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Unique Tidal Regime

Started by jmkassak, November 11, 2010, 06:40:19 PM

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I am modeling a large estuary with an "unconventional" tidal/sea level situation.  In the estuary, there is a relatively small daily tidal range, but a significant (approx. 1 ft) difference in sea level over the course of the year.  Specifically, sea level is about 1 foot higher in October than it is in May.  Thus, traditional mean tide level datapoints tend to mask the true extent of saline inundation/exposure in the area.   We are in the process of determining how best to accommodate this situation in SLAMM.

The most obvious issue that this creates in SLAMM is that the habitats in the region are not going to conform to SLAMM's predicted habitat ranges.  (see my previous post for an example of how this plays out in a model run).  It seems there are two options for dealing with this:
1. run the elevation analysis and revise the ranges to better reflect reality
2. rather than using the "sanctioned' calculations for the various tidal data, use proxies of other tidal statistics that would have the same operational meaning of the requested tidal data and would fit the habitat types better.  (e.g., define MLW as whatever the lower extent of the tidal flats are, rather than calculating it as you normally would).

Option 2 was suggested by individuals in the region prior to me becoming familiar with the elevation analysis.  It now seems that Option 1 would work well, and is a much more straightforward way to deal with this.

Other than providing a limit for some of the habitat types, how else does SLAMM utilize the user-provided tidal data?  Are there other tweaks that might need to be made to accommodate this situation?


Jonathan S. Clough

Wow, this does not sound like an easy site to model with SLAMM at all.  For one thing, extreme microtidal regions sometimes cause problem with the SLAMM relationships between tide ranges and wetland elevations. 

For example, if your tide-range is 1cm and your elevation data has a 10cm vertical accuracy then all of your statistics are going to be thrown off.  If you are within 5cm of the conceptual model (usually a desirable place to be) you are having errors of up to 500%. 

Also, winds become very important relative to a microtidal regime.

Secondly, SLAMM does not account for these seasonal variabilities, are they a function of seasonal winds?

Regarding options 1 and 2 above, probably option 1 would be a better choice and you may wish to change SLAMM conceptual model's categories to be in units of meters, rather than "half tide units" given the considerations mentioned above. 

Then you will have your habitat ranges as a function of mean tide level and as mean tide level changes and these habitats move vertically due to accretion, different predictions will take place.  This seems like a valid approach to me.

I cannot think of any other model modifications that would be required off-hand, but as results become available (and if they seem unreasonable) we could consider other model modifications that may be required.

Good luck!  -- Jonathan