Calculation of the time of concentration

The Time of concentration application allows estimating the “concentration delay” at the basin scale using some empirical formulas widely used in professional practice.

The calculation relationships use some geomorphological data of the watershed and provide as output “the time, expressed in hours, required for a generic drop of rain falling at the most hydraulically distant point to reach the outlet section of the basin”. Using the same input data, some basin shape indices are also derived.

Per gli applicativi desktop per idrologia e idraulica visita: geostru.eu/idraulica

                                                              
The empirical formulas used in the following application to estimate the time of concentration are:

Giandotti = (4 · A ^0.5 + 1.5 · L) / (0.8 ·(Q_med – Q_min) ^0.5)
Kirpick = 0.000325 · (L · 1000) ^0.77 · i_b ^-0.385
Johnstone and Cross = (3.258 · (D_max / i_b) ^0.5) / 60
California Culvert Practice = ((11.9 · (L · 0.621371) ³) / ((Q_max – Q_min) · 3.28084)) ^0.385)
NRCS SCS = (((1000 / CN) – 9) ^0.7 · (D_max · 1000) ^0.8) / (441 · (i_b · 100) ^0.5)
Pezzoli = 0.055 · L / i_a ^0.5
Puglisi = 6 · L ^{2 / 3} ·(Q_max – Q_min) ^{-1 / 3}
Ventura = 0.1272 · (A / i_a) ^0.5
Tournon = ((0.396 · L) / (i_a) ^0.5) · ((A / L ²) · (i_a) / i_b) ^0.5) ^0.72
Pasini = 0.108 · ((A · L) ^{1 / 3}) / (i_a ^0.5)
Viparelli=L/(3.5·1.5)

Where:

A [Km2], Area of the watershed.

L [Km], Length of the main channel.

Qmax [m], Maximum elevation of the watershed.

Qmed [m], Mean elevation of the watershed.

Qmin [m], Minimum elevation of the watershed.

CN [0-100], Curve Number.

Dmax [Km], Maximum distance between the drainage divide and the outlet section.

ib [m/m], Average slope of the watershed.

ia [m/m], Average slope of the main channel.

V [m/s], Average flow velocity within the channels (suggested values are between 1 and 1.5 m/s; the online application assumes the upper limit of 1.5).

P [Km], Perimeter of the watershed.