the elusive one percent chance flood, why does it keep ... · pdf filethe elusive one percent...
TRANSCRIPT
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.