Drainage & Overland flow

Drainage modelling is an approximation of how systems perform under different rainfall scenarios. We analyse 5–10 year minor events against the drainage network, then address discharge gaps in major events by directing flows through intersections, rivers, or detention basins. Sag pits are integrated into larger stormwater systems to avoid localised flooding. We help councils strengthen networks, close discharge gaps, and build infrastructure resilient to major events.

Feature	Sag Pit (Low Point Inlet)	Grade Pit (On-Slope Inlet)
Location	Placed at the lowest point (sag) of the road where water naturally collects.	Installed along the grade of a street where water flows past.
Hydraulic Demand	Handles both on-grade flow and ponded water — very high capacity demand.	Only intercepts part of the flow; excess continues downstream.
Flooding Risk	High — if blocked, localised ponding and flooding occur.	Lower — water can bypass to next pit if blocked.
Design Challenge	Must connect to a reliable major drainage system to avoid failures.	Spacing along the road is critical to manage spread width.
Interesting Fact	Often designed with multiple side-entry and grate combinations to reduce blockage risk.	Acts like a “safety net,” catching flow progressively along the kerb.

• Austroads Guide to Road Design Part 5A: Drainage Design
• Australian Rainfall & Runoff (ARR) 2019 – Book 9: Runoff in Urban Catchments
• AS/NZS 3500.3: Plumbing and Drainage – Stormwater Drainage
• VicRoads Drainage Design Manual
• Stormwater Australia – Guidelines & Resources

Wall Type	Description	Best Suited For	Interesting Fact
Timber Sleeper	Economical and quick to install using treated timber posts and sleepers.	Low-height walls in landscaping, residential or temporary works.	Can be prefabricated and installed in tight access areas.
Reinforced Concrete	Cast-in-place or precast concrete with steel reinforcement for strength.	Permanent structures needing durability and higher load capacity.	Often designed with architectural finishes for urban aesthetics.
Gabion	Wire mesh baskets filled with rock, providing mass and drainage.	River training, erosion control, flexible foundation conditions.	They adapt to ground movement and allow vegetation growth.
Sheet Pile	Interlocking steel or vinyl sheets driven into the ground for cut-off and support.	Deep excavations, waterfronts, or where space is limited.	Can be used temporarily and extracted for reuse.
Mechanically Stabilized Earth (MSE)	Soil reinforced with layers of geogrid or metal strips, faced with panels or blocks.	High walls, bridge abutments, or embankment support.	Cost-effective for very tall walls compared to conventional RC walls.

References

• Austroads Guide to Geotechnical Investigation and Design
• AS 4678: Earth-retaining Structures
• ICE Manual of Geotechnical Engineering
• Civil’s Guide – Retaining Wall Design Overview
• Geosynthetica – MSE Wall Design Resources