Post on 17-Jul-2020
Floodways - The Original Intent
Cooperating Technical Partners Information Exchange
ASFPM Flood Science Center September 19, 2019
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ASFPM Mapping and Engineering Standards Committee
Cooperating Technical Partners Subcommittee
Co-chairs: • Brooke Seymour, P.E., CFM - bseymour@udfcd.org
Colorado's Urban Drainage and Flood Control District• Rita Weaver, P.E., CFM - rita.weaver@state.mn.us
Minnesota Department of Natural Resources
Goals:• Identify common concerns• Provide opportunities for information exchange• Identify training needs• Promote and document the value of CTPs
Agenda
Introduction - Alan Lulloff (ASFPM Flood Science Center)
Floodway Surcharge: History and Impacts - Alan Lulloff
Mapping Floodways: Basic, Better, NAI - Alan Lulloff & Dave Carlton (dkcarlton and associates)
Mapping Floodways: MN Best Practice - Ceil Strauss (MN DNR)
Managing Development in Floodways: Basic, Better, NAI - Alan Lulloff & Dave Carlton
Managing Development in Floodways: WI Best Practice - Chris Olds (WI DNR)
Questions/Discussion
FloodwaysThe Original Intent
Presented by:Alan R. Lulloff, P.E., CFMDave Carlton, P.E., CFM
AcknowledgementsAdvocates for Change
Larry Larson
Mark Riebau
Lynn Lovell
John Ivey
Brian Varella
Jeff Sickels
Del Schwalls
Bill Brown
Dave Carlton
Acknowledgementsfor this project
FEMA
Rhonda Montgomery Rick Sacbibit Rachael Sears
Advisory Committee
Mark Riebau Mark Forest Jerry Murphy Brian Varella Carey Johnson Dave Knipe Steve Story Heidi Hansen
One-foot surcharge floodway
What is this thing called surcharge?
What was the original intent?
Matching the original intent
Addressing Cumulative Impacts
Title 44 CFR Part 60, Section 59.1
“A floodway is defined as the channel of a river or other watercourse and the adjacent land areas that must be reservedin order to discharge the base flood without cumulatively increasing the water-surface elevation by more than a designated height.”
NFIP Floodway Definition
Title 44 CFR Part 60, Section 60.3
… the community shall
60.3 (d) (2) Select and adopt a regulatory floodway based on the principle that the area chosen for the regulatory floodway must be designed to carry the waters of the base flood, without increasing the water surface elevation of that flood more than one footat any point;
Equal Degree of Encroachment – Cumulative Impacts
Floodway Surcharge
Why was FW surcharge concept established?
James Goddard 1978 Report:
Origin and Rationale of Criterion Used in
Designating Floodways
Tennessee Valley
Why was FW surcharge concept established?
According to Goddard:
“It was to be a minimum criterion intended as a regional standard, recognizing that there were urbanizing areas …(with) existing development where … a much smaller rise might be appropriately considered.”
States that allow less of a surchargeWisconsin 0.00
Illinois 0.1 (measureable amt.)
Indiana 0.1 (measureable amt.)
Michigan 0.1 (measureable amt.)
New Jersey 0.2
Minnesota 0.5
Montana 0.5
Colorado 0.5
Quotes from some States that do not allow a measureable surcharge
Illinois –
“the overbank fp of most streams in state quite flat. A small increase can significantly expand the width of the fp. Unreasonable economically to allow any increase that subjects previously ‘safe’ structures to flood waters.”
Indiana –
“there are few topographic restraints on development in Indiana, so there is no real need to view floodplains as the only developable area.”
Quotes from some States that do not allow a measureable surcharge
Community established higher floodway standards
Charlotte/ Mecklenburg, NC
Future conditions
0.1 ft Surcharge Floodway
ASFPM Floodway Study 2013
Comparison between1 ft & 0 surcharge
Stream
Avg. width Decrease
(%)
Velocity Increase (ft/sec)
Average Velocity Inc. (%)
Increase floodplain
(%)Pine Creek 59 2.28 to 3.69 62 6
Patterson Creek 68 1.11 to 1.61 45
Stevens Branch 39 4.98 to 5.82 18
Sugar River 50 1.57 to 2.07 32 14
Four Mile Creek 43 2.58 to 3.25 26
Cypress Creek (100) 48 1.92 to 2.46 22
Cypress Creek (172) 64 1.18 to 1.71 45
Plum Creek 32 7.15 to 8.31 16
Avg. all 8 reaches 50 2.85 to 3.62 33 10
Stream
Avg. width Decrease
(%)
Average Velocity Inc. (%)
Increase floodplain
(%)
Avg. all 8 reaches 50 33 10
Comparison between1 ft & 0 surcharge
One-foot surcharge FWs enable filling half the natural FW
Effects of Encroachment due to Surcharge
Floodwaters rise to higher levels causing properties that were once flood‐free to now be flood‐prone
Rise in floodwaters increases velocity of flood waters and therefore increases the potential to erode stream banks
Floodplain filling reduces floodplain’s ability to store water
Poll Question
Floodways with a one-foot surcharge pinch-in the natural floodway by what percentage?
