Hazard Insufficient preparation for flooding
Floods and flood waters can present significant hazards to responders and communities. Floods are often predictable and fire and rescue services should use weather and tide forecasting to inform their response to incidents. This may not always be possible as some types of flood may occur with little or no warning. Fire and rescue services should identify the potential causes of flooding in their area and plan accordingly.
Sustained heavy rainfall, melting snow or ice can cause heavy run-off, causing water to rise above or break the banks of a river. Fast flows and increased hydrostatic pressure can overcome flood defences causing flooding in areas where it had not been anticipated by forecasting agencies.
Ground conditions in catchment areas can affect the likelihood of flooding. For example, areas with clay soil or developed areas with paved or hard surfaces will not absorb water, increasing the risk of flooding regardless of conditions prior to rainfall.
Rural catchment areas or those with more permeable soil will allow a certain portion of water run-off to be absorbed, decreasing peak flow into channels and absorbing water from flooded channels. This reduces the likelihood of flooding, but sustained periods of rainfall will cause the ground to become saturated and increase the likelihood of flooding.
Flood plains are generally low-lying sections of land that can be flooded deliberately to prevent flooding in other areas. This may be achieved naturally or by reversing locks and activating water management systems. However, even unpopulated flood plains may be used as grazing land or contain informally established temporary dwellings such as tents or caravans.
Obstructions and breaches
Blockages in channels or drainage systems may result in flooding. This can include major flooding caused by landslides or a build-up of debris in channels. Blockages in channels are often linked to narrowing of channels around man-made structures such as bridges or water management systems. More localised events where blockage of drainage systems can cause flooding of properties in the immediate vicinity.
Breaches in flood defences can result in the creation of fast moving and destructive flows of water. As the size of the breach increases the pressure and speed of flow will reduce. Depending on the size of the affected channel or body of water resulting flooding may be severe. Although regular inspections and reports on the condition of flood defences are completed the location, size and number of breaches are difficult to predict.
Breaches or failures of dams can result in significant and unexpected flooding.
Storms increase the tidal range beyond their normal levels. Low pressure and high winds increase the size of waves and the water level of bodies of water. If wind direction, atmospheric conditions and high tides align it can result in major flooding across a significant area and cause river flooding upstream. The damage caused by storm surges can be extreme. They may affect several areas and possibly countries at once and have a long-lasting impact, stretching resources and requiring a longer-term response.
Burst water mains, failures of water management systems and breaches in flood defences can cause unexpected flooding. The severity and scale will be dictated by the size of the leak and how much water is contained. Most leakages will be localised but the effects on residents of affected properties may be severe. Leakages of foul water systems may require responders to consider the risk of contamination.
Floods can create fast-moving waters, static or semi-static conditions. Because of the additional risks presented by floods, all flood water should be treated as moving water.
Ground water flooding will often be localised, occur quickly and may recede quickly after weather conditions change.
Flash floods rise quickly and often recede relatively rapidly, but can create dangerous conditions for responders as flow rates are likely to be higher and conditions will alter rapidly.
Slow moving or semi-static flooding will provide a greater lead time to response.
The rate at which flood water recedes will vary and depend on ground conditions, drainage and weather conditions in the local area. Upstream and downstream conditions will also affect the duration and effects of a flood. Pooling may occur in dips or low areas and in some cases these areas may require pumping out.
Most flood types can be modelled and forecasters should be able to provide accurate scenarios for likely development of conditions.
Hazards related to flooding
Floods are moving incidents that follow physical rather than administrative boundaries such as areas represented by resilience forums. They are often long duration, multi-agency events.
Hazards in flooded areas can include fast flowing water, entrapment hazards, displaced street furniture, contaminated water, flood waters affecting electrical facilities or water treatment plants, limited access and rising waters affecting egress.
Assessing the quality of flood water requires professional assistance. Without confirmation from a responsible body, flood water should be treated as contaminated as it may contain biohazards and hazardous materials. Flood water may also affect biosecurity as non-native species can move freely.
See National Operational Guidance: Environmental Protection – Biosecurity.
Working durations during flood events may be extended and risks to personnel include fatigue and physiological stress.
See National Operational Guidance: Operations – Physiological stress
Displaced or submerged street furniture and waterborne objects can present a range of hazards to responders. Drain covers may be lifted, creating trip hazards and the risk of falling into an obscured hole. Waterborne objects may affect responders; hydrostatic pressure may lift large objects like rocks, trees or vehicles. Railings and other semipermeable objects will become strainers when submerged presenting a risk of limb entrapment.
The following legislation and guidance is applicable to flood incidents and relevant to emergency planners and responders:
- Control measurePrepare for flooding