Note: this information is a general guide, and local conditions need to be assessed before designing surface water management structures. Managers should use specialist advice for their site.
Where grass cover is very good and suitable groundcover can be maintained, water velocities can exceed 1.5 metres per second (m/s). See Table 1.
For flows over easily eroded soils, whether they are bare or vegetated, reduce velocities by 20% (that is, multiply suggested velocity by 0.80).
For flows on slopes greater than 5%, reduce velocities by 15% (that is, multiply suggested velocity by 0.85).
| Maximum flow velocity (m/s) | Maximum flow velocity (m/s) | Maximum flow velocity (m/s) | |
|---|---|---|---|
| Material | Bare to light grass cover1 | Medium grass cover | Very good grass cover |
| Sand | 0.4 | 0.7 | 1.2 |
| Sandy loam | 0.6 | 1.2 | 1.5 |
| Clay loam | 0.75 | 1.3 | 1.8 |
| Medium to heavy clay | 1.2 | 1.4 | 2.0 |
| Clay loam/coarse gravel mix | 1.2 | 1.4 | not applicable2 |
1Grass cover is defined as:
- bare to light grass cover — bare soil or poor annual grasses; use for temporary waterways
- medium grass cover — annual grass and clover/medic mix with height and groundcover maintained; use under average Western Australian grainbelt conditions
- very good grass cover — annual and perennial grass mix in higher rainfall areas with complete groundcover; use for high rainfall areas (more than 600mm annual rainfall) of the south-west of Western Australia.
2Coarse gravel is less than 60mm in diameter. A clay loam and coarse gravel mix is unlikely to form very good grass cover.