Bridge Hydraulics Lecture Notes

Flashcards covering the fundamentals of bridge hydraulics, scour mechanisms, design requirements, hydraulic analysis formulas, and protection types.

How is a bridge defined from a hydraulic perspective?

A structure built over a depression or obstacle for passageway, forming part of a stream crossing system that includes the approach roadway across the floodplain and any openings.

What are the key hydraulic considerations for bridge planning and design?

Discharge patterns, water levels, slopes, cross-sections, velocities, roughness, erosion and sedimentation, and scour.

What types of vortices are associated with local pier scour around a cylindrical element?

Horseshoe and Wake Vortices.

What is the formula for the approximate river width, $B$?

$B = (c) Q^{3/4}$ where $Q$ is discharge and $c$ is a coefficient ranging from $0.5-0.8$ determined by floodplain obstruction.

What is the formula for calculating the desirable minimum bridge span length ($L$)?

$$L = 20 + 0.005Q$$

What is the minimum required freeboard for rivers carrying debris?

Not less than $1.50\,m$.

What is the minimum required freeboard for bridges over rivers not carrying debris?

Not less than $1.00\,m$.

According to HPCG/CG-8, what is the formula for vertical clearance for navigable channels?

$\text{Vertical Clearance} = \text{HWL} + \text{HV} + K$ where $\text{HWL}$ is the highest water level, $\text{HV}$ is the height of the vessel, and $K$ is a constant $1\,m$ allowance.

What are the two sections of hydraulic analysis for bridges?

1. Analysis under the design flow to determine bridge configuration. 2. Analysis of the selected configuration to define flood conditions for different return periods.

What is Manning's Formula for determining velocity ($V$)?

$V = \frac{1}{n} R^{2/3} S^{1/2}$ where $n$ is the roughness coefficient, $R$ is the hydraulic radius, and $S$ is the slope of the energy grade line.

How is the hydraulic radius ($R$) calculated?

$R = \frac{A}{P}$ where $A$ is the area and $P$ is the wetted perimeter.

Define Ordinary Water Level or Normal Water Level (OWL/NWL).

The height of the water level in the river under normal conditions.

Define Maximum Flood Level (MFL).

The highest recorded flood level, which may be based on anecdotal observations if gauging is unavailable, and must be validated by a hydrologist.

What are the design and check flood frequencies for bridge scour on a National Road according to Table 3.2.5-1?

The Scour Design Flood is the $100\,yr$ flood and the Scour Check Flood is the $500\,yr$ flood.

What are the hydraulic design and scour check flood frequencies for an Expressway?

$100\,yr$ for the Hydraulic Design Flood and $500\,yr$ for the Scour Check Flood.

List the common types of bridge pier protection mentioned.

Loose boulder apron, gabions and mattresses, and articulated blocks.

List the common types of bridge abutment protection mentioned.

Boulder riprap, gabions, grouted riprap, and reinforced concrete.