levish pa leo flood hydrology and flood hydrology

8/3/2019 Levish Pa Leo Flood Hydrology and Flood Hydrology

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Paleoflood Hydrology

AndFlood Hydrology

Daniel Levish, Ph.D.Technical Service Center

June 2011


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Reclamation Dam Safety

350 Storage Dams

17 Western States

Constructed 1900s - 1980s

Water Storage and Hydropower


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The Problem


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PMF 50 feet over Pleistocene terrace


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About 10 percent of the PMF would removethe Pleistocene terrace (>10,000 years old)


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In the absence of sufficient rainfall data, it is

fortunate that geology furnishes, in many

instances, a clue to the probable heights offlood-waters. The clue referred to is afforded

by flood-plains and terraces, which often give

evidences of heights of water greater thanthose which would be indicated by the

computations from greatest recorded

rainfall. American Society of Civil EngineersFinal Report of the Special Committee

on Floods and Flood PreventionJanuary 19, 1916


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Paleoflood Hydrology

Uses principles of geology, hydrology, and fluiddynamics to take advantage of long-lastingrecord of large floods

Extend the record of large floods from hundredsto thousands of years

Provides context for large historical floods

Provides information on magnitude andfrequency of floods of greatest interest to Dam

Safety Reconstruct floods

Establish non-exceedance bounds

A key piece of data


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Similar to Earthquake Hazard Analysis

Since at least the 1970s seismologists have usedpaleoseismic evidence for earthquake hazard analysis

Geologic evidence of past earthquakes that ranges fromhundreds to thousands of years is combined with

seismological information, the gage record ofearthquakes, to develop earthquake magnitude andfrequency

What is so different about hydrologists combining geologicevidence of floods, or the lack of floods, with the gagerecord of floods to develop flood magnitude andfrequency?


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What if you missed something?


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Evidence of Past Floods


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Record of Past Floods


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Record of 1969 Flood


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Limits of Floods


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Why Not Just Bound the Size of Floods?

Similar to paleoseismic studies

Circumvent the what if you missed one?

Bounds are great value to frequency analysis

Provide high-assurance for dam safety Bounds span 100s to 1000s of years on nearly every

river in the West

Simply quantify channel capacity as bounded by soil

stratigraphic and geomorphic evidence Not very different from the goal of non-exceedance

now have physically-based associated probability


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7600 Year Old Volcanic Ash

S bl S f


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Stable Surface


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Example From North Platte RiverEmploys Non-Exceedance and Reconstructed Floods


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Upstream SnowmeltNon-Exceedance Bounds


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Radiocarbon Dated Stratigraphy


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Snowmelt TributaryNon-Exceedance Bound


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2-D hydraulic model results indicate there has not beena discharge of this magnitude in nearly 7000 years


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Downstream ThunderstormsMany Floods Larger Than Historical Floods


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R d f P t Fl d


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Record of Past Floods


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Paleoflood Chronology

1600

1400

1200

1000

800

600

400

200

0

Lower stage

flood

GG2

1flood

5floods

2fl

oods

1flood

2floods

1flood

5floods

3floods

2floods

1flood

3floods

5floods

1flood

1 Flood

2 Floods

3 Floods

5 Floods

GG13

GG12

GG12

GG12

GG11

GG11

GG8

GG7

GG7

GG4

GG4

GG1

a

oc

ar

on

ge

cayrs

N h Pl Ri Gl d


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North Platte River at Glendo


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levish pa leo flood hydrology and flood hydrology

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