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Maine marsh erosion rates

Started by caroline, May 08, 2013, 03:07:56 PM

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Hi Jonathan,

I am doing some SLAMM modeling near Eastport Maine (on the Passamaquoddy Indian Reservation). I read the Moosehorn and Petit Manan NWR reports to find local parameters. The closest accretion data I could find was in Cobscook Bay from the referenced journal article in the NWR studies by Goodman et. al. Nowhere in the reports does it say where the erosion rate parameters are from. Marsh, swamp and tidal flat erosion respectively used throughout the subsites are 1.8, 1 and 0.5 m/yr. Where did these come from? I am assuming they are for this local region?  Thanks.


Jonathan S. Clough

I don't think we found any data for that region and probably used rough model defaults as a result.  -- J


Ok. Thanks Jonathan,

Another question. If there is no accretion/erosion data for my region how do we go about field collecting the data to put in to SLAMM? Is there a technique/s (or references) you suggest for collecting these parameters- marsh, swamp and tidal falt erosion rates, and salt marsh, brackish marsh and tidal fresh accretion rates?


Jonathan S. Clough

This is an excellent question.

The best data we have found and used so far is historical shoreline change data (under conditions of minimal SLR, as SLR inundation effects are considered to be independent of erosion effects within the SLAMM model).  In some projects we have separated out the coastline into many "subsites" with different baseline erosion rates that are assumed to continue into the future.  Even in this circumstance you are generally applying an average erosion rate to a beach rather than predicting which specific cells will disappear.

I would caution against using the Bruun rule, which applies to the "ocean beach" category only, as many researchers have abandoned this in favor of extrapolating forward with historical erosion rates.  The erosion formulation in SLAMM is quite simple and could stand to be replaced by a more mechanistic model in the future but we are searching for the best choice to implement there.  You can turn off the Bruun rule in the "Execute" window.

One more note, marsh and swamp erosion is only assumed to occur at the marsh and swamp to open water interface (fringing tidal flats and beaches are assumed protective, at least until they have eroded away).  Also, marsh and swamp erosion is only assumed to occur in locations with adequate wave setup (the current assumption is 9km of fetch).  So if you have shoreline change data for marshes and swamps, you don't want to average erosion rates where erosion is occurring and erosion is not occuring, but rather use measured erosion rates in locations where erosional loss is actually occurring as the model will then apply these rates only in areas where erosion is predicted to occur.

Finally, I recommend a sensitivity analysis in which low and high erosion rates are tested and the results are compared to get an idea of the model-result uncertainty as a result of uncertainty in erosion rates.

If you have suggestions for how to improve this rather simple model, we'll be happy to hear them!   

Best regards