EngiMetric Arena / Design Challenge
Size an infiltration trench for a 1,200 m2 car park
Competency Design and sizing primary Quantitative reasoning supporting Systems and balances supporting
What you should take away: Convert a catchment runoff volume into the physical length of a storage trench, accounting for the void space in the aggregate.
The situation
What is happening
A 1,200 m2 car park drains to an infiltration trench. The design storm is 32 mm, the runoff coefficient is 0.9, the trench is 1.0 m wide and 1.2 m deep, and the aggregate void ratio is 35 %. Size the trench length required to store the runoff.
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Catchment area | 1 200 | m2 | Car park draining to the trench. |
| Design rainfall | 32 | mm | Design storm depth. |
| Runoff coefficient | 0.9 | - | Paved surface. |
| Trench width | 1.0 | m | Fixed by the site strip. |
| Trench depth | 1.2 | m | Fixed by the invert and cover. |
| Void ratio | 35 | % | Storage voids in the aggregate. |
Assumptions this challenge makes
- The trench is rectangular with vertical sides and a flat base.
- The design storm volume is stored before infiltration is credited; infiltration only affects drain-down.
- The void ratio applies uniformly to the aggregate fill.
Your call
Your answer
Hints
- First compute the runoff volume: catchment area x rainfall depth x runoff coefficient. Watch the millimetres-to-metres conversion.
- The trench volume is not all storage. Divide the runoff volume by the void ratio to get the gross trench volume.
- Divide the gross trench volume by the cross-section (width x depth) to get the length.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- Runoff volume is 1 200 x 0.032 x 0.9 = 34.56 m3.
- The aggregate stores 35 % of its gross volume, so the trench volume is 34.56 / 0.35 = 98.74 m3.
- At 1.0 m wide and 1.2 m deep, the cross-section is 1.2 m2. The required length is 98.74 / 1.2 = 82.3 m.
- The infiltration rate does not reduce the storage length in this calculation; it determines how quickly the trench drains after the storm. That drain-down check is separate.
- Rounding the length up to a constructible value is a site decision, but the hydraulic requirement is about 82 m.
What this challenge does not establish
- Infiltration during the storm is not credited; the full runoff volume is stored. This is conservative.
- The trench is treated as a clean rectangular void with uniform aggregate; clogging and sediment build-up reduce storage over time.
- Groundwater level, separation to the water table and infiltration-rate testing are not assessed.
- The design storm is a single depth, not a full rainfall hyetograph or a site-specific return-period analysis.