- 10%
- 20%
- 33%
- 50%
FEMA tote at ASFPM in Cleveland
Potential Impacts of Fill in the Natural Floodway
Hydraulic/Hydrologic Impacts
Increased channel velocities
Higher hazard (depth & velocity)
Aggradation –raised water surface
Degradation –bank failure
Review of guidelines and CFR
Guidelines – FEMA Nov 2016
Floodway Coordination Meeting
CFR – 60.3 (d) (2) the community shall select and adopt a regulatory floodway
(Minnesota best practice)
Minnesota Floodway Best Practices
September 19, 2019
“The limits of the floodway shall be designated so that permissible encroachments on the floodplain will not cause an increase in stage of the regional flood of more than 0.5 feet in any one reach or for the cumulative effect of several reaches of a watercourse. If the increase in flood stage will materially increase the flood damage potential, the commissioner may require that such increases be less than 0.5 feet.”
Interpreted as: No (0.00’ foot) increase allowed if existing insurable structures impacted.
Legal Background ‐ MN Rules adopted 1970
MN Rules 6120.5700, Subp. 4, A.
• Technical guidance on how to meet the 0.5’ maximum stage increase
• Encouraged designating entire floodplain as floodway
• Process for community selection of floodway; community designates floodway based on their needs, but it’s verified to not cause more than 0.5’ increase or impact existing buildings
1977 Technical Report on Floodways
Example of Floodway in Community With Existing Development
• Example 1986 Flood Boundary and Floodway (FBFW) map
• Floodplain all floodway where possible
• Mainly current development shown as flood fringe and rest of floodplain as floodway
Note on FBFW maps:• Floodway is white with heavy dashed boundary
• Flood fringe is gray shading
•Many communities and watershed districts have community or district‐wide modeling, and enforce higher standards
• Local studies are leveraged and used as supporting data in updated FEMA Flood Insurance Rate Maps
•Some studies assume storage areas will be retained (i.e., XP‐SWMM)
•Those storage areas are shown as “Floodway” on the FIRMs and are administered with the more restrictive floodway ordinance standards.
More Recent Use of Floodway For Modeling
Thank You!
Ceil StraussCeil.Strauss@state.mn.us
651‐259‐5713
Basic
FEMA G&S FW Coordination Mtg.
CFR Have community select a
floodway – i.e. a surcharge.
Promote zero-surcharge FW---------------------------
Require community to notify owners if structures are impacted (similar to CLOMR)
Better
Risk Assessment Provide a future conditions (fully
encroached) map that shows the additional inundation caused by the surcharge
Provide zero-surcharge FWs as the default
Allow a surcharge if no structures are impacted
Require freeboard for new structures in flood fringe
Matching Original Intent -Mapping
Matching Original Intent -MappingNo Adverse Impact
Require easements be obtained from property owners impacted by the surcharge (WI Best Practice)
Risk Assessments (e.g. HAZUS) should include both existing (BFE) and future conditions (BFE + surcharge) so that community can see the potential additional damages caused by new development as fringe is filled
In your opinion, if made aware of the impacts – what percentage of communities would opt for zero-surcharge floodways?
- No communities would opt for a zero-surcharge floodway
- 10% of communities
- 20% of communities
- 30% of communities
- 50% of communities
Poll Question
In your opinion, if made aware of the impacts – what percentage of communities would opt for zero-surcharge floodways?
Poll Results
Managing Development in Floodways:Wisconsin Code
Chris OldsState Floodplain Engineer
Wisconsin Department of Natural Resources
Floodway Definitions
• State Statute 87.30
• Administrative Code NR 116
• Model Floodplain Ordinance
• LOMC Guidance
Statute 87.30
NR 116
NR 116
NR 116
NR 116
State Model Ordinance
Easement Example
WI DNR LOMC Guidance
Chris OldsWisconsin Department of Natural ResourcesChristopher.Olds@Wisconsin.gov608-266-5606
Require cumulative impact analysis for proposed encroachments
Based upon legal precedent concept
Consistent with historic FEMA Community Assistance Series No. 4 (1974)
Managing Development in the Floodway ---- Better
61
Use better tools to analyze the impacts
2D models
Unsteady models
Limit increases in velocity in addition to loss of conveyance
Managing Development in the Floodway ---- Better
Best Practice City of Brevard, Transylvania County, NC
No Adverse Impact certification instead of a No Rise certification
No increase in elevation, velocity or erosion
Managing Development in the FW No Adverse Impact
Loss of flood storage should be included in the analysis
Impacts on floodplain habitat should be analyzed (ESA)
Managing Development in the FW No Adverse Impact
ASFPM No Adverse ImpactHow-to Guides
Questions&
DiscussionAlan Lulloff, PE, CFMalan@floods.org
Dave Carlton, P.E., CFMdave@dkcarlton.com
Ceil Strauss, CFMceil.strauss@state.mn.us
Chris Olds, P.E.christopher.olds@wisconsin.gov
ASFPM Flood Science Center
Cooperating Technical Partners
Information Exchange
Poll Question
Please rate this webinar.
Future CTP Webinar Topics Poll
https://www.surveymonkey.com/r/CTP-Webinar-Topics-Poll-2019
Check your chat panel for this link.
• Certified Floodplain Managers are eligible for 1 CEC for participating in this webinar.
• You must have registered individually and indicated you are a CFM at time of registration.
• Eligibility for CEC is dependent on your participation in poll questions and time spent viewing the webinar, as determined by the webinar software.
• Attending this webinar in a group setting or only viewing the recording is NOT eligible for CEC.
Continuing Education Credits
• To suggest future CTP webinar topics, please contact Alan Lulloff at alan@floods.org or type a suggested topic into the Questions panel today.
• ASFPM CFM CECs will be automatically applied.
• Certificates of Attendance will be emailed, please contact cfm@floods.org with any certificate issues.
• Follow-up email with link to slides and recording will be sent next week
Thank You for Joining Us!
Closing Comments