More accurate criterion for the terminal of backwater will get a greater extreme value of backwater length; while rough criterion for the terminal backwater may lead extreme value in the range of backwater length variation to vanish.

• 回水末端的位置精度越高,回水长度极值就越大,当末端标准精度较低时,在一定范围内回水极值现象可能不再出现;