A tidal bore is a sort of hydraulic jump that propagates up (i.e., upstream) a river estuary. The upper part of Figure 4.7 shows such a bore in the local rest frame of the Earth. The bore is propagating at the velocity up a river of uniform width, and depth , that is flowing downstream at the velocity . The flow behind the bore is of depth , and is flowing upstream at the velocity . The lower part of the figure shows the same phenomenon in the rest frame of the bore. In this frame, we observe a stationary hydraulic jump with an upstream depth and flow velocity and , respectively, and a downstream depth and flow velocity and , respectively. Making use of Equations (4.44) and (4.45), we obtain
Tidal bores are found in river estuaries where a funneling effect causes the speed of the incoming tide to increase to such a point that the flow becomes super-critical. For example, bores can be observed daily on the River Severn in England.