The upper surface of a weir over which
water flows is known as
Crest
Nappe
Sill
Weir top
Sill
The sheet of water flowing through a
notch or over a weir is known as nappe or vein.
The bottom edge of the notch or the
top of a weir over which water flows is known as sill.
The height above the bottom of the
tank or channel is known as crest height.
Hence, the correct option is (C).
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