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Geology and Floodplain Management
        A Concept Whose Time Has Come




Kyle House
Nevada Bureau of
Mines and Geology
It appears that the current system has room for improvement….




      Why not improve it with an infusion of reality/geo...
Which statement is most
         persuasive?
• Our model output predicts that your
  property is within the inundation lim...
The Role of Geology in Floodplain Management—
Establishing the physical context of flood hazards

• Surficial geologic map...
Geologic Insights into Poor Urban Planning
                      or
How the heck did the Reno-Tahoe airport get flooded?!
...
Geologic Insights into Fluvial Dynamics

                                          East Fork of the
                      ...
Geologic Insights into
Fluvial Dynamics
Paleochannel patterns on
the Humboldt River
Floodplain near Battle
Mountain, Nevad...
External Geologic controls on flood hazards

                           •   Low sun-angle photo (ca.
                     ...
Why Evaluate Alluvial Fan Flood Hazards With
Geological Information?
• Alluvial fans are landforms composed of geologic de...
FEMA’s New Three-Step Approach to
 Assessing Alluvial Fan Flood Hazards
1. Determining whether the area under study is an
...
Role of Geology and Geomorphology in
  the New FEMA Recommendations



Recognizing and   • Is the landform composed of all...
Role of Geology and Geomorphology in
    the New FEMA Recommendations

  Defining Active and
  Inactive Areas of     • Wha...
Role of Geology and Geomorphology in
   the New FEMA Recommendations

 Defining the 100-     • Method of analysis: determi...
Detailed field studies:
Anatomy of an alluvial-fan flood
                 Total extent of 10,000 cfs flood on Wild Burro F...
Mapping Hazards on a small alluvial fan
       Buckbrush Wash, Nevada




1938                               1997
Suburban development on Buckbrush Wash, Nevada




   1938                              1997
Buckbrush Wash, Nevada, 1997
Surficial Geology in the Vicinity of Laughlin, Nevada




• Active fan surfaces

• Inactive fan surfaces

• Relict fan sur...
Complex alluvial fans—real and really over-simplified:
Dripping Springs Wash near Laughlin, NV
Geologic Floodplain (GFP) compared to Regulatory
Floodplain (RFP) in Laughlin, Nevada
Total piedmont flood
hazard extent:
GFP: 39%
RFP: 65%
Error Components
36% of Non-GFP in RFP
23% of GFP not in RFP



• 59...
Piedmont Flood Hazard Assessment

• Geologic studies should be first step

• Extent of Holocene alluvium (deposits/surface...
Geologic Mapping and Floodplain Management
           on Desert Piedmonts

   • Provides a scientific basis
      – Consti...
Paleoflood Hydrology
• The science of reconstructing the magnitude
  and frequency of large floods using geologic
  eviden...
The Verde River, Arizona
Verde River Flood Stratigraphy
Verde River Flood Stratigraphy
Translate stratigraphy to flood discharge via hydraulic modeling
Verde River paleoflood data structure
Flood Frequency Analysis:
                                                                                                ...
Constraining the Holocene Flood History of the Verde River

                             • Using the Quaternary history of...
Truckee River               1090 m



• Closed basin
   – 150 mile link between
     two large lakes
• Total drainage area...
Lower Truckee River

• Flood stratigraphy
• Stream Gage
• Abandoned terraces
• 1997 high-water
  marks
• Bedrock control
Lower Truckee River: Paleoflood Data Structure and
      Comparison to the Systematic Record

                           5...
Comparative Flood Frequency Analysis: Truckee River at Farad, CA
                       40000

                           ...
Flood Frequency Analysis: Lower Truckee River
• 4 FFA scenarios         Q100 est.      Recurrence Intervals
              ...
Recommendations
• Geologic studies are essential and should be performed as a
  matter of course, not as a novel add-on
  ...
Closing Thought

• Judicious (mandated?) inclusion of relevant geologic
  information into the arena of floodplain managem...
Shameless self-promotion…
in the interest of science




 New Book!
 Published by the
 American Geophysical Union
 Washing...
Geology and Floodplain Management
Geology and Floodplain Management
Geology and Floodplain Management
Geology and Floodplain Management
Geology and Floodplain Management
Geology and Floodplain Management
Geology and Floodplain Management
Geology and Floodplain Management
Geology and Floodplain Management
Geology and Floodplain Management
Geology and Floodplain Management
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Geology and Floodplain Management

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Garish rant about using geology in floodplain management in the western US. Given at Flooplain Management Association Conference in 2000.

