Water Maths at the Barn Fire
You are incident commander at a barn fire 6 km from the nearest hydrant. Your pump carries 1,800 litres and is fed by a water tender holding 9,000 litres. Each attack line flows 250 litres per minute, and you should allow 15% extra for losses through hose and fittings. The next appliance able to shuttle is 20 minutes away; once it arrives, a full round trip from the fire to the fill point and back takes 22 minutes, delivering 9,000 litres per trip. The barn holds 200 tonnes of baled straw and a tractor with a full 180-litre diesel tank. A stone cottage stands 8 m downwind, occupied by two adults, one of whom uses a walking frame.
1. Calculate how long the on-scene water will last with one line and with two lines, and say what the figures tell you.
Read the worked answer
Water available: 9,000 + 1,800 = 10,800 litres.
One line: 250 × 1.15 = 287.5 L/min → 10,800 ÷ 287.5 = 37.6 minutes (about 37 min 34 s).
Two lines: 500 × 1.15 = 575 L/min → 10,800 ÷ 575 = 18.8 minutes (about 18 min 47 s).
What the numbers mean: the second appliance is 20 minutes away. Two lines empty every drop before the first shuttle load can possibly arrive, so a two-line attack commits me to running dry at the exact moment the fire is least controlled - a worst-case position, because a pump with no water and hose full of air has to be re-primed while the fire grows. One line leaves roughly 17 minutes of buffer after the shuttle arrives and keeps a reserve for the cottage exposure. The arithmetic, not the fire's appearance, decides the number of lines.
2. How many tenders must be in the shuttle to sustain one line and to sustain two? Show your working and note what changes before the second appliance arrives.
Read the worked answer
Delivery per tender in the shuttle: 9,000 L ÷ 22 min = 409 L/min.
One line needs 287.5 L/min. One tender delivers 409 L/min, which covers it with 121 L/min spare. One tender in the shuttle is sufficient for one line; the on-scene tender then acts as the buffer tank and stays full.
Two lines need 575 L/min. 575 ÷ 409 = 1.41, so two tenders in the shuttle (818 L/min) are needed, plus the tender on scene as buffer - three appliances in total.
Before minute 20: there is no shuttle at all. Only the 10,800 litres on scene exist, so the sustainable flow is effectively zero after 18.8 minutes on two lines. That gap is the decisive constraint: any plan relying on two lines is a plan to be out of water before the first refill, and the commander should say so out loud rather than discover it at minute 19.
3. Choose a tactical plan on these numbers and state what you would tell the two adults in the cottage.
Read the worked answer
Plan: one attack line, positioned to protect the cottage and the tractor, not to extinguish 200 tonnes of straw.
Reasons: (1) straw in bales burns deep-seated and will not be extinguished by one line at 287 L/min - water on the surface is wasted; (2) the cottage is 8 m downwind with two occupants, one with limited mobility, so the exposure is the life risk and it is protectable; (3) one line is the only flow the water supply can actually sustain, and running dry to look aggressive is not aggression, it is theatre.
Actions: one line cooling the cottage wall and roof space, request the second tender before committing to any second line, treat the diesel tank as a hazardous exposure and keep crews upwind, and plan for the straw to be left burning under control rather than fought frontally.
What I tell the occupants: plainly, that the fire is in the barn, that the wind is carrying smoke and embers towards the house, and that they should leave now with medication, keys and the walking frame - not because the house is on fire, but because leaving later will be harder. I offer assistance and record their decision if they decline.