Hydraulics & drainage · Flooding & scour
Flood freeboard, hazard and loads on a building calculator
For a building site in a flood-prone area: depth and depth × velocity, the ARR 2019 combined hazard category (H1–H6), the minimum habitable floor level (defined flood level + freeboard) against the proposed level, and the flood actions on a wall (hydrostatic, hydrodynamic drag and debris impact) per the ABCB flood standard.
Standard: ARR 2019 Book 6 Ch 7 hazard categories H1–H6 · ABCB Construction of Buildings in Flood Hazard Areas (2012) · local planning freeboard
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Defined flood level (e.g. 1 % AEP) | 4.2 | m AHD |
| Natural ground level | 3.1 | m AHD |
| Proposed habitable floor level | 4.8 | m AHD |
| Required freeboard | 0.5 | m |
| Flood velocity at the site | 0.5 | m/s |
| Wall width facing the flow | 10 | m |
| Drag coefficient | 2 | |
| Debris mass | 1000 | kg |
| Debris impact duration | 1 | s |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Flood depth at the site | 1.10 | m |
| Depth × velocity | 0.55 | m²/s |
| ARR 2019 hazard category | H3: unsafe for vehicles, children and the elderly | |
| Minimum habitable floor level | 4.70 | m AHD |
| Margin above the minimum | 0.10 | m |
| Hydrostatic force on the wall | 59 | kN |
| Hydrodynamic drag force | 2.8 | kN |
| Debris impact force | 0.5 | kN |
- ✓ Floor level ≥ DFL + freeboard — 0.10 m
- ✗ Hazard ≤ H2 (safe access for small vehicles excluded) — H3
- ✓ Hazard below H5 (ordinary construction acceptable) — H3
Method, formulas and assumptions
- ARR 2019 combined hazard curves (Book 6 Fig 6.7.9): H1 D·V ≤ 0.3, D ≤ 0.3, V ≤ 2; H2 D·V ≤ 0.6, D ≤ 0.5; H3 D·V ≤ 0.6, D ≤ 1.2; H4 D·V ≤ 1.0, D ≤ 2; H5 D·V ≤ 4, D ≤ 4, V ≤ 4; H6 otherwise.
- Hydrostatic Fh = ½γw·d²·B; hydrodynamic Fd = ½ρ·Cd·V²·d·B; debris impact F = m·V/Δt (ABCB flood standard).
- Freeboard is set by the local planning scheme (commonly 300–500 mm for habitable floors).
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.