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Flood (and drought) prediction. ERS 482/682 Small Watershed Hydrology. Floodplain management. Any action to reduce future flood damages Flood control Structures to control floods May reduce flood frequencies at certain locations May increase flood frequencies at other locations
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Flood (and drought) prediction ERS 482/682 Small Watershed Hydrology
Floodplain management • Any action to reduce future flood damages • Flood control • Structures to control floods • May reduce flood frequencies at certain locations • May increase flood frequencies at other locations • Tend to lead to increased development bigger floods
Floodplain management • Any action to reduce future flood damages • Flood control • Structures to control floods • May reduce flood frequencies at certain locations • May increase flood frequencies at other locations • Tend to lead to increased development bigger floods • Damage control • Policies to reduce flood damage
Flood prediction • Statistical analysis of historical streamflows • Construct annual series of peak flows • Calculate exceedence probability • Non-parametric • Parametric • Normal distribution • Log-normal distribution • Gamma (Pearson Type III) distribution • Gumbel’s extremal distribution • Predict flows for extreme events
Flood prediction at ungaged sites • Flood magnitude-frequency relations • Relate magnitude of floods to measurable basin characteristics (see example in Box 10-9) • USGS has relationships for all 50 states • Apply rainfall-runoff models • Rational method • SCS Curve Number method • Etc. ASSUMPTION:Ungaged basin isthe same asgaged basin
Drought definitions • Low flows • Frequency analysis with minimum flows • Annual minimum flows averaged over consecutive days • High exceedence probability (e.g., 95%) • Example: 7Q10 10% probability that the lowest 7-consecutive-day average flow will be less than the 7Q10 value • See Table 10-18
Drought definitions • Extended severe dry periods • Must have duration • Must be a significant departure from normal • Can be defined in terms of • Precipitation • Streamflow • Groundwater level
Define drought threshold severity Figure 10-36: Dingman (2002)
6 8 5 8 Average = 709 cfs D 5 years