CHANGES IN FLOOD CHARACTERISTICS AFTER A MAJOR EVENT: RE EVALUATING - PowerPoint PPT Presentation

CHANGES IN FLOOD CHARACTERISTICS AFTER A MAJOR EVENT: RE ‐ EVALUATING THE EFFECT OF HURRICANE FLOYD ON FUTURE FLOOD RESPONSE Jennifer Arrigo, ECU Geography Hurricane Floyd Research Symposium Sept. 18,2009

Outline � Immediate Impacts/Measures of Floyd � Led to monitoring, modeling and policy changes � Despite Urbanization, a relatively underdeveloped and unregulated watershed � Generally know large floods can alter flood characteristics � 10 years later allows a quantitative look at this

Immediate Effects of Floyd � Historic flood heights � Flood water estimated at 95% Pamlico Sound volume (Bales, 2003) � Many location exceeded 24hr, 100 year rainfall

Legacies of Floyd � North Carolina Flood Mapping Project � demonstration project under the NWS Advanced Hydrologic Prediction Service (AHPS � Landmarks and Histories � Is there a physical or hydrologic legacy?

Eastern North Carolina � Nearly level physiographic area � Slow surface water (Carbone and Hildore 2008) Eimers et al, 2000

Eastern North Carolina � Nearly level physiographic area � Slow surface water (Carbone and Hildore 2008) Eimers et al, 2000

Channel Characteristics � Urban Stream System � Impervious surfaces cause channelization/incision � Ditching and Drainage � Floodplains themselves � Did the Hurricane Floyd Flooding change channel characteristics? � If so, we should re ‐ evaluate established relationships

Stage – Discharge Relations � Based on long term monitoring � Power law curve � Breakpoint where disc ↑≠ stage ↑ � Established relationships used for flood forecasts and modeling

Methods: Examining Relationships � Three Gage Sites based on NWS flood modeling program � Tar River @ Greenville ( USGS 02084000) � Tar River @ NC97, Rocky Mount ( USGS 02082585 ) � Tar River @ Tarboro ( USGS 02083500)

Rocky Mount � Upstream of Tar Reservoir (since 60s) � Inundation model based on gauge height

Previous SDC (1993 ‐ pre 9/99) 4 x 10 3.5 3 D 2.5 i S 2 C h 1.5 a 1 R g 0.5 e cfs 0 0 5 10 15 20 25 30 35 Stage (ft)

Variations in Stage ‐ Discharge 14000 12000 Post-Floyd Data 10/16/99 - 10/31/99 6/7/06 - 6/12/06 10000 DISC 8000 cfs 6000 4000 receding 2000 0 0 5 10 15 20 25 Stage (ft)

Stage Discharge Relations 30 R² = 0.9948 R² = 0.9846 25 20 Pre 15 Post Power (Pre) Power (Post) 10 Tar River at Tarboro 5 0 0 5000 10000 15000 20000 25000 30000

Partial Distribution Series: Greenville Gauge 35 30 25 20 Pre 1999 stage dist Post 1999 stage dist. 15 10 5 0 0 2000 4000 6000 8000 10000 12000

Suggestive Results in SDC � Rocky Mount SDC shows: � Variability � Possible hysteresis � Moderate rises vs. Extreme rises � Tarboro/Greenville Gauges not as different � Greenville PDS shows decrease in occurrence of moderate stage levels