Why bridges get hit — a river pilot’s perspectiveMar 27, 2017 01:36 PM
Brian Gauvin photo
An assist tug and barge tow pass under the U.S. 190 bridge in Baton Rouge, La., during high water in 2011. Faster currents during flood stage multiply the risks for pilots navigating spans on the Lower Mississippi River.
This year’s bridge-hitting season is coming up soon, and let’s hope it’s not like the last one. Wow, what a doozy! Annual spring rains almost always cause river stages to spike, and with that comes bridge allisions and the accompanying damage to equipment, profits and nerves.
With any accident investigation there is the question of why, but when there are multiple allisions at a particular bridge in a very short time period, it borders on hysteria. “Why does this bridge keep getting hit?” I actually get asked this question a lot, since virtually all of my acquaintances know that I have been a river pilot for many years. It comes up every time a bridge allision makes it into the news. It is tempting to just say, “Because it’s not wide enough.” That is probably true of the bridges that get hit most often, but incidents involving the narrowest bridges are rarely the ones in the news. Why is it that the bridges on the Lower Mississippi River generally have the widest spans, but disproportionately cost the inland marine industry so much more in terms of equipment damage, lost revenue and damaged reputation? The short answer to that question is that the width of the span is not the only factor to consider. Here is the rest of the story.
Bridge allisions are truly non-discriminatory. They happen to large tows, small tows, experienced pilots and inexperienced pilots. They happen in daylight and dark, good weather and bad. So is it just a matter of luck? Hardly, but let me say at the outset that it is much harder to transit a bridge than most people realize, especially during high water.
First of all, on the Lower Mississippi you are dealing with the faster currents of a free-flowing river, and in many cases you are also dealing with downbound heavy tows made up of 30 to 48 loaded barges in configurations covering up to seven acres. When viewed from this perspective, it would seem a better question to ask is, “How are so many bridge transits successful?” It would appear that bridge allisions are just the answer to the question of what happens when an unstoppable force meets an immovable object. However, river pilots, like most pilots, are highly creative individuals and over the years they have developed techniques to manage the risks involved very effectively. Most of the time, experienced pilots make bridge transits look as easy as driving a car. But that all depends on having conditions that are reasonably predictable.
In today’s environment, post-accident investigations almost always land on some form of human error — it’s just a matter of finding out who and how much they are to blame. When investigators start rounding up the usual suspects, what they expect to find first is a flaw in the planning stages of a bridge transit. So it is no small wonder that when they look at the planning, lo and behold, there it is: There is nothing in this plan that includes hitting this bridge. It is very easy to say, “I guess you didn’t plan for this,” but that’s like telling someone who was struck by lightning, “I bet you didn’t see that coming.”
In my experience, the problem is not so much with the plan as it is the execution. Everything can be going just hunky-dory and then all of a sudden it’s not. It occurs so suddenly that you’re left wondering what just happened. Next, in the absence of any mechanical failure on board, the hammer of blame goes looking for a nail and everyone begins to point toward the pilot. The pilot, in turn, perhaps having only made a minor mistake in judgment, tends to believe that there is something new occurring in the approach, something that he or she never experienced before. To some extent that is true because the conditions have changed, but then again, they are always changing — that is the nature of alluvial rivers. More to the point, could he or she have reasonably predicted those changes?
Although I have never had a major marine incident (thank the Lord), I have had enough close calls to know that a relatively minor mistake in judgment can have major consequences, especially when it comes to bridge transits. Without going into all of the details of every accident and the hazards for every bridge on the Lower Miss, it might be useful to look at some of the variables that pilots are constantly analyzing in the process of “making” a bridge.
Since marine navigation is a two-dimensional activity, it would seem that all you would have to do is line up on the green lights and “let her rip, potato chip.” That is true if you have a straight approach and the current is running straight in that approach. However, that is almost never the case.
Bridges, especially older ones, tend to have been built in areas where there is a fairly sharp bend. There were actually valid reasons for this. River channels tend to be narrower and more stable where there is a sharp bend; with the limited engineering and resources available from the 1800s to the mid-1900s, engineers often had to choose areas where the channels were narrow and stable. Needless to say, it was good from an engineering standpoint but bad from a pilot’s standpoint. Engineering has since reached the point at which bridges can be built without restricting navigation, but that does not solve the immediate problem for pilots dealing with existing bridges.
Since bridges tend to be on or just below sharp bends, the approach is almost always a curving approach. Turning causes your momentum to carry your tow to the outside of your turn and the sharper you turn, the more that momentum causes you to “slide” across the surface of the water. Also, since momentum is a function of speed and weight, the faster you are going or the heavier your tow, the more momentum you are carrying and the harder it is to overcome.
Another variable is the direction and speed of the current. Since your vessel is moving on the surface of a liquid that has a movement of its own, you also have the added element of set (pilots actually call it a draft) to one side or the other. That gives us four primary variables affecting our course under a bridge. There are other factors such as wind, available horsepower and tow configuration, but the following are the four biggies:
- Sharpness of the turn in the approach.
- Size and weight of the tow.
