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erosion parameter sensitivity

Started by cindyt, April 29, 2011, 09:24:28 AM

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I'm conducting a basic sensitivity analysis and  have some questions about the erosion parameters.  I'm increasing or decreasing the erosion parameters by 50% and comparing the output with the baseline model output (using the same values for all 3 erosion parameters).  The study area is a subset of Mobile Bay, in the southern mainland of Mobile County.  Accretion parameters are defined as 5 mm/yr.  The model output for 2100 (A1B Max scenario) is affected very little by varying the erosion parameters.  I tried tripling the baseline value (1.74 m/year) I'm using for the erosion rate, and there are only a few more pixels of tidal flats converted to open water at the land/water interface, compared to the baseline model.   Otherwise the outputs are nearly identical.  I expected variations in the erosion parameter to have a stronger affect on the outcome.

The SLAMM technical documentation says that there must be a minimum of 9 km of fetch for the erosion model to kick in.  There are some useful posts in this forum that say a minimum of one cell must be designated as open water next to the shoreline so that the model can recognize the land-water interface.  If the model encounters a blank cell as it moves across the water surface, it assumes that the fetch is infinite from that point.  All my input layers have the same spatial extent and resolution, and extend out over 1 km into open water.  In this particular region, the dominant wetland type is irregularly flooded marsh, which transitions directly to open water.  Does anyone have ideas as to why the erosion parameter is having very little affect on the model outcomes?   Thank you!

Jonathan S. Clough

Apologies for the long delay in response. 

You wrote, I tripled the baseline value and there are only a few more pixels converted.

How big are your pixel sizes?  How many years are you running your simulation for?  You can do some quick math to double-check the model calculations.  Adding 3 m/year of horizontal erosion for 100 years, you would expect 300 additional meters of erosion or 10 cells at 30 meter resolution.  So I can see why you may be concerned.

What is your erosion rate for marshes as opposed to beach/tidal flat?  Have you tripled all of them?  When irregularly flooded marsh is subject to inundation it converts to regularly-flooded marsh in the SLAMM conceptual model, then tidal flat.  The direct conversion of irregularly-flooded marsh to open water is not generally supported by the model decision tree, but by setting the elevation range for regularly-flooded marsh to a very narrow range you can, in effect, produce these predictions. 

If you would like to email me some maps, I can try to help you figure out what may be going on.

-- Jonathan

Jonathan S. Clough

Mystery solved to some degree.

When calculating "fetch" for wave setup, when SLAMM gets to the edge of the map with OCEAN WATER being the last cell, it assumes Max Fetch is infinite.

When SLAMM gets to the edge of a map with ESTUARINE OPEN WATER being the last cell, it doesn't feel that it can assume an infinite max fetch.  This is because, in an estuary, there is no way of knowing whether or how the water is enclosed by barrier island or a far shoreline. 

So in earlier posts there was a simplification there that the model assumed that "open water" at the edge of a map equals infinite fetch.  It's only "ocean water" at the edge of a map.  While it seems a reasonable assumption to some degree, it would be nice to have the assumption be editable (e.g. a check box for "assume infinite fetch at maps edge.").   At a minimum the assumption should be more transparent (i.e. better documentation)!

Thanks for your patience.


Just to clarify, on page 30 of the 6.0.1 tech documentation, it states that " Tidal flats are assumed subject to erosion regardless of the extent of
wave setup." This has led to a lot of streaking in the output for inlets that contain tidal flats. I bring this up because this is not very clear in the documentation.

Jonathan S. Clough

That's interesting.  I've never seen tidal flat results be streaky on the basis of erosion.  What is the cell size you are using?  Is your direction off-shore set correctly?  Feel free to email me the streaky maps and I'll take a look at them.  -- Jonathan