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Sunny day flooding increases fecal bacteria in coastal waters

Historically, heavy rainfall has been considered the primary culprit for introducing contaminants into coastal waters. However, a recent study underscores the changing dynamics of water quality. Researchers found that “sunny day flooding,” driven by rising sea levels, can also lead to increased levels of fecal bacteria, prompting a re-evaluation of water safety protocols.

What is sunny day flooding?

Sunny day flooding, also known as nuisance flooding or high-tide flooding, occurs when high tides – often influenced by the moon’s gravitational pull – rise higher than usual and inundate low-lying coastal areas.

This can include streets, storm drains, and even some properties. The key point here is that this flooding happens on days with clear skies and no rain.

The moon’s gravitational pull exerts a significant influence on Earth’s oceans, causing them to bulge outwards on the side closest to the moon and on the opposite side. These bulges create high tides, while the regions between them experience low tides.

Twice a month, during new and full moons, the gravitational forces of the sun and moon align, leading to particularly strong high tides known as spring tides.

Frequency of sunny day floods

Sea levels are rising due to climate change, primarily caused by the burning of fossil fuels. This rising water level effectively raises the baseline for the ocean, meaning that even on days with normal high tides, water levels can reach higher than before, inundating areas that previously remained dry. This is why sunny day flooding events are becoming more frequent and severe.

While they may not be the dramatic, raging floods we typically picture, sunny day flooding events can still have significant consequences, as this study demonstrates. The floodwaters can overwhelm storm drains, leading to backups of potentially contaminated water.

Contamination from sunny day flooding

The new study by North Carolina State University found that “sunny day flooding” isn’t just an inconvenience. These events introduce a range of contaminants into coastal waters, most notably fecal bacteria, raising concerns about potential health risks.

While these elevated bacteria levels typically dissipate within a few hours, scientists caution that the situation warrants attention. With rising sea levels, sunny day flooding is expected to become a more frequent and widespread issue, potentially leading to longer periods of water contamination.

How does this happen?

“Historically we see the highest levels of fecal bacteria contamination in coastal waterways after it rains, because the rain washes contaminants into the waterways,” said Dr. Natalie Nelson, a lead researcher on the study.

Rising sea levels create a surprising issue: street flooding even on perfectly sunny days. High tides push water back up through stormwater systems. This causes shallow, contaminated puddles that are often dismissed.

Sunny day flood bacteria

To track just how bad these sunny day floods are, Dr. Nelson’s team spent two summer months collecting daily water samples in coastal North Carolina. They were particularly interested in what happened during the “perigean spring tides” – super-strong tides due to the moon’s position.

Sure enough, the floodwaters were contaminated. “We found that the floodwaters themselves had relatively high levels of fecal bacteria,” said Dr. Nelson.

The impact on larger coastal areas was not as clear. Sometimes the receding tide seemed to cause a spike in bacteria, but other times it didn’t; and the spikes usually fizzled out within a few hours.

Public health and safety

While your beach vacation likely isn’t in jeopardy, there’s still cause for concern. This study focused on a specific coastline, and water conditions can indeed vary. However, the fact that sunny day flooding led to any increase in bacteria highlights a potential risk that scientists like Dr. Nelson want to further investigate.

“Sea levels are going to continue rising for the foreseeable future. So we are definitely going to see more sunny day flooding, and those floods will be getting worse,” said Dr. Nelson.

“We need to continue studying the impact that these tidal floods have on our water quality, because the more we understand, the better able we will be to make informed decisions about public health and safety.”

The next time you see street flooding on a sunny day, don’t think “just seawater.” Be mindful that the water is probably more polluted than it looks, especially for a few hours after the tide goes out.

While scientists are still figuring out the exact risks, it’s good practice to avoid letting kids play in this water, and thoroughly rinse off any gear (or yourself) that comes into contact with it.

The study is yet another reminder of the complex ways that climate change can affect our everyday lives.

The study is published in the journal GeoHealth.


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