This document provides an overview of floodplain mapping processes for riverine and coastal studies. It discusses how FEMA identifies and funds mapping studies, the typical study timeline and products. For riverine studies, it describes the level of detail for different flood zones and the methodology for approximate zone A mapping using hydrology, cross sections and HEC-RAS modeling. For coastal studies, it reviews the history of guidelines and methodology, components of a flood insurance study, and how to interpret risk-based and wave-inclusive special flood hazard areas on flood maps. Contact information is also provided.

1. Floodplain Mapping for
Design Professionals
April 7, 2016
2016 Annual RIFMA Conference
Incentivizing Actionable Resilience
To Flooding

2. 111
Agenda
 Kerry Bogdan, FEMA Region I
 Greg Stewart, USGS
 Brian Caufield, CDM Smith

3. 112
STUDY PROCESS
Kerry Bogdan, FEMA Region I

4. 113
FEMA Regions

5. 114
Identifying and Funding Studies
 Identify Priorities
• Utilize FEMA’s Community Needs Management System (CNMS) to
identify age and validity of study
• Incorporate information contained within Hazard Mitigation Plans
 Funding allocation
• FEMA Region I receives some of the lowest funding from FEMA HQ
• Funding is provided in Allocation Buckets that have to be used on
projects pertaining to that Bucket
 i.e., funding for a Coastal Study cannot be used to fund a Riverine
Study
 Receive final PALT

6. 115
Study Timeline
 Activities
 Project Timeline
 Products

7. 116
RIVERINE STUDY
Greg Stewart, P.E., USGS

8. 117
Example: Pawcatuck/Wood Watershed, RI

9. 118
Level of Study
 Coastal Zones AE and VE
 Riverine Zone AE (Detail Study)
 Riverine Zone AE (Limited Detail Study)
 Riverine Zone A (Approximate Study)
 Redelineation (Zone AE or Zone A)

10. 119
Riverine Zone A (Approximate Study)
Example: Pawcatuck/Wood Watershed, RI

11. 120
New Method: Example Reach
The watershed area for the
selected reach is 8.8 mi2
which is within the
acceptable range for Rhode
Island State Regression (4
to 404 mi2)

12. 121
Old Two Point Method
Cross sections are
drawn and elevations
are picked from both
sides of the floodplain.
The water surface
elevation is the average
of the two points.
327.5
329.6

13. 122
Two Point Method Issue
• This method really
breaks down when
the old effective
floodplain is shifted
or does not conform
to the LiDAR
topography at all.
• At this point it is
even more of an art
and completely
dependent on what
we see on the
LiDAR.

14. 123
Selected Reach with Old Floodplain

15. 124
Calculating Hydrology
Points are generated
along the reach
centerline at all
confluence nodes and
hydrology is
calculated 20 feet
below and above the
confluence to
determine drainage
area difference. If the
drainage area is less
than 50% of a
downstream flow
change location then
it is kept as a flow
change location.

16. 125
Flow Change Locations
For this reach, the
only points kept for
calculating flow is
the most
downstream point
and one flow change
location seen as a
black triangle.
The table below
shows the calculated
flows for both points
using Rhode Island
State Regression.

17. 126
Bank Lines
Bank lines are drawn using the most current ortho-
photography and LiDAR.

18. 127
Reach with drawn cross sections
Cross sections are
drawn
at elevation
changes, upstream
and downstream of
structures, and
directly downstream
of flow change
locations.

19. 128
HEC-GeoRAS Export
• We have incorporated the MA bankfull
equations for estimating depth
• The cross sections are export from HEC
GeoRAS for HEC-RAS modeling.

20. 129
Final mapping

21. 130
Final mapping results at map scale (1:6000)

22. 131
New and Old Floodplain boundaries
Old floodplain (transparent green)
over the new approximate modeled
floodplain (dark blue)

23. 132
Summary
New Zone A with hydrology and hydraulic
information to back up the flood lines.
• No BFE available
 Structures are not included
 Hydrology assumptions (outside the bounds of equations)
 Many hydraulic assumptions must be made (channel depth, n-values)
without field data
• Data will be uploaded and stored in the MIP
Questions?

24. 133
COASTAL STUDY
Brian Caufield, CDM Smith

25. 134
History of Coastal Floodplain Mapping
1968
1973
1977
1979
1981
1986
to
1988
1988
1989
1995
2002
2005
2007
NFIP
Authorized
Hb = 3'
NAS Develops
Wave Analysis
Methodology
FEMA Adopts NAS
Methodology
Runup
Methodology
Accepted
Wetland Vegetation
Muddy Bottom
Erosion Analysis (540SF)
PFD
CERC
TR 89-15
G&S for Wave Elevation
and V Zone Mapping
Appendix D
Proc. Memo
No. 37
Atlantic Ocean and Gulf
of Mexico Guidelines
Update (Final Draft)
2008
Proc. Memo
No. 50

26. 135
READING AND INTERPRETING
FLOOD HAZARDS

27. 136
Components of a Flood Insurance
Study
 Hydrologic study
How much water will there be?
 Hydraulic study
How high will the water get?
 Topographic analysis
What areas will the water cover?

28. 137
Order of Importance – Coastal
Study
 Flood Insurance Rate Map
• Some Special Flood Hazard Areas are risk based
• Some Special Flood Hazard Areas include wave action
 Summary of Stillwater Elevations table in FIS

29. 138
Risk Based Special Flood Hazard
Areas
 Primary Frontal Dunes
• Identification of a geomorphologic feature on the profile
• Base Flood Elevation associated with flood zone may be lower than actual ground
elevation
 Wave overtopping
• Primarily occurs in areas of coastal structures
• Mapping is associated with a distance back from the slope break

30. 139
Wave Action Inclusive Special
Flood Hazard Areas
 In areas of overland wave propagation
 Topographic limit of Special Flood Hazard associated with SWL and
wave setup
 Flood zones at the boundary may contain wave action
 ‘Hidden AE’ zones

31. 140
Limit of Moderate Wave Action
LiMWA
• Areas subject to wave
heights greater than 1.5 feet
• Defines Coastal A Zone
• Recommendation of building
to V zone standards
• It is regulatory in Rhode
Island

32. 141
Coastal A Zones
 Portion of the coastal AE Zone located between the VE Zone and the
LiMWA, where the waves are between 1.5 and 3 feet
 Also known as the Moderate Wave Action or MoWA

33. 142
FEMA Standard 614
 FEMA will only use whole foot BFEs for LOMA or CLOMA
determinations where effective flood hazard areas are the result of
coastal flood hazard analysis.
 For example:
• SWEL – 8.8 feet NAVD 88
• BFE shown on map is AE EL 9
• LAG is 8.9 feet NAVD 88
• Structure would not be removed from SFHA as 8.9 is lower than 9

34. 143
NGS Benchmarks
• Obtained from National Geodetic Survey website: http://www.ngs.noaa.gov/
• FEMA requires that Benchmarks possess stability of C or higher
• Benchmarks shown on old FIRMs may not possess stability of C or higher and
thus will not be shown on the new DFIRMs

35. 144
Contact Information
 Kerry Bogdan, Kerry.Bogdan@fema.dhs.gov
 Greg Stewart, gstewart@usgs.gov
 Brian Caufield, CaufieldBA@cdmsmith.com
Links:
FEMA Map Service Center: https://msc.fema.gov
National Geodetic Survey website: http://www.ngs.noaa.gov/
More information is available on line at: http://www.fema.gov/flood-hazard-
mapping-frequently-asked-questions