom:  Thanks for the thoughtful post.

The code does attempt to account for erosion that exceeds one cell width in a given time step.  I did step through this in the compiler's "debug mode" and thought everything was working properly at the time.  However, you are right in that  we often run with large enough cell sizes and small enough erosion rates that we may not have noticed if there was a problem given a prediction of multi-cell erosion in a single time step.  I'll look into that portion of the code again ASAP and see if I can identify a problem there.  

We have tested the sensitivity of the model to erosion parameters and found the sensitivity to be smaller than the other parameters, but certainly not zero.  

You have an interesting point about the marsh to open water interface.  If a tidal flat is produced between the marsh and open water interface it is assumed to be protective of the marsh in terms of erosion until the tidal flat itself is inundated or erodes away.  This further desensitizes the model to marsh erosion parameters.  I like your suggestion of allowing marshes within the plus or minus 1 HTU range to experience erosion regardless of what is in front of them (omitting a dike, of course!)  I will look into making this an option in the model for future model versions.  

I'll let you know when I get a chance to test the code further with respect to both multi-cell erosion and the marsh-to-open water interface.  We are looking for opportunities to continue to make the erosion portion of the model more sophisticated in general.

Best regards -- Jonathan

Tom

Hi Jonathan,

Thanks for your quick reply! I look forward to hearing more from you.

Best regards,
Tom

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