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Connectivity in regularly flooded marsh

Started by lsheehan, August 15, 2013, 10:05:20 AM

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lsheehan

Hi,

I ran two accretion scenarios for SLAMM and got some interesting results (attached). It seems like the marsh will move up slope, when using the connectivity function, only if it is connected to a tidal flat or channel- is this correct?  Does marsh not move up slope if it is connected to regularly flooded marsh? In my situation, it appears that marsh is only moving into undeveloped dry land when the accretion is low enough that the surrounding marsh is converted to tidal flat.  However, when the accretion allows the marsh to keep up with sea level rise, the marsh doesn't move up slope at all.

I'd like to still use the connectivity feature for some other areas of the site, but is there anyway to edit it so that if dry land is touching regularly flooded marsh it is considered connected? Or am i interpreting how this feature works incorrectly?

Thanks,
Lindsey

marco.propato

Hi Lindsey,
I believe that what is happening is the following.

When connectivity is on, SLAMM looks also if the cell is connected to open water and not just if it is below a reference elevation (e.g. below salt elevation). In other words, if there is a mountain in between, water will not be able to inundate the cell even if it lies below the water line.   

Therefore, in the first simulation, the dry land becomes connected to water through some paths that were initially occupied by the marsh and now have fallen very low in the tidal frame. Accretion in this case is not able to keep up with SLR.

In the second case, since the salt marsh is accreting at a rate that is comparable to SLR, water does not have a path to reach the dry land. In fact, the marsh continuously builds up enough elevation to act as a "protecting dike" for the land behind. This is also confirmed by the fact that around the dry land the marsh area remains regularly flooded and is not converted to tidal flat meaning that its elevation with respect to MTL is still high enough.

Hope it helps,
Marco


lsheehan

Thanks Marco.

I guess I'm still confused about why regularly flooded marsh would act as a protecting dike for the land behind it.  If it is flooded regularly, wouldn't that allow water to reach the dry land, assuming the dry land is low enough in the tide range? At least during high tides?

Thanks,
Lindsey

marco.propato

Hi Lindsey,
it really depends on the salt elevation assigned for the area and the elevations connecting the dry land cells to the open water.

You can look at connectivity and elevation maps at 2100 to better understand where the water is coming from (or not coming at all) using the SLAMM Map Menu (or save the elevation rasters).

In addition, for SLAMM regularly flooded marsh can only convert when drowning but it continues to exist even when its elevation goes above being regularly flooded (for example with a very high constant accretion rate ... a more realistic model is to set the accretion rate as a function of the elevation, see the technical documentation). 

Marco

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