EngiMetric Arena / Design Challenge
How much detention storage does the storm need?
Competency Systems and balances primary Design and sizing supporting Quantitative reasoning supporting
What you should take away: Convert catchment rainfall to peak inflow, subtract the permitted release and integrate over the storm duration to size storage.
The situation
What is happening
A 12 ha catchment has a runoff coefficient of 0.6. The 45 minute design storm has a rainfall intensity of 40 mm/h, and the permitted discharge is 0.15 m3/s. What detention storage volume is required?
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Catchment area | 12 | ha | Contributing area. |
| Runoff coefficient | 0.6 | - | Composite catchment value. |
| Storm duration | 45 | min | Design storm duration. |
| Rainfall intensity | 40 | mm/h | Design intensity. |
| Permitted discharge | 0.15 | m3/s | Allowable release to the receiving system. |
Assumptions this challenge makes
- The Rational Method peak inflow is held constant over the storm duration.
- The permitted discharge is constant during the storm.
- Infiltration and evaporation during the storm are neglected.
Your call
Your answer
Hints
- Use Q = 2.78 x C x i x A with area in hectares and intensity in mm/h to get peak inflow in L/s.
- Convert the inflow to m3/s, then subtract the permitted release. The difference is the net flow that must be stored.
- Multiply the net flow by the storm duration in seconds. Keep the duration and flow units consistent.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- Peak inflow is 2.78 x 0.6 x 40 x 12 = 800.64 L/s, or 0.80064 m3/s.
- The net flow to storage is 0.80064 - 0.15 = 0.65064 m3/s.
- Over 45 minutes (2 700 s), the storage is 0.65064 x 2 700 = 1 756.7 m3.
- At a 2.5 m pond depth, the footprint would be 1 756.7 / 2.5 = 702.7 m2. That footprint is a geometry consequence, not an independent design choice.
- The calculation uses a constant peak inflow over the storm. A full routing model would use a hyetograph and a stage-discharge relationship.
What this challenge does not establish
- The Rational Method peak is held constant over the storm, which is conservative but not a full hydrograph.
- The permitted discharge is assumed constant and available throughout the storm.
- Infiltration, evaporation, outlet control and emergency spillway design are not modelled.
- Local drainage criteria and the actual stage-storage relationship must be checked.