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Steps of graphical flood frequency method

Short answer

Arrange annual maximum floods in descending order, compute return period using Weibull T=(N+1)/m, plot on suitable probability paper, draw best-fit line, and extrapolate to desired T to read flood magnitude. Reliability decreases for T much longer than record length.

Mention the steps for the computation of flood of return period T using graphical method.

Graphical (plotting-position) method for flood of return period T

  1. Collect the annual maximum flood series for N years.
  2. Arrange the floods in descending order and assign the rank m (m = 1 for the largest).
  3. Compute the return period of each flood by a plotting-position formula, e.g. Weibull T=N+1mT=\dfrac{N+1}{m} (or the exceedance probability P=m/(N+1)P=m/(N+1)).
  4. Choose suitable probability paper (semi-log, Gumbel, or log-normal paper) so that the data fall nearly on a straight line.
  5. Plot the flood magnitude (vertical axis) against T or P (horizontal axis) for every point.
  6. Draw the best-fit straight line (or smooth curve) through the points.
  7. Extend the line (extrapolate) to the required return period T and read the flood magnitude from the vertical axis.
  8. Report the flood, noting that reliability falls when T is much larger than N.

✓ Answer checked by Claude Sonnet 5.5

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