The Land Drainage Act 1991, Section 23 states:
23 Prohibition on obstructions etc. in watercourses
(1)No person shall—
(a) erect any mill dam, weir or other like obstruction to the flow of any ordinary watercourse or raise or otherwise alter any such obstruction; or
[F1(b)erect a culvert in an ordinary watercourse, or
(c)alter a culvert in a manner that would be likely to affect the flow of an ordinary watercourse,]
without the consent in writing of the drainage board concerned.