Abstract
Understanding the nature of frequent floods is important for characterising channel morphology, riparian and aquatic habitat, and informing river restoration efforts. This paper presents results from an analysis on frequency estimates of low magnitude floods using the annual maximum and partial series data compared to actual flood series. Five frequency distribution models were fitted to data from 24 gauging stations in the Great Barrier Reef (GBR) lagoon catchments in north-eastern Australia. Based on the goodness of fit test, Generalised Extreme Value, Generalised Pareto and Log Pearson Type 3 models were used to estimate flood frequencies across the study region. Results suggest frequency estimates based on a partial series are better, compared to an annual series, for small to medium floods, while both methods produce similar results for large floods. Although both methods converge at a higher recurrence interval, the convergence recurrence interval varies between catchments. Results also suggest frequency estimates vary slightly between two or more partial series, depending on flood threshold, and the differences are large for the catchments that experience less frequent floods. While a partial series produces better frequency estimates, it can underestimate or overestimate the frequency if the flood threshold differs largely compared to bankfull discharge. These results have significant implications in calculating the dependency of floodplain ecosystems on the frequency of flooding and their subsequent management.
Highlights
Flood frequency estimates are of prime importance in many water resource planning and management projects such as design of infrastructure, flood insurance studies, floodplain management and ecological studies [1,2]
Catchments in the wet tropical region are subject to frequent flooding because of frequent and intense rainfall and high antecedent soil water conditions [39]
It can be seen that the difference in estimates of flood magnitude between the annual maximum (AM) and BF series is relatively small for a wet tropical catchment (Figure 9a) compared to a dry tropical catchment (Figure 9b)
Summary
Flood frequency estimates are of prime importance in many water resource planning and management projects such as design of infrastructure, flood insurance studies, floodplain management and ecological studies [1,2]. Research on flood frequency has focused on the estimation of extreme flood events because of their large and often dramatic impacts on society and visible economic costs. The importance of frequent, low-magnitude floods is often overlooked. Despite their high erosive power, large floods transport a relatively small proportion of total sediment loads to the marine environment because they occur less frequently. Floods occurring on average once a year account for 50% of total sediment loads [3]. While large floods are usually responsible for channel avulsions, levee breaches, and transporting large bed-load sediments, frequent floods (e.g., recurrence interval of one or two years) are primarily responsible for controlling channel morphology because of their frequent nature and ability to erode
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