Paradigm Shift in Flood Hazard Analysis

Swamp Stomp

Volume 15, Issue 6

The number of Americans living in flood-prone areas increases each year due to continuing urban expansion. Subsequently, the cost of direct flood damage also continues to grow. In 2012, this number reached almost half a billion dollars. The contributing factors of flood risk, however, remain largely un-understood, and as a result, river basin management and flood insurance premiums may often be erroneous.

Michael Singer, an associate researcher at the Earth Research Institute at the University of California Santa Barbara, and his colleagues released a new study in Geophysical Research Letters that presents a paradigm shift in flood hazard analysis. This shift may allow for more accurate flood predictions, and change the way that risk is assessed.

Current analysis procedures associate flood hazard with how often high water flows occur. They don’t yet take into consideration the ability of river channels to facilitate them. Singer’s method, however, presents an innovative technique that compares the effects of channel capacity and stream flow on flood hazard frequency, as well as documenting how flood hazard has changed over time in over 400 streams across the U.S.

“Our results demonstrate that changes in river channel boundaries directly impact flood hazard trends across the U.S.,” Singer claimed. “We show that in order to accurately calculate flood hazard and insurance premiums for river basins, channel capacity needs to be considered jointly with stream flow.”

Lead author of the report, Louise Slater, a lecturer at Queen Mary University of London, collected U.S. Geological Survey data that was recently digitized, and reduced the original dataset of 11,000 stations to 401 that were well distributed across the country. She then developed a procedure for separating the effects of hydrology and geomorphology on flood hazard. The study data averaged between 40 and 60 years for each location.

In order for the new method to work, the researchers needed to find the relative magnitude of two separate data components, and then see how they may interact with each other. The first component, water quantity, is also known as the flow frequency effect. The second component is channel size and conditions, or morphology, otherwise referred to as the channel capacity.

Singer said, “If there’s more water coming from the watershed but the channel gets enlarged somehow, that would offset the increased water flow. These two factors potentially interacting could have no change or they could increase the amount of change in one direction or another.”

The study found that important trends in channel morphology through time were three times more common than those related to water quantity. This demonstrates that the geometry of the channel tends to offset increases in water flow. Singer claimed, “That raised alarm bells. It suggests that a lot of areas that we might not have considered to have trends in flood risk actually do.”

The Pacific Northwest is an example. The increase in flood hazard there is identified with the channel capacity effect because the channels are filling with sediment or vegetation that impedes the water flow. “The channel morphology has a big impact on flood hazard,” Singer said, “making this an area where flood hazard has been underestimated.”

He continued, “The opposite is happening in the Mississippi River Valley. This is an area where people might overestimate the impact of increased stream flow because the channels are adjusting to accommodate an accelerated hydrologic cycle.”

According to the findings, 10-year trends in channel capacity considerably impact long-term flooding frequency, and flood hazard is changing extensively at the majority of the sites studied. Singer concluded, “Based on our analysis, we argue that in order to develop appropriate management strategies or to set flood insurance premiums for any location, you need to consider the flow frequency and channel capacity effects of flood hazard.”

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