Campaigner and civil engineer Ramzi Nasayev said the proposed scheme would only provide “a one in 20-year return resilience” despite its £17m cost.
He said a higher level of protection should be possible and argued the project should reduce the flood risk further.
He said the stream currently supported species including European eels and sticklebacks and warned that sewage discharges could damage the habitat.
Referring to children who played in the water near the beach, he said: “This will just be another way of getting more pollution onto the beach.”
The Portreath Flood Resilience Action Group, also said the project replaced an earlier proposal that focused on storing water upstream to reduce flood risk before it reached the village.
They believed that approach would have offered better protection while also improving water quality and protecting wildlife.
