In light of sobering IPCC report, how can we address increasing flood risks?

As a civil engineer specializing in water resources, my profession is tasked with creating resilient solutions to the challenges of riverine, urban and coastal flooding. Simultaneously, we must never lose focus that our responsibility is to the protection of public health, safety and welfare, as well as the natural environment.

Along with many others, I have been concerned for some time with the increasing volume of rainfall and frequency of extreme events that we have seen over the past decade. The recently released Intergovernmental Panel on Climate Change (IPCC) report confirms that what we have been seeing is indeed caused by the change in climate. It is daunting to realize that the increased heavy precipitation and flood surges is expected to continue, and, if anything, will get worse for large parts of the United States and the world. The use of the term “irreversible” is sobering, however, that is not a reason to abandon efforts on reversing this trend.

So, how will these extreme climate changes, as predicted by the IPCC, affect our current water management infrastructure, and what do we need to do? My perspective on this was informed by two projects I was involved with through the American Society of Civil Engineer. The first explored the causes of the failure of the New Orleans Hurricane Protection System during Hurricane Katrina. The second was a task committee following Superstorm Sandy, which was charged with assessing whether we had learned or just observed lessons that had been paid for at such a high cost and to make recommendations to the nation.

In my opinion, the first action we need to take is to understand how the climate is changing and what protection we can expect from our current water infrastructure. Without this understanding we cannot effectively implement decisions and projects to reduce our exposure, nor meet our obligation to inform our community what their risk is and will be in the future. Clearly, this is not a new challenge as community risk was not understood by the public in New Orleans before Katrina, nor the coastal areas decimated by Sandy.

We design our water infrastructure based upon a set risk. When projecting flooding along a river, many communities use the term 100-year flood plain elevation. What this means is that there is a 1 percent chance that the flood depth will be exceeded every year, so there is always the possibility of a larger storm. For other systems, we accept a larger risk, and design for smaller events.

For example, we assign a larger risk to urban drainage and expect these systems to flood more often, since the damage is considered minor and building the system to carry the larger storm is simply too expensive. Stream banks are formed for smaller storms that have a 50 percent chance of being exceeded every year (two-year storm). Dams and other structures are generally designed for the Probable Maximum Precipitation, a much higher threshold, while it is commonly understood that flood surges follows a similar approach.

The rainfall that occurs during a 100-year storm is tied to a specific location, developed from long-term, historical local climate records. A 100-year storm in Philadelphia is different than in Pittsburgh, as the drivers of the weather are different for a city closer to the coast, then one further west.

As discussed in the IPCC report, our heavy rainfall volumes are now expected to continue to increase in many areas. So, what is the effect of extreme climate change on our water infrastructure? While the storm volumes or flows used in the design will not change, the risk will be greater, how much will depend on the region.

For example, inlets and culverts designed for 25-year events, may now handle a 20-year or 15-year storm. To maintain the 100-year designation, the flood plain will need to expand and riverine sediment and erosion processes should be expected to increase changing river geometry.

After we understand and communicate what our increased risk is, what can we do? Probably the most essential tool that we have is communication and evacuation. Staying in an area when flooding is imminent can only increase your risk. Funding spent on evacuation pathways can literally save lives.

An obvious step is increased flood mitigation infrastructure, prioritized by our understanding of what the flood risk is. Our work at Villanova has demonstrated that the use of natural processes such as green stormwater infrastructure in concert with the more traditional structural components can aid in the solution.

A benefit is these green solutions also protect our natural environment that is also subject to changes in climate. Creating more green space from unneeded impervious surfaces can reduce impacts by reintroducing infiltration area.

For other areas, flooding will unfortunately occur more often than what we normally accept. For these areas, we need to build in the abilities to recover quickly following a flooding event. An example of this resilience is floodproofing or moving the control circuits to higher elevations for hospitals or subway systems.

There will be hard decisions on whether or now we should rebuild in areas that we cannot protect. For example, in the aftermath of the 1993 flooding of the Mississippi, one town in Illinois was moved to a higher elevation. This may be a path forward for some communities.

Clearly, collaboration at all levels of government is needed to make sure funding decisions for water infrastructure is prioritized. Land use or zoning decisions must consider flood risk, and not work against our mitigation efforts. That coupled with evacuation planning and communication of risk is best accomplished at the local level. 

While many areas are looking at resilience in their communities, the message presented in the IPCC report is clear: Time is rapidly running out. While I remain optimistic and expect that we will address and reduce the growing impacts of climate change, we need to be proactive and resolute in reducing our exposure to flooding. Actions that we take now can reduce the consequences of future extreme events. Avoidance or delays are not an option. 

Robert G. Traver (Ph.D.,P.E., DWRE) is director of the Villanova Center for Resilient Water Systems, and the former Daylor chair of Civil Engineering at Villanova University. He is an expert in water resources and green stormwater infrastructure. Follow him on Twitter: @Doc_Stormwater