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Bruun rule and beach erosion

Started by Elena V, November 15, 2012, 04:12:53 PM

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Elena V

In some of the SLAMM documentation I read that the Bruun rule for ocean beaches is optional in SLAMM. How do I implement this optional rule? Also (this may or may not be related), I'm noticing that all the beaches in my study area disappear in 2020 for all my model runs, even when my input erosion rates are low (0.2) and wouldn't account for that much erosion by 2030. The beaches are present in the 2005 and 2010 maps, which makes me think this is not a conflict between the SLAMM map and topography.
Thanks!



Jonathan S. Clough

At this point we are not in the practice of using the Bruun Rule.  In the Execute window is an option to disable the Bruun rule which is probably the recommended approach.  It is a radio button at the lower left labeled "Use Bruun Rule for 'ocean beach' Erosion."

My guess is that your simulation is using the Bruun rule which is resulting in this extensive beach erosion that you are seeing.  The other possibility is that your beach elevations are all just above the lower elevation boundary for beach.  This means even a small amount of SLR will result in complete loss.  You can examine this by using the profile tool within SLAMM.  You may also want to run an elevation analysis and see where the elevations of ocean beach are with respect to the conceptual model (where SLAMM expects them to be.)

Don't hesitate to ask further questions.

Thanks -- JC

Elena V

Thanks for the quick response! I do not see the "Use Bruun Rule for 'ocean beach' Erosion" button on the execution screen (see attached screenshot). I'm using version 6.0.1 beta -- is this the wrong version to be using? I thought this was the most up-to-date version.

According to the elevation analysis, essentially all of the "Ocean Beach" elevations are above minimum elevation for Ocean Beach (see attached figure), so I don't think that's what's causing the erosion...

On the other hand, I'm not sure that the Bruun rule is causing this either. According to the output .csv file there is 0.26 meters of SLR by 2030. The Bruun rule would cause 100*0.26 meters = 26 meters erosion, right? In some places the beach is eroding >200 meters between my 2010 and 2030 output rasters. At least half of this beach is well above the highest observed water level.

Is there something else that might be causing this?
Thanks for your help!
Nena

Jonathan S. Clough

Hmm, apparently that option was added post 6.0.1.   Thanks for the screen captures and the careful attention to detail.

SLAMM 6.2 will come out in early December and will be 64 bit.  Just need to finalize some documentation.

I will email you an interim version which allows Bruun to be turned off to see if that solves the problem.

The only thing I can think of is if your beach is on a thin strip of land that SLAMM sees as a barrier island (<500 meters between the front edge and open water or back-edge marsh) and your frequency of barrier island overwash is set fairly high.  That would erode a lot of beach.

Unless you send me your study files directly (email to jclough@warrenpinnacle.com) I can't help you debug your problem.  Hopefully the version of SLAMM I send you will help shed some light as you can turn Bruun off and see if it's still happening.

Cheers -- Jonathan

Elena V

Unfortunately I'm not using the overwash function (frequency of overwash = 0), so I don't think that could be causing the beach to disappear.

Thanks for the files - I'll get the new version installed and respond with what I learn. Thanks!

Elena V

I installed the updated version you sent me, and I no longer have the problem of the beach disappearing in 2030. It still disappeared when I kept the "use Bruun rule" box checked, so that was indeed the problem. I have some follow up questions but I'll start a new question. Thanks a lot!

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