Adding grid cells to an existing grid

The grid system can be extended by creating a second grid system of the area you want to add and then sewing it onto the first grid system using the cadpts.dat and fplain.dat file. The trick is to get the two grid systems to match exactly.

Step 1
Open the original project in the GDS and zoom in on the area where you want to add the new cells. Turn on the grid element numbers using the view menu. Take a screen shot of that image and print it out. Also, copy down the x y coordinate of your upper left grid element in your original grid system. You can get this exactly from the CADPTS.DAT file. Keep in mind it may not be the 1st element.

Step 2
Open the GDS again so you can build the add-on grid system. Choose to create a new project using the define extents option. Make your project extents cover both the old and new grid system. When you create the grid you have the option to set the upper left coordinate. You copied the reference coordinate in step 1. You may need to shift your new reference point to make room for the new grid but that is simple as long as you make the shift as a multiple of your grid element width.

For example, if your upper left coordinate is 154000 234500 and your grid element size is 25 ft. You want to shift the grid system to the left you will reduce the x coordinate by multiples of 25. 154000 to 153975 will shift the entire grid left 25 ft. This ensures your grid system is still aligned to your original grid system and this is a key point.

Don't forget to use a grid element width that matches your original grid system. If your old system was 25 ft x 25 ft that is what your new system has to be.

Step 3
Make a computational boundary that will encompass your new addition only. Use the image you printed to help align the boundary cells to the original grid system. This is simple so long as you under stand that the boundary elements of the new system will lay exactly on top of the grid elements along the split. This works both ways. The red boundary elements of the original grid system will lay exactly on top of the grid elements of the new system.

Step 4
Load your elevation data for the new system and interpolate. You might also want to establish your manning n values at this point.

Step 5
Click File/Create FPLAIN.DAT and CADPTS.DAT. Click yes to save the Manning n values and then you will be given an option to renumber your grid system. Enter a value that is the number of grid elements in your old system plus 1. You just start the numbering from the end of your original system.

Step 6
Open the CADPTS.DAT files for both systems. Add the new data onto the end of the old data and remove any blank lines. Save that file and close you are finished with this file. You can use WordPad or UltraEdit to do this because they are text files.

Step 7
Here is the tricky step. FPLAIN.DAT file looks like this:

1 0 2 200 0 0.040 3500.25
2 0 3 201 1 0.040 3500.36

GE GE to north GE to east GE to south GE to west N-value Elevation

Note that boundary elements are 0 values. So, you have to replace the 0 values of both grid systems with the neighboring grid elements. The easiest way to do this is make another screen shot print out of you new grid system with the grid element numbers turned on. Cut the red boundary off of each picture and tape them together so that they are in alignment.

Take the data in the new FPLAIN.DAT file and paste it onto the bottom of the data in the original FPLAIN.DAT file. Then use your image to edit the boundary elements. This is how you sew your floodplains together.

Step 8
Now you can run a program called CHECKER.EXE. It should be in the FLO-2D folder. Just copy it to the folder with your new FPLAIN.DAT and CADPTS.DAT. It will check your files and tell you if there are any mistakes. It writes the mistakes to the ERROR.CHK file.

Once checker is clean you can put your new FPLAIN.DAT and CADPTS.DAT files in a project folder with the other original data files (i.e. CHAN.DAT, LEVEE.DAT) and open the whole grid system in the GDS for further editing and quality control. Everything from the old model will be in place because you did not make any changes to the original floodplain data.