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Erosion issue

Started by MSub, May 11, 2015, 12:19:33 PM

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MSub

We are having an issue with Erosion in SLAMM that we are running in Galveston Bay. SLAMM is not triggering marsh and beach erosion in areas where wetlands are adjacent to water and the maximum fetch threshold is reached. We ran a sensitivity analysis varying erosion rates (marsh, swamp, and tidal flat) by +/- 15%, which resulted in no change to model output from the base case. Following the post in SLAMM forum, we attempted to force SLAMM to trigger erosion by converting the Estuarine Open Water class to Open Ocean since SLAMM will assume max fetch is infinite when the last cell on the edge of the map is Open Ocean, but erosion still isn't occurring. Similarly, we also tried to run the model with the reclassified water and run the sensitivity analysis with the reclassified water, but it also does not make much difference. We wanted to make changes on SLAMM code to see if decreasing the fetch length from 9 km may be useful. Can you please point me to the place in the code where I can make changes to it?

Thank you for your help.

MSub

In Transfer.inc, I found a procedure called Procedure Erode(Cell: PCompressedCell; Var Erosion: ErosionScale) where erosion is defined based on Knutson et al. 1981 (I also could not understand why SLAMM has used those ranges for the fetch from the paper.) I tried changing the range of the fetch within the procedure, but it did not make any difference in the erosion map of my study area. I am wondering if there is any other procedure where I need to make changes. Thank you in advance.

marco.propato

You are right, the Procedure Erode is the function to change the fetch.

However, one reason that observed marsh erosion is limited may be due to the fact that when marsh is eroded, land cover is converted to tidal flat. As a result, the marsh behind will not be connected to open water, and thus not eroding anymore, until the fringe of tidal flat is gone (permanently submerged or eroded away). So you may want to increase tidal flat erosion rate or change the conversion rule for marshes to Estuarine Water instead to Tidal Flat; change the line Convert(Cell,Clss,Tidalflat,-99) to Convert(Cell,Clss,EstuarineWater,-99) in the  Procedure ErodeMarsh.

Hope it helps. 

MSub

Erosion is still not occurring in all places where we presume it should be even after I made the following changes in the code:
1. defined Erosion = Moderate if AdjMaxFetch <= 9 and got rid of "Erosion = Little" as I did not see the use of "Little" in the code
2. changed the line Convert(Cell,Clss,Tidalflat,-99) to Convert(Cell,Clss,EstuarineWater,-99) in the Procedure ErodeMarsh based on your suggestion

I could not attach the pdf file due to size limitation here but have uploaded it on dropbox (https://app.box.com/s/rm83kqlfoxi06slr7oqvd4jlmnvm64g1). The pdf file has the resulting maps and figures before and after making changes in the code. E.g. we think there should be erosion in an area shown by purple rectangle on page # 3 of the pdf file . For our test run, we've used A1T max SLR scenario with 25 years time step. There's also a screenshot of the site parameters that we've used in our test run. Can you please take a look on the maps and help us understand if we are are overseeing or missing something? Thank you.

marco.propato

Hi,
I do not know what is the scale of your map, but it is possible that inside the purple rectangle there is less than 9 km fetch. Your code change has not modified much in that sense since marsh erode only if erosion> moderate and moderate is still <=9km as it was before. So what you want to do is to define: Erosion=Moderate if AdjMaxFetch <= 1,2,3,4... (whatever the fetch you want) and Erosion=Heavy > 1,2,3,4,.... In that case marsh will start to erode with fetch greater than 1,2,3,4....

On a side note, it looks that the dryland now is converting to estuarine beach meaning that the fetch is >Heavy (Severe) there. Not sure why. Look at the code changes (and search for the words Heavy and Moderate in transfer.inc to see the implications).

Let us know. M

MSub

Hi Marco,
Thank you for your suggestion. I do see the erosion being triggered now after I redefine AdjMaxFetch <= 1 km. However, I see that the maximum erosion is 10 m on the legend of both marsh and beach erosion maps. For this run, the marsh erosion, swamp erosion and tidal flat erosion was 0.589788, 0.997603 and 1.26461 m/yr respectively. For another test run, we increased the erosion rate to 10 m/yr for all three erosion types. But, we get the same legend again for marsh and beach erosion with the maximum of 10 m. While we compare the output csv files of these two runs, we see significant changes in the marsh area.

Actually, we have not seen the legend going beyond 10 m in any of our test runs. However, I found that it has gone to 30 m in the report that Warren Pinnacle had prepared for TNC on Galveston Bay (http://www.slammview.org/slammview2/reports/Galveston_Report_6_30_2011_w_GBEP_reduc.pdf). Do you think it's something we should be worried about?

Thanks again.

marco.propato

The erosion map shows the amount of eroded land in each cell. Therefore it will not exceed the cell size which I suspect must be 10 m x 10 m in your project.

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