FLOOD08298

• County: Statewide
• Basin: Statewide
• Title: Computer Determination of Flow Through Bridges
• Date: 2/1/1979
• Prepared For: US
• Prepared By: COE
• Floodplain - Doc Type: Educational/Technical/Reference Information

The momentum flux at Section B can therefore be given as:

2 2
-S:. - - - -S:.
~ .. ~ + VB .. Ai'3 - A.e3YP3 + gA3

(4)

Assuming that llllY f'orce exerted by bridge piers is negligible between

Sections A and B, the momentum flux will be constant at these and all

intermediate sections. That is

MA .. ~ .. ~ (5)

where M2 .. momentum nux at a:ny intermediate section between Sections A
..f. -
and B, .. gA + Ai'2
2
EqUAtions (3), (4) and (5) can be combined to give the f'ollowing relation:

- - Q2[ Cn 1..f. -
A1Yl . "'P1Ypl + gAl Al - :'2"'Pl .. sA:2 + Ai'2 .

2
- - -S:.
Ai'3 . "'P3YP3 + gA
3

(6)

EqUAtion (6) contains three expressions f'or the manentum nux in the

constriction. The expressions are illustrated by the three curves in

Figures 2&, b and c, which relate the momentum f'lux ~ ~ constriction
to 'depths upstream f'rom, within, and downstream from the constriction,

respectively. These curves are t'unctions solely of' the discharge and the

geometry of' the cross sections and piers.

Six low-flow conditions can occur in the constriction, as illustrated

in Figure 3. In Figures 3&, b and c, the constriction is in a reach where

f'law would be sub-critical if the presence of' the bridge piers were ignored.

6