Risk-based design of flood protection

Posted in: Municipal, Water

Current methods of flood prediction are often inadequate

Any flood protection must be established on a sound technical and scientific basis to be effective.

Conventional design of flood protective measures have been based on a “design flood”. This means that the measures are designed to be effective up to that high-water level. For example, a one-percent-annual-chance (100-year return period) flood event is often the protective objective pursed. But does this make sense to subject all regions to the same flood probability? Or should the cost of the protection system be in balance with the value of the protected area?

Traditionally, water resources professionals quantify flooding to a specific standard based on a frequency analysis of historical data and apply this to obtain flood hazards. This approach tends to assume that events in the future are predictable based on the experience of the past. While these methods are the industry norm, they often do not properly quantify the future flood hazard.

The U.S. Army Corps of Engineers has transitioned from being a traditional, standards-based agency to being risk-informed. The transition of the Corps followed Hurricane Katrina and the failure of levees that were part of the New Orleans Hurricane Protection System. The USACE recognizes that they must understand uncertainties and how their investments will perform in the future.

Why adopt a risk-based approach

Risk-based analysis is an analytical process that provides information about or quantifies probabilities and consequences of a flood event. It allows engineers, scientists, planners and community leaders to identify these uncertainties and quantify their effect on decision-making processes. Over the years, risk-based design methods have proven to be useful to obtain a balanced level of protection and helps minimize the overall risk much more efficiently.

In risk-based design, the design of flood protection is determined by balancing the cost of construction against the reduction of the flooding risk obtained. The benefit of a risk-based approach is it deals with outcomes. It enables intervention options to be compared on the basis of the impact that they are expected to have on the frequency and severity of flooding in a specified area. The risk-based approach gives decision makers informed choices based on comparison of the expected outcomes and costs of alternative courses of action. This is different from a standards-based approach that focuses on the severity of the loading that a particular asset has been design for.

Flood events are bound to occur, and they have existed and will continue to exist. One must shift from overly conservative protection against flood hazard towards management of the risk and living with floods.

Reducing flood risk makes it possible to move away from the paradigm of keeping people and water separated and towards an integrated flood management approach that balances the costs and benefits of living near bodies of water.

Karen Wiemeri

About the Author

Karen Wiemeri, P.E., is a project manager in the Water Group. She provides design and technical support for structural flood mitigation measures. Through her work with structural flood mitigation and stormwater management, Karen has an increased interest in more sustainable approaches to flood risk management that can complement existing defenses.

Read more posts by Karen Wiemeri

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