Formo Fibreglass manufactures corrosion resistant GRP Parshall flumes that have all the benefits of GRP products.
Venturi flumes are metering devices for measuring flow of water or waste water in open channels.
They operate on the principal, that if a restriction of specific shape and form is placed across a channel, a rise in upstream liquid levels will result, which is a function of the rate of flow through the restricted section. A standard level measuring device can then be used to record the flow.
Typical dimensions of Parshall flumes can be found here.