- Speed through the water.
- Direction and speed of the current in the approach.
All of these are fairly straightforward estimations that pilots are constantly making in the course of daily navigation and all are reasonably predictable, except for the last one. The direction and speed of the current can change dramatically, sometimes in a matter of hours. The direction of the current changes based on the fact that it is encountering a different set of obstructions depending on whether the river stage is high or low. The speed of the current changes dramatically depending on whether the river is rising or falling and how high or low it is overall. The changing speed of the current has little impact on your course if it is running straight — in other words, parallel with your approach to the bridge — but when it is hitting you from the side, the speed of the current has an enormous effect. The more current that is coming from the side in the approach to a bridge, the more difficult it is to accurately predict how much that current will affect your tow and to compensate accordingly.
Changing currents greatly affect the approach to the railroad bridge and the adjacent Interstate 20 bridge in Vicksburg, Miss. The railroad bridge was hit five times in less than two weeks during high water in January 2016, according to the U.S. Coast Guard.
Pat Rossi illustration/Capt. Gregory L. Smith
When conditions start changing fast, there are just too many variables to accurately predict. At this point, pilots have to switch from “predictive navigation” to “reactive navigation.” Instead of following a plan, you are forced to react to the circumstances as they happen. In my view, unpredictable side currents are the single largest contributing factor in bridge allisions and the real reason why some bridges are hit much more often than others despite having wider spans. It is “numero uno” in the sense that it causes the first big crash.
However, there is another factor that has a huge bearing on bridge allisions: the psychological factor. After a particular bridge pier is struck, the next pilot will have a natural tendency to change his approach to compensate for what he or she believes caused the first incident. Sometimes, the result is that they overcompensate and strike the opposite pier. Then the cumulative effect of multiple accidents begins to take on a life of its own. It starts to skew your perspective as a pilot and you start overanalyzing and second-guessing yourself to the point of chaos and confusion. If you want to know why a particular bridge gets hit multiple times on different piers with different approaches, here is your primary culprit. At this point, only the most experienced pilots have the self-confidence necessary to get the job done.
Now that we know a little more about why bridges get hit, the question then becomes how do we manage that risk? Having trained many pilots, I am often tasked with coaching them on how to minimize the risk of hitting a bridge. With the exception of using a sharp pencil and putting them off on the other watch, I recommend that pilots try to develop bridge approaches with the following in mind:
- Never start an approach without having your mind completely made up about which span you will run. If the thought ever crosses your mind to change spans while you are in the approach, then you will probably succeed in making both spans with different fragments of your tow.
- Develop an approach that doesn’t require you to use the outer limits of your vessel’s capabilities — horsepower is not a good substitute for skill. By all means if you need it, use it, but if you are constantly relying on all of your horsepower to stay in shape, then that alone should serve as a warning that your approach may not be a good one.
- Minimize the impact of as many variables as you can. Manage traffic so that it doesn’t affect your approach, wait for daylight, flank if the turn is too sharp or if the current is too swift for the tow size that you have, etc.
- Maintain a positive outlook. Instead of focusing on what others are doing wrong, remember that most transits by far are successful. Focus on what others are doing right.
However, there is more that can be done to prevent bridge allisions than just better pilot training. The more obvious solutions are the ones we are already implementing, like restrictions on tow size and horsepower, making daylight transits, and traffic management when the rivers are rising. Perhaps it is high time that those of us engaged in the inland marine industry start pushing harder for some engineering solutions. I would suggest the following:
- Consider bringing the Truman-Hobbs Act to bear on some of these bridges. The fact that we have tow-size restrictions because of a particular bridge is prima facie evidence that the bridge may be an unreasonable restriction to navigation. We need to begin making plans now to build better bridges with less restrictive spans.
- Consider modifications to the channel where there are hard side drafts in the immediate approach to a bridge. The U.S. Army Corps of Engineers is tasked with maintaining navigation as part of its mission, and some bridge approaches could be dramatically improved just by straightening the current in the immediate approach. This would remove much of the guesswork for pilots when calculating how much a side draft will affect them.
- Consider glide path buoys. Use the ECS track lines of multiple vessels that have transited safely to set outside parameters, then set fast water buoys to mark the edges of those parameters and provide a visual glide path.
The obstacle to these solutions is funding. These engineering solutions all require action on the part of either the Coast Guard or the Corps of Engineers, and they recently have had their budgets hammered down to a subsistence level. However, with a new administration in power, and hopefully a renewed emphasis on infrastructure, perhaps the time is ripe for making some headway on these issues. So the ball is back in our court. Ultimately, we will need the help of Congress to fund an analysis so we can project how cost-effective these solutions would be and prioritize accordingly. That will require appeals to members of Congress from our industry.
In the meantime, pilots will just have to do what we always do: Predict what we can, react to what we can’t, and carry a spare pair of clean underwear.
Capt. Gregory L. Smith is a veteran of the inland marine industry with 40 years of experience training pilots. He is the author of the textbook River Navigation and is currently a heavy tow captain on the Lower Mississippi River. For information on obtaining his book, email firstname.lastname@example.org.