Drainage Improvement Calculator

1. Enter the catchment area that drains to your inlet or pipe.
2. Use a rainfall intensity that matches your design return period.
3. Select a runoff coefficient that fits the surface conditions.
4. Add infiltration rate and factor if you expect soak-in benefits.
5. Provide slope, pipe diameter, and roughness for capacity checking.
6. Apply clogging and safety factors to reflect real site risks.
7. Press Calculate to view results above the form.

This tool supports early planning. Confirm final designs with local standards, field data, and professional review.

• Peak runoff (Rational Method): Q = 0.00278 × C × I × A, where I is mm/hr and A is hectares.
• Infiltration allowance: Q_inf = 0.00278 × f × i × A, where f is infiltration factor and i is mm/hr.
• Design flow: Q_design = (Q − Q_inf) ÷ CF × SF, with clogging factor CF and safety factor SF.
• Pipe capacity (Manning, full flow): Q = (1/n) × A × R^2/3 × S^1/2.

Capacity is a simplified full-flow estimate; entrance losses and partial depth are not modeled here.

Example values are illustrative only. Use local rainfall data and verified site measurements for decisions.

Peak runoff is estimated with the Rational method, using Q = 0.00278 × C × I × A. Here, A is converted to hectares, I is the selected intensity in mm/hr, and C represents surface response. Typical planning values range from 0.20–0.40 for vegetated soils, 0.50–0.70 for mixed residential lots, and 0.80–0.95 for paved areas. Record your assumption to keep comparisons consistent across all options tested.

The calculator subtracts an infiltration allowance based on an infiltration rate and an effectiveness factor. This supports early comparisons between “convey more” and “absorb more” options. Even modest infiltration can reduce net runoff when the contributing area is large and the intensity is moderate. Use field tests where possible and keep the factor conservative when soils are compacted.

Existing conveyance is checked using full-flow Manning capacity, Q = (1/n) × A × R^(2/3) × S^(1/2). Capacity is highly sensitive to slope because it depends on the square root of S. For smooth pipe, n near 0.010–0.015 is common; rougher materials or biofilm may justify higher n, lowering capacity.

Real sites rarely behave like clean laboratory conditions. The clogging factor reduces effective flow capacity to reflect leaves, sediment, and partial inlet blockage. A safety factor then increases design flow to cover uncertainty in rainfall selection, future surface changes, and measurement error. Together, these parameters help avoid under-design when improvements must perform reliably.

When capacity is below the design flow, the tool suggests the next standard diameter that meets the target flow at the chosen slope. This is a screening output, useful for comparing alternatives such as upsizing a reach, adding a parallel pipe, improving inlet capture, or reducing runoff with swales and storage. If length and unit cost are provided, a rough materials estimate supports budget discussions with stakeholders.

1) What rainfall intensity should I enter?

Use an intensity from local IDF curves for your chosen return period and storm duration. If unsure, run multiple intensities to see how sensitive sizing and adequacy results are.

2) How do I pick a runoff coefficient?

Select C based on surface type and compaction. Paved areas are highest, lawns and landscaped soils are lower. Mixed sites often fall between 0.50 and 0.70 for planning.

3) Why is there an infiltration factor?

Not all soil area infiltrates effectively during a storm. The factor accounts for compaction, ponding limits, and bypass flow. Keep it conservative unless you have test data and clear source-control features.

4) Does the pipe capacity assume full flow?

Yes. The Manning check assumes full-flow conditions and does not model inlet losses, partial depth, bends, or surcharging. Use it for screening, then refine with detailed hydraulics if needed.

5) What do clogging and safety factors represent?

Clogging reflects debris, sediment, and partial blockage risk. Safety covers uncertainty in rainfall selection and future changes. Lower clogging or higher safety increases design conservatism and suggested sizes.

6) When should I trust the suggested diameter?

Use it as a starting point for upgrade discussions. Confirm with site grades, outlet constraints, materials, and local design standards. Complex networks, backwater, or ponding require a more detailed analysis.