Personnel should be aware that people in a water environment may not remain static. It is important for the agencies in attendance to quickly limit the scope of the search by identifying and placing controls around the places where a person may be identified. The water entry point (WEP), point last seen (PLS) or last known position (LKP) can be used as the initial planning point (IPP), if they are the same. However, more recent sightings will update the PLS and can be used to update a tactical plan. The time of these sightings will help to establish the starting point of a search; this information should be established and recorded.
Once the WEP, PLS or LKP times are known, a judgement should be made on the movement of water between then and the start of the search, to ascertain how far a person may have travelled.
Improved situational awareness will help to clearly define the search area. This can include:
- Information on the physical environment, such as:
- Water flow
- Tide times
- Visual information, such as footage from drones
- Requesting resources that may assist in defining the area of search should be considered at the initial stages; for more information refer to Specialist resources: Water search and rescue
In channels with a directional flow, resources should be sent to a point beyond this to establish a downstream safety system. To establish the area to be searched and prevent any people in water from travelling beyond this point, the speed of flow should be estimated.
When searching in water, it may be necessary to determine a search box. This defines the likely area that the person will be in, based on factors such as:
- Riverbanks
- Physical barriers
- Water speed
When calculating the size of the search box, it should be assumed that the person is travelling where the water is at its fastest, usually the centre of the channel. It is then feasible to calculate the possible distance travelled, by multiplying the estimated speed of water by the time that has passed since entry.
This calculation should include any time required to reach the point downstream to close off the search box. For example:
- A river that is flowing at 1m per second (60m per minute) will in 30 minutes potentially move the person in water 1800m from the water entry point (WEP)
- If the travel time for personnel to the WEP is 10 minutes, the person in water will have travelled a further 600m
- Therefore, the total distance to close off the search box is 2400m
Physical barriers
Physical barriers should be considered as a useful tool in identifying a search box; in the water environment, these may be features such as:
- A strainer feature that will not let a person pass through it
- Water process features, such as pumping stations
- Shallow areas of flow that would not support the movement of a person
These features should be considered on a case-by-case basis, with the potential for the person to bypass them assessed.