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Exceedance Probability (%)

FIGURE 2.10 Flow duration curves for two rivers in Michigan, showing the high flow discharge (Q0.05) that is exceeded only 5% of the time, and the low flow discharge (Q0.95) that is exceeded 95% of the time. Because the two watersheds are of similar area, discharge was not normalized to drainage area. Graphs were constructed from daily records for 1990-2000.

comparisons distinguish "flashy" from stable streams, as Figure 2.10 illustrates for locations in Michigan. The relatively flat curve of the Au Sable River indicates stable flow throughout the year compared to the much more variable Black River.

Estimating the probability of an extreme annual flood, such as one that might occur on average once in 10 or 50 years, also is useful. Typically one estimates the probability of a "1-in-N-year'' flood event of a given size or larger. Thus, a 1-in-100-year flood has a 1% likelihood of occurring in any year and the average recurrence interval is 100 years between two floods of that magnitude or larger. Flood probability (P) and average recurrence (T) are reciprocals:

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