Recovery Diving in Baltimore
We’ve probably all seen news stories about the dramatic collapse of the Francis Scott Key Bridge in Baltimore recently. Six individuals were lost; as of now, two have been recovered. At California Recovery Divers, we’ve fielded a lot of questions, and we’re very familiar with some of the challenges and tools used in these searches, so it seems like a good opportunity to share some of this information more widely.
Side Scan Sonar
Side scan sonar is a widely used tool for underwater search and recovery. It works by emitting sound waves across the bottom of the water, and then measuring the time it takes for the sound waves to return. This information can be used to create a detailed image of the underwater terrain, and to identify objects that may be of interest.
We use side scan sonar as our primary tool in many circumstances. It lets us search large areas for images of human remains, identifiable by the reflection and shadow in the sonar imagery. This tool has some serious drawbacks in a situation like the Baltimore bridge collapse, unfortunately.
The proper sonar device for this application is a towed torpedo-like unit that “flies” a few feet above the bottom, and transmits image and other data along the cable to analytics software on the surface. In an area filled with bridge debris, this towfish can easily become entangled in the debris. We sometimes have a similar challenge in California reservoirs: they’re filled with live oak trees, which make great fish habitats, but can easily ensnare a towed sonar unit. And it can be quite a challenge to remove the unit once it’s snagged.
The second serious problem is that the debris casts shadows and reflections that can make it nearly impossible to see what’s behind, or within, the tangled structure. It’s sort of like a flashlight shining through a smoky room: many objects make it extremely difficult to differentiate one from another, and one object can easily hide another.
This image shows a small boat on the bottom of a bay. The Baltimore debris would be very complex.
ROV (Remotely Operated Vehicle)
A remotely operated vehicle (ROV) is a submersible robot that can be controlled from a remote location. ROVs are equipped with cameras, lights, sonar, and other sensors that allow them to explore underwater environments. They also sometimes have a remote arm to grab or manipulate objects. We typically use an ROV to make recoveries of bodies at great depths that would be too dangerous for divers.
This is a terrific tool if you know where your recovery target is located, but it’s less useful to search large areas. One of the key tenets of good public safety diving is creating a reproducible search: you need to know what has been searched, as well as what hasn’t been searched. Because an ROV operates at depth, connected to a tether, you’re not exactly sure where it is, even with some newer electronics to assist. When searching for something small, like human remains, the sonar is also typically only useful for a limited distance from the ROV, and the images can be more difficult to interpret than those coming from towed sonar. In water like that in Baltimore harbor or most of our search areas, there’s also extremely limited visibility (maybe only a few inches), so the lights and camera aren’t much use. That’s why we really only use this tool to investigate or recover objects at known locations.
Another difficulty with ROV use in the Baltimore situation is the tidal current. ROVs don’t work well with current. Even units with extra propulsion really behave like butterflies in the wind. Our tides in San Francisco Bay might be similar, and anywhere with tidal influence only has a few minutes of slack tide with no tidal current 4 times a day. This really isn’t enough time to mount a search.
The unit here is our go-to instrument for retrieving objects from depths up to 100 meters.
It might be possible to use an ROV with a lot of weight, to “drop in” at one location and look around with sonar, then move to a different location, but then it’s pretty immobile, and quite tedious. The teams in Maryland may be doing this to get some mapping of what’s going on under the collapsed bridge structure.
Divers
Divers are an essential part of any underwater search and recovery team. Divers are able to go where ROVs cannot, and they can use their hands to search for and recover victims and evidence. There’s a reason “divers” is part of our name.
Generally, divers conducting a hand search should also follow the rule of reproducibility. It’s pointless, and dangerous to conduct a search in a haphazard way. This means divers, using SCUBA or surface-supplied-air are generally attached to a tether that goes to the surface. A “tender” at the surface directs them in their search, to methodically cover a target area. This means keeping the tender at a fixed location and keeping complete records of where the diver has been, based on the tender’s location and the tether line’s length and direction. This can be complicated by bridge debris or any other obstructions. From a diver’s perspective, this is done without any visibility, feeling by hand, using a life support system to breathe.
My understanding is that the two victims that have been recovered so far were recovered by divers from a vehicle that went into the water with the bridge collapse. My best guess is that the vehicle was located using sonar or other technology, and divers either lifted the vehicle with the victims in it, or removed the victims from the vehicle. We generally lift vehicles with the victim inside, using inflatable “lift bags”, but circumstances vary.
Searching for the other victims, divers really aren’t a good search option, unfortunately. Visibility is extremely low, current will pull a diver off their location, and there’s a lot of unstable debris that could easily trap, injure, or even kill searchers.
Constraints of Underwater Search and Recovery
We should all be thankful for those involved with the search and dive operations at the Francis Scott Key Bridge. Underwater search and recovery is a challenging and dangerous task. There are a number of constraints that can make it difficult and dangerous to find and recover victims and evidence, including:
Visibility: Visibility underwater is very limited, which makes it difficult to see objects.
Currents: Currents can make it difficult for divers to stay in one place, and they also move ROVs around and make bridge debris downright dangerous.
Temperature: The temperature of the water in Baltimore for this time of year is around 50 degrees F. This can make it difficult for divers to stay warm, even with good thermal protection (e.g. good drysuits).
Depth: Because diving beyond 2 atmospheres (66 ft) goes through air quickly, especially with exertion and low temperatures, this would be a very dangerous search environment. It’s too easy to get trapped and run out of air.
We’re not in Baltimore, but we’re grateful to these teams. They’re doing work we’re very familiar with, and that’s often a bit of a mystery to everyone else. Be safe, everyone.
