floodways, do we still need them?

Floodways, Do We Still Need Them?

Presented at the TFMA Region 6 Inaugural Regional South Central Luncheon/Seminar- June 12, 2014, San Antonio, TX

T. Lynn Lovell, PE, CFM, D.WRE – Halff Associates, Inc.Bill Brown, PE, City of Arlington, TxJohn Ivey, PE, CFM – Halff Associates, Inc.Mike Moya, PE, CFM – Halff Associates, Inc.

Outline of Presentation

• Introduction – Reasons for Paper

• Floodway Concept and History

• Floodway Issues/ Impacts /Questions• Case Studies

• Findings and Conclusions

TFMA Spring Conference in Irving

President and Vice-Presidentof TFMA Frolic in Floodway Fringe

J.B..

D. C.

Joe F.

Key Question of the Day:

Is the continued use of the “Floodway Concept”, as a floodplain management tool, appropriate for sustainable, resilient, and prudent floodplain-related decisions and polices, now and in the future?

Introduction – Reasons for PaperLots Attention of Late:

• ASFPM White Paper on “The Floodway Encroachment Standard: Minimizing Cumulative Adverse Impacts” Lulloff, et al, (May 2013)

• ASFPM/NAFSMA Paper on Unsteady and Two Dimensional Models: Issues for Regulatory Use (April 2014) – Forum at Seattle last week.

Introduction – Reasons for PaperLots Attention of Late:

• 2014 - Success of NCTCOG/Corps’ Trinity River Corridor Development Certificate (CDC) program in preserving “Valley Storage” along river

Introduction – Reasons for Paper

• Long time feeling that the floodway concept was not prudent floodplain management (since 1971)

• Continued Trend: “Fill floodway fringe to the Limits”

Introduction - WWBD

Bill Brown has strong opinions regarding floodways:

– A floodway is an administrative tool that has no physical meaning.

– The concept is antiquated and a historical tool whose usefulness is diminishing.

– Becoming obsolete with modeling paradigm shift to dynamic models.

Introduction - WWBD

Bill Brown has stronger opinions regarding floodways (continued):

– Floodways need to go away and we should focus on regulatory concepts where conveyance, zero rise, channel stability, and flood storage preservation are the primary factors in cases where encroachment is necessary. Otherwise, don’t encroach into the floodplain!

Introduction - WWBDBill Brown FINAL WORDS regarding floodways :

– A floodway is to hydraulics what the Rational Method is to hydrology …

– They both were useful tools whose time has passed.

I sure wish he would make his OPINIONS clear on floodways!

Introduction - WWBD

Floodways Just Suck!

Some DefinitionsTHE REGULATORY FLOODWAY:The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the base flood (1% chance) can be carried without substantial increases (Max. of 1.0’ ) in flood heights.

NO ADVERSE IMPACT :NAI is a philosophy that looks at the impacts of land use decisions, identifies adverse impacts, and mitigates them through a variety of actions.

Some “No Adverse Impact” Floodway Factors

• 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).

The “Natural” Floodway Concept

FLOODWAY

FLOODPLAINLIMITS

Essentially No Encroachment Into “MOVING WATER”

Discussed by Tom Lee,Wisconsin DWR, ASCEPaper, August 1971

The FEMA Floodway Concept

Floodway Schematic (FEMA, 2012)


Equal Loss of Conveyance onBoth Sides

Conveyance is a function ofArea, Wetted Perimeter,

And Roughness

Maximum 1.0’ Rise

Floodplain

Floodway RegulationsCurrent 8 States that enforce more restrictive floodways (state law):

Illinois (0.1’) Montana (0.5’)

Indiana (0.1’) Michigan (0.1’)

New Jersey (0.2’) Colorado (0.5’)

Minnesota (0.5) Wisconsin(0.01’)<<<

Early TFMA Floodway Target Selection Discussion+2.0’RISE!!

1.0’ 0.5’0.01’

MINUS 1.0’ RISE!!

