Empirical Method: ( 12- 05- 2011 L: 5 )

The maximum flood discharge Q_p(m³/s) from a catchment area  A(km²)is given by the following empirical formula as

a) Dickens Formula (1865): Q_p = C_dA^3/4, where C_d Dicken’s constant (6~30).

b) Ryres Formula:  Q_p = C_RA^2/3, where C_R Ryres constant.

c) Inglis Formula (1930): 

d) Fuller’s Formula (1914): Q_TP = C_fA^0.8(1+0.8logT)

Where, Q_TP = Maximum 24hr flood with a frequency of T year in m³/s,

And C_f = constant (0.8~1.88).

e) Envelop curves Formula: 

 Fig: Enveloping curves for Indian Rivers

US- soil conservation service curve number method (US-SCS-CN)

The SCS-CN method is based on the water balanced equation and two fundamental hypothesises. The first hypothesis states that the ratio of actual amount of surface runoff to the maximum potential runoff is equal to the ratio of the cumulative amount of actual infiltration to the amount of potential maximum retention. The second hypothesis states that the amount of initial abstraction is some fraction of potential maximum retention. Mathematically the water balance equation and the hypothesis can be expressed as follows:

                          P = I_a + F + Q

Where, P = Potential rainfall, I_a = Initial abstraction, F = Cumulative infiltration excluding I_a, Q = Direct Runoff, S = Potential maximum retention

Hypothesis -I:  

                     

Hypothesis -II:

                                  I_a = 0.2S 

 The SCS expressed S as a function of CN

Where, S in inches, CN = curve number values from 0 ~ 10

 

Paved Area,

                    S = 0 ; CN = 100

Areas where no surface runoff occurs,

                                                       

The number is derived from soil characteristics, vegetation including crops, landuse of soil as well as intensity of use.