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Hazard

Hydrology

Hazard Knowledge

The hazards created by water will vary greatly, depending on whether the water is static or flowing, rising or receding and its temperature, speed and depth. To properly assess an appropriate course of action personnel should understand water hazards and hydrology.

The force of water is directly related to the speed of the flow. Doubling the water speed will quadruple the force. Small volumes of water at sufficient velocity may be enough to cause responders to lose their footing. Standing water will exert an upward pressure against an object which may cause it to lift and a lateral pressure that can cause movement of structures, vehicles and people.

The local microgeography in a water environment can create radically different water movement and hydrological features over very short distances, even less than one metre. The risk to responders will need to be constantly reviewed on scene by the teams themselves, with autonomy of decision making devolved to team leaders where appropriate based on individual competence.

Moving water in a channel

It can be useful to consider a moving body of water as a series of connected layers rather than a single body. In flowing water, the layer in the middle of the channel generally moves fastest, the speed decreasing closer to the edge or bottom of the channel.

An object or person in moving water may be swept from the bank into the centre of the river. Water near the surface moves more quickly than water near the river bed. When a river curves, water on the outside of the curve will travel faster than water on the inside. Frictional resistance, obstructions and changes in direction will reduce the speed of flow. Moving water will behave similarly in any channel, although the scouring that increases depth and shapes bends will not have occurred in impromptu channels created by flooding. These features will increase the number of sudden turns the flow takes, slowing the flow but also creating turbulence and generating strong forces on any object in its path. People or responders caught in the flow may be subject to impact as the water collides with structures or objects in its path.

Moving water will erode underwater surfaces such as mud and stone. This erosion can be unseen, particularly where the substrate beneath the surface is softer than that above it. Areas that are subjected to continuously greater forces generated from the flow such as a waterfall or bend in a river are also more prone to erosion. Erosion or undercutting can make river banks unstable, collapsing when weight is applied. Sub-surface areas that have been subject to undercutting can also generate eddies that pull objects, casualties and responders into holes and gaps beneath the surface.

Recirculation

When water passes over a vertical drop it accelerates and then recirculates downstream of the drop. This can cause a casualty or object to be held by the recirculating water. The strength of the recirculation will vary depending on water levels, angle of descent and speed of flow.

Recirculations may create tow back, an area of water that moves back against the direction of the flow, pulling an object or casualty back towards the hazard.

As water travels over a drop and recirculates it becomes aerated, reducing its density and making swimming, paddling and the use of powered engines less effective. People can be caught in recirculations, either being flushed out further downstream or held below the surface. Whether they are held can depend upon the shape of the hazard, design of man-made structures and whether undercutting has occurred. It is also possible to be caught again by the tow back and recirculated. Recirculations that hold or pin casualties, responders or objects are often referred to as hydraulics, holes or stoppers. The release of air where the recirculation meets the main flow creates an area of aerated water that bubbles as if it were boiling. This ‘boil line’ may not always be present and its absence should not be assumed to indicate a lack of recirculation.

The aeration of water may affect the buoyancy provided by personal protective equipment (PPE) and buoyancy aids. Because the aeration of water reduces the density and makes swimming less effective, coupled with the strength of tow back, it may not be possible to swim out of a recirculation.

Eddies

Where flowing water passes static or slower moving water, it causes the area of static water to rotate in the opposite direction to the main flow. This recirculated water, or eddy, is slower than the main flow. The reduction in speed causes debris to be deposited, reducing water depth around eddies. Areas of slower flow and shallower water, such as those around eddies, can be an area of relative safety in the water.

Eddies form more frequently as water speed increases and may form behind obstacles in flowing water where channels increase in width suddenly and where a narrow channel of flowing water enters a wider, static body of water.

Tidal water

Tidal conditions are predictable and can be anticipated and prepared for. However, tidal water can rise quickly, isolating people and resources. This represents a significant hazard to those who are unprepared for tidal changes. Incidents involving tidal water have additional hazards from currents and waves. Some rivers, inlets and estuaries are also influenced by tides. The depth of water can change rapidly as the tide turns.

Care should be taken to avoid being cut off or isolated from egress routes. This may occur over a very short space of time, at least twice a day. Where ingress and egress of tidal water is restricted, for example around a tunnel, water levels can change dramatically and unexpected hazards may form quickly, including recirculations. If teams are made available for mutual aid deployments out of their area, they should be trained and equipped to operate in all foreseeable water environments including tidal waters.

Flood water

Flood water should be considered in a similar way to moving water. The principles of operating in moving water apply even when the conditions appear to be still. Similar to tidal conditions, water levels can rise rapidly during periods of excessive inundation and flood.

Obstructions in the water

Rocks or other debris below the water surface or partially submerged may pose entrapment hazards to personnel or entangle rescuers’ lines. This is particularly hazardous in flowing water where the force of water may also cause a loss of balance. Poor water clarity will make it difficult to identify sub-surface objects.

Debris suspended in water: Objects such as branches, rubbish or other suspended debris may entangle or otherwise harm casualties and personnel.

Pins: A risk of pinning exists where fast-moving water flows against a solid object such as a bridge pillar. Most objects will tend to be flushed around the obstacle, but a person or boat could be pinned with considerable force.

Strainers: Anything that allows water to pass through but traps solid objects. They can include tree roots, fences, gates, and cars. A person or object may be drawn against the strainer and trapped by the force of water passing through it.

Siphons: A sub-surface gap or hole in a barrier or structure that allows water to flow through. Strong currents can pull objects, including people, down towards the siphon.

Undercuts: Areas where the bank has been worn or cut away by the movement of water below the surface. Often found on the outside of a bend where the current has worn away the bank beneath the surface of the water or around recirculations.

Flooded environments will create entrapment hazards that may not be expected such as displaced drain covers and or submerged street furniture.