Spill or overflow weirs are pretty common on the canal network, they are a means of managing water levels by providing an "escape" for overflow water if the water level gets too high. 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 this type of 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.