Much has been written (incorrectly) about how to deploy boom in order to protect, deflect or contain oil. We have long promoted using math and science when deploying spill countermeasures not only because it works, but because it’s faster, safer, more effective and it saves time, money and impact. What we’ll focus on in this article is the where of boom deployment.
When it comes to understanding where to correctly deploy boom during an oil spill incident, it’s important to understand that booming locations are a tradeoff – some of the best booming sites may be inaccessible while some of the most accessible sites may be poor booming locations. Additionally, understanding the relationship booming has to oil skimming and recovery is critical to the success of both booming and recovery. The foundation of oil recovery with a skimmer begins with effective and appropriate booming; without this, oil recovery will be challenging at best and nearly impossible at worse.
Knowing where to deploy boom requires an understanding of two additional sciences; namely, hydrology and topography. In simple terms, hydrology is the science of the movement of water whereas topography is the science and description of earth landforms (both natural and man-made) including, mountains, hills, valleys, ponds, lakes, rivers, streams, irrigation ditches, swamps, other hydrographic features, roads, trails, railroads, buildings and much more.
When determining where to deploy boom, whether reading rivers or understanding currents and wind effects in coastal and open water environments it is important to think of water as energy. Flowing water travels downhill due to gravity. As it travels, because of its mass, it seeks to maintain its velocity unless it is acted upon by such things as the channel bottom and banks, obstructions such as rocks or plants, curves and bends in the channel, etc. Based on these forces, when reading rivers or other water flowing channels you can expect the following:
- Water flows in path of least resistance
- Energy is concentrated on outside bends
- Obstructions redirect flow to less resistance
- Velocity is a function of resistance (banks, bottom and obstructions)
- Narrow points accelerate flow
- Shallow areas accelerate flow
- Immediately after shallow water is typically turbulent
- Immediately before shallow water velocity accelerates
When determining where to deploy countermeasures remember, for containment booming the goal is to minimize the impact area and prepare for effective and efficient recovery. This requires effective and appropriate booming tactics and techniques in order to:
- Keep oil away from (exclusion) poor recovery sites or from sensitive locations;
- Move oil (deflection) to a suitable collection site; and
- Collect and concentrate the oil (containment) for recovery.
Responders must understand that booming and recovery tactics and techniques are closely interrelated. If appropriate and effective booming is not implemented in the proper environment, it is likely that oil recovery will not be effective, efficient or in some cases, possible.
Major river environments that influence effective booming include:
- Riffles: Rocks that penetrate the surface of the water and are generally found in shallow areas
- Runs: Deeper areas of swift current, usually just upstream or just downstream of a riffle
- Pools: Areas of relatively still water between riffles and runs
- Outside of Bend: Where erosion typically occurs; water is typically deeper and faster
- Inside of Bend: Where deposition typically occurs; water is typically shallower and slower
The following drawing shows the cross section of a channel and displays the water velocity throughout a channel. Note how the channel bottom and banks slow the water due to increased resistance. This understanding is important when dealing with submerged oils as well as floating oil. Submerged oils will generally travel slower than the main water flow due to this increased resistance. Floating oils will be more concentrated in the center of the channel due to its increased velocity. Furthermore, this demonstrates beyond the physics of oil entrainment why responders must angle deployed boom correctly – using a “U” configuration (which is common but incorrect) will actually intensify oil entrainment; whereas directing the oil to the shoreline for recovery will slow the relative velocity minimizing or eliminating oil entrainment. Just as the appropriate calculation can give you the effective tactic, knowing and understanding the science of where to boom effectively can also provide the effective tactic.
When water within a channel flows at the same speed, little effect would be noted; however, since there is friction caused by the bottom and sides of the channel the water velocity is different in different areas of the channel allowing centrifugal forces to come into play.
Simply stated, centrifugal force is an object in motion will likely continue in a straight line – in this case water in a channel being forced to the outside of the channel. Floating materials (i.e. oil) will be forced with the surface water to the outside of the channel. Additionally, circulatory currents within the channel will also keep the oil directed to the outside of the channel (in a spiraling or screw type manner.) Finally, water will accelerate around a bend or obstruction to maintain constant velocity because the water on the outside of the curve must travel a greater distance than the water on the inside of the curve to maintain flow within the channel.
In addition to shallow, fast moving water (riffles and runs), the outside of a bend and inaccessible locations, poor booming / collection sites include:
Eddies: Eddies are created when water flows past an island or other obstruction and causes a general swirling or reverse current downstream of the island or obstruction. Eddies are often times found too far from the shoreline making recovery and booming difficult, if not impossible.
Slough: This is a wetland, or more often a swamp or shallow lake backwater to a larger body of water that is often stagnant or may flow more slowly than the main channel. Slough habitats generally contain more plant and animal species vulnerable to oil impact and should be boomed to exclude oil impact.
Channel Confluence: Channel confluences may have increased flow and velocity, unpredictable currents such as eddies and debris or other confluence structures. Additionally, these sites rarely require booming since side channel flows will prevent oil from entering the side channel. A short section of deflection boom may be placed upstream in the main channel ensuring oil does not get hung up in eddies, debris or other confluence structures. Oftentimes, many resources (equipment, personnel, time, money, etc.) are wasted on side channel protection.
The following photograph is an example of not properly using the river’s energy for boom deployment in order to deflect oil from the outside of the channel to a suitable collection point in a calm area downstream. Note how the boom is deployed within the fastest part of the channel (the outside curve – red star), causing the oil to accelerate through the river curve; this, along with an inappropriate boom angle is causing the oil to entrain (red arrow).