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Dredging Increases Salinity and Storm Surge

Dredging Increases Salinity and Storm Surge Credit: Florida Times-Union

For decades, the St. Johns River has been deepened, straightened and manipulated to allow bigger and bigger ships to access Jacksonville's port.

The Florida Times-Union recently released a special report, As the Ocean Creeps In, that takes a look at the impact these alterations to the river have had on saltwater intrusion, tide levels, and storm surge.

According to the report, “The U.S. Army Corps of Engineers, one of the nation’s oldest federal agencies and the overseer of these changes to the St. Johns River, has never studied how more than a century of work [dredging] might make the city more vulnerable to storm surge and flooding."

We urge you to read this outstanding special report in its entirety, but here are some of the highlights:

Saltwater Creeps In

"The river is now a superhighway for Atlantic Ocean water, which in the event of a major storm could present greater risk to downtown and the neighborhoods around it even though they are miles away from the river's mouth."

"In chasing its dreams of a deepwater port, the city has brought the Atlantic Ocean to its doorstep."

"The saltwater transition zone — where Atlantic Ocean saltwater and river freshwater begin to mix — has moved further upstream through the decades. That zone is thought to have been near the Acosta Bridge in downtown in the 1950s....Today the salinity zone shifts between the Buckman Bridge near Orange Park and the Shands Bridge near Green Cove Springs, depending on rainfall levels, meaning it has moved between about 10 and 25 miles south of downtown — about 50 miles from the ocean — over the past six decades."

"'I remember that I used to regularly see grass beds around Sadler Point north of NAS JAX on regular… surveys during 1994-2000,” said Gerard Pinto, a Jacksonville University scientist who helps compile annual checkups on the state of the St. Johns. 'Those areas are now typically bare of vegetation.'"

"Scientists are certain dredging is one of among several man-made reasons behind salinity intrusion in the St. Johns River."

Increased Storm Surge

“The Army Corps acknowledges in its own analysis that the latest 7-foot dredging project could increase water levels in a 100-year storm surge by 3 to 6 inches in the main stem of the river, and by 8 inches in areas closer to the ocean. But the agency — which has nearly unchecked latitude in making such determinations — has, for reasons that are unclear, deemed those potential increases insignificant."

Stefan Talke and Ramin Familkhalili are Portland State University professors who are "studying the cumulative effect that years of deepening projects can have on tides and storm surge intensity within estuaries." Their research of the Cape Fear River in North Caroline discovered that the tidal range — the difference between high tide and low tide — has increased due to decades of dredging.

“The results of the study, the two wrote, ‘suggest a simple but profound lesson: locations in which tide waves have been amplified are also vulnerable to increases in storm surge and flood risk …’”

"The increased tidal changes Talke and Familkhalili tracked are less pronounced along coastal areas and more pronounced further inland, suggesting, they argue, that the cause of those changes lies within man-made alterations to the river rather than driven by large-scale ocean changes like sea-level rise, itself the result of human activity warming the climate. That also means any increase in storm surge intensity would be felt in those more inland areas, too."

"An 1879 annual report from the chief of engineers makes reference to the tide range in downtown Jacksonville measuring about one foot. Today, the range is about 1.8 feet — nearly double. Like Cape Fear, the increased tide range near the coast is much smaller than that, suggesting that man-made activities like dredging are the source of the change."

Flood Waters Rising

"The St. Johns Riverkeeper — a well-known local environmental watchdog — late last year amended an existing federal lawsuit seeking to stop the 7-foot dredge to include an argument that the Army Corps failed to properly study storm surge intensity and its flooding impact."

“A major point of disagreement between the Riverkeeper and the corps is whether the agency’s storm surge study — which acknowledges that the 7-foot dredge could increase storm surge height by as much as 8 or 9 inches in parts of the river close to the dredging project — downplays the potential risks. The Army Corps’ concluded that the potential increases in storm surge are insignificant. It’s not clear in the report, however, why the Army Corps came to that conclusion."

"A 2016 storm surge study for the Charleston project found potential storm surge water level increases in the harbor ranging between about 1.2 inches to a maximum of about 3.6 inches — the lower end of what the Army Corps expects could happen in Jacksonville. In Jacksonville, the Army Corps dismissed the potential increases in water levels without indicating the need for a more refined study."

"After Irma, the agency doubled down on its position. It said the flooding issues 'do not constitute significant new circumstances or information relevant to environmental concerns bearing on the project or its impacts.'"

A City Unprepared

"The city sends mixed signals about how urgently it views sea-level rise and resiliency issues. It’s not clear what Curry’s views are on climate change, and his office did not respond to a request for an interview."

"Last year, the city began working on a state-mandated change to its 2030 master plan that acknowledges in writing the need to prepare for sea-level rise and establishes a working group to start studying the issue within the year. That change faced skepticism from the city Planning Commission — which recommended the City Council not pass it based on fears it would be onerous to business owners."

"At minimum, Hurricane Irma was a wake-up call, but it’s not clear the public or government are galvanized to tackle Jacksonville’s flooding problems."

"'The lesson that is taught by all of this, communities need to be far more engaged and far more informed,' said Davis, the Tulane professor." 

Read comments submitted to the Army Corps of Engineers by Lisa Rinaman, the St. Johns Riverkeeper, on December 29, 2017, regarding the Corps' failure to fully assess potential flooding impacts from the propsed dredging of the St. Johns River.

Click here to learn more about the latest dredging proposal and the potential impacts to the St. Johns.  

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