HISTORY OF FLOODWAYS

Flood Hazard Studies Timeline1950 19901960 1970 1980

1960’s – 1970’s

TVA Flood Hazard Information StudiesDefined “Regional” and “Maximum

Probable Floods”

1968 – 1980’s

FIA – National Flood Insurance Study effortFHBM, FIS-FIRM, FBFM

1960-1980

USACE Floodplain Information Reports (FPI’s)

1950’s, 1960’s and early 1970’s

Flood Hazard Studies/ Flood Prone QUAD’sTVA, USACE, USGS, SCS

2000

’s –

MA

P M

OD

and

RIS

K M

AP

FLOODWAY CONCEPT/IMPLEMENTATION

History

• 1951 flooding in Kansas and Missouri resulted in a study to determine the feasibility of a flood insurance program. The study, funded by the insurance industry, did not favor such a program.

• 1955 and 1956 floods resulted in Congress enacting the Federal Flood Insurance Act of 1956.

History• Tennessee Valley Authority (TVA) – 1950’s First

floodplain maps; first floodways

• 1960 - Maryville, TN adopted the 1st Floodplain Ordinance with Floodway Map

• FEMA Floodway Concepts, based on 1% Chance (100-year frequency) flood, since the early days of the NFIP

History

• 1950’s & 60’s TVA Flood Hazard Information studies defined “Regional” and Maximum Probable” floods

• 1958 seven states (CN, IN, IO, MA, NJ, PA and WA) had enacted and were enforcing state floodplain management regulations

• 1959 TVA report on reducing national flood damage

HistoryHousing and Urban Development Act of 1968 … gave birth to the NFIP and …

“A Unified National Approach for Floodplain Management”

The 1968 Act established floodplain and floodway regulations but community enforcement was voluntary

History1969 - The Water Resources Council revised the Flood Hazard Evaluation Guidelines and defined the floodway as we know it today.

History – 1977 - EO 11988

“to provide for the discharge of the base flood so the cumulative increase in water surface elevation is no more than a designated amount (not to exceed one foot as set by the NFIP)”

“to avoid to the extent possible the long and short term adverse impacts associated with the occupancy and modification of floodplains”.

History – 1977 EO 11988“Base flood is that flood which has a one percent chance of occurring in any given year” (also known as the 100-year flood) and “to provide for the discharge of the base flood so the cumulative increase in water surface elevation is no more than a designated amount (not to exceed one foot as set by the NFIP)”

History - Floodway

• James Goddard’s Report to the FIA 1978

“Origin & Rational of CriterionUsed in Designating Floodways”

History - Goddard 1978

• The technical requirements must often be modified by practical, economic, social, and related considerations. (i.e. COMPROMISE!)

• The “Zero” rise approach could result in inefficient use of the flood plain. (Was this a valid statement?)

1977 or 1978 Corps’ Floodway Study

• Based on 2,390 cross-sections (Nation-wide)• Floodway Target was 1.0’

– Mean increase in water surface was about 0.7 foot.

– Increase at many points was less than 0.4 foot.– Average width of floodway was about 55 percent

of the 100-year flood plain width.

Nine States with More Stringent Floodways (1978)• State Allowable Rise (feet)• Illinois 0.1• Indiana 0.1• Maryland* Zero• Michigan 0.1• Minnesota 0.5• Montana 0.5• New Jersey 0.2• Ohio* 0.5• Wisconsin 0.1

*Not in current list.

History - Floodway Guidelines - 1979

• FIA published “The Floodway – A Guide for Community Permit Officials” HUD-529-FIA

1984 - Floodway Boundary “Cast in Stone”

Once Floodways are designated by a community, usually when the FIRM is referenced by ordinance, floodways can only be changed by LOMC or FEMA’s remap efforts. (1984)

1987 – HEC Report to FIA

“Floodway Concept Applications in Unique Situations” (Draft Report - February 1987)• Covered floodway concepts such as:

– split flow, low levees, alluvial streams– Supercritical flow, developed floodways– Issues with bridges/culverts– Alternative methods of computation

2007 - ASFPM – “National Flood Programs and Policies in Review”

• “A no-rise Floodway, with no impact on water surface and velocity should be required, so that only those areas of insignificant hydraulic conveyance could be filled.”

