Evaluating Precipitation Options in GSSHA

CEEn-531 Dr. Nelson

Overview

The same types of options for defining spatial and temporal variations of rainfall input are used for distributed models like GSSHA.  The mechanics of how it is done are somewhat different, but the same general principles apply.  You should remember that GSSHA wants all rainfall data in mm and not inches, so you will have to do some conversions of your data values.

For this assignment you will need to get a basic model of GSSHA going.  While you can probably use one of the previously saved Judy's Branch projects, I recommend that you begin again from scratch (or at least from the basic delineated model that was suggested you save from the previous assignment).

To develop your basic GSSHA project:

• Delineate the Judy's Branch watershed

• Create a GSSHA grid with 100m sized grid cells.

• Define a uniform index map with a roughness of .03 (be sure to never save the project with an index map grid selected)

• Eliminate all Digital Dams (be sure to input a file name for the new elevations and to replace the current elevations when clean dam finishes)

• You will probably want to run with some uniform precipitation over a given time period to establish

• Define channel routing using trapezoid cross sections with the following dimensions: Bottom Width 5ft, depth 4ft and Manning's n 0.027. After you add channel, do not forget to redistribute the vertices (Map Module, Feature Objects | Redistribute, use 50 as your spacing). Then smooth the channel (Map module, select your model to be GSSHA, GSSHA | Smooth Stream arcs) you might want to refer GSSHA tutorial back if you do not remember these things (Page 41).

• Save this as your base GSSHA modeling project

Creating a GSSHA Gage File with the spreadsheet macro

The most challenging part of this assignment will be to create a gage file for a GSSHA simulation.  Until now we have only defined a uniform rainfall on GSSHA.  In order to define a depth and temporal distribution you must create a Gage input file, something that WMS does not do automatically.  However you can download this spreadsheet, which contains instructions and macros for generating a file from spreadsheet data.

In order to create a gage file with an SCS distribution as assigned you can follow these steps:

1. Open WMS
2. Go to HMS modeling and into the Meteorological Model
3. Define a user defined hyetograph and open the XY Series Editor
4. Import the .xys file that has all of the SCS distributions defined
5. Select the distribution you want to use (like Type II)
6. Turn on the show dates fields
7. Cut and paste the time and normalized depths to a blank spreadsheet
8. Use the Data | Columns to Text option in Excel to expand the date column into separate columns for year, month, day, hour, minutes, seconds.
9. Open up the GSSHA gage file formatting excel macro spreadsheet
10. Follow the instructions using the data copied from the XY series editor into the scratch area of the formatting macro to create the gage file.

Creating a GSSHA Gage Using a WMS Rain Gage Coverage

You can also create a WMS rain gage coverage that can be used to set up a GSSHA gage file.  To do this you can follow these steps:

1. Right click on the coverages folder in WMS and create a new coverage.
2. Make sure this coverage is a rain gage type coverage and name it something that identifies the type of event you intend to model (i.e. Uniform, TypeII, etc.)
3. Select the Create feature point tool and create a new gage in the middle of the watershed when you only need a single gage to set up an SCS storm or other basin average.  If you will have multiple gages then click anywhere in the WMS window and edit the coordinates being sure to define the longitude (x value) as negative.  Repeat for other gages and when gage entry is complete you can cover the coordinates of that rain gage coverage only from geographic NAD83 to UTM NAD83.
4. Change the tool to select feature points and double click on the gage.
5. Change the type from HEC-1 to GSSHA.
6. Select the Define button for rainfall
7. Enter the incremental values of rainfall in mm
   1. You can load the standard WMS file with different SCS distributions
   2. Copy the rainfall distribution which is cumulative and adds to 1.0 (it is normalized)
   3. Paste in a spreadsheet
   4. Use your spreadsheet to convert values to mm
   5. Change the cumulative values to incremental
8. Go to the GSSHA Precipitation command and turn on Gage
9. Select the WMS coverage, or event you wish to simulate.
10. Save the GSSHA project
11. Run GSSHA

Note:  Unfortunately this GAGE coverage is not saved correctly by WMS.  However if you load your base model you can "import" the GSSHA gage file it will recreate the coverage.

Assignment

Once you have the basic setup of your GSSHA model established you should perform the following analyses (don't forget to enter rainfall depths in mm):

• Determine the storm total from the St. Louis gage for July 28-29 and run GSSHA using this depth with:
  • A uniform intensity over a 24 hr duration (probably a pretty small value)
  • An SCS Type II Distribution
  • The actual Distribution
• Change the single gage file to include the arithmetic average of the July 18 storm with the Type II distribution and run GSSHA.
• Now modify your GSSHA gage file to include all four gages, using the  July 18 storm totals and the Type II distribution for each
  • Thiessen interpolation
  • IDW interpolation

To turn in:

• Compare the results of the different temporal distribution patterns.
• Compare the results the single gage vs multiple gages with the different interpolation methods.