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Accretion or surface elevation change

Started by Karin, August 30, 2010, 02:30:40 AM

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Karin

Hi Jonathan,

For our study in south-east QLD, Australia, we have access to field data for both accretion rates and for surface elevation changes (i.e. accretion minus subsidence caused by e.g. soil compaction). The two values differ a lot, especially for saltmarsh which has an accretion rate of ca 2 mm/year but a negative surface elevation change (relative to a fixed subsurface benchmark).

Do you think it would be better to use the values for surface elevation change than the pure accretion values when running SLAMM?

Thanks,
Karin   

Jonathan S. Clough

Thank you for this interesting question.

Just to start with some background.  The SLAMM conceptual model considers three factors that affect marsh elevations in relation to sea level.  The first factor is deep subsidence, the second being shallow subsidence or soil compaction, the third being marsh accretion or the accumulation of inorganic sediment and organic matter.

The SLAMM conceptual model accounts for deep subsidence as a function of either the historical SLR rate (as compared with eustatic historical SLR rate) or if spatial data are available these can be input as a raster "Uplift/Subsidence File."

The other two values are combined within the wetland accretion rate.  Therefore based on your question, the surface elevation change, if it does not account for deep subsidence (such as that measured by a SET table) would be the better set of data.

I would also note that, in my understanding, shallow compaction processes can be quite temporally variable due to short-term processes such as peat collapse or compaction of sediments following levee or seawall construction.  If the surface soils are relatively porous and subject to short-term compaction there is a point where the soils will become more dense and such a high rate of compaction will no longer be possible.  For this reason, it may not always be appropriate to take a short-term measurement of soil compaction and project this rate into the future.

I recommend using professional judgment and acknowledging uncertainty in these accretion rates and testing the effects of these parameters with sensitivity and/or uncertainty analyses.

Good luck.  -- Jonathan

Karin

Thanks for your (very informative) reply, Jonathan.

I will discuss your comments further with the wetland specialist in our group, and also run scenarios with both sets of values (accretion or surface elevation change) to see what effect it has on long-term wetland changes as you suggested.

Cheers, Karin

Karin

Hi again Jonathan,

Just to follow up - I discussed this with the wetland specialist in our research group (assoc prof Catherine Lovelock) and she confirmed that using the surface elevation change (accretion minus shallow subsidence) would indeed be the more sensible thing to do here. The deep subsidence is close to negligible in this region. The shallow subsidence varies with distance to water channel, so we will use the accretion model built into SLAMM and parameterise it with the surface elevation change data instead.

Also, Catherine mentioned that she is developing a research proposal and would be very interested in discussing the possibility to use and potentially adapt/develop the SLAMM model to other key coastal/marine environments in Australia. I'll forward your email address to her if that's OK so that she can contact you about this directly.

Cheers, Karin

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