FLOOD07955

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: Hydraulic Design of Bridges with Risk Analysis
• Date: 3/1/1980
• Prepared For: USDOT
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

HYDRAULIC DESIGN OF BRIDGES WITH RISK ANALYSIS

By Verne R. Schneider and Kenneth V. Wilson

INTRODUCTION

The hydraulic design of bridges involves an evaluation of the
flood hazard to the highway and the effect of the proposed
highway on the hazard to lives, property, and stream stability.
In specific terms, this evaluation results in the selection of the
size 'and location of bridges, culverts, and other drainage structures
and the determination of the embankment fill height. This hydraulic
design previously included a flood of a specific frequency where the
flood frequency was selected for types of roads depending upon their
importance. structures were designed to safely pass this specific
flood, and various safety factors such as embankment freeboard were
added to protect the structure and ensure the free passage of traffic
under all but the most severe flooding conditions. The effect of
the proposed highway on the flood hazard to lives, property, and
stream stability were considered using various criteria. Attempts
were made to limit inundation of the roadway embankment, to limit
embankment erosion and scour, to minimize the backwater caused by
the embankment fill, and to minimize the disturbance to the
natural stream.

The stream crossing design analysis used in this report is based on
the premise that the total stream crossing, including the approach
fills in the flood plains and all necessary waterway openings, should
be designed and constructed for the least total expected cost to the
public. The total expected cost to the public during the service life
of the highway includes the capital investment in the highway, expected
replacement and repair costs as a result of flood dpmages, expected user
costs from traffic interruptions and detours, and expected backwater
damages. Economic and engineering analyses of alternative designs
provide information for selecting a design of least total cost to the
public or an optimum crossing design.

Briefly, an optimization procedure involves assessing all expected
costs during the life of a facility, including capital costs, using a
common time frame such as present worth of future expenditure and
capital expenditures or annual cost of capital investment and expected
future annual expenditures, and choosing the alternative which costs the
least. Thus, the least total cost facility is the optimum solution and
makes the most effective use of public monies. To illustrate, in figure 1,

1