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This page contains an archive of the web seminar given in January, 2009. To view this archive, first download and install the windows player from AT&T and WebEx. This player can be downloaded here. After downloading the player, download the recording, start the player and open the recording in the player program (Select File | Open from the menu and then open the Webinar ending in ".wrf").

  • Download and install the player directly from here
  • Download the web seminar here (30 MB)
  • Also download the web seminar as a windows media file here (146 MB, it is not necessary to install the player if downloading this version)

Webinar Questions and Answers

How can we use gridded radar precipitation data?


In the case of a long-term simulation (i.e. 2 years), how can I enter the rainfall data?

I created rainfall gages, but managing either rainfall or discharge gages is not easy. Is there any way I didn't discover to better manage these items?

Can we calibrate and run the model with real time obtained rain data?


What determines grid size of watershed? Is it max of inputs?

The area of the watershed and the processing capability of the computer being used.

Can the grid size be smaller than the stream width?

The grid size can be smaller than the stream width. Water will enter the channel in the cells described as stream cells, without regard to the defined width of the stream. This may or may not be important, depending on the goals of the study, see:

What is the minimum size of a tributary for WMS?

There is not necessarily a minimum size of a tributary in WMS. Tributaries are conceptualized by using stream arcs on the Drainage coverage. Stream arcs are created by:

  1. digitizing the streams as vector data
  2. automatically digitizing streams using raster data (DEM)
  3. importing GIS data (shapefile) or CAD data (DWG, DXF, or DGN files)

For GSSHA it is generally recommended that the minimum tributary size be 0.5X - 1.5X the size of the grid cell resolution.

If having any tributaries that are comprised of only one or two vertices or are contained within a single grid cell, these can be deleted from the model. The node at the end of the tributary should be converted to a vertex.

How does WMS treat higher resolution data in terms of resampling for the model grid when they differ?

Specifying grid spacing means resampling the grid resolution?

Is define and smooth an analog to fill DEM, flow direction, flow accumulation, etc in GIS?

What about snowmelt, rain-on-snow events, and longer-term time steps (weeks-months)?

Does GSSHA have a snowmelt component?

Is velocity a standard output parameter?

Velocity is not a standard output option except in the 1D hydraulic channel network, but depth and flow in the channel and the overland are.

Can I show runoff generation areas during an event?

There is currently no option to display which cells are generating runoff, except overland flow depth, which is indicative of the cells producing runoff.

Can the program show inundation?

Yes, GSSHA outputs depth from both the 2D overland flow and 1D hydraulic components of the model.

Are other output formats other that Google Earth (ESRI, ESRI 3-D…) available?

Datasets output by GSSHA can also be exported to AVI files, Arc/Info ASCII grid files (.asc), and to text files.

Can you get EPA/USGS NHDPlus datasets using the web services in WMS? Can WMS pull the layers from the USGS NHDPlus datasets?


With the seamless data there is a file size limitation on downloads. How does WMS address this?

WMS displays a message if attempting to download seamless NED data for an area (file size) that is too large.

It is my understanding GSSHA is not currently approved by FEMA, is there a plan to for FEMA approval?

Is GSSHA accepted by FEMA for floodplain studies?

We are still in the process of obtaining FEMA approval. We anticipate it will be approved as more flood and hydrology studies use the GSSHA model.

Here is a quote from a FEMA document available here: "In accordance with Title 44 of the Code of Federal Regulations (44 CFR), Subparagraph 65.6(a)(6)(i) of the National Flood Insurance Program (NFIP) regulations, any computer program used to perform hydrologic and hydraulic analyses in supporat of a map revision must meet the following criteria:

  • The model must have been reviewed and accepted by a governmental agency responsible for the implementation of programs for flood control and/or the regulation of floodplain lands.
  • For computer programs adopted by non-Federal agencies, certification by a responsible agency official must be provided which states that the program has been reviewed, tested, and accepted by that agency for purposes of design of flood control structures or floodplain land use regulations.


"Hydrologic and hydraulic models developed by Federal agencies responsible for the implementation of flood control programs, floodplain regulation, and/or flood hazard analysis clearly meet the criteria stated in Paragraph 65.6(a)(6)(i)."

Are there any State/Federal agencies that have approved the use of GSSHA on projects? If so do you have a couple of examples of projects this has been used on?

Other than FEMA, state and Federal agencies don't usually approve models for general use but on a study by study basis. The model has been used at a variety of sites. Some are listed on the wiki, see

a more complete list is available in the reference section

Can you simulate systems with pipes/culverts, or just open channels?

The current release version of GSSHA can simulate culverts, weirs, and dams with rating curves and release schedules. Sub-surface drainage networks are currently being tested in a research version of the code.

Can you add multiple hydraulic structures (bridges/culverts) in the stream network?

Single or multiple hydraulic structures can be added to any stream node.

What is the 1-D hydraulic model used for those hydraulic structures in stream?

