| i (mm/hr) | C | A (ha) | Wc (m) | S0 (%) | Sx (%) | n | T allow (m) | Q design (m³/s) | Q cap (m³/s) | T req (m) | d req (m) | L max (m) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 75 | 0.75 | 0.25 | 20 | 1.00 | 2.00 | 0.016 | 2.50 | 0.0391 | 0.0398 | 2.484 | 0.050 | 127.1 |
T is spread (m), S0 is longitudinal slope (m/m), Sx is cross slope (m/m), n is Manning’s roughness, and K ≈ 0.375 in this simplified SI form.
Depth at curb is approximated as d = T · Sx.
L_max = (Q_cap·10000)/(0.00278·C·i·Wc).
- Select a mode: Area check for a known drainage area, or Spacing estimate to size inlet spacing from an allowable spread.
- Enter rainfall intensity from your local IDF data and choose a runoff coefficient matching surface type.
- Provide slopes and Manning’s n. Small changes in slope or roughness can noticeably change capacity.
- Set allowable spread and curb depth limits (curb height minus freeboard). These represent your floodable street performance targets.
- Click Calculate. Review pass/fail checks and adjust inputs or add inlets if needed.
- Use Download CSV and Download PDF to attach results to design notes and submittals.
Floodable streets are planned corridors that safely convey storm runoff when underground systems reach capacity. Instead of avoiding surface flow entirely, the design controls where water goes, how deep it becomes, and how far it spreads. A practical early-stage check is to pair a runoff estimate with a gutter capacity limit based on allowable spread and curb depth. This calculator supports that workflow with transparent assumptions and fast iteration.
The runoff side uses the Rational method, which is commonly applied for small drainage areas and short response times. Choose a runoff coefficient that reflects your surface conditions and storage. For example, a mostly paved catchment may use C = 0.75, while landscaped areas may be lower. Rainfall intensity should come from local intensity–duration–frequency data for your chosen storm. Once Q is established, the street section is checked using a simplified triangular gutter relationship. Longitudinal slope, cross slope, and Manning’s roughness govern conveyance. Even small slope changes can noticeably increase or reduce capacity, so enter values that match the grading plan.
Use the Area check mode when you already know the contributing drainage area to a segment. The tool reports the required spread and curb depth for the computed flow, then compares them to your limits. If the spread exceeds the allowable width, typical fixes include adding inlets, reducing tributary area by splitting drainage, improving surface smoothness, or adjusting grades while meeting accessibility criteria. Depth should also stay below the curb height minus freeboard to protect properties and critical entrances.
Use the Spacing estimate mode to approximate inlet spacing for a uniform contributing width. In this mode, the calculator solves a maximum spacing so the runoff between inlets does not exceed gutter capacity at your allowable spread. This is useful for preliminary layouts and comparing alternatives before detailed inlet capture modeling.
Example: With i = 75 mm/hr, C = 0.75, A = 0.25 ha, S0 = 1%, Sx = 2%, n = 0.016, and allowable spread T = 2.5 m, the design flow is about 0.0391 m³/s. If curb height is 0.15 m with 0.03 m freeboard, allowable depth is 0.12 m. In spacing mode with Wc = 20 m, the computed maximum spacing is roughly 58.0 m for the same spread limit.
This calculator is intended for conceptual and preliminary checks. Final design should follow your adopted roadway drainage criteria and include inlet capacity, bypass flow, and system hydraulics.
1) What is “allowable spread” in a floodable street?
Allowable spread is the maximum width of water across the pavement you permit during the design storm. It helps protect travel lanes, sidewalks, and driveways while still allowing controlled surface conveyance.
2) When should I use Area check versus Spacing estimate?
Use Area check when you know the contributing drainage area to the segment. Use Spacing estimate when you want a preliminary inlet spacing based on a uniform contributing width and an allowable spread limit.
3) How do I choose the runoff coefficient?
Select a coefficient that reflects surface type and storage. Paved areas typically range from about 0.70 to 0.95, while landscaped areas can be much lower. Use local guidance and calibrate with site observations when possible.
4) What does Manning’s n represent here?
Manning’s n is a roughness factor that influences gutter conveyance. Smoother surfaces carry more flow at the same depth. Use values consistent with your pavement type, condition, and expected debris or texture.
5) Why can spread fail even when depth looks acceptable?
Spread is sensitive to cross slope and longitudinal slope. A shallow cross slope can create wide, thin sheets of water. You may need steeper crossfall, more frequent inlets, or a smaller tributary area to control spread.
6) Does the spacing result replace inlet capture calculations?
No. The spacing estimate is a planning-level approximation based on capacity at a chosen spread. Final design should verify inlet capture, bypass flow, clogging allowances, sag behavior, and downstream system capacity.
7) What freeboard should I use?
Freeboard is a safety margin below curb top. Choose a value aligned with your standard and risk tolerance, often a few centimeters. Higher freeboard reduces allowable depth and may require more inlets or grading adjustments.