Hydrology of Currents: What Public Safety Divers Should Know
by Darrell Adams:
Some of the most challenging environments a public safety diver (PSD) can encounter are ones that deal with the forces of Mother Nature. At times she can be so unpredictable. However, with proper training, pre-planning, hazard/risk assessment, and incident evaluations, PSDs may be able to alter the tides of the unknown to establish a strategically planned operation. This operation would create a mitigation plan for problematic factors that promotes a greater margin for success and safety. In the PSD arena we all know that there are many factores that cause unique problems, but one of the most challenging and dangerous ones are those that deal with currents. Whether it involves the rivers of the Appalachian Mountains, canals of the metropolis areas in California, or the surfs of the New England coast line, teams responding to these environments need to understand the risk related to operations pertaining to “current hydrology.”
When evaluating PSD operational environments with regard to currents, the determination of the “mode of the response” should be part of the primary assessment. Will the team’s efforts be channeled into a rescue mode or that of a recovery? We can all agree that when operating in recovery mode, the time of recovery may not be as crucial as it would be in a rescue. As mentioned before, there are many different environments in which PSDs are likely to encounter currents: rivers and streams, canals, floods waters, surf, open oceans, and river deltas just to name a few. Each of these different environments possesses unique current hydrology hazards that need to be recognized and for which mitigation efforts must be implemented. In-depth understanding of any such hazards found within a team’s jurisdiction requires the team to spend a considerable amount of time educating members and getting plenty of water time to develop the skill sets needed to operate more efficiently while fostering reduced risks through effective hazard mitigation efforts.
Moving water is a very powerful force. There are several things that go into factoring the power of moving water like: speed, volume, bottom contours/gradient, tides, and winds. The current hydrology characteristics of rivers, streams, and canals deal with the volume of the waters flowing, the speed, and the bottom/side topography. The speed of the current when increased has a direct correlation to its force. For example, when the speed of a moving body of water is doubled, the force of that water imposed on the diver or object in the water is quadrupled. So waters moving at 2.5 knots can impose about 34 pounds of force on a diver, but when the water speed increases to 5 knots it can impose approximately 136 pounds of force on the same diver. The speed of moving waters in a river, stream, canal, or flood scenario are in direct relation to the size of the passages the water is flowing through and the effects of gravity based on the gradient at which the water is moving. The current speed of a section of moving water can be estimated by timing an objects passage between two points and referencing a chart which may be expressed in feet per second (fps) or meters per second (mps). With the speed now factored, consider the volume of water that is moving. This can be sometimes calculated or derived from monitored sources like USGS for which cell phone applications have been developed. Water volume is typically expressed in cubic feet/second (cfs) or cubic meter/second (cms). So let us imagine we have a stream bed that is 20 feet wide and 15 feet in depth, and water is flowing at 5 fps. That would come up to 1500 cft. of water moving by every second. Since a cft of fresh waters weighs 62.4 lbs., let us imagine about 1500 basketballs flowing by every second that weigh 62.4 lbs. What kind of power that would be?
This force of water can cause complications in regard to operations in certain environments. The act of swimming, walking, or diving into a current can be very dangerous. There are mainly three options when determining the dive platform: shore based, boat based, or bottom based. In a river-type situation, shore based searches in currents may result in a diver’s movement being unpredictable and search pattern management may become unreliable due to the speed a diver or swimmer is moving with the current. The task loading of swimming against a current may also result in a loss of focus and result in increased air consumption rates. Tender operations may become problematic because of issues with line management and the effectiveness in location accuracy as a result of line deflection. Therefore, operations on a boat anchored by a static highline or anchor, and bottom based searches tend to result in a more effective operation. These platforms tend to address some of the previously mentioned issues and establish something more manageable. One issue that may arise in these operations is the planning of a tethered diver. Variables relating to this would include the speed of the water’s movement, the length of tethered line, and the weight of the diver. Line length and weight may be adjusted to achieve effective search positioning. PSDs have to be cautious in over-weighting along with the streamlining of one’s gear. Another problem is object drift. Depending on force, bottom composition/contour and debris, the item(s) for which a diver is searching may move or be covered up, making item discovery more tedious.
Now if we throw in the other hazards associated with these environments like hydraulics caused by objects in the pathway of water flow that may be permanent. These objects may include but are not limited to rocks and boulders, low head dams, ditches on flooded roadways where water is moving across, and/or movable objects like vehicles in a river. Other hazards can be loads (objects in motion in the water column), strainers and sweepers (things in which divers or objects can become entrapped that are on the surface and below the surface of the water), pollution, and entrapments/entanglements. With so much to consider in regard to the hazard and risk assessment for operations in these environments, one can understand the need for proper equipment selection and training in many different technical rescue disciplines. It is highly encouraged that teams operating in moving waters like rivers and streams be trained in technical rope rescue applications, surface water operations for moving water, and small boat operations before commencing operations in these types of moving water environments. ERDI has training programs that can assist teams with achieving these goals.
PSD operations in ocean environments offer their own unique problems as well. Teams that are conducting rescues and recoveries in surf/open ocean and river delta environments may be dealing with a variety of current hydrology issues that if not considered, may complicate or jeopardize the safety of team members. Some of the ocean forces to be addressed are tidal currents, longshore currents, rip currents, undertows, surges, and waves. The problems posed by these forces can be lessened by pre-planning dive operations with a team’s jurisdictional support resources and an effective incident size-up. Also, by training in these environments teams will have a better clarity in trying to determine the “point last seen” (PLS) of victims and/or objects being recovered. These types of environments are always changing and can be problematic for operations that span a timeline across these changes. For example, the bottom topography in a surf zone is always changing due to the waves, surges, and tides and that may tend to cover objects on the bottom. Situations such as this may make it more difficult to locate or even move the object(s) from their original PLS. Some of the same issues with searching in river type incidents also may apply to ocean-based scenarios like pattern management, air consumption, weighting, streamlining of gear, and equipment selection.
Public safety divers have many factors with which they must contend in regard to the performance of standard operational activities. The number one job for any PSD is to get home at the end of the day. For this reason, risk assessment and hazard mitigation do take center stage in the operational game plan. With regard to “current hydrology,” individuals need to remember that there are no walls in the ocean to hold on to when you get tired, and you cannot call a time out when problems arise. Bret Gilliam stated once, “He who prepares and anticipates his adversary, whether narcosis or the great white shark, will handle the situation well. And likewise, he who hesitates… is lunch!” Therefore, as PSDs, the key to success involves training, to train again, and to train some more. This cannot be further from the truth when dealing with currents. ERDI programs offer building blocks for success for teams dealing with moving waters as well as other hazards.
– Darrell Adams
SDI/TDI/ERDI Instructor Trainer with Air Hogs Scuba in Garner, NC, Captain with the Harnett County Underwater Search & Recovery Dive Team and Technical Rescue Instructor for NC Fire and Rescue Commission