• Paraphrase: “…Allowing current FEMA floodway standards (1’ Rise), causes increased flood peaks, additional flood damage, and promotes filling of riparian zones…”

2010 - Survey from ASFPM

Floodway Issues, Impacts and Some Questions

Regulatory and Economic Issues

• Constitutional/Legal (taking issues)

• Economics: Floodway fringe fill is a “Carrot”

• Balance between Technical requirements and practical, economic, social, and environmental considerations (Remember Goddard’s quote, i.e. COMPROMISE).

Technical Issues

• Equity in encroachments allowed (equal loss of conveyance)

• Depths and velocities at edges of encroachments (danger)

• Existing development upstream, downstream• Other required permits (Sec 404, wetlands,

unique, historic)

Regulatory and Economic Issues

• Public’s difficulty in understanding concepts

• Costs for more detailed studies/floodway computations

• Freeboard (Elevate fill and finished floors)

• Political and land development pressures

Technical Issues

Islands

Technical Issues

Some Floodway EncroachmentsBridges Levees/Treatment Plants Mobile Homes in Floodway

Technical Issues

• Adequate topographic mapping (accuracy and detail)

• Effort/judgment needed for more intensive modeling of floodways

• Unusual structures show up in floodways!

Parking Lot Built Out Over Floodway

Technical Issues• Urbanization of watersheds will eventually

create higher flood elevations for floodways developed with “existing” conditions watersheds. (Old studies vs. New)

• Complex flow patterns (i.e. split flow, diversions, etc.) create floodway difficulties

• Delineated floodway on FEMA maps do not match hydraulic models (Map Mod saw this)


HEC-2 STYLE

A FEMA “Density Floodway” Concept (Circa 1981)

Was this a good idea?Any Adverse Impacts here?


HEC-RAS STYLE

Technical Issues

XPSWMM 2-DFloodway

???

Questions:

• “Do the economic benefits of additional development justify the physical encroachments of the floodway?”

• “Does the floodway concept work in a changing paradigm of Unsteady and Two Dimensional hydrologic-hydraulic modeling?”

Questions:

• “Can a floodway-dominated stream achieve No Adverse Impact?”

• Do we really need floodways in our floodplain management programs?

• Is it realistic to think that the floodway

concept can be eliminated?

FLOODWAY CASE STUDIES

Colleyville, Tx Floodway Case Study

Case Study – Colleyville, TX• A 1992 study for Colleyville, TX determined

that in certain reaches of Big and Little Bear Creeks, the FEMA floodway encroachments reduced the valley storage by up to 45% and increased peak 100-year discharges from 25% to 42%.

• These increased discharges would result in increased expected flood elevations by as much as 2.9 feet.

Urbanization Colleyville, TX

Little Bear Creek

1971 Corps’ FPI

2009 FEMA Map Mod

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

Note the effect of floodway encroachments on hydrology!

+ 37%+ 25%

+ 7%

+ 11%

+ 11%

Observations and Conclusions

The most significant factors related to increased flood peaks and floodway impacts in Colleyville study:

1) The changes, over time, in watersheds due to the effects of urbanization.

2) Decrease in available “valley flood storage”

Case Study – Village Creek

2009 Map ModUpdated DFIRM

Village Creek in Kennedale – 1.4 miles

Of Stream*

* Drainage AreaAt US 287 = 121 SM

I.H. 20

Lake Arlington


• Most development in floodway prior to incorporation into city

• June 1970 Corps’ FPI Report – few structures

U.S. 287


FLOODWAY

US 287


Most Development is Industrial, Mobile Home Parks, Limited Residential


• Impacts/Factors in Floodplain/Floodway– 2012 Flood Study - 133 structures in 100-yr

floodplain, many in 5-yr, most in Floodway

– blight, lower real estate values

– Stream degradation, pollution

– Upstream watershed is about 50% developed - Kennedale is at downstream end!

