In log-normal frequency analysis of annual maximum flood series, the flood magnitude QT at return period T is computed as:
Choose the correct answer
QT = mean(Q) + K x sigma(Q)
log(QT) = mean(log Q) + K_T x sigma(log Q), where K_T = standard normal variate at probability P = 1/T
QT = median x T
QT = mode + T x CV
Correct Answer
B. log(QT) = mean(log Q) + K_T x sigma(log Q), where K_T = standard normal variate at probability P = 1/T
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Log-normal: transform Q to Y = log(Q). Y follows normal distribution. Y_T = mu_Y + z_T x sigma_Y, where z_T = standard normal variate for exceedance probability 1/T. QT = antilog(Y_T). Coefficient of skewness of log Q should be near zero for log-normal to be appropriate.
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