Abstract:Traditional flood-frequency analysis involves the assumption of homogeneity of the flood distribution. However, floods are often generated by heterogeneous distributions composed of a mixture of two or more populations. Differences between the populations may be the result of a number of factors, including seasonal variations in the flood-producing mechanisms, changes in weather patterns resulting from low-frequency climate shifts and/or El Niño/La Nina oscillations, changes in channel routing owing to the dominance of within-channel or floodplain flow, and basin variability resulting from changes in antecedent soil moisture. Not recognizing these physical processes in conventional flood-frequency analysis probably is the main reason that many frequency distributions do not provide an acceptable fit to flood data. In this paper, we use long-term hydroclimatic records from the Gila River basin of south-east and central Arizona in the USA to explore the extent and significance of mixed populations. First, we discuss the probable causes of heterogeneity in the frequency distribution of annual flood and present evidence of its occurrence. Second, we investigate the implications of using various popular homogeneous distributions for predicting peak flows for basins that exhibit mixed population characteristics. Third, we demonstrate how alternative frequency models that explicitly account for floods generated by a mixture of two or more populations are both hydrologically and statistically more appropriate. We illustrate how the selection of the most plausible distribution for flood-frequency analysis also should be based on hydrological reasoning as opposed to the sole application of the traditional statistical goodness-of-fit tests.
In the stream culvert discharge design guidelines of the Forest Practices Code (FPC) of British Columbia (BC), the 100-year instantaneous flood (Q100) is assumed to be three times as large as the mean annual flood (Q2) regardless of basin characteristics and location in the province. A regional linear moment analysis of annual maximum flows is used to demonstrate that this assumption is invalid and that Q100/Q2 ratios vary substantially with basin area and climate. For the snowmelt-dominated peak flows in the Columbia and southern Rocky Mountains, Q100/Q2 decreases slightly with increasing drainage area, from 2.3 (1 km2) to 1.9 (100 km2). For the flood peaks generated by rainfall and rain on snow in coastal BC, this range is 3.12.6. In the semi-arid Interior Plateau region, variability in Q100/Q2 ratios is most dramatic. For a 10-km2 basin, the calculated Q100/Q2 ratio of 4.9 is 1.6 times the assumed factor of 3, while for a 1-km2 basin Q100/Q2 is 7.5 or 2.5 times this factor. Underestimating Q100/Q2 may lead to underdesign and early failure of road culverts, and therefore, current FPC guidelines for estimating the 100-year instantaneous flood may have serious adverse economic and environmental consequences in small Interior Plateau watersheds.
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