1. Bridge
A bridge is a structure that connects two roads across a waterway.
Head losses occur while water passes through the bridge between its walls.
Head losses should not exceed 10 cm.

2. Longitudinal section in a bridge.
≤ 10 cm
Water depth d

3. Cross section in a bridge
0.75 m
Cross section in a bridge

4. Hydraulic Design
Hydraulic design is conducted to obtain the number of vents (n) and the span of the vent (s)
The selection is made such that the head losses does not exceed 10 cm.
A simple design can be achieved following the next steps:

5. Assume n, then calculate S as follows:
Adjust S to a practical value (S ≤ 5.0 m)
Calculate the velocity through the bridge
Where Q is the discharge in m3/sec, V is the velocity in m/sec and Aw is the water area.

6. Aw = n . S . d
Where d is the water depth.
Head losses is calculated as follows
Where Hl is head loss (m), g is gravitational acceleration (9.81 m/sec2), C is a constant.

7. C = for S ≤ 2.0 m
C = for S ≤ 4.0 m
C = for S > 4.0 m

8. Example:
Design a bridge across a waterway having a discharge of 18 m3/sec. The bottom width is 15 m and the top width is 21 m and the water depth is 3 m.
Sol.
Assume n = 5
S = (21-3)/5 = 3.6 m
Aw = (15+21)/2 * 3 = 54 m2

9. v = 18/54 = 0.33 m/sec
Av= 5 * 3.6 * 3 = 54 m2
Check of Heading Up
Since Aw = Av Hu = 0
Hu < 0.1 m … OK