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

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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

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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

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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

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Immediate Effects of Floyd

Historic flood heights Flood water estimated at 95% Pamlico Sound

volume (Bales, 2003)

Many location

exceeded 24hr, 100 year rainfall

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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?

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Eastern North Carolina

Nearly level physiographic area Slow surface water (Carbone and Hildore 2008)

Eimers et al, 2000

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Eastern North Carolina

Nearly level physiographic area Slow surface water (Carbone and Hildore 2008)

Eimers et al, 2000

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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

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Stage – Discharge Relations

Based on long term

monitoring

Power law curve

Breakpoint where disc↑≠

stage ↑

Established

relationships used for flood forecasts and modeling

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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)

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Rocky Mount

Upstream of Tar

Reservoir (since 60s)

Inundation model

based on gauge height

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Previous SDC (1993 ‐pre 9/99)

5 10 15 20 25 30 35 0.5 1 1.5 2 2.5 3 3.5 x 10

Stage (ft) D i S C h a R g e

cfs

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Variations in Stage‐Discharge

5 10 15 20 25 2000 4000 6000 8000 10000 12000 14000 Post-Floyd Data 10/16/99 - 10/31/99 6/7/06 - 6/12/06 receding

Stage (ft) DISC cfs

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Stage Discharge Relations

R² = 0.9948 R² = 0.9846 5 10 15 20 25 30 5000 10000 15000 20000 25000 30000 Pre Post Power (Pre) Power (Post)

Tar River at Tarboro

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Partial Distribution Series: Greenville Gauge

5 10 15 20 25 30 35 2000 4000 6000 8000 10000 12000 Pre 1999 stage dist Post 1999 stage dist.

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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

f moderate stage levels