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disgreen

Depression Polygon Mask

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Hello,

I'm attempting to construct a GSSHA model of a low-relief watershed containing a significant number of natural topographic depressions. I'm uncertain about how to build and apply the depression polygon mask within WMS. I have a polygon shapefile of the depressions for this watershed. Can someone either point me in the direction of a tutorial on this process, or provide general steps within the answer to this post?

Thanks,

David

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David,

There's not a specific tutorial that describes how to maintain depressions to WMS.  But you should be able to use your polygon shapefile to define your depressions fairly easily. You need to make sure you're using the most recent version of WMS on our web site since there was a bug in a recent release of WMS.  Setup your basic model, but don't run the cleandam program until you've added your depression polygons. Here is what you do (after you have defined your initial model):

1. Read your polygon shapefile using File | Open.

2. Make sure the GSSHA coverage you're using with your model is the active coverage by selecting it.  Go to the GIS Module.  Select Mapping | Shapes->Feature Objects. Select Yes to convert all your shapefile data to map data.  Step through the wizard to convert your data.  Don't worry about mapping any attributes.

3. Go to your GSSHA coverage, which should now contain your shapefile polygons.  Select the select polygon tool and select all your depression polygons.  Select Feature Objects | Attributes and change the polygon type to "Depression Mask".

4. You should now be able to continue building your model and run the "cleandam" program before running GSSHA.  The cleandam program is accessed from the GSSHA menu in the 2D Grid module (Clean Digital Dams) or from the Hydrologic Modeling Wizard (the Clean Up Model step).  There's a display option to show digital dams in the 2D Grid Data display options.  The digital dams inside your depression polygons should be maintained after running cleandam and the other digital dams should be removed.

There are actually some WMS tutorials on our web site that have some discussion of digital dams.  A tutorial you might be interested in going through is called "Updating a GSSHA Model using the MWBM Wizard" and can be accessed by going to our tutorial page here and clicking on the Distributed Hydrology tutorial tab.

I hope this helps,

Chris

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Hi Chris,

Thank you for the prompt reply and detailed instructions. I've worked through most of the tutorials on WMS, but have yet to fully make connections between parts of the platform. As I mentioned in my post, we are modeling depressional storage and fill-spill processes on varied low-relief watersheds (i.e. think the prairie pothole region). I know that these depressions exist on the landscape, but am uncertain about whether they are retained in WMS when building a GSSHA model. For instance, are all depressions resolved when running TOPAZ?, and if so, does the elimination of these features show up in the model grid? How can I be certain that I am retaining depressional features on the landscape, and ensuring that GSSHA models their fill-spill processes correctly? 

Thanks in advance for your assistance.

David

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David,

The depressions are retained on the DEM and in your model after running TOPAZ.  The purpose of TOPAZ, when used with WMS, is to find the areas of high flow accumulation in your watershed.  WMS can then determine stream locations from these accumulation values.  TOPAZ itself does change your elevations to accomplish its purpose, but the DEM elevations in WMS are not changed.  So because of this, TOPAZ may actually create areas of high flow accumulation (streams) that do not actually exist in a landscape like the one you are working on.  You can always edit the streams created by WMS to more accurately represent aerial photograph or topographic map data for your area.

So the answer to your question is that no, TOPAZ does not change your elevation data in WMS, but the flow accumulation values and stream network generated from running TOPAZ may not represent what actually happens in your watershed.

The elevation values on your grid are changed when you run the "Clean Digital Dams" command from the GSSHA menu on your 2D Grid.  This is when your depression polygons are used to define which elevation values you don't want changed.  You can be certain any elevation values on your 2D Grid inside depression polygons are not changed, and you can confirm this by turning on the option to display Digital Dams in the 2D Grid display options.  GSSHA takes significantly longer to run when you have lots of depressions like you have, but if this is what you're trying to do, the extra time is probably worth it to you.

Hope this helps, you might want to contact Cody Alberts here at Aquaveo.  He has had some experience in modeling areas with lots of depressions like yours using GSSHA.  Let me know if I can help with anything else.

Chris

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