text: For shallow water waves, such as tsunamis and hydraulic jumps, the characteristic velocity  is the average flow velocity, averaged over the cross-section perpendicular to the flow direction. The wave velocity, termed celerity , is equal to the square root of gravitational acceleration , times cross-sectional area , divided by free-surface width :
  so the Froude number in shallow water is:
  For rectangular cross-sections with uniform depth , the Froude number can be simplified to:
  For  the flow is called a subcritical flow, further for  the flow is characterised as supercritical flow. When  the flow is denoted as critical flow.