Calculator inputs
Use this form to build a synthetic inflow hydrograph for site drainage studies.
| Area (km²) | Runoff C | Intensity (mm/hr) | Duration (min) | Tc (min) | Shape | Δt (min) |
|---|---|---|---|---|---|---|
| 1.25 | 0.55 | 55 | 30 | 40 | Triangular | 5 |
Formula used
Peak inflow is estimated with the Rational approach: Qp = 0.278 × C × i × A, where C is runoff coefficient, i is rainfall intensity (mm/hr), and A is catchment area (km²).
A synthetic hydrograph is then built by distributing discharge through time. The triangular option rises linearly to Qp at Tp and recedes to zero at Tb. The dimensionless option scales a typical smooth curve using Tp and Qp.
Runoff volume is computed by numerical integration using the trapezoidal rule on the discharge–time series.
How to use this calculator
- Enter the catchment area, runoff coefficient, and rainfall intensity.
- Provide storm duration and time of concentration for realistic timing.
- Select a hydrograph shape and choose a time step Δt.
- Use auto mode for a conservative time-to-peak estimate.
- Press calculate to view the chart, summary, and table above.
- Download CSV or PDF to attach results to design notes.
Technical article
1) Inputs that drive the inflow hydrograph
The calculator combines catchment area (km²), runoff coefficient (0–1), and rainfall intensity (mm/hr) to estimate peak inflow. For example, A=1.25 km², C=0.55, and i=55 mm/hr produces Qp ≈ 0.278×0.55×55×1.25 = 10.52 m³/s before any scaling factor.
2) Timing controls for construction drainage
Hydrograph timing is governed by time of concentration (Tc) and the selected storm duration. In auto mode, the calculator sets Tp = max(Tc, 0.6×duration). This helps avoid unrealistically sharp peaks that can underpredict temporary storage requirements and overstate conveyance capacity.
3) Shape selection and what it implies
The triangular option rises linearly to Qp at Tp and decays to zero at Tb = factor×Tp. A base factor of 3.0 yields Tb = 3Tp, which is often suitable for quick checks. The dimensionless option produces a smoother recession, which can better reflect delayed catchment response.
4) Volume, detention, and reporting
Runoff volume is calculated by trapezoidal integration of the discharge series. Volume is the key input for detention sizing and for checking if temporary basins have adequate freeboard. Report Qp, Tp, Tb, time step Δt, and volume together so reviewers can reproduce the curve. Always state the selected return period clearly in reports.
5) Practical QA checks before design use
Typical small-site studies use C between 0.30 and 0.90 and intensities from 25–150 mm/hr, depending on climate and return period.
Confirm units, then compare Qp to local experience for similar drainage areas. Reduce the loss/scaling factor below 1.0 only when you have documented abstractions or partial contributing area. Finally, run a sensitivity check by varying C by ±0.10 and Δt between 2–10 minutes to see how peak and volume respond.
FAQs
1) What does Qp represent?
Qp is the estimated peak inflow rate at the outlet for the selected rainfall inputs. It is used to check culvert, ditch, and inlet capacity during the critical part of the storm.
2) Why use auto time to peak?
Auto mode sets Tp = max(Tc, 0.6×duration) to avoid overly sharp peaks. This generally yields more defensible timing for temporary drainage and storage checks.
3) When should I choose the triangular shape?
Choose it for preliminary sizing, fast comparisons, or when you need a transparent curve. It is easy to explain and verify because it rises and falls linearly around the peak.
4) What does the base-time factor do?
For the triangular option, Tb = factor×Tp. Larger factors lengthen the recession tail and increase runoff volume. Typical factors for quick site checks are 3 to 5.
5) What is the loss/scaling factor used for?
It scales peak inflow to reflect partial contributing area, diversions, or abstractions when supported by documentation. Keep it at 1.0 unless you have a justified adjustment.
6) Does switching to cfs change the calculation?
No. Only the reported discharge units change. Internally, flows are computed in m³/s and converted for display; runoff volume is always reported in cubic metres.
7) How do I pick an appropriate time step Δt?
Use 2–10 minutes for most construction drainage studies. Smaller steps give smoother curves and a better peak estimate, while very large steps can hide the true peak timing.