Spill or overflow weirs are pretty common on the canal network, they help manage water levels by providing an "escape" for overflow water if the water level gets too high. Much rarer though are circular spill weirs, several still remain on the Cotswold Canals such as the one shown below at Ebley Wharf on the Stroudwater Navigation.
With a circular spill weir water from the canal rises up in the outer ring and when it reaches a certain level overflows into the central hole. The water then is carried away through a culvert. In the case with the weir here to the river Froome.