Average Breach Width

[MTC]

Equation 6-1 in the DAMBRK User’s Manual specifies that the terminal breach width (b) is equal to the average width (bbar) times one-half the slope (Z) times the dam height (hd), or

b = bbar – 0.5 Z hd.

Many sources, including the NWS DAMBRK Model: Theoretical Background/ User Documentation by Fread, 1988, as well as basic geometry, suggest that

b = bbar – Z hd (not 0.5 Z hd)

Is this a miss print, or does the user manual define the average base width differently, such as a time dependent average?

I assume the average base width is equivalent to the average width of a trapezoid, or 1/2 times the top width minus the bottom width. Is this correct?

[Chris Maeder]

I researched this, and it appears that the equation within the User Manual is in error.

The terminal breach width (b) equals to the average width (bbar) minus the slope (Z) times the dam height (hd), or:

b = bbar – (Z * hd)

Also, the average width of a trapezoid is ½ times the top width plus the bottom width.

Note that I have included computations to illustrate this. See the attached GIF image.