Geology and Floodplain ManagementA Concept Whose Time Has Come

Kyle HouseNevada Bureau ofMines and Geology

It appears that the current system has room for improvement….

Why not improve it with an infusion of reality/geology ?

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

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.

1939 1994

Geologic Insights into Poor Urban Planningor

How the heck did the Reno-Tahoe airport get flooded?!

East Fork of the Carson River near Gardnerville, Nevada

Geologic Insights into Fluvial Dynamics

Range frontfault

Avulsed channel locations

One mile

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

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

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

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)

Recognizing and CharacterizingAlluvial FanLandforms

•Is the landform composed of alluvium or debris-flow deposits?•Does the landform have a fan-shape?

•Is the landform located at a topographic break?•Where are the lateral boundaries of the landform?

Role of Geology and Geomorphology in the New FEMA Recommendations

• These questions are of an entirely geologic nature.• Detailed surficial geologic mapping addresses each

of these issues as a matter of course.

Defining Active andInactive Areas ofErosion andDeposition

•What parts of the fan are still active?•What parts are inactive but still subject to flooding?

Role of Geology and Geomorphology in the New FEMA Recommendations

• 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.

Defining the 100-Year Flood Withinthe Defined Areas

•Method of analysis: deterministic, probabilistic, geomorphic•To what extent is flooding occurring in the defined area?

Role of Geology and Geomorphology in the New FEMA Recommendations

• 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?

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

1997

Mapping Hazards on a small alluvial fan Buckbrush Wash, Nevada

1938

1938 1997

Suburban development on Buckbrush Wash, Nevada

Buckbrush Wash, Nevada, 1997

Surficial Geology in the Vicinity of Laughlin, Nevada

• Active fan surfaces

• Inactive fan surfaces

• Relict fan surfaces

• River Terraces

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% 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

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

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)

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

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

100-year Floo

d Estim

ates140,000

205,000

240,000

25 35 120

168,000

0.10.20.5125

66

Recurrence Interval of 1993 Flood (years)

Percent Chance Exceedance

0.10.20.51251020304050

Pea

k D

isch

arge

, m3 /s

0

100000

200000

300000

Paleoflood Data

Gaged Data

Gaged and Historical Data

Gaged and Historical Data through 1992

January, 1993 Discharge Estimate

Paleoflood Data

Gaged Data

Gaged and Historical Data

Gaged, Historical Data through 1992

Flood Frequency Analysis:Lower Verde River, Arizona

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

Truckee River• 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

1090 m

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

Water Year

-220

0

-200

0

-180

0

-160

0

-140

0

-120

0

-100

0

-800

-600

-400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Pea

k D

isch

arge

, ft

3 / s

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000 Threshold 1: 45,000 cfsExceeded once in 7000 years

Threshold 2: 26,000 cfsExceeded 3 times in 700 years

Threshold 3: 24,000 cfsExceeded twice in 135 years

USGS Prediction of unregulated Qpk

1997 flood Qpk

Water Year1900 1920 1940 1960 1980 2000

Pea

k D

isch

arge

, ft

3 / s

0

5000

10000

15000

20000

25000

Composite systematic record

Comparative Flood Frequency Analysis: Truckee River at Farad, CA

0

5000

10000

15000

20000

25000

30000

35000

40000

0.0010.010.11

Annual Probability

Pea

k D

isch

arg

e (c

fs) 28-yr regulated, LP-3

98-yr regulated, LP-3

98-yr unregulated, LP-3

5000-yr, GLO

5000-yr, LP-3, censored

5000-yr, LP-3, uncensored

REGULATED 1997 DISCHARGE

UNREGULATED 1997 DISCHARGE

"100-year floo

d"

Flood Frequency Analysis: Lower Truckee River

• 4 FFA scenarios

1. Existing record

• 42 years

2. Composite record

• 98 years

3. Paleoflood record

• 700 years4. Paleoflood record

• 4000 years

Q100 est. Recurrence Intervals 1997 Qusgs

1. 24,800 ~60 years ~900 yr

2. 17,900 ~230 years >>1000 yr

3. 19,700 ~140 years ~1000 yr

4. 20,800 ~110 years >1000 yr

Q97 at Nixon: 21,200 cfs; Qusgs: 42,500 cfs

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.

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

New Book!

Published by the

American Geophysical Union

Washington DC

Available November 2001

Shameless self-promotion…in the interest of science