‘Invisible Infrastructure’ Important To Mississippi River

By Sean Duffy

Big River Coalition Executive Director

The Mississippi River Ship Channel Deepening to 50 Feet Project has shed a unique light on the importance of our transportation infrastructure and what I refer to as “invisible infrastructure.”

Defining the term “invisible infrastructure” is complex. Sometimes components of our invisible infrastructure are clearly visible, yet they are completely overlooked until they fail. Then, in an instant, the whole world is focused upon them.

For example, consider the power lines that closed the Mississippi River Ship Channel when they were knocked down in Hurricane Ida. Thousands of ships passed under those lines each year, but no one thought much of them until they fell and closed the ship channel. As a nation, we speak a lot about physical infrastructure, while often overlooking critical components like invisible infrastructure.

Invisible infrastructure also includes things like air gap sensors, sediment, pipelines, navigation technology, aids to navigation, current meters, fog sensors and, perhaps the hardest to understand, geodetic datums.

The historic effort to deepen the Mississippi River Ship Channel to 50 feet has encountered some delays related to pipeline challenges, namely the inability of survey equipment to locate pipelines buried under hard sediment (mostly sand). Although the first 195 statute miles of the ship channel have already been deepened beyond 50 feet, pipelines in key areas have not been successfully surveyed to show their depth below the river bottom. Channel survey data focuses on how many feet of water are above the river bottom, but identifying the depth of pipelines below bottom is also critical. In March 2021, questions first arose about the depth of a few pipelines in the Venice Corridor (Mile 12 AHP to Mile 11 AHP), an area 30-plus miles inland from the jetties at Southwest Pass.

Despite that issue, the Lower Mississippi River recently passed a significant waypoint in the efforts to reach a draft of 50 feet: the first vessel with a draft greater than 47 feet just departed the Ship Channel. The Associated Branch Pilots of the Port of New Orleans (Bar Pilots) increased their maximum draft restriction to 49 feet (freshwater) on December 17, 2021, and three days later, the Crescent River Port Pilots Association increased their maximum draft recommendation to 48 feet (freshwater). The maximum controlling draft of the ship channel remains 48 feet at this time, but adjustments toward 50 feet should be expected.

This was the first draft increase specifically related to the current channel deepening project—and the first since the 1980s. The pilots are actively reviewing all data, including channel surveys, because decisions to advance a 50-foot channel are a top priority.

A second issue is the need to “marinerize” technology in use on the Ship Channel. Take current meters as an example. Sensors that track the speed of the current on the Mississippi River Ship Channel measure it in “feet per second.” Imagine driving a car today where the speedometer was measured in feet per second. Would you know how fast you were going? Or would you be able to easily convert it to miles per hour? Since the Mississippi River is arranged according to mile markers, measuring current in miles per hour would benefit mariners and better promote navigational safety.

Vertical clearance at bridges, another example of invisible infrastructure, may not be on everyone’s infrastructure radar, but it should be. A report from the National Transportation Safety Board (NTSB) on the allision with a crane barge and the Sunshine Bridge (Burnside, La.) documented challenges in calculating vertical clearance. The calculations used are complicated, represent close approximations to the actual clearance at best, and are typically found by subtracting the established height of the low steel of the bridge and the closest river gage: i.e, 

174 feet minus the noted river gage in feet equals vertical clearance, or air gap. However, with modern technology readily available that provides accurate measurements every six minutes, why are we approximating clearances along critical navigation channels while risking our supply chain economics? It’s now 2022, and we are calculating bridge heights the same way the Egyptians did.

The NTSB’s Marine Accident Brief (No. DCA19FM003) on the allision with the Sunshine Bridge and crane barge detailed some of the truly antiquated formulas that are often used on the fly and sometimes based on one’s own training, job history, time available to calculate and resources. 

The following quotes are reproduced from the Marine Accident Brief:

“However, the NTSB reviewed the available data for the Sunshine Bridge and other bridges with multiple spans on the Lower Mississippi River and found inconsistencies in the vertical clearances provided for all navigable spans. …”

and,

“Navigational aids should provide mariners with a simple and precise way to navigate and not increase workload or cause confusion. Mariners would be better equipped with the correct information if all of the information was included on the chart and any additional data sources were clearly stated so as to ‘steer’ the mariner to the map books, river gages, and/or other Corps of Engineers information. …”

After reading the NTSB’s Marine Safety Brief, I decided to compile the formulas used to estimate bridge heights for the seven bridges that cross over the Mississippi River Ship Channel. I noticed an error for the documented formula for the Baton Rouge I-10 Bridge. The two federal agencies had listed the wrong river gage in the formula to subtract for the Baton Rouge I-10 Bridge air gap calculation. After bringing this to their attention, it was quickly agreed that I had found an error, and the information was immediately updated to list the proper gage.

I share this without specific details because the information is important and expands on inconsistencies noted by the NTSB. In that instance, I reached out to the government agencies, pointed out what I had uncovered, and they agreed and made the proper adjustment quickly. In all honesty, I myself had looked at that bridge data numerous times before, but never with the scrutiny that I did to prepare this document. For me to notice the error, it took listing what the pilot association, National Oceanic and Atmospheric Administration, U.S. Army Corps of Engineers and U.S. Coast Guard used to calculate the bridge height or air gap.

There is another reason that focusing on invisible infrastructure is key to properly restoring, rebuilding and reconfiguring our physical infrastructure. The size of modern-day vessels continues to increase, and the need for deeper channels and accurate bridge clearances is a key component of modernizing our visible and invisible water commerce infrastructure. These larger vessels will test the bottoms and the tops of our channels and highlight the need for accuracy and intelligent forethought as we try to promote maritime commerce and remove or repair the weak links across our supply chain.

This nation became an economic world leader because of a national commitment to build our transportation infrastructure to promote trade and create jobs after the Great Depression and World War II. Like our forefathers did, we today must commit ourselves to a similar endeavor.

As we advance a national effort to repair, restore, revitalize and reinvigorate our physical infrastructure, we must also embrace modern technologies to ensure that the funding spent on infrastructure is not doomed because we lost focus on our invisible infrastructure.