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Transcript of "Geology and Floodplain Management"

  1. 1. Geology and Floodplain Management A Concept Whose Time Has Come Kyle House Nevada Bureau of Mines and Geology
  2. 2. It appears that the current system has room for improvement…. Why not improve it with an infusion of reality/geology ?
  3. 3. Which statement is most persuasive? • Our model output predicts that your property is within the inundation limits of our other model’s design discharge. • Physical evidence indicates that your property has not been flooded in the last 10,000 years
  4. 4. The Role of Geology in Floodplain Management— Establishing the physical context of flood hazards • Surficial geologic mapping – Understand the distribution of flood hazards from the basis of physical evidence – Understand related hazards and external geologic controls • Paleoflood hydrology – Extension of flood records in real time over 100s to 1000s of years.
  5. 5. Geologic Insights into Poor Urban Planning or How the heck did the Reno-Tahoe airport get flooded?! 1939 1994
  6. 6. Geologic Insights into Fluvial Dynamics East Fork of the Carson River near Range front Gardnerville, fault Nevada Avulsed channel locations
  7. 7. Geologic Insights into Fluvial Dynamics Paleochannel patterns on the Humboldt River Floodplain near Battle Mountain, Nevada 2000-year old meander-belt 2000-year old floodplain surface Overlying 12,000-year old Meander-belt One mile
  8. 8. External Geologic controls on flood hazards • Low sun-angle photo (ca. 1972) accentuates fault scarps • Faults exert some control on extent of flood hazard • Faults and potential for future offset complicates hazard management in unforeseen way • Indication of need for multi- hazard management in region Buckbrush Wash, Nevada
  9. 9. Why Evaluate Alluvial Fan Flood Hazards With Geological Information? • Alluvial fans are landforms composed of geologic deposits – They are mappable by virtue of their geologic characteristics – Active and inactive alluvial fans are distinguishable from the basis of geological characteristics • The deposits comprise a stratigraphic and morphologic record of flood occurrence over a large range of time scales – A natural, objective event chronology over time scales including and far in excess of planning considerations. • Despite its obvious relevance, geologic mapping is relatively inexpensive and thorough – all surficial deposits are mappable, not just those associated with principal drainages. • Geologic mapping and related studies can provide additional insights into prevailing hazards and external controls
  10. 10. FEMA’s New Three-Step Approach to Assessing Alluvial Fan Flood Hazards 1. Determining whether the area under study is an alluvial fan. 2. Identifying which portions, if any, of the area are characterized by or subject to active and/or inactive alluvial fan flooding, and 3. Defining the base (1-percent-annual-chance) flood within the areas of alluvial fan flooding identified on the alluvial fan ―GUIDELINES FOR DETERMINING FLOOD HAZARDS ON ALLUVIAL FANS‖ http://www.fema.gov/mit/tsd/FT_afgd.htm (1999)
  11. 11. Role of Geology and Geomorphology in the New FEMA Recommendations Recognizing and • Is the landform composed of alluvium or debris-flow deposits? Characterizing • Does the landform have a fan-shape? Alluvial Fan • Is the landform located at a topographic break? Landforms • Where are the lateral boundaries of the landform? • These questions are of an entirely geologic nature. • Detailed surficial geologic mapping addresses each of these issues as a matter of course.
  12. 12. Role of Geology and Geomorphology in the New FEMA Recommendations Defining Active and Inactive Areas of • What parts of the fan are still active? Erosion and • What parts are inactive but still subject to flooding? Deposition • These questions are also of an entirely geologic nature • Detailed surficial geologic mapping and related field studies can directly address them as a matter of course.
  13. 13. Role of Geology and Geomorphology in the New FEMA Recommendations Defining the 100- • Method of analysis: deterministic, probabilistic, geomorphic Year Flood Within • To what extent is flooding occurring in the defined area? the Defined Areas • Paleoflood Hydrology can greatly improve confidence in estimates of the so-called ―100-year‖ flood • Extent of flooding is largely confined to extent of Holocene alluvial deposits. – Rely on 10,000 years of flood history or anticipate that the unprecedented will occur?
  14. 14. Detailed field studies: Anatomy of an alluvial-fan flood Total extent of 10,000 cfs flood on Wild Burro Fan, Arizona Mapped in 1990-1991 by K. Vincent, P. Pearthree, K. House, and K. Demsey
  15. 15. Mapping Hazards on a small alluvial fan Buckbrush Wash, Nevada 1938 1997
  16. 16. Suburban development on Buckbrush Wash, Nevada 1938 1997
  17. 17. Buckbrush Wash, Nevada, 1997
  18. 18. Surficial Geology in the Vicinity of Laughlin, Nevada • Active fan surfaces • Inactive fan surfaces • Relict fan surfaces • River Terraces
  19. 19. Complex alluvial fans—real and really over-simplified: Dripping Springs Wash near Laughlin, NV
  20. 20. Geologic Floodplain (GFP) compared to Regulatory Floodplain (RFP) in Laughlin, Nevada
