FLOOD07468

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: The Design of Encroachments on Floodplains Using Risk Analysis
• Date: 10/1/1980
• Prepared For: USDOT
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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u.s. DEPARTMENT OF TRANSPORTATION
FEDERAL HIGfMAY ADfHNISTRATION

THE DESIGN OF ENCROACHMENTS ON FLOOD PLAINS USING RISK ANAL YS IS

Prepared by M. L. Corry, J. S. Jones, and P. L. Thompson

1.0 INTRODUCTION

The design of all flood plain encroachments should include an evaluation of
the inherent flood related risks to the highway facility and to the surrounding
" property. When this evaluation indicates that the risk warrants additional
study, a detailed analysis of alternative designs is necessary in order to
detennine that design with the least total expected cost (LTEC) to the public.

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The purpose of this manual is to provide guidance in the appl ication of the
LTCC design decisionmaking process. The LTEC design process is basically one
of optimization, where economic and engineering analyses of alternative
designs provide the basis for decisionmaking.

An essential ingredient in the LTEC design concept is risk analysis. Risk
analysis provides the vehicle for analyzing the losses incurred for the
various design strategies due to possible states of natures (floods events).
All quantifiable losses are included in the risk analysis. These may involve
damage to structures, embankments, surrounding property, traffic related
losses and scour or stream channel damage. The product of the ri sk analysi s
is the annual economic risk associated ~lith each design strategy.

The sum of the annual economic risk and the annual capital costs, the total
construction costs multipl ied by a capital recovery factor, results in the
total expected cost (TEC) for each design strategy. Comparison of the TEC's
for all design strategies allows the desigder to select the LTEC or optimum
design.

Although, the elaphasis in this manual is on bridge crossi ngs, the LTEC
decisionmaking process concept is applicable to other drainage features.
For exar<lple, it may be utilized in the design evaluation of culverts,
longitudinal encroachments, countenneasllres and foundation elevations.
Bridge, culvert and spur dike design problems are presented in appendices
A, B, and C.

1.1 LTEC and Traditional Design Concepts

Risk is defined as the consequences associated with the probability of flooding
att ributab 1 e to an encroachment. Therefore, regardl ess of the desi gn process
utilized, traditional or LTEC, there is a level of risk associated with every
flood plain encroachment. The manner of assessing project risk and the way in
which risk influences decisionmaking illustrate the basic differences between
the LTEC and traditional design processes.

In the traditional design process, the level of risk is seldom quantified but
is implied through the application of predetermined design standards, e.g.
design frequency, backwater limitations, limiting velocity, etc. Its role
in decisionmaking is impl icit and limited to the degree that risk was considered
in establishing the design standards.

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