RationalMAC estimates the peak stormwater discharge from a drainage area. You supply the runoff coefficient, the time of concentration, and the area; the tool reads rainfall intensity straight off an NOAA Atlas 14 IDF curve at your chosen return period, then solves the Rational equation.
1What the tool does
The Rational Method is the standard hand-calc for peak flow from small catchments. Its three terms are:
Q — peak discharge, in cubic feet per second (cfs).
C — dimensionless runoff coefficient (the fraction of rainfall that becomes runoff).
i — rainfall intensity, in inches per hour, taken at a storm duration equal to the time of concentration.
A — drainage area, in acres.
Because an acre-inch per hour is almost exactly one cubic foot per second (1.008 cfs), the equation is used directly as Q = C·i·A. RationalMAC applies the 1.008 factor for you.
2Design rainfall (IDF)
The first panel sets where your rainfall intensity comes from. Two modes:
Map · Atlas 14
Click a point anywhere in Texas, or use the search tool, and RationalMAC pulls the live NOAA Atlas 14 (Vol. 11) point precipitation frequencies for that grid cell. It converts each tabulated depth to an intensity (i = depth × 60 ⁄ duration) across every available duration and return period.
Search — the magnifier icon opens a box that accepts lat, lng coordinates or a Texas place name.
Basemap — the diamond icon switches between street, aerial, hybrid, and gray tiles.
Status line — confirms the grid cell and which return periods loaded.
Manual entry
Switch to Manual entry to type intensities yourself into the seven-cell grid (5, 10, 15, 30, 60, 120, and 180 minutes) for a single return period. Use this for jurisdiction-specific IDF equations or values from a local drainage manual. Values must fall between 0 and 40 in/hr and decrease as duration increases; cells that break that rule are flagged.
3Rational parameters
Enter the three catchment values and choose the design storm:
Runoff coefficient — C
Choose C for the land cover
Time of concentration — Tc
The time for runoff to travel from the hydraulically most distant point of the catchment to the outlet. The Rational Method assumes the peak occurs when the whole area is contributing — i.e. at a storm duration equal to Tc — so Tc is the duration at which intensity is read. Values of 5 to 180 minutes are expected; shorter Tc means higher intensity and a larger peak.
Drainage area — A
The contributing area in acres. The Rational Method is customary for areas under about 200 acres; above roughly 90 acres, a hydrograph or level-pool routing method is generally preferred, and the tool will note this.
4Reading the results
Three cards report the answer, and the formula strip shows the numbers substituted in:
Design storm — the return period, in years.
Rainfall intensity — i at duration = Tc, interpolated log–log between the tabulated durations.
Peak discharge — Q = C·i·A, in cfs (the headline number).
Enter at Tc on the horizontal axis, rise to the curve, read i on the vertical axis.
5The interactive IDF curve
Below the results, the intensity–duration–frequency chart plots rainfall intensity against storm duration on log–log axes — one line per return period, the way NOAA's own PFDS presents them.
Hover or tap the plot — a guide line follows your cursor and a readout lists the intensity of every return period at that duration.
The Tc marker — the bright dot and dashed cross-hairs sit at duration = Tc on the selected curve, labelled with the i feeding your Q.
Legend chips — tap a return period below the chart to make it the active design storm; the results and marker update instantly.
In map mode all of the Atlas cell's return periods are drawn; in manual mode the single curve you entered is shown.
6Assumptions & limits
Rainfall is assumed uniform over the area and constant over the storm duration; the peak coincides with a duration equal to Tc.
Runoff coefficient C is constant through the storm and does not capture antecedent moisture or losses that vary in time.
Intensity is interpolated log–log between tabulated durations; a Tc outside the tabulated range is clamped to the nearest end and flagged.
Best suited to drainage areas under ~200 acres. For larger or more complex catchments, use a hydrograph method.
Atlas 14 point estimates carry statistical uncertainty (NOAA publishes 90% confidence bounds); RationalMAC uses the central estimate.
Professional judgment Results are preliminary and must be checked against the design criteria your reviewing agency requires. This tool is intended for use by, or under the supervision of, a registered professional engineer.
