UM-19 : Flood frequency analysis on a microcomputer with basic language

Abstract

Flood frequency analysis approach is a very useful technique for estimation of design flood from the limited data. In general, in frequency analysis approach the sample data is used to fit frequency distribution, which in turn is used to extrapolate from record events to design events either graphically or by estimating the parameters of frequency distribution. Efficient and unbiased estimation of parameters for various frequency distributions with the help of calculator is somewhat tedious. Many State Irrigation Departments of the country do not have the main fame computer facilities, so far, though micro computers are available. One of the micro computer systems which is quite versatile and is available in different parts of the country is BBC micro computer system with BASIC language compiler.

Commonly used probability distributions for flood frequency analysis include log normal two parameters, log normal three parameters, extreme value type I distribution, Pearson type III distribution and log Pearson type III distribution and parameters are estimated using method of moments and method of maximum likelihood to compute 2, 5, 10, 25, 50, 100 years return period floods using annual flood series data.

Seven computer programmes for flood frequency analysis viz. (i) MEAN.BAS, for preliminary statistics of annual peak discharge series, (ii) LN2.BAS, for log normal two parameter distribution, (iii) LN3.BAS, for log normal three parameter distribution, (iv) EV1.BAS, for extreme value type I distribution, (v) PT3.BAS for Pearson type III distribution, (vi) LP3.BAS, for log Pearson type III distribution, and (vii) SER.BAS for computation of standard error of various distributions in fitting the annual flood series, were written in BASIC language and implemented and tested on BBC micro computer system.

This user's manual provides input and output specifications for these programmes along with an illustrative example. The programme can be run on micro computers other than BBC micro computer having BASIC compiler after making suitable minor software modifications to suit the requirements of the particular system.