Calculator Inputs
Example Data Table
Use this example to verify your setup and unit selection.
| Input | Example value (SI) | Notes |
|---|---|---|
| Rainfall intensity, i | 75 mm/hr | Design storm intensity |
| Runoff coefficient, C | 0.90 | Impervious surface assumption |
| Contributing width, W | 8 m | Equivalent tributary width |
| Longitudinal slope, S | 1.0% | Along-gutter grade |
| Cross slope, Sx | 2.0% | Pavement cross fall |
| Manning’s n | 0.016 | Typical for rough asphalt |
| Allowable spread, T | 2.5 m | Maximum spread criterion |
| Inlet method | Grate inlet | Efficiency estimated from velocity |
| Grate width / length | 0.6 m / 0.6 m | Representative catch basin grate |
| Splash-over velocity, Vo | 1.8 m/s | Common reference value |
Formulas Used
This tool combines a simple runoff-per-length estimate with gutter hydraulics and inlet interception.
Depth at curb: d = Sx · T, Area: A = 0.5 · Sx · T², Velocity: V = Q/A.
- Grate inlet: Efficiency from frontal/side components, then Qi = E · Q.
- Curb opening: Required length for 100% interception: LT = K · Q0.42 · S0.3 · (1/(n·Sx))0.6. If L < LT, E = 1 − (1 − L/LT)1.8.
- Fixed efficiency: Qi = (E/100) · Q.
- Manual capacity: Use your own Qi directly.
How to Use This Calculator
- Select your unit system and enter the rainfall intensity and runoff coefficient.
- Set contributing width, slopes, Manning’s n, and allowable spread.
- Pick an inlet method that matches your catch basin type.
- Click Calculate to see spacing and hydraulics immediately.
- Apply optional min/max spacing limits if your agency requires them.
- Enter a project length to estimate the total number of basins.
- Export CSV/PDF for documentation and field coordination.
Catch Basin Spacing Design Notes
This calculator sizes inlet spacing along curb-and-gutter by matching intercepted inlet capacity to runoff generated per meter or foot of roadway. Enter site hydrology and geometric controls, then compare computed spacing with local criteria for lane encroachment, maintenance access, and downstream pipe capacity.
1) Why spacing controls roadway performance
Catch basin spacing is a safety and serviceability decision. Longer spacing increases gutter flow, widening spread into traffic lanes and increasing curb depth. Using an allowable spread target helps keep drainage performance measurable and consistent across roadway segments.
2) Inputs that most affect the result
Spacing depends directly on rainfall intensity, runoff coefficient, and tributary width because they set inflow per unit length. Grade and cross slope also matter because triangular gutter flow scales strongly with S, Sx, and allowable spread T. For pavement, Manning’s n commonly ranges from about 0.013 to 0.016.
3) Interception method selection
When inlet performance tables or manufacturer data exist, “Manual inlet capacity” keeps spacing aligned with published values. The grate and curb-opening options estimate efficiency from flow conditions and geometry, which is useful for preliminary design and concept layouts. Recheck efficiency where debris, clogging, or steep grades are likely.
4) How to interpret the spacing output
The spacing result follows L = Qi/qL. “Raw spacing” is the direct hydraulic balance; optional minimum and maximum limits let you apply agency constraints while keeping a record of the underlying hydraulic recommendation. Review the computed flow at spacing against lead connections and downstream capacity.
5) Practical checks before finalizing plans
Confirm the design storm intensity matches the selected return period and your time-of-concentration basis. Verify slopes from grading plans and keep units consistent. At sag points, consider separate ponding checks and sump-appropriate inlet methods. Use the export files to document assumptions and support reviews.
FAQs
1) What does “allowable spread” mean?
Allowable spread is the maximum width of water on the pavement measured from the curb. It is typically limited to keep lanes, shoulders, bike facilities, and pedestrian areas functional during the design storm.
2) Should slopes be entered as percent or ratio?
Either works. Use percent when reading directly from roadway plans, and ratio when you already have decimal slopes. The calculator converts percent to ratio internally before hydraulic computations.
3) How do I select a runoff coefficient (C)?
Select C based on surface type and imperviousness. Pavement is often 0.85–0.95, while landscaped or mixed areas may be lower. Align values with your local drainage manual or project hydrology method.
4) When should I use “manual inlet capacity”?
Use it when you have inlet capacity values from standards, agency tables, or manufacturer data. It is also helpful for specialized inlet types not represented by simplified efficiency relationships.
5) Why does spacing change when cross slope changes?
Cross slope changes the triangular flow geometry and curb depth for a given spread. Higher cross slope generally increases gutter capacity and can improve interception, which may allow longer inlet spacing.
6) What do minimum and maximum spacing limits do?
Limits bound the computed spacing to meet policy, maintenance, or constructability constraints. The tool computes raw hydraulic spacing first, then applies optional min/max values to report the final recommended spacing.
7) Does this replace local design guidance?
No. It supports sizing and documentation. Always verify storm selection, inlet type, and special conditions (sags, ponding, debris) using your agency’s drainage manual and project-specific criteria.