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Quinton White - JU MSRI

River Life: Hurricane season, Jacksonville and the St. Johns River

June 07, 2022
It's All about the timing and tides.
Hurricanes are part of living in Florida and regretfully, when combined with the St. Johns River, can make a destructive and even deadly combination. And timing is critical.
A key component concerning hurricane damage is the timing of the storm and the tides. Hurricanes are massive low-pressure areas with swirling winds at speeds over 74 miles per hour. The low pressure allows water to rise and the wind, depending on speed and direction, can push water into the St. Johns River. Then there are the tides. All of this combines to cause flooding.
The tides in the St. Johns River are semi-diurnal, meaning two highs and two lows over a 24.8-hour cycle. That 24.8-hour cycle is a lunar day, as opposed to the 24-hour solar day. 
And to make it slightly more complex, the relative position of the sun and moon in relation to the earth is critical. When the three are aligned during the new moon or full moon, we get the highest high tides, but we also get the lowest low tides then too. Just 6.2 hours later. Those are spring tides. Spring does not refer to the season but comes from a German word meaning to “jump up.” 
We also get neap tides that are the lowest highs, and highest lows, when the sun, moon and earth are at right angles to each other.  Here in Northeast Florida, our tidal range, which is the difference between the high tide and low tide is about 5 feet at Mayport. However, tidal influence on the St. Johns River extends over 160 miles to Lake Monroe. Even in Welaka, the tidal range is about 6 inches.
In other words, it is a matter of timing. If we get a hurricane, or even a strong storm, the tidal height when it arrives can have a huge impact on the amount of flooding and damage that occurs.  Remember too that the St. Johns River flows north. So rainfall, especially massive rainfall associated with hurricanes, falling in Central Florida can impact us too.  That is effectively what happened during Hurricane Irma. Wind and tide had pushed water south into the St. Johns River basin and it began to flow back north when the next tidal cycle started pushing even more water into the St. Johns River. That harmonic effect caused the massive flooding in Riverside with the iconic photo of the Memorial Park statue “Life” surrounded by water.
Over the last few years, hurricane forecasting has improved. However, it is still an inexact science, especially when predicting more than three to five days in the future. That is one reason to be prepared. You need to be able to move to safety quickly. You may not have the luxury of time.  So, pay attention to the evacuation notices and know your risks. You can find your zone with this link www.floridadisaster.org/knowyourzone.
In addition to the normal hurricane preparations for evacuation, you should also use your cellphone and make a video of your house and belongings. The video will provide a record of any losses you experience due to flooding or wind damage. Video both inside and outside your home to give yourself a visual record of what is there before the storm.  And then pay attention to the forecast and be prepared.
Glad you asked River Life
What is the ugliest creature you have seen in the St. Johns River?
It may well be the armored catfish (Pterygoplichthys multiradiatus). This somewhat prehistoric-looking fish has a disc-shaped sucker mouth that points down with bony plates running along its back.  It can grow to 20 inches and weigh 3 pounds. They were originally imported from South America as algae eaters for aquaria. But when they outgrew their aquaria, people released them into ponds, rivers and streams.
They are an invasive species that prefers shoreline and riverbank habitats where they can cause erosion. You should never release aquaria animals into the natural environment. They compete with native species and upset the natural balance.
Source:  Florida Times-Union
River Life runs the first Tuesday of each month in The Times-Union. Email Quinton White, executive director of Jacksonville University’s Marine Science Research Institute, with questions about our waterways at qwhite@ju.edu. For more on the MSRI, visit ju.edu/msri.