A direct analysis of flood interval probability using approximately 100-year streamflow datasets

Received XXXX; accepted XXXX Abstract Series of observed flood intervals, defined as the time intervals between successive flood peaks over a threshold, were extracted directly from eleven approximately 100-year streamflow datasets from Queensland, Australia. A range of discharge thresholds were analysed that correspond to approximately 3.7 months to 6.3 year return periods. Flood interval histograms at South East Queensland gauges were consistently unimodal whereas those of the North and Central Queensland sites were often multimodal. The exponential probability distribution (pd) is often used to describe interval exceedance probabilities, but fitting utilizing the Anderson Darling statistic found little evidence that it is the most suitable. The fatigue life pd dominated sub-year return periods (<1 year), often transitioning to a log Pearson 3 pd at above-year return periods. Fatigue life pd is used in analysis of the life time to structural failure when a threshold is exceeded and this paper demonstrates its relevance also to the elapsed time between above-threshold floods. At most sites, the interval medians were substantially less than the means for sub-year return periods. Statistically the median is a better measure of central tendency of skewed distributions but the mean is generally used in practice to describe the classical concept of flood return period.