Lecture ERS 482/682 (Fall 2002) Flood (and drought) prediction ERS 482/682 Small Watershed Hydrology

Lecture ERS 482/682 (Fall 2002) 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

Lecture ERS 482/682 (Fall 2002) Figure 11-8: Dunne and Leopold (1978)

Lecture ERS 482/682 (Fall 2002) 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

Lecture ERS 482/682 (Fall 2002) Figure 11-6: Dunne and Leopold (1978)

Lecture ERS 482/682 (Fall 2002) Figure 16.37: Viessman and Lewis (1997)

Lecture ERS 482/682 (Fall 2002) Figure 11-29: Dunne and Leopold (1978)

Lecture ERS 482/682 (Fall 2002) Figure 10-29: Dingman (2002)

Lecture ERS 482/682 (Fall 2002)

Lecture ERS 482/682 (Fall 2002) 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

Lecture ERS 482/682 (Fall 2002) 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: ASSUMPTION: Ungaged basin is the same as gaged basin

Lecture ERS 482/682 (Fall 2002) 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

Lecture ERS 482/682 (Fall 2002) Drought definitions Extended severe dry periods duration –Must have duration significant departure –Must be a significant departure from normal Can be defined in terms of –Precipitation –Streamflow –Groundwater level

Lecture ERS 482/682 (Fall 2002) Figure 10-36: Dingman (2002) Define drought threshold severity

Lecture ERS 482/682 (Fall 2002) Average = 709 cfs D  5 years