Inputs
Choose a method, enter dimensions or stage areas, and optionally check runoff sizing against available storage.
Formula used
- Rectangular (vertical): V = L × W × D
- Rectangular (sloped sides): V = LbWbD + (Lb·sW + Wb·sL)D² + (4/3)sL·sW·D³
- Circular (vertical): V = π × (D/2)² × Depth
- Circular (sloped sides): V = (πh/12)(Db² + Db·Dt + Dt²), Dt = Db + 2s·h
- Stage-storage: V = Σ((Ai + Ai−1)/2) × Δh
- Runoff required: Vreq = Area × StormDepth × C
How to use this calculator
- Select a method: geometric for a quick shape model, or stage-storage for surveyed areas.
- Choose units, enter depth, and add freeboard to reserve the top portion.
- For geometric mode, pick a shape and fill only the fields related to that model.
- For stage-storage, enter at least two stage rows with increasing depth/elevation.
- Optionally enter sediment reserve and runoff sizing inputs to check adequacy.
- Press Submit to view results, then export CSV or PDF for documentation.
Example data table
Sample inputs and outputs to illustrate typical retention basin checks.
| Scenario | Method | Key Inputs | Computed Total Volume | Runoff Check |
|---|---|---|---|---|
| Sloped rectangular | Geometric | Lb=20 m, Wb=12 m, D=2 m, slopes 3H:1V, freeboard 0.3 m | ≈ 684.000 m³ | 2 ha, 25 mm, C=0.60 → 300.000 m³ |
| Stage-storage survey | Stage | (0 m, 120 m²), (1 m, 260 m²), (2 m, 420 m²) | ≈ 530.000 m³ | 1.5 ha, 20 mm, C=0.50 → 150.000 m³ |
| Vertical circular | Geometric | Diameter=18 m, Depth=1.5 m, freeboard 0.2 m | ≈ 381.704 m³ | 1 ac, 1 in, C=0.80 → 83.419 m³ |
Storage intent and design checks
Retention basins reduce downstream peak flow by temporarily storing stormwater and releasing it at a controlled rate. This calculator helps you estimate the storage volume you can provide from proposed geometry or measured stage areas, then compare it to a simple runoff volume check. Use the output as a screening step before detailed routing and outlet design.
Geometric modeling for early layouts
When a basin is at concept stage, plan dimensions are often known before a survey surface model exists. The geometric options cover rectangular and circular shapes with vertical or sloped sides. For sloped sides, top dimensions increase with depth based on the side slope ratio, producing a more realistic storage estimate than a single plan area assumption.
Stage-storage for surveyed basins
Stage-storage is preferred when you have surface areas at multiple elevations from grading plans or a terrain model. The calculator integrates area over depth using trapezoidal segments, which is consistent with common engineering practice for storage curves. Enter at least two stages; more stages improves accuracy where side slopes change or benches exist.
Operational allowances and reporting
Practical storage is typically reduced by freeboard and sediment allowance. Freeboard protects against wave action, construction tolerances, and blockage risks, while sediment reserve preserves long-term capacity between maintenance cycles. The results separate total, usable, and available storage and provide unit conversions to support submittals and quick peer review.
Example data for a quick sanity check
Use the following example values to verify your workflow, then replace with project data:
- Geometric (sloped): Lb 20 m, Wb 12 m, depth 2.0 m, slopes 3H:1V, freeboard 0.3 m → total ≈ 684 m³.
- Stage-storage: (0 m, 120 m²), (1 m, 260 m²), (2 m, 420 m²) → total ≈ 530 m³.
- Runoff check: 2 ha, 25 mm, C 0.60 → required ≈ 300 m³, compare to available storage.
FAQs
1) Which method should I choose?
Use geometric for concept layouts with known dimensions. Use stage-storage when you have areas by elevation from grading plans or surveys. Stage-storage better captures benches and changing side slopes.
2) What does “available storage” mean?
Available storage is usable volume minus your sediment reserve. It represents the capacity you expect to rely on for storm events while maintaining long-term performance between cleanouts.
3) How is freeboard applied in the results?
Freeboard reduces the effective depth used for storage. The calculator scales the total volume by the ratio of usable depth to total depth, giving a practical screening estimate for the reserved top zone.
4) Is the runoff check a full hydrologic design?
No. It is a simplified volume check using Area × Storm Depth × C. Final design typically requires hydrograph routing, outlet control sizing, and local regulatory criteria for release rates and drawdown.
5) What runoff coefficient should I use?
Choose C based on site surfaces and guidance from local manuals. Lower values suit permeable soils and vegetation; higher values suit pavement and roofs. Use conservative values when uncertainty is high.
6) Can I enter mixed units?
Yes, but stay consistent within each input group. Length units apply to dimensions and depths, while area units apply to stage and custom areas. Runoff inputs include their own area and depth unit selectors.
7) Why do my geometric and stage results differ?
They may represent different shapes or assumptions. Stage-storage reflects actual surface areas at elevations, while geometric shapes idealize the basin. Differences are expected when the basin has benches, irregular edges, or variable slopes.