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Chesapeake Bay prj

Started by cpapiez, July 09, 2010, 02:18:23 PM

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As you know we are working on running SLAMM6 for Maryland's Chesapeake Bay and we've been analzying the previous model run by NWF.  We have a few questions listed below.  Any help is greatly appreciated.  Thanks!

We are working on refining our diked data from the NWI classifications.  I've been talking to our wetland ecologist about the function SLAMM uses to address diked wetlands, but I'm not exactly sure how it works.  Diked wetlands are not allowed to accrete even after over 2 meters of rise over tops them, is this correct?  Why are all dikes considered to be 2-meters high, can this be adjusted? 

Time step
Are sea-level rise, accretion and other parameters, with a value per year added only at each time step or are they added every year?  i.e. if we choose 25 year time step does the accretion value only get added every 25 years?

Great Diurnal Tide Range(m)
We noticed that the input for SLAMM 5 included Tide Range Ocean, Tide Range Inland, Mean High Water Spring, MHSW Inland and Water Depth but the latest version only has Great Diurnal Tide Range (m).  How do we figure out what that is for each subsite?  Is this NOAA tidal stations data?

What is the reasoning to use and not use overwash?  NWF study had an overwash of 25 years for all subsites but it wasn't clear why this number was chosen or why overwash should be included for the Bay vs. outer coast. 

Jonathan S. Clough

Diked-- I think you are correct that accretion remains zero after overtopping, however this situation almost never occurs or occurs on the last time-step of a two meter simulation so it hasn't been much concern to us and has had no effect on the results of simulations we've run in the past. 

The 2 meter high limit is an artifact of very old code and off-the-cuff decisionmaking and is rather arbitrary.  Fortunately, it's rarely exceeded which means that, for the most part, diked areas remain protected.  We just assume that "the dike owner will maintain dikes up to 2M of local SLR but not beyond" and leave it at that.

With a high quality, high horizontal resolution DEM, you can use the connectivity algorithm to calculate dike overtopping.  Alternatively, the 2M limit could be changed within the code very easily, as could the accretion after overtopping assumption.

Time-Step -- Accretion values are added at each time-step and normalized to the length of the time-step that occurs.  That is, if a simulation is run for 10 years with 1 mm/year of accretion predicted 10 mm of accretion would be assumed to occur.

GT -- Based on NOAA tidal stations primarily though can be ascertained from other sources (tide tables, for example).  If the tide range is spatially variable ("inland" and "oceanic") multiple sub-sites should be utilized to demarcate these differences.  The "inland" and "oceanic" designations were really simple ways to try to specify a tidal gradient but the model uses a more sophisticated approach now so they are no longer useful.

Overwash -- This is a simple model of what may occur on barrier islands in the event of overwash.  This model is very flexible and different effects can be specified in different locations.  The overwash model has no effect within the bay as there are no barrier islands in this location.