We are using SLAMM version 6.7 and wanted to make sure the following holds on this version:
this version considers historical SLR rate or Uplift/Subsidence raster and marsh accretion rate in order to calculate the marsh elevations in relation to sea level as explained in Accretion or surface elevation change. So, it means the elevation output of SLAMM incorporates both these rates in marsh area and the first rate (historic SLR rate or Uplift/Subsidence raster) in open water and upland area?
unlike mentioned on Elevation output, the elevation output is converted back into NAVD88 on this version?
Tidal ranges affect SLAMM in many ways. Certainly the model predicts significantly enhanced resilience to SLR for larger tidal range sites.
For one thing, a fixed change in tide levels (due to SLR) is a much higher percentage of the tide range in a microtidal site than a macrotidal. This means that a marsh (that exists within the "tidal frame") can have much more elevation capital in a macrotidal site.
But with regard to the question at hand. The SLAMM model does not directly model sediment inputs but those inputs come into account in the relationship between marsh accretion rate and marsh elevation. See for example Figure 4 in this document. https://www.sciencedirect.com/science/article/pii/S1364815216302705 This figure was derived based on the calibration of a model that does explicitly model sediment inputs (MAM3)
We recognized that a model such as MAM3 generally predicts a single parabolic accretion-rate response to SLR (if you hold sediment inputs and tide range constant over time). (The parabola has accretion rates on the y axis and marsh elevation relative to MTL on the x axis) Therefore we allow for an input of this type of parabola to SLAMM. A site with a larger tide range has more elevation capital to work with and will generally accumulate more sediment over a period of SLR. MAM3 also predicts increased sedimentation rates in sites with larger tide ranges.
To generate the required parabola a site-specific empirical data analysis or application of a site-specific mechanistic model should be utilized.
Hope this is useful and sorry about the long delay in response
Last post by Pat Prado - April 04, 2018, 06:42:19 AM
I was wondering if someone could clarify if SLAMM takes into account that higher sediment inputs seem to happen in systems with greater tydal ranges, which could lead to enhanced resilience to SLR compared to systems with lower tydal ranges. Is the model calibrated to simulate this effect?
The input files look good but there was one key issue. The CRS units for SLAMM projects must be meters. I am sorry if that has not been made clear enough through the GUI and/or users guide. I will specify that on the file-input GUI for the next version .
Last post by pse1999 - February 24, 2018, 10:25:56 PM
I think one way you can do it is when you convert the polygon (NWI shp) to a raster through the tool you can set the processing extent the already created DEM raster through the "environments.." button to match your other rasters. There should be a box you can click where it will snap to the raster.
Last post by pse1999 - February 24, 2018, 10:21:41 PM
I am curious if anyone else who has used SLAMM to evaluate elevations has noticed when a cell converts to regularly flooded marsh that elevation decreases exponentially more than other classification types? data derived from the below example is sourced from the GCPLCC dem and NWI ASCII files
"8" is classified as "Regularly flooded marsh" and as seen it gradually decreases at first to essentially falling down a hole in the latter time steps. Is this common? is it the time step interval that is too large?
If you are using a dike layer I would turn that off as a test.
Next thing to do is look at the elevation analysis -- Set Map Attributes, Elevation Analysis button towards middle of Analysis Tools tab. Then "Run Elevation Analysis (This Site) Double click on the category names to sort, sorting by n cells can be useful. The 5th percentile for wetland classes should be around the minimum elevation.
See the help file text for more information on interpreting the matrix on that page. Also you can see this reference: