The Elusive One Percent Chance Flood, Why Does It Keep Changing? Association of State Floodplain Managers, Annual Conference, June 9-14, 2013 – Hartford, CT T. Lynn Lovell, PE, CFM, D.WRE – Halff Associates, Inc. Walter Skipwith, PE, D.WRE – Halff Associates, Inc. Craig Loftin, PE, CFM – Corps of Engineers, Fort Worth District

Outline of Presentation

• Introduction – ASCE Paper, Original Idea, WIP • History of 100-Year (1% Chance) Flood

Standard • Causes of Changes • Case Studies – Changing 100-Year (1%) Flood • Observations and Conclusions

100-Year (1% Chance) Flood Definition

• Definition: “1% Chance Flood” is an event of a magnitude which could be expected to be equaled or exceeded once on the average over a 100-year period (recurrence interval).

• “The 1% chance flood standard was a compromise from the beginning.”

REDUCING FLOOD LOSSES: Is the 1% Chance Flood Standard Sufficient?, 2004 Gilbert F. White National Flood Policy Forum, Washington, D.C.

100-Year (1% Chance) Flood Definition

• In the Corps of Engineers’ Flood Plain Management Services program, the term “Intermediate Regional Flood” (100-year) was used for floodplain planning in FPI Reports (Mostly in 1970s-1980’s).

• FEMA officially adopted the term “1% Annual Chance Flood” in the updated flood insurance studies (tables and DFIRMs), beginning with the Map Modernization (Map Mod) program in 2003.

History of Use of Term: 100-Year Flood • 1923 – California Dept. of Public Works – 100

year probable frequency for 140 streams in CA • 1955 – Book “FLOODS” by Hoyt and Langbein

• In 1971 the FIA (administered by HUD) issued

a rule setting the 1% or 100-year flood as the mandatory minimum regulatory standard for the NFIP.

Causes of Changes or Revisions in Computed Flood Data

• Urbanization • Changes in the stream geometry • Encroachments • Obstructions (such as dams, aerial utility

crossings, new bridges) • Topographic data • Computational methods and procedures

Causes of Changes or Revisions in Computed Flood Data

• Modeling software • Changing historical flood records • Technical judgment and assumptions • Changes in floodplain characteristics • Changes in policies regarding reservoir

operation • Climate Change? Rainfall changes.

Urbanization

• Key cause of increased runoff

• Flood discharges at least 250% higher in urban watersheds compared with forested catchments in Texas and New York after similar storms

Reference: Rising Waters (Brody, 2011)

Urbanization

1992

2006

14 Year - Comparison of Urbanization

in DFW Metroplex 1992-2006

(Melinda Luna, TNRIS)

Urbanization Colleyville, TX

Little Bear Creek

1971 Corps’ FPI

2009 FEMA Map Mod

Encroachments

• Encroachments within a floodplain or a stream, such as structures or fill, reduce the flood-carrying capacity, generally increase flood heights and velocities and often increase flood hazards in areas beyond the encroachment itself.

• “FEMA Floodway fringe” encroachments have long been the cause of reduction of natural or existing valley storage (and changes in floods).

Physical Changes in Stream and Floodplain Geometry

From Pima County, AZ

Encroachments

FEMA Floodways Levees/Treatment Plants

Dallas, TX

Mobile Homes

Haltom City, TX

Encroachments

• A 1992 study for Colleyville, TX determined that in certain reaches of Little Bear Creek the FEMA floodway encroachments reduced the valley storage by up to 30% and increased peak 100-year discharges from 25% to 42%.

• These increased discharges resulted in increased expected flood elevations by an average of 2.9 feet.

Physical Changes in Stream and Floodplain Geometry

Obstructions

Floodplain and Stream Vegetation

Over time the density of vegetation within a channel or in the overbanks of a floodplain can change and decrease the conveyance or capacity of the stream, resulting in generally higher flood levels and changes in the peak discharges.

Floodplain and Stream Vegetation

Revised Topographic Information

Changes in the topographical nature of a given watershed have several causes: channelization or other modifications of streams, erosion and sedimentation processes, filling of floodplain fringes for development, diversions and introduction of new obstructions such as dams and roadways.

Changes in Topographic Data

1999 = Blue 2009 = Orange

Computational Methods, Modeling Software, and Engineering Judgment

Another significant reason for changes or differences in flood discharges/elevations for studies in the same watershed or stream is the wide range of methods/technology of hydrology and hydraulics.

Different Methods • OLD SCHOOL – Pre-computers: Planimeters,

slide-rules, mechanical calculators, paper graphs, nomographs and charts, hand backwater

• GIS-based modeling tools • 2-D Dynamic models

Different Models/Software • LRD-1 • HEC-1 • HEC-2 • TR-20 • WSP • HEC-HMS • HEC-RAS • Unsteady and Two-Dimensional Models • MANY OPTIONS TO CHOSE FROM!

Model Selection Discussion

HEC-2

HEC-RAS WSP XPSWMM-2-D Rod Serling’s 4-D

Engineering Judgment/Assumptions • Basic selection of modeling software to use • Delineation of drainage areas • Hydrologic parameters • Location/spacing/alignment of cross-sections • Selection of roughness values • Bridge/culvert modeling • Effective/ineffective flow limits • Rainfall data

0 500 1000 1500 2000204

206

208

210

212

214

216

218

220

222

Floodway Determination - Example 6 Plan: Method 5 Encroachment 4/5/2005

Upstream Boundary Cross Section

Station (ft)

Elev

atio

n (ft

)

Legend

EG PF#2

WS PF#2

EG PF#1

WS PF#1

Ground

Levee

Bank Sta

Encroachment

.1 .14 .04

.14

Example of Wide Variation in 1% Chance Flood in Hill County, Texas

5,000 cfs/sm

800 cfs/sm

Changes in Historical Records The accumulation of historical rainfall and flood data, whether in the form of collected stream gage information (stage and measured flow) or high water marks, impacts the results of hydrologic and hydraulic studies.

Changes/Cycles in Hydrologic Records

Historical River Flow

Historical Rainfall

Graphs from Freese and Nichols

CASE STUDIES All in Texas

Note: Red Highlights = Maximum Value

Green Highlights = Minimum Value

West Fork of Trinity River

Oct 1981- July 1988 FIS(GP)

COE REIS 1987

Dallas County

Aug 2004 FIS

NCTCOG CDC

V.4 - July 2009

Draft Map (2010) Mod

FEMA RAMPP (2012)

Draft Revised CDC – USACE 2013

LOCATION

D.A (sm)

Software or Method FFA HEC-1 HEC-1/FFA HEC-1 HEC-1/FFA

Peak FQ HEC-HMS

Year of Land Use 1977

Exist in

1985? Future Exist 2050 2055

Above Elm Fork 3,045 88,500 87,800 92,800 87,800 103,100 At Beltline Road Grand Prairie Gage 3,070 79,000 75,100 86,200 92,300 90,000 57,520 106,300

At Highway 360 2,572 76,500 78,000 87,100 93,900 87,100 107,400 At Fort Worth gage

2,614 53,000 53,000 47,000* 58,700 47,000 52,300 69,400

TABLE 1 – Comparison of 100-Year Floods – West Fork Trinity River

West Fork of Trinity River TABLE 1 – Comparison of 100-Year Flood Elevations – West Fork Trinity River

Location CDC Manual 2nd Edition -

SEP 1998

CDC Manual 3rd Edition

2003

CDC Manual 4th Edition JULY 2009

Revised CDC Manual 4th

Edition 2010 (USACE 9/09)

REV. USACE CDC Study - DRAFT NOV

2012

REV-USACE CDC Draft May 2013

Diff. May 2013 Rev CDC-4th Ed July 2009

SH 360 463.99 463.89 463.89 464.12 465.8 465.4 1.28

Belt Line Road 440.88 438.71 438.71 438.77 441.7 441.2 2.43

(Gage)

Macarthur Boulevard 436.11 436.01 436.01 436.09 437.2 437 0.91

Current Effective Corridor Development Certificate Elevations

West Fork of Trinity River

Probable reasons for changes in West Fork Flood Discharges include: • Changes in watershed urbanization

• Different hydrologic analyses/software

• Complex hydraulic modeling

• Changing historical record

Little Bear Creek

COE FPI 1971

1982 FIS

January 1993

Tarrant County FIS

August 1995 Tarrant County

FIS

Tarrant County Map Mod 2009

LOCATION D.A (sm)

Software or Method

Hand Calcs NUDALLAS NUDALLAS NUDALLAS

At Mouth 24.1 13,700 13,700 13,700

At SH 121 20.2 14,000 12,500 12,500 12,500 12,500

At SH 26 11.16 7,500 7,500 7,500 7,500

TABLE 6 – Comparison of 100-Year Floods – Little Bear Creek

This table demonstrates the problem of outdated hydrology that is used over long periods of time

(27+ years).

Little Bear Creek TABLE 7 – 100-Year Flood - Little Bear Creek (1992 Colleyville Study - Halff)

Note the effect of floodway encroachments on hydrology!

+40%

Cottonwood Creek – Collin County Changes from 1997 FIS* to 2009 FIS (Map Mod)

-71% -78%

Source: Hydraulics TSDN for Collin County Map Mod Revisions (June 15, 2005)

-81%

-85%

-87%

Discharge Change

(1)

(2)

(1) Maximum difference in elevation (BFE) = 7 feet (2) Difference in elevation (BFE) = 0 feet (Wow!)

* Original FIS (USACE) H & H from 1980’s

*

Cottonwood Creek – Collin County

Source: Hydrology TSDN for Collin County Map Mod Revisions (March 31, 2005)

Cottonwood Creek – Collin County

Possible Reasons for Radical Reduction (Changes) in discharges on Cottonwood Creek:

• Different hydrologic methods/parameters

• Differences in drainage areas (8%-11%)

• Different urbanization/land use data + changes

Cottonwood Creek – Collin County Possible Reasons for Radical Reduction (Changes)

on Cottonwood Creek: • Major errors in original hydrology

• Major errors in revised hydrology

• Elevation differences – bridge modeling?

• Calibration methods differ?

Colorado River

COE FPI 1975

January 2000 Travis County

FIS

Travis County Map Mod, date

Difference

LOCATION D.A (sm)

Software or Method

Frequency Analysis

Frequency Analysis

Frequency/Models

Year of Land Use

Lake Travis 38,130 716.0(1) 722.0(1) 4.0

Austin Gage 38,400 170,000 170,000 90,100 79,900

At Onion Creek

38,500 170,000 90,000 79,900

TABLE 10 – Comparison of 100-Year Floods – Colorado River

(1) Stillwater elevation at dam

Colorado River Reasons for change in flood releases and Lake Travis elevation changes are due to extensive restudy/modeling/Period-of-record simulation/frequency analysis of Colorado River basin and reservoir flood operation.

• HEC-HMS /Unsteady HEC-RAS • HEC-5 (later HEC-ResSIM) • SUPER • FFA

Trinity River (Dallas)

LOCATION

CORPS 1990

1957 Land Use

HALFF 1991

1991 Land Use

Difference*

Downstream of Loop 12 401.90 401.90 0.00

Texas & New Orleans R.R. (downstream) 406.7 408.72 2.02

U.S. 75 South Bound 406.89 409.30 2.41

Central WWTP near Peak Flow Storage Lagoon 408.41 410.80 2.39

Central WWTP near Dallas Peak Flow Pump Station

409.00 412.19 3.19

A.T. & S.F. RR 411.90 415.52 3.62

Commerce Street 413.92 417.24 3.32

TABLE 11 – Comparison of 100-Year Flood Elevations – Trinity River at Dallas

*Note: Differences due primarily to floodplain encroachments and vegetation growth. Current Corps’ studies will again revise these values.

Guadalupe River

LOCATION Comal County 2009

Map Mod, ft-msl 1983 Effective

FIS, ft-msl Difference

feet

Horseshoe Falls 759 751 8

Dam #5 744 732 12

Private Road (BR 22) 720 704 16

River Road (3rd Crossing) 710 704 16

River Road (2nd Crossing) 684 674 10

TABLE 12 – Comparison of 100-Year Flood Elevations – Guadalupe River

Major differences primarily caused by changed historical flood record (such as 2002 flood), changes in channel and overbanks, and more detailed study.

Observations and Conclusions The most significant factors for changing 1% Chance discharges and elevations seem to be: 1) The changes, over time, in watersheds due to the effects

of urbanization and historical rainfall/flooding records.

2) Floodplain encroachments and changing vegetation.

3) The various methods/assumptions and models that are used for the hydrologic and hydraulic computations

Observations • One disturbing observation from the Texas

Case Studies is the “non-changing” 1% Chance flood discharges, over long periods of time, in many watersheds, using outdated hydrology (Little Bear Creek example).

• At the regional (Southwest) level there seems to be a diverging from traditional (primarily USACE) hydrology methods (example: Cottonwood Creek – Collin County)

Observations

A significant issue with changing flood data and floodplains is the impact that it has on cities and counties as they attempt to prudently manage their floodplains and adjoining development.

Conclusions It is a difficult task for floodplain administrators to explain all of this to the layman or developer who often asks this question: “Why are the flood elevations and floodplains changing?” Sometimes we just don’t really know the answer.

?

Conclusions

• There is an obvious need to analyze the hydrologic (and hydraulic for that matter) impacts of the FEMA floodway concept, as it is utilized in streams and rivers.

• Even better would be to eliminate the floodway concept entirely, such as the City of Waco is proposing.

Final Conclusion In conclusion, changes are inevitable in watersheds, streams and floodplains due to urban growth, land modifications, changing vegetation, climate change, and other physical changes.

QUESTIONS?

The Elusive One Percent Chance Flood, Why Does It Keep Changing? Association of State Floodplain Managers, Annual Conference, June 9-14, 2013 – Hartford, CT T. Lynn Lovell, PE, CFM, D.WRE – Halff Associates, Inc. Walter Skipwith, PE, D.WRE – Halff Associates, Inc. Craig Loftin, PE, CFM – Corps of Engineers, Fort Worth District

The Elusive One Percent Chance Flood, Why Does It Keep Changing?

Association of State Floodplain Managers, Annual Conference, June 9-14, 2013 – Hartford, CT T. Lynn Lovell, PE, CFM, D.WRE – Halff Associates, Inc. Walter Skipwith, PE, D.WRE – Halff Associates, Inc. Craig Loftin, PE, CFM – Corps of Engineers, Fort Worth District

Travis County, Texas – 100-Year Peak Regression

2,400 CFS/SM

1,100 CFS/SM

Walnut Creek (Austin)

COE FPI 1972

COE XFPI May 1980

1989 COE HEC-2 Model

January 2000 Travis County

FIS Travis County FIS, Sep 2008

LOCATION D.A (sm)

Software or Method

Hand Calcs HEC-1 NUDALLAS NUDALLAS HEC-HMS

Year of Land Use 1972 1979 1989 +/- 2006 +/-

Mouth 56.53 20,500 33,170 33,170 33,350

US SPRR 52.86 15,780 15,790

DS of MLK 51.50 26,900 15,600 15,600

US of MLK 51.50 26,300 33,270 33,270 32,210

DS Little Walnut Cr

50.93 21,300 32,550 32,550 32,440

US Little Walnut Cr

36.29/37.55 16,000 25,400 25,400 27,410

At Dessau Rd 25.71 16,000 20,200 24,900 24,900 26,940

DS Lamar 20.48 15,200 13,900 21,400 21,400

TABLE 9 – Comparison of 100-Year Floods – Walnut Creek (Austin)

Mary’s Creek

COE FPI 1975

1986 COE HEC-2 Model

January 1993 Tarrant County

FIS Tarrant County Map Mod 2009

Preliminary Fort Worth Study

2011(1)

LOCATION D.A (sm)

(1)Prel. , Being

revised. Software or Method

Hand Calcs NUDALLAS NUDALLAS NUDALLAS HEC-HMS

Year of Land Use/Cover

Exist in 1974

1984 1984 1984 2010

At Mouth 56.04 44,400 42,800 42,800 42800 29,210

Benbrook Rd(USGS gage)

43.31 37,500 37,500 37,500 32,450

Above Walnut Cr

43.28 36,500 37,000

DS Loop 820 40.83 37,900 37,900 37,900 29,520

US South Marys Cr

26.21 26,100 27,300 30,700 30,700

DS South Marys Cr

17.05 22,600 22,600 19,900

TABLE 8 – Comparison of 100-Year Floods – Mary’s Creek

Mary's Creek

Probable reasons for changes in Mary’s Creek Flood Discharges include: • Changing historical record at gage?

• Hydrologic/hydraulic analyses/assumptions

• Changes in watershed urbanization

Big Fossil Creek

COE FPI 1974

1986 COE HEC-2 Model

Halff Sep 1988

COFW

January 1993 Tarrant County

FIS

2005 COE Restudy

Tarrant County Map Mod, 2009

FEMA RAMPP (2012)

LOCATION D.A (sm)

Software or Method

Hand Calcs

NUDALLAS NUDALLAS NUDALLAS HEC-HMS HMS Peak

FQ/HMS Year of Land Use/Cover

Exist in 1974

2005

At West Fork 76.64 41,250 45,100 43,380 DS Little Fossil Cr

73.77 41,250 45,240 45,240

US Little Fossil Cr

56.1 29,300 33,440 36,400 33,444 38,240

At SH 121 55.0 29,300 33,810 36,700 33,813 At Belknap St (US 183)

52.8 54.75(2)

29,300 33,820 36,650 33,820 40,540

At SLSW RR at R.M. 6.30 (30945)

43.95 41,300 29,400 31,660 36,010(3)

At IH35 21.14 20,310 20,310 22,800 20,310

At FM 156 16.33(4) 21,000 19,490 20,050 18,600

TABLE 2 – Comparison of 100-Year Floods – Big Fossil Creek

More Big Fossil Creek – Comparisons of Methods

TABLE 3 – Big Fossil Creek at IH 35W (TxDOT Study By H&H Resources - 2012)

Even More Big Fossil Creek Comparisons

TABLE 4 - Comparison Table From FEMA - RAMPP STUDY (July, 2012) Restudy Versus Effective FEMA (Parentheses)

100-Year

Big Fossil Creek

Probable reasons for changes in Big Fossil Creek Flood Discharges

• Different H&H methods and models

• Hydrologic/hydraulic analyses/assumptions

• Changes in watershed urbanization

Big Bear Creek

COE FPI 1971

Oct 1981 Grand Prairie FIS(1)

Sep 1990 DFW

Airport Study

January 1993 Tarrant County

FIS 2007

NCTCOG

Tarrant County FIS,

2009

LOCATION D.A (sm) NUDALLAS

Software or Method

Hand Calcs

HEC-HMS

Year of Land Use

1970 1990 2007

Mouth 93.8 42,000 30,230

US Beltline Rd 82.22 33,510 30,730 30,180*

Hwy 183 74.89 77.13

40,000 34,000 38,100 34,000 31,080 31,080

DS Little Bear Cr 68.2 67.1

36,100 34,100 28,140 35,000

US Little Bear Cr 44.6 25,000 22,700 22,800 20,220 23,100

At Trib BB1 43.2 21,200 22,000 19,670 19,920

Davis Blvd 17.9 15,800 14,810 14,810

TABLE 5 – Comparison of 100-Year Floods – Big Bear Creek

*Between Beltline and Shady Grove Road – Dallas Draft Map Mod

TxDOT History of Use of Term: 100-Year Flood • 1962 – TX HWY DEPT – Drainage Manual

– Design Criteria for “River Crossings” = 10- to 100-Year or Highest historical flood

• 1970 – TX HWY DEPT – Drainage Manual

– Design Criteria for “River Crossings” = 10- to 50-Year • 1985 - TxDOT– Drainage Manual

– Design Criteria for “River Crossings” = 10- to 50-Year • 2004-2011- TxDOT– Drainage Manual

– Design Criteria for “Major River Crossings” = 25- to 50-Year, Check 100-Year Flood (for all drainage structures!)

Buildings as Encroachments in Floodway Fringe

Travis County, Texas – 100-Year Peak Regression

2,400 CFS/SM

1,100 CFS/SM

Little Bear Creek

COE FPI

1971

1982 FIS

1992 Colleyville M

Plan

1993 Tarrant

County FIS

August 1995 Tarrant County

FIS

Tarrant County Map Mod 2009

LOCATION D.A (sm)

Software or Method

Hand Calcs NUDALLAS HEC-1 NUDALLAS NUDALLAS NUDALLAS

Year of Land Use 1971 1990 At Mouth 24.1 13,700 13,700 13,700

At SH 121 20.2 14,000 12,500 12,500 12,500 12,500

At SH 26 11.16 7,500 7,500 7,500 7,500

Above LB-2 8.85 7,700 At SL & SW Railroad

8.2 7,400 7,400 7,400

At FM 1938 (Davis Blvd)

4.6 4,500 4,500 4,500

TABLE 6 – Comparison of 100-Year Floods – Little Bear Creek

Floodway Schematic (FEMA, 2012)

Obstructions

Obstructions to flood flow along streams and rivers include man-made structures such as dams, aerial utility crossings, roadways and bridges.

Hydraulic Encroachment?

Floodplain and Stream Vegetation

Examples of Different Methods

• Historical frequency analysis, SCS, Snyder’s Unit Hydrograph Method

• OLD SCHOOL – Hand backwater computations, gradually varied flow (one-dimensional steady-state)

• Backwater computer models, steady or unsteady and/or 2-D models

Engineering Judgment/Assumptions – Modelers … So Many Choices!

0 500 1000 1500 2000202

204

206

208

210

212

214

216

218

Multiple Openings - Example 5 Plan: Modified Conditions Geom: Culvert Group + Relief Bridge Flow : Beaver Cr. - 3 Flow s

RS = 5.39 Dow nstream of bridge embankment Run for Velocity checks

Station (ft)

Elev

atio

n (ft

)

Legend

EG 100 yr

WS 100 yr

0 ft/s

1 ft/s

2 ft/s

3 ft/s

4 ft/s

5 ft/s

6 ft/s

7 ft/s

Ground

Ineff

Bank Sta

.15 .04 .2 .06 .2

• Hydrologic modeling parameters • Hydraulic modeling parameters • Model selection (appropriate?)

Figure 2 – 1975 FPI Report on Colorado River and Country Club Creek Note: The 1936 Flood has been considered to be approximately the 100-year Flood.

1936 Flood Hydrographs

Cottonwood Creek – Collin County

Comparison of Changes and Methods

Source: Hydrology TSDN for Collin County Map Mod Revisions (March 31, 2005)

Conclusions

It is possible that standardization of modeling methods and techniques could eventually help minimize the widely varying differences in computed 100-year flood discharges and elevations.