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ApI' \3 = obstructed areas at upstream and downstream sections, respectively Yl, Y2, Y3 - vertical distance from water surface to center of gravity of AI' A2' and A3' respectively Ypl' ~2 - vertical distance from water surface to center of gravity of ApI and Ap3' respectively Q = discharge g - gravitational acceleration (c) The three parts of the momentum equation represent the total momentum flux in the constriction expressed in terms of the channel properties and flow depths upstream, within and downstream of the constricted section, respectively. If each part of this equation is plotted as a function of the water depth, three curves are obtained, representing the total momentum flux in the constriction for various depths at each location. The desired solutions (water depths) are then readily available for any class of flow. If the water surface profile has been computed to the section at the downstream end of the pier, as is the usual case for subcritical flow, then the downstream depth is known. If the momentum flux for the constriction based on this downstream depth is greater than the momentum flux for the constriction based on critical depth, and the downstream depth is above critical depth, the flow is Class A, and the upstream depth is determined by the use of Yarnell's energy equation since the momentum method does not take into account an exit loss. The depth within the constricted section is determined by solving for the depth of flow which will provide a momentum flux equal to the downstream momentum 'flux. If the downstream momentum flux is less than the momentum flux for the constriction at critical depth, and the downstream depth is above critical, the flow is Class B, and the water surface elevation in the constriction is at critical depth. A new downstream depth (below critical) and the upstream depth (above critical) can be determined by finding the depths whose corresponding momentum fluxes equal the momentum flux at the constriction for critical depth. If the upstream depth is known, as is usually true for supercritical flow, and the momentum flux for the constricted section based on the upstream depth is greater than the momentum flux for the constricted section at critical depth, and the upstream depth is less than critical, the flow is Class C, and the downstream depth and the depth within the bridge section are found by deter- mining depths corresponding to a momentum flux in the constriction based on the upstream depth. If, however, the computed momentum flux for the constricted section based on the upstream depth is less than the momentum flux for the constricted section at critical depth, the flow is Class B and the upstream depth is the depth (above critical) corresponding to the momentum flux for the constricted section at critical depth. The water surface profile must 10