A flood wall or berm blocks flood water from a building or garden by standing between them and the water. Earth berms are the simplest, gabion walls suit banks and tight spots, and sandbags are the emergency option. Build the barrier 0.5-1 m above the highest flood level you expect, because water pushes on a wall with a force that grows with the square of its depth: about 4.9 kN on every metre of wall at 1 m deep, and four times that at 2 m.
Part of Flood Control and Drainage
A flood barrier is a raised wall or bank that stops floodwater from reaching what lies behind it. The choice among earth, stone-filled wire and sandbags depends on how long it must last.
Water Pushes Harder Than It Looks
Standing water presses on a wall at every point, harder the deeper the point. The total push on a wall of depth h, for each metre of its length, is 1/2 x density x gravity x h squared. For water, density 1,000 kg/m3 and gravity 9.81 m/s2:
- At 1 m: 0.5 x 1,000 x 9.81 x 1^2 = 4,905 N, about 4.9 kN per metre of wall.
- At 2 m: 0.5 x 1,000 x 9.81 x 2^2 = 19,620 N, about 19.6 kN per metre.
Doubling the water depth quadruples the push, making it four times as large (19.6 / 4.9 = 4). A flood barrier must resist this and also keep water from soaking through or flowing around the ends.
Your Three Options
- Earth berms — a compacted mound of earth along the edge of a flood-prone area. Cheap and durable. See Earth Berms.
- Gabion walls — wire cages filled with rock. They allow slow seepage, flex with the ground and last long; excellent for bank stabilization and tight spaces. See Gabion Walls.
- Sandbags — a temporary wall for an imminent flood. See Sandbag Deployment.
What You Need
- The highest flood level you expect — from Historical Flood Marks.
- Earth, rock or bags — and labour to place them.
- A drain plan — rain that falls behind the barrier has to go somewhere.
Method: Plan A Barrier
Step 1: Set the height. Build 0.5-1 m above the highest expected flood level. That margin is the only protection against a flood bigger than the last.
Step 2: Choose the material. Earth for a long, lasting barrier on open ground. Gabions where space is tight or a bank needs holding. Sandbags only for emergencies.
Step 3: Close the ends. A wall with open ends lets water flow around it. Tie the ends into rising ground.
Step 4: Plan the inside drainage. Water that falls or seeps behind a barrier cannot escape on its own. Provide a drain with a way to close it during flood.
Step 5: Protect the surface. Seed earth with grass immediately. Rock or stone protects the water side.
Common Mistakes
- Building to the flood level, not above it — no margin means overtopping.
- Leaving open ends — water goes round.
- No inside drainage — rain and seepage fill the dry side behind the barrier.
- Bare earth — it erodes quickly.
- Assuming a barrier is permanent — inspect it every season, and after every flood.
Safety
Barriers fail suddenly, and people drown in the breach
A flood barrier can fail with little warning by overtopping, by water soaking through the base or by the base sliding, and the water behind it then comes through all at once. Never stand or shelter on the dry side of a barrier with water high against the other side. Never walk along the crest in a flood, since the surface can give way. About 15 cm of fast-flowing water can knock an adult off their feet. Move people and stock away from the line before the water arrives.
What’s Next
- Learn next: Earth Berms — the simplest permanent barrier
- Learn next: Sandbag Deployment — an emergency barrier
- Combine with: Flood Planning — design to the highest recorded flood