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This International Standard deals with the
measurement of flow in rivers and artificial
channels using flat-V weirs under steady or slowly
varying flow conditions. The standard flat-V weir is
a control structure, the crest of which takes the form
of a shallow “V” when viewed in the direction of flow.
The weir can be used in both the modular
(undrowned) and the drowned ranges of flow. In the
modular (undrowned) flow range, discharges
depend solely on the upstream water levels and a
single measurement of the upstream head will
suffice. In the drowned flow range, discharges
depend on both the upstream and the downstream
water levels and two independent head
measurements are required. For the standard flat-V
weir, these are
a) the upstream head;
b) the head developed within the separation
pocket which forms just downstream of the crest.
BS 3680-4G: 1990 ISO 4377:1990 : Measurement of liquid flow in open channels — Part 4: Weirs and flumes — Part 4G: Flat-V weirs