For this case, the assumption is made that the flow on the flood plain at the approach section remains on the flood plain as it flows under the bridge. Similarly, the flow in the main channel at the approach section remains in the channel under the bridge. Accordingly, the following relationship will hold true for flows on either the right or left flood plain subsections for the approach section (1) and the bridge section (2):
Q1 = Q2 or q1 * W1 = q2 * W2 (4b)The discharge, Q, in any cross-section subarea (channel, overbank area) is computed as:
Q = q * W (5)Where:
The average velocity is
(Va)i=Qi/AiWhere (Va)i is the velocity at sub-area i, Qi is the discharge under bridge of the sub-area, Ai is the area of the sub-area.