The Weir Design component provides:
Water surface data/profile along the weir crest.
An envelope of weir heights/lengths which accomplish the desired weir discharge.
Below is a table which provides an envelope of weir heights/lengths for one desired weir discharge. i.e. if there is a need to spill (over the weir) a discharge of 4529 cfs, you can have a range of weir lengths and heights which accomplish this discharge.
A weir height of 9' was selected and BAS computed a weir length of 198.1