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Hydrological hazards

Hazard Knowledge

Hydrology is the science concerned with the properties of water, and especially its movement in relation to land. The study of water movement and its behaviour relating to the topography, allows personnel to assess the hydrological hazards that can exist in moving or static water.

This website provides some descriptions and diagrams of hydrological hazards featured in this guidance. Examples include:

  • River characteristics
  • Water flow
  • Terminology
  • River orientation
  • Water hazards, including:
    • Strainers
    • Foot entrapments
    • Recirculations
    • Water force
    • Undercuts
    • Siphons

The hazards of water will vary greatly, depending on whether the water is static or flowing, rising or receding, 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 and volume of the flow; doubling the water speed will quadruple the force. Small volumes of water at sufficient velocity may be enough to cause personnel 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. For more information relating to vehicles in water refer to Person in a road vehicle in water.

Micro geography in a water environment can create radically different water movement and hydrological features over very short distances, even less than 1m. The risks to personnel will need to continuously be reviewed by the teams, with team leaders being responsible for decision-making, based on the level of competency of their team members.

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, with the speed decreasing closer to the edge or bottom of the channel.

People or personnel caught in the flow may be subject to impact, as the water collides with structures or objects in its path. The noise of moving water can affect communication between people, whether in the water or on land.


When water passes over a vertical drop it accelerates and then recirculates downstream of the drop. This can cause a person 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, which can be both vertically and horizontally, pulling an object or person back towards the hazard. They can aerate the water, which will affect the buoyancy provided by personal protective equipment (PPE) and buoyancy aids. Because the aeration of water reduces the density and makes swimming, boat paddles and propellers less effective, coupled with the strength of the water, it may not be possible to swim, row or navigate a boat out of a recirculation hydrological feature.

Boil line

This is a line of boiling or bubbling water that delineates the water going downstream from the water that is flowing back towards a hydraulic. Usually, the further away a boil line is from the hydraulic, the more hazardous the feature.


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.

Undercut riverbanks

Moving water will erode underwater materials, 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 riverbanks unstable, collapsing when a load is applied. Underwater areas that have been subject to undercutting can also generate eddies that pull objects, people  and personnel into holes and gaps beneath the surface.

Inland waterways

An inland waterway can be termed as any body of water that is not coastal. Artificial waterways may incorporate mechanisms that could present additional hazards.

People who have fallen into an inland waterway may become involved with the features present, such as sluice gates, locks, and debris screens. These features can produce the same hydrological hazards experienced in a natural river environment and the operation of such mechanisms may result in injury or death. For more information refer to On-site machinery: Water management systems.


Floodwater should be considered in a similar way to moving water when considering a water rescue. The principles of operating in moving water apply, even when the conditions appear to be still. As with tidal conditions, water levels can rise rapidly.

An additional hazard is that floodwater is likely to be contaminated by sewage, fuel and other substances. For more information refer to Geophysical hazards.

Flooded built environments will create entrapment hazards that may not be expected, such as displaced drain covers or submerged street furniture. These items can create hydrological hazards, like those found in the natural environment.

Tidal water

Tidal conditions are usually predictable, which can be anticipated and prepared for. However, tidal water can rise quickly, isolating people and resources, and the depth of water can change rapidly as the tide turns. 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 influenced by tides.

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 may be 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.

Sea foam

Sea foam is a common natural occurrence, along the coastline in small quantities, which may result in a fire and rescue service responding to a coastal rescue.

On rare occasions it can accumulate in very large quantities, often due to wind, water currents and waves pushing the foam towards land features that trap it, for example, coves, gullies, and harbour walls. It has been recorded that foam can reach up to 3m in depth.

There are specific circumstances where it can present significant risks. It is often difficult to assess the level of risk, as the composition of the foam is unknown in the early stages of the incident. This could include health hazards, such as infectious diseases. For more information refer to Operations – Infectious diseases.

The foam may present risks including:

  • Low buoyancy compared to water
  • People and rescuers will sink through it
  • Powered rescue craft will have issues with buoyancy and oxygen starvation to engines
  • Restricted visibility in the foam, making it difficult to estimate its depth or see submerged obstacles or hazards
  • A lack of visual reference, which increases the risk of slips, trips and falls

Any dynamic wave action or currents below the surface of the foam will be ‘dampened’; the surface of the foam may be static while there is water movement below it.

Obstructions in the water

In a moving body of water, hazardous debris and materials including large objects can affect personnel or compromise safe systems of work. Debris may be on the surface, suspended in the water or rolling along the bottom.

Rocks or other debris, such as branches or rubbish, underwater or partially submerged, may present entrapment or entanglement hazards. 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 obstructions in the water.


Fire and rescue services can affect biosecurity if facilitating the transfer of material from one open water source to another. This could be as a result of equipment, vehicles or PPE being contaminated while carrying out a water rescue.

Unless otherwise confirmed by a responsible person, bodies of water should be treated as if they are contaminated, as they may contain biological hazards. This could include harmful substances, such as sewage or industrial chemicals. Bodies of water may also contain invasive species.

For more information refer to Environmental protection – Biosecurity.