Case Study – Village Creek• Impacts/Factors in Floodplain/Floodway

– Kennedale is at downstream end of watershed

– The city is trying to restore the floodway and stream for open space, parks, water quality benefits

Case Study – Village Creek• What Can Happen in Floodplain/Floodway?

– If upstream floodway fringe is filled, potential impacts from lost “Valley Storage”…increased flood elevations/more floodplain/more damages

– What could happen if valley storage loss and flood discharges increases similar to the Colleyville (Little Bear Creek) pattern and percentages ?


BASELINE – EXISTING – NO FLOODWAYS

$6.3 MillionFor 100-year flood

*


INCREASE Q BY 10% W/FLOODWAYS

+ $1.150 MillionFor 100-year flood


INCREASE Q BY 23% W/FWAY

+ $2.0 MillionFor 100-year flood


INCREASE Q BY 42% W/FW + $3.4 MillionFor 100-year flood

Conclusion: Case Study – Village Creek

• Using the Colleyville pattern: decrease in upstream valley storage, increase in discharges and corresponding flood levels, and more flood damages.

• Result: Even a 10% increase, due to upstream floodway fringe filling will increase damages significantly!

Case Study – Mountain Creek

• Pre-Flood Control Dam – Hydrology and Hydraulics (includes floodway)

• Post- Flood Control Dam – Revised and lowered flood discharges, profiles, and floodway

• Floodway included split flow, created “islands”

Case Study – Mountain Creek• Pre-Flood Control Dam – Hydrology and

Hydraulics (includes floodway)- 1982 FBFW*

Mountain Creek

Floodway Limits

* Not a FIRM

Case Study – Mountain Creek• Post- Flood Control Dam – Revised and lowered flood

discharges, profiles, and revised the floodway (with “islands” *)

Floodway Limits

**

Current DFIRM

Case Study – Mountain CreekCompare Pre- and Post- Flood Control Dam/Revised Floodway

Floodway Limits

Floodway Limits

• 1982 Floodway = 2,800’ in Corporate Limits• 2014 Floodway = 1,050’ in Corporate Limits• 63% Reduction in FW Width

Case Study – Mountain Creek Impacts and Issues

• Post- Flood Control Dam – Revised and lowered flood discharges, profiles, and floodway (reduced width by 63%)

• Lack of access (traffic, emergency vehicles)• Reduced Floodway Width and “islands” and

Increased “developable” area• Created development issues for FPA at city• Adverse impacts?

Case Study – Shoal Creek - Austin• Case study shows impacts of historical

development/encroachment along urban stream near downtown

• City uses the 25-year floodplain as regulatory Floodway (i.e. no FEMA floodway on FIRMs)

• Some development took place before floodplain/floodway information available

Case Study – Shoal Creek - Austin

W. 6 th Street

June 1993 Travis County FIRM

SHOAL CREEK

W. 6

th Stre

et

Shoal Creek – June 15, 1935

Shoal Creek – Memorial Day 1981


Blue = 25-Year Floodplain = FLOODWAYGreen = FLOODWAY FRINGE

W. 6

th Stre

et


Blocked out buildings for hydraulics

W. 6 th Street


W. 6

th S

tree

t


• Buildings encroached on theoretical Floodway (25-year floodplain)

• If blocks (development) were not there, 100-year WSEL would be up to 2 feet lower.

• What do you do about that?

Regional Case StudyNCTCOG – Corridor Development Certificate

Case Study – Trinity River CDC in DFW

The Corridor Development Certificate (CDC)– Trinity River and Major Tributaries in DFW area

– Objectives – reduce future flooding with strong floodplain development standards, including “no rise”, “preservation of valley storage”, and “no cumulative impacts”

Regional Case StudyNCTCOG – Corridor Development Certificate

• Trinity River and Major Tributaries in DFW Metroplex – 9 cities, three counties, 2 river authorities – Adopted 1st CDC Criteria Manual on May 23, 1991

• 1991-2014 Cities permitted over 100 floodplain reclamation or transportation projects

Technical Criteria From Record of Decision/CDC Manual

1. No rise (0.0’) in the 100-year flood or significant rise in the SPF.

2. Maximum allowable loss in valley storage capacity for 100-year flood and SPF discharges will be 0% and 5% respectively.

