On the other hand, it involves the expense of a wide foot-bridge for raising the frames, and wide and high river piers, especially for the navigable passes where the bridge has to be raised high enough to afford the regulation headway for vessels at the highest navigable flood-level (fig.
The addition of a foot-bridge greatly facilitates the raising and lowering of these shutter weirs, and also aids the regulation of the discharge; but it renders this form of weir much more costly than the ordinary frame weir, and where large quantities of drift come down with sudden floods, the frames of the bridge are liable to be carried away, and therefore boats must be relied on for working the weir.
The frames hang vertically from the bottom of the overhead bridge, and rest against a sill at the bottom when the weir is in operation, the openings between the frames being closed below the water-level by rolling-up curtains or sliding panels, which are lowered or raised by a travelling winch carried by a small foot-bridge formed by hinged brackets at the p ack of the frames, and situated a little above the highest floodlevel.
Apart, hinged to a masonry apron on the bed of the river and carrying a foot-bridge along the top, from which the actual barrier, resting against the frames and cross- weir.
The weir is raised again by pulling up the shutters to a horizontal position by their bottom chains from a special boat, or from a foot-bridge on movable frames, together with their trestles and the props which are replaced in their shoes.