GSSHA has its own hydraulic model, see

Are the 1-D hydraulic computations in GSSHA based on Manning's equation or is it a backwater analysis similar to HEC-RAS?

GSSHA uses the same algorithm for both 1-D channel routing and 2-D overland flow, which is a two step diffusive wave computation of the dynamic flow equations, see

In the webinar, it was stated that from the results of the 1-D component of the calculations, the water surface elevation can be used to determine bridge height requirements. Has this ever been done in practice (I guess what I'm looking for is a single model from which I can develop flows and stream crossing characteristics)? It seems that Manning's equation would be too simplistic to determine stream crossing requirements, unless the plan was to completely span the design flood event.

We not aware of an example of using GSSHA for this particular practice, but it may be possible using the combination of embankments and a culvert hydraulic structure. The channel routing formulation is a diffusive wave solution of the flow equations, see

Is GSSHA able to simulate underdrain flow? A couple years ago they were working on it but there were some issues with it. Is it fixed yet?

This component of GSSHA is still being verified in watershed studies and is not available in the release version. WMS does have tools for conceptualizing sub-surface drainage networks.

Can we input a separate shape file for Manning's n?

Manning's n values can be mapped from a shapefile containing arcs to arcs on a GSSHA coverage.

I am going to drain a dam. I have the discharge hydrograph and sediment graph. Can I model the sediment plume downstream? Does it consider deposition and scour?

GSSHA simulates sediment flow in the overland flow and the streams, including deposition and scour. GSSHA may not be the best choice for this type of simulation, which is really a riverine application. Consider a riverine sediment transport model. A description of the sediment transport in GSSHA is contained in the manual, this information does not reflect many recent improvements to the sediment transport routines.

Can WMS/GSSHA handle release rate analysis in a watershed for establishing release rate zones?

Although we are not familiar with release rate analysis, obtain hydrograph data at any point in the 1D channel network within a GSSHA model. GSSHA models can incorporate the effects of land use change locations and hydraulic structures in the channel network. GSSHA can simulate reservoirs with different outlet structures, see

Does groundwater transfer between grid cells?

Groundwater transfers laterally in two dimensions between grid cells, according to the free surface flow equations, see

Are multiple soil layers (i.e. layers of different soil types) allowed on top of a geological layer?

Multiple layers can be simulated in the unsaturated zone, depending on the type of infiltration that is selected for the model. The saturated groundwater model is a single layer.

How does the groundwater-surface water exchange occur between channel and cells

Darcy's Law is used to compute the interaction between the groundwater and surface water in both 2D overland cells and 1D hydraulic channels.

Since watershed boundaries aren't necessarily important to groundwater flows, how are groundwater boundary conditions defined at the watershed/model boundary?

The boundaries for a GSSHA study do not have to conform to watershed boundaries. If the actual watershed boundary is not a good groundwater boundary the model can be extended to where appropriate boundaries are available.

How is the percent impervious area per each grid cell accounted for?

The percent impervious area for each cell can be specified with an index map. Alternately, the impervious area in each grid cell can be accounted for in the distributed parameters required for the processes that are modeled in the GSSHA simulation.

What about water quality and contaminant modeling? How does pollutant runoff work?

Does Aquaveo have a reservoir simulation model?

GSSHA does simulate reservoirs.

WMS also has tools for setting up and viewing output from CE-QUAL-W2 models.

Please tell about the link to CE-QUAL-W2.

A link has been established to populate CE-QUAL-W2 input parameters using GSSHA output, but this linkage is still being researched.

Is there a linkage to HEC-RAS?

At one point there was a linkage established between GSSHA and HEC-RAS within WMS, but this functionality has not been tested or used in quite some time. It is possible to use output from GSSHA as inflows to a HEC-RAS model, and vise versa.

Is GSSHA a third party software add-in to WMS?

GSSHA is developed and maintained by the U.S. Army Corps of Engineers (USACE) Environmental Research Development Center (ERDC). Tools in WMS are developed in collaboration with GSSHA developers in order to facilitate pre- and post-processing of GIS data for GSSHA model development.

What are the minimum hardware requirements to run GSSHA on a PC?

There are not really any hardware requirements for running GSSHA on a PC, but the hardware is related to the number of cells and processes that can be simulated in a GSSHA model. GSSHA tends to be processor limited; WMS tends to be memory limited.

Does Aquaveo now provide support for registered GSSHA users (non-Corps)? You didn't when we first purchased GSSHA/WMS.

Aquaveo provides support for WMS 8.1/GSSHA 3.0b and above.

is a new version of TOPAZ needed for the upgraded version of WMS?

WMS provides a version of TOPAZ with the installation. Alternatively, write the TOPAZ files and run your own version of TOPAZ.

Are CEU's being given out for this webinar? Will you be providing PDH certificates for this webinar?

Please send an email to lwhite@aquaveo.com to request a certificate.