3. No erosive velocities (on or off site)

1990 Corp’s Recon StudyEstimated Average Annual Damages

1. Without CDC Criteria in full force AAD = $684 M

2. With CDC Criteria in place AAD = $194 M

3. Average Annual Benefit = $490 M

2013 CDC H&H UPDATE

Urbanization of Dallas-Fort Worth

Storage Accountability

0 50000 100000 150000 200000 2500000

5000

10000

15000

20000

25000

30000

Preferred

1995 Storage2012 Storage2040 100-yr

Discharge (cfs)

Stor

age

Volu

me

(acr

e-ft)

0 20000 40000 60000 80000 100000 120000 1400000

500010000150002000025000300003500040000

Typical

1995 Storage2012 Storage2040 100-yr

Discharge (cfs)

Stor

age

Volu

me

(acr

e-ft) Inherent redistribution of

storage

Preserved Valley Storage– Trinity River

After 24 Years ofReclamation

From 2013 Corp’s Updated CDC H&H Report

Re-distributed Valley Storage Trinity River

After 24 Years ofReclamation

From 2013 Corp’s Updated CDC H&H Report

100-Year

FindingsNCTCOG – CDC Case Study

1. The strict CDC criteria allows floodplain reclamation and still preserves the needed Valley Storage,

2. The FEMA floodway is often altered, but there is a “no rise” result, due to valley storage mitigation.

3. Significant flood damages are prevented.

Floodway - Overall Observations

• In North Texas, there is a definite trend to fully utilize the floodway fringe, filling to the limits, on both large rivers and small streams.

• It would be very prudent for communities to use “extra” freeboard to account for future changes in flood elevations.

• It is clear: Floodways have ADVERSE IMPACTS!

Floodway - Overall ObservationsIt is a difficult task for floodplain administrators to explain all of this to the layman who often asks these questions: • “What does a floodway look like? • “Why are the floodplains/floodways

changing?” ?

Conclusions

• There is an obvious need to analyze the hydrologic, as well as hydraulic impacts of the FEMA floodway concept, as it is utilized in floodplain management (specifically FIS).

• Even better would be to eliminate the floodway concept entirely, such as many cities have done or proposed.

[email protected]

QUESTIONS?

Floodways, Do We Still Need Them?

Presented at the TFMA Region 6 Inaugural Regional South Central Luncheon/Seminar- June 12, 2014, San Antonio, TX

T. Lynn Lovell, PE, CFM, D.WRE – Halff Associates, Inc.Bill Brown, PE, City of Arlington, TxJohn Ivey, PE, CFM – Halff Associates, Inc.Mike Moya, PE, CFM – Halff Associates, Inc.

Floodway - Overall Observations

• One disturbing observation from the Texas Case Studies is the “non-changing” FEMA 100-year flood discharges, over long periods of time, in many watersheds, due to use of outdated “existing” hydrology and hydraulic models.

• There is a definite trend to fully utilize the floodway fringe, filling to the limits.

• It would be very prudent for communities to use “extra” freeboard to account for future changes in flood elevations.

Summary of Texas Floodplain Management Criteria

• The Texas Floodplain Management Association (TFMA) has conducted an annual Freeboard Survey since 2004 to encourage Texas cities and counties to adopt and enforce higher floodplain management standards. The following is a summary:

2013 TFMA Freeboard Survey

• 255 of 1225 Texas communities participated• 226 enforce +1’ or higher freeboard (Max. 4’)• 70 enforce BFE’s based on fully developed

Floodplains and Floodways• 63 require detention (NAI)• 60 participate in CRS

• DEM has incorporated the Freeboard Survey into the State of Texas Mitigation Plan

floodways, do we still need them?

Documents

paper floodway concept

floodway fringej

floodway encroachment

introduction wwbd floodways

river introduction reasons

flood heights

floodplain management

asfpmnafsma paper