Stormwater drainage design manages runoff from rainfall events to protect property, infrastructure, and safety. In Australia, design is based on Australian Rainfall and Runoff (ARR), the national guideline for flood estimation and drainage design.
Design Storm Selection
Drainage systems are sized to an Average Recurrence Interval (ARI) or, in current ARR terminology, an Annual Exceedance Probability (AEP):
| System | Typical Design Standard |
|---|---|
| Minor drainage (pits, pipes) | 5–10 year ARI (10–20% AEP) |
| Major drainage (overland flow path) | 100 year ARI (1% AEP) |
| Culverts/bridges (varies by consequence) | 20–100+ year ARI |
Minor/major drainage philosophy: the piped ("minor") system handles frequent storms; larger, rarer storms are managed by a defined overland flow path ("major" system) that safely conveys excess flow without unacceptable property flooding.
Rational Method
For small-to-medium catchments, peak flow is commonly estimated using the Rational Method:
$$ Q = \frac{C i A}{360} $$Where:
- $Q$ = peak flow (m³/s)
- $C$ = runoff coefficient (dimensionless, land-use dependent)
- $i$ = design rainfall intensity (mm/h) for the time of concentration and selected AEP
- $A$ = catchment area (ha)
Runoff Coefficients
| Surface Type | Typical $C$ |
|---|---|
| Roofs | 0.85–0.95 |
| Paved/sealed roads | 0.80–0.90 |
| Grassed areas (flat) | 0.15–0.25 |
| Grassed areas (steep) | 0.25–0.35 |
| Undeveloped/bushland | 0.10–0.20 |
Time of Concentration
$$ t_c = t_{overland} + t_{pipe/channel} $$Rainfall intensity $i$ is read from Intensity-Frequency-Duration (IFD) data (via the Bureau of Meteorology) at a duration equal to $t_c$.
Pipe Design
Pipes are typically sized using Manning's equation for gravity flow:
$$ Q = \frac{1}{n} A R^{2/3} S^{1/2} $$Where:
- $n$ = Manning's roughness coefficient (~0.013 for concrete pipe)
- $A$ = flow area
- $R$ = hydraulic radius
- $S$ = pipe grade
| Pipe Material | Typical Manning's $n$ |
|---|---|
| Concrete (RCP) | 0.013 |
| uPVC | 0.010 |
| Corrugated steel | 0.024 |
Minimum Pipe Grades and Velocities
| Requirement | Typical Value |
|---|---|
| Minimum self-cleansing velocity | 0.6–0.9 m/s |
| Minimum pipe size (public infrastructure) | 300–375 mm diameter |
| Minimum cover | 600 mm (traffic areas) |
Pit Spacing and Inlet Capacity
Stormwater pits (inlets) are spaced based on:
- Allowable spread of water across the trafficable road surface
- Inlet capture capacity (kerb inlet, grated inlet, or combination)
- Sag points (low points) always require an inlet, regardless of spacing rules
On-grade kerb inlet capacity depends on approach flow, gutter cross-slope, and inlet length: typically determined from Austroads/local authority design charts rather than closed-form calculation.
Detention and Retention
Where downstream capacity is limited, on-site detention (OSD) attenuates peak flow:
$$ V_{req} \approx \text{(Inflow hydrograph volume)} - \text{(Permitted outflow hydrograph volume)} $$| System | Function |
|---|---|
| On-site detention (OSD) | Temporarily stores runoff, releases at a controlled (pre-development) rate |
| Retention/infiltration | Retains runoff on-site, reducing volume via infiltration/reuse |
| Bioretention/WSUD systems | Combines treatment and flow control (see Water Sensitive Urban Design) |
Practical Notes
- The minor/major system philosophy must be checked together: a pipe network sized correctly for the minor storm is not sufficient if the major storm overland flow path is blocked by buildings or has no safe outlet.
- Runoff coefficients should reflect the developed (post-construction) catchment condition, not existing conditions, unless assessing an existing system.
- Self-cleansing velocity, not just capacity, should be checked at flat grades: an oversized, under-graded pipe can silt up even though it has adequate hydraulic capacity.