YARA PILBARA NITRATES – DRAIN EXTENSION
Ground Civil Pty Ltd (GC) has been engaged by KW Civil & Construction (KW Civil) to design existing drain extension
and culvert crossing at Yara Pilbara Nitrate site, Burrup.
Manning’s Equation was used to determine the flow rate with the following assumptions:
- Steady Flow – flow variables remain constant over time
- Uniform Flow – flow variables remain the same across every part of the channel.
- Voids in the rocks are excluded from calculation as it is a standard engineering practice to design channel
- without the rocks, having the rock protection boxed out.
- Drains were constructed as per the IFC Drawings, no construction deviation.
- Rock size within the drain was not provided. It is estimated in Section 2.2.
- It is assumed that the channel is designed to be flowing full (no freeboard).
Channel’s Manning’s roughness (n) will greatly influence the flow rate calculated. The document “Use of Rock in
Waterway Engineering” (2020) provided guidance on Manning’s roughness based on d50 & d90 rock size parameter.
The d50 & d90 being rock size for which 50% and 90% of rocks are smaller.
Based on the calculations, the following are observed.
- Channel capacity variation across diffrent Sections.
- Diffrent channel section for flatter parts.
- Diffrent Channel cross section to transition to daylight.
- Based on the above, a design flow rate considered appropriately for the system.
