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Section 3: Evaluation of Risk

As with other natural phenomena, occurrence of flooding appears to be governed by chance. The chance of flooding is described by statistical analysis of flooding history in the subject watershed or in similar watersheds. Because it is not economically feasible to design a structure for the maximum possible runoff from watershed, the designer must choose a design frequency, or inversely the Annual Exceedance Probability (AEP) of a flood appropriate for the structure. (See Chapter 4 for an explanation of AEP). Once a design AEP is selected, the structure should be designed to provide adequate capacity to appropriately convey the discharge associated with that probability. In this process the designer sets the level of conservatism by the selection of the design AEP. This is in contrast to the conservatism associated with structural design elements, which is typically based on safety factors in loading and structural capacity.

The design AEP can be established by standards or limited by factors such as economic considerations. Numerous methods have been developed to assist the engineer in assessing the risk involved in choosing the design flood and the check flood. For the purposes of this manual, risk is defined as the consequences associated with the probability of flooding attributable to the project, including the potential for property loss and hazard to life during the service life of the highway. A project can be fully compliant with policy and standards yet still incur an inappropriate level of risk. Consequently, all sources of potential risk should be considered as part of the H&H investigation for hydraulic structures in order to determine whether modified site-specific standards or criteria are appropriate.

If the consideration of risks appears to warrant a design criteria more or less stringent than the standard, a risk assessment should be conducted. As described below, the assessment of risk can be either qualitative or quantitative in nature. If the results of the assessment show that lower standards are warranted, the assessment will be documented through the design exception process and coordinated with the District Hydraulics Engineer (DHE) and the Hydraulics Branch of the Design Division (DES-HYD).

Most projects will require only a qualitative risk assessment. A qualitative risk assessment may be determined appropriate or inappropriate based on such considerations as the presence or absence of structures that could be impacted by the project, the perceived economic impact of temporary road closures, the environmental impact, or the cost of the roadway facility itself.

Highly complex, expensive projects or those with particularly high levels of risk may justify detailed and quantitative risk analyses. A quantitative risk analysis provides a detailed economic comparison of design alternatives using expected total costs (construction costs and maintenance costs plus risk costs such as the economic cost of an extensive and long-duration detour in the event of a failure, the cost of repair, etc.) to determine the alternative with the least total expected cost to the public. A quantitative risk analysis supports the appropriate design discharge and criteria based on the economic comparison of alternatives rather than a set of predetermined design AEPs and criteria such as those presented in this manual.

HEC-17, FHWA 1981 is a publication that provides procedures for the design of encroachments on floodplains using risk analysis. HEC-17 describes a quantitative assessment method called Least Total Expected Cost (LTEC). Least total expected cost refers to the result of a detailed economic analysis that attempts to account for all viable costs associated with a project. The analysis is ideally based on actual cost data.

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Risk Assessment Forms

The TxDOT form titled “ Economic and Risk Assessment for Bridge Class Structures” is a helpful resource in developing a qualitative risk assessment. The form has an associated worksheet to assist in developing a simplified estimation of the annual risk cost and annual capital cost. The form provides guidance on when a more detailed analysis following the HEC-17 LTEC approach is justified.

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