Posted on May 23, 2017
By Joel Hogue, Elastec
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.
Now, let’s look at coastal and open water booming. There are a number of forces along an ocean and other large bodies of water shorelines both planners and responders must be aware of. Each of these forces can significantly affect the behavior of spilled oil should it encounter the shoreline.
Surface Current: As the wind blows across the water, waves develop based on the wind speed, distance and duration. When the wind speed is fast and blows for a long time over a long distance, large waves can be expected; conversely, when the wind speed is fast for a short time period and distance, small waves can be expected. When the base of a wave encounters the sea floor near the shoreline, the waves break onto the shoreline.
Longshore Current: When waves break at an angle to the shoreline, they create longshore currents. These currents generally run parallel to the shoreline.
Rip Current: When breaking waves are prevented from flowing directly back into the ocean from an underwater structure such as a sandbar or pier, a rip current is generated. These currents generally run perpendicular to the shoreline and can be very strong and fast, but they last a relatively short period of time.
Upwelling: This occurs when the wind blows the surface water away and displaces it allowing deeper water to rise and replace what has been blown away.
Downwelling: This occurs when the wind blows the surface water toward a barrier such as the shoreline and forces the surface water to sink.
TIDAL CURRENT AND TIDES: Tidal currents (horizontal movement) are a result of the rise and fall of the water level due to tides (vertical movement). In coastal areas, there are two low tides and two high tides every lunar day, or 24 hours and 50 minutes.
Flood Flow: The tide is coming in and the water entering needs to overcome slow water to move forward toward the shoreline. The time this occurs will be different for different points based on their distance from the ocean as the tide comes in.
High Stand: The highest water level is reached at a specific point, but flood flow continues toward other points toward the shoreline (high tide).
High Slack Water: When High Stand is reached at all locations near the shoreline and the current stops.
Ebb Flow: The tide is going out and the water leaving needs to overcome slow water to move back toward the ocean. The time this occurs will be different for different points based on their distance from the ocean as the tide is going out.
Low Stand: The lowest water level is reached at a specific point, but ebb flow continues toward other points toward the ocean (low tide).
Low Slack Water: When Low Stand is reached at all locations near the shoreline and the current stops.
The sequence of tidal flow at any point along a shoreline varies between any two points at different distances from the ocean. Rivers flowing into the ocean slow the movement of tide into the river, causing the Flood Flow to be slower. Conversely, when flowing into the ocean, rivers at Ebb Flow may be significantly faster.
When deploying oil spill countermeasures, different tactics must be employed in order to effectively protect the shoreline and provide oil containment for recovery. Boom should never be deployed parallel to the shoreline and islands should never be encircled with boom. Encircling an island or other feature in the presence of current or wind will nearly always be ineffective; booming angles must be determined based on site current, wind and oil vectors. A sacrificial oil landing area on the shoreline must be identified as oil is excluded or diverted from sensitive areas; continuous monitoring is required based on the current, wind and oil vectors – simply deploying boom without monitoring and adjusting it over time is not an effective tactic.
As shown in the following photograph, the yellow line represents the correct exclusionary / diversionary boom angle against the Oil Vector and the black arrow represents the calculated Oil Vector based on current and wind. Using this tactic ensures effective booming angles and allows the current and/or wind to move oil past an island to an area for effective and efficient recovery. This is an effective tactic and requires significantly fewer resources to deploy and maintain. Remember, booming in open water is the same as booming a river – you boom against the Current, Wind or Oil Vector (note the failed boom and oil impact to the island).
There is much more to reading rivers and other bodies of water than presented here, but for planners and responders this simplified introduction will allow countermeasure deployment to be much more effective. It is important to understand the following when considering response options:
– Local responders (public and private) must know their specific response area – no amount of classroom training will teach you where to deploy boom, what to protect, etc.
– Look to future conditions as well as current conditions (upstream rain events, snow melt, shifting tides, etc.)
– What is debris load (certain channels will have a very high woody debris load where others will have very little or none)
– Anchor points and moorings must hold boom, current forces, oil and debris load
– Whether or not there is flow within a channel, no boom should be deployed at 90°, regardless of current speed. Doing so causes boom to “belly” in mid-stream where oil will collect, making collection difficult or impossible. Additionally, entrainment of oil is likely. By angling the boom into the current, the oil will move to the shoreline for collection
– Regardless of the calculated boom deployment angle or regardless of the Oil Escape Velocity used to determine the boom deployment angle, it does not guarantee entrainment won’t occur. Constant monitoring and adjustment of deployed boom is required based on individual site conditions as well as efficient recovery of contained oil as early as possible
– If the oil isn’t recoverable due to a poor containment site or entrainment, then any countermeasure deployed has been a waste of time, money and other resources. It’s likely that if appropriate and effective booming isn’t accomplished the first time, there won’t be enough time or equipment to deployment them correctly a second time
To understand more about other forces that effect water and how these forces impact oil spill countermeasures, ask about our other articles:
– Seiches and Their Effect on Non-Submerged Oils
– Oil Vector Determinations: Critical to Booming but Frequently Overlooked
Sources and References:
– USGS Topographic Map Symbols Booklet
– “Meandering River Channels,” Colorado Water Resources, Colorado State University
– “Fluvial and Submarine Morphodynamics of Laminar and Near-laminar Flows: A Synthesis,” Lajeunesse, et al, December 2009
– “Oil Spill Response in Fast Currents, A Field Guide”, U.S. Coast Guard, October, 2001
– U.S. Army Corp of Engineers, http://mvs-wc.mvs.usace.army.mil/arec/Basics_Weirs…
– “Barrier Failure by Critical Accumulation of Viscous Oil,” Gerald A.L. Delvigne, Delft Hydraulics, 1989
– The Lo
Source: Elastec
