Rational Formula in Hydrology – Program

The use of the rational formula or rational method is far from one of the most used methods for estimating the maximum flow associated with a design rain, mainly because it does not require a large amount of hydrometric data for the determination of maximum flood flows, yes, it is always important to take into account that the rational method is intended for not very extensive basins (basin concentration time (Tc) < 6h).nnn

Calculation of Growth Flow using the Rational Method

nnn Runoff coefficient C: ntttt
Rain intensity I [mm/hour]: ntttt
nttttDrainage area A [Ha]:
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nnn

nnnNote: Does not apply an adjustment to the runoff coefficient.n

Explanation of rational formula or rational method

Where:nnnC: It corresponds to the runoff coefficient of the rational formula, represents the degree to which we will have surface runoff on the ground from precipitation.I: Corresponds to the intensity of the design rain, in [mm/h]nA: Basin area in [ha]nCu: Corresponds to the unit conversion coefficient, in this case for the units given corresponds to 1/360.nCf: Adjustment of runoff coefficient to adjust infiltration reduction and other losses in high intensity precipitation.nnNote: Usually Cu and Cf are applied in a single coefficient generally designated as “1/K”n

Coefficient runoff types

nThe runoff coefficient is basically the relationship between falling precipitation and runoff, it is the most complex variable to estimate, since it represents the interaction of many complex variables, so an averaged value is used to facilitate the achievement of this parameter.nnWhen a study area is composed of several areas with different runouts, the following relationship must be performed:nnbegin{equation}

Simplified table of runoff values for the rational method (See references below)

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Area	Runoff coefficient, c
Grass	0.05 – 0.35
Forest	0.05 – 0.25
Cultivated soil	0.08-0.41
Prairie	0.1 – 0.5
Parks, Cemeterys	0.1 – 0.25
Unimproved areas	0.1 – 0.3
Grassland	0.12 – 0.62
Residential area	0.3 – 0.75
Business areas	0.5 – 0.95
Industrial areas	0.5 – 0.9
Streets with asphalt	0.7 – 0.95
Streets with bricks	0.7 – 0.85
Ceilings	0.75 – 0.95
Concrete streets	0.7 – 0.95

Runoff adjustment

nValues in which runoff must be adjusted

Return period

Runoff adjustment factor

10 years or less

25 years

1.1

50 years

1.2

100 years

1.25

The intensity of the design rain (I)

The design rain intensity corresponds to the average rate of rain in millimetres per hour for a basin or sub-basin of drainage studied. The intensity is selected based on the duration of the design rain and the return period, in such a way that:

-begin{equation-ni-frac{P_{Tr-t-sun-sun}end{equation}

nWhere:n $i$ : Design rain intensity (mm/hr)n{P_{Tr}: Design precipitation (mm)n $D$ : Design duration (hr)n

The design duration is equal to the concentration time for the drainage area under consideration, while the return period is set using design standards.

It is assumed that the runoff reaches its maximum in the time of concentration (it is the time required by a drop of water to flow from the point most remet in the basin to the point of interest or design), since it is the time when the entire basin contributes to the flow of output.

For the determination of design precipitation, IDF curves are usually used.

nnn

la fórmula racional — the rational formula

nnReferences and BibliographynChin, David A. 2000. Water-Resources Engineering. Prentice-Hall.nChow, Ven Te, David R. Maidment, and Larry W. Mays. 1988. Applied Hydrology. McGraw-Hill.nCorbitt, Robert A. 1999. Standard Handbook of Environmental Engineering. McGraw-Hill. 2ed.nLindsley, Ray K., Joseph B. Franzini, David L. Freyberg, and George Tchobanoglous. 1992. Water-Resources Engineering. McGraw-Hill. 4ed.nMcCuen, Richard H. 1998. Hydrology Analysis and Design. Prentice-Hall. 2ed.nSingh, Vijay P. 1992. Elementary Hydrology. Prentice-Hall.ncCuen.oregon.govNo, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no. no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no,drdbthompson.net

Rational Formula in Hydrology – Program

Calculation of Growth Flow using the Rational Method