  21. 21. Total piedmont flood hazard extent: GFP: 39% RFP: 65% Error Components 36% of Non-GFP in RFP 23% of GFP not in RFP • 59% of the piedmont mischaracterized • Flood Control structures reduce extent of the GFP by approximately 25% • Geologic mapping costs a fraction of one flood-control structure
  22. 22. Piedmont Flood Hazard Assessment • Geologic studies should be first step • Extent of Holocene alluvium (deposits/surfaces <10,000 yrs old) as the extent of the geologic floodplain is conservative • In developed areas, the geological approach is partially hindsight, but its value is clearly indicated • Combine geologic data with engineering approach to iteratively develop the best characterization of flood hazards • Promote development consistent with topography and drainage • Disallow development in GFP
  23. 23. Geologic Mapping and Floodplain Management on Desert Piedmonts • Provides a scientific basis – Constitutes a test of regulatory models • Objective – Basic goal is to understand natural processes • Comprehensive scope – Coverage of large areas • Inexpensive – Relative to comprehensive engineering analyses (for which it can provide tighter focus)
  24. 24. Paleoflood Hydrology • The science of reconstructing the magnitude and frequency of large floods using geologic evidence – Physical evidence of floods • Flood-related sediments and landforms • Stratigraphic chronology of floods – Physical evidence of landscape stability • Sediments, soils, and landforms that preclude flooding • Paleohydrologic bounds—time interval over which a flood discharge has not been exceeded
  25. 25. The Verde River, Arizona
  26. 26. Verde River Flood Stratigraphy
  27. 27. Verde River Flood Stratigraphy
  28. 28. Translate stratigraphy to flood discharge via hydraulic modeling
  29. 29. Verde River paleoflood data structure
  30. 30. Flood Frequency Analysis: Recurrence Interval of 1993 Flood (years) Lower Verde River, Arizona 25 35 66 120 300000 240,000 100-year Flood Estimates 205,000 Peak Discharge, m 3 /s 200000 168,000 January, 1993 Discharge Estimate 140,000 100000 5 2 1 0.5 0.2 0.1 Paleoflood Data 0 Gaged Data 50 40 30 20 10 5 2 1 0.5 0.2 0.1 Percent Chance Exceedance Gaged and Historical Data Gaged, Historical Data through 1992
  31. 31. Constraining the Holocene Flood History of the Verde River • Using the Quaternary history of the river to constrain its flood history: – Holocene flood stratigraphy – Evidence for landscape stability Evidence converges on maximum flood magnitudes in the Holocene
  32. 32. Truckee River 1090 m • Closed basin – 150 mile link between two large lakes • Total drainage area: – 1827 mi2 / 4730 km2 • Primary runoff sources head in Sierra Nevada • Largest floods due to winter rain-on-snow scenarios 1899 m
  33. 33. Lower Truckee River • Flood stratigraphy • Stream Gage • Abandoned terraces • 1997 high-water marks • Bedrock control
  34. 34. Lower Truckee River: Paleoflood Data Structure and Comparison to the Systematic Record 50000 Threshold 1: 45,000 cfs Exceeded once in 7000 years 45000 Threshold 2: 26,000 cfs USGS Prediction of unregulated Qpk Exceeded 3 times in 700 years 40000 Threshold 3: 24,000 cfs Peak Discharge, ft3 / s 35000 Exceeded twice in 135 years 30000 25000 1997 flood Qpk 20000 15000 10000 Composite systematic record 25000 Peak Discharge, ft / s 5000 3 20000 15000 0 10000 -2200 -2000 -1800 -1600 -1400 -1200 -1000 -800 -600 -400 -200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 5000 0 1900 1920 1940 1960 1980 2000 Water Year Water Year
  35. 35. Comparative Flood Frequency Analysis: Truckee River at Farad, CA 40000 UNREGULATED 1997 DISCHARGE 35000 30000 28-yr regulated, LP-3 Peak Discharge (cfs) 98-yr regulated, LP-3 25000 98-yr unregulated, LP-3 5000-yr, GLO 20000 5000-yr, LP-3, censored 5000-yr, LP-3, uncensored 15000 REGULATED 1997 DISCHARGE 10000 "100-year flood" 5000 0 1 0.1 0.01 0.001 Annual Probability
  36. 36. Flood Frequency Analysis: Lower Truckee River • 4 FFA scenarios Q100 est. Recurrence Intervals 1997 Qusgs 1. Existing record 1. 24,800 ~60 years ~900 yr • 42 years 2. Composite record 2. 17,900 ~230 years >>1000 yr • 98 years 3. Paleoflood record 3. 19,700 ~140 years ~1000 yr • 700 years 4. Paleoflood record 4. 20,800 ~110 years >1000 yr • 4000 years Q97 at Nixon: 21,200 cfs; Qusgs: 42,500 cfs
  37. 37. Recommendations • Geologic studies are essential and should be performed as a matter of course, not as a novel add-on – has greatest scientific value early in process – reality check throughout process – Can elucidate unforeseen hazards / physical controls • Alluvial fan hazards – Extent of Holocene alluvium (deposits/surfaces <10,000 yrs old) should be considered the extent of the geologic floodplain • Flood record extension / model testing – paleoflood information should be collected to corroborate, check, repudiate empirical/theoretical flood magnitudes when record length is short and related project is moderate to high-risk.
  38. 38. Closing Thought • Judicious (mandated?) inclusion of relevant geologic information into the arena of floodplain management is essential for realistic, effective management. • Ignorance or dismissal of relevant geologic information is irresponsible if that information can be demonstrated to bear directly on the problem at hand. Photo by C. Fenton
  39. 39. Shameless self-promotion… in the interest of science New Book! Published by the American Geophysical Union Washington DC Available November 2001
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