Rational method
Peak discharge of a small catchment from its runoff coefficient, the rainfall intensity and its area, with the frequency factor for rarer storms.
From the rain to the peak, on your iPhone
Hydrology Pro works out how much of a storm runs off a catchment, how fast it gathers and how high it peaks, with the chart and a PDF report to go with it. For iPhone and iPad.
Coming soon to the App Store
Eight methods
The everyday calculations of drainage and flood design, each with a note on what it was derived for.
Time of concentration
Kirpich, Kerby, the NRCS lag equation, the FAA equation and Bransby Williams, to suit overland flow, small channels or larger rural catchments.
Runoff depth
The NRCS (SCS) curve number method: runoff depth and volume, the retention and the initial abstraction.
Unit hydrograph
The NRCS triangular and curvilinear hydrographs: peak discharge, time to peak, and a table of ordinates.
Rainfall intensity
An intensity-duration-frequency curve with your coefficients, with the intensity and depth at standard durations.
Detention storage
A first estimate of the volume that holds an inflow peak back to an allowable release.
Risk and return period
The chance that a design event is exceeded in a design life, or the return period to design for at a risk you accept.
Frequency analysis
The Gumbel distribution of annual maxima: the design value at any return period, with a table of standard ones.
Made for the desk and the site
- Metric and English units. Millimetres and hectares or inches and acres: switch on any form and what you typed is converted.
- Charts. The hydrograph, the IDF curve or the runoff curve with every result, with the solved state marked.
- PDF reports. The inputs, results, notes, chart and tables on paper, to share or file.
- Saved calculations. Keep a calculation under a name and open it again later.
- Works offline. Nothing leaves your device: no account, no tracking, no network.
The results are for preliminary design and checking. Coefficients and curve numbers depend on the catchment: confirm them from local data and standards, and have the design reviewed by a qualified engineer.