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

1. Geology and Floodplain Management
A Concept Whose Time Has Come
Kyle House
Nevada Bureau of
Mines and Geology

2. It appears that the current system has room for improvement….
Why not improve it with an infusion of reality/geology ?

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. 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. Geologic Insights into Poor Urban Planning
or
How the heck did the Reno-Tahoe airport get flooded?!
1939 1994

6. Geologic Insights into Fluvial Dynamics
East Fork of the
Carson River near
Range front Gardnerville,
fault Nevada
Avulsed channel locations

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. 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

11. 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

12. 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)

13. 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.

14. 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.

15. 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?

16. 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

18. Mapping Hazards on a small alluvial fan
Buckbrush Wash, Nevada
1938 1997

19. Suburban development on Buckbrush Wash, Nevada
1938 1997

20. Buckbrush Wash, Nevada, 1997

23. Surficial Geology in the Vicinity of Laughlin, Nevada
• Active fan surfaces
• Inactive fan surfaces
• Relict fan surfaces
• River Terraces

24. Complex alluvial fans—real and really over-simplified:
Dripping Springs Wash near Laughlin, NV

25. Geologic Floodplain (GFP) compared to Regulatory
Floodplain (RFP) in Laughlin, Nevada

26. 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

27. 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

28. 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)

30. 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

31. The Verde River, Arizona

33. Verde River Flood Stratigraphy

35. Verde River Flood Stratigraphy

39. Translate stratigraphy to flood discharge via hydraulic modeling

40. Verde River paleoflood data structure

41. 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

42. 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

43. 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

44. Lower Truckee River
• Flood stratigraphy
• Stream Gage
• Abandoned terraces
• 1997 high-water
marks
• Bedrock control

45. 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

46. 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

47. 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

48. 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.

49. 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

50. Shameless self-promotion…
in the interest of science
New Book!
Published by the
American Geophysical Union
Washington DC
Available November 2001