Floodplain Boundary Standard
A Coastal Perspective
May 23, 2012
Mark Zito, GISP, CFM
CDM Smith
Alex Sirotek, CFM
CDM Smith
RSC 1 Lead
Floodplain Boundary Standard
• The Floodplain Boundary Standard (FBS) was formalized with the
release of FEMA’s Procedure Memorandum (PM) 38.
– Originally issued in 2005, revised in October, 2007
• PM38 formalized the concepts of Risk Classifications and vertical
tolerances for floodplain boundaries.
– The PM came with an accompanying document, Floodplain
Boundary Standard Audit Procedures, outlining the process to
complete a self-certification.
Coastal Floodplain Boundary Standard
Floodplain Boundary Standard Workflow
Coastal Floodplain Boundary Standard
Floodplain Boundary Standard Tolerances
Delineation Reliability of the
floodplain per study methodology
Risk
Class
Characteristics
A
High population and densities within the
+/- 1.0 ft/ 95%
floodplain, and/or high anticipated growth
+/- ½ contour 95%
B
Medium population and densities within
the floodplain, and/or modest anticipated
growth
+/- 1.0 ft/ 90%
+/- ½ contour 90%
C
Low population and densities within the
floodplain, small or no anticipated growth
+/- 1.0 ft/ 85%
+/- ½ contour 85%
D
Undetermined Risk, likely subject to
flooding
NA
NA
E
Minimal risk of flooding; area not studied
NA
NA
Coastal Floodplain Boundary Standard
Detailed
Approximate
Floodplain Boundary Standard
• The guidance document was primarily written for riverine
– Applying the guidance to coastal floodplains does not translate well.
• FBS Audit Procedures Draft Version 3 from January 2010
addressed some coastal issues, but it was not finalized
Coastal Floodplain Boundary Standard
FBS Result
Riverine FBS
Coastal Floodplain Boundary Standard
!
FAIL
!
PASS Horizontal
!
PASS Vertical
Coastal Floodplain Boundary Standard
FBS on a Coastal Study
• PM states that “the computed flood elevation and the ground
elevation at the mapped floodplain boundary [must] match within
a tolerance set for a flood risk class.”
– For coastal, this can only apply to the landward Floodplain Boundary
• It can’t address zone breaks, or AE vs. VE zones
– Additional boundary issues compared to riverine flooding
•
•
•
•
Wave overtopping and splash zones
Runup Extent
Primary Frontal Dunes
Reduction of zones due to width at mapping scale
Coastal Floodplain Boundary Standard
Current Coastal FBS Guidance and Issues
• FBS Audit Procedures Version 2.0 suggests developing a water
surface model using Coastal Transects, but doesn’t provide exact
guidance
– Map Mod Schema does not capture an elevation in this file
– Mapping is not a straight interpolation between transects
• Draft Version 3.0 proposes two methods
– Compare the static Base Flood Elevation (BFE) to the ground surface
• Static BFE is an integer, and comparing it to a more precise value could
result in failing points due to rounding
– Compare a still water elevation TIN to the ground surface
• The draft guidance doesn’t say what the data source for this TIN is or
how to create it
Coastal Floodplain Boundary Standard
Suggested Improvements
• Water Surface Comparison – Use two methods everywhere
– FBS Audit Proc. Version 3.0 guidance says to create a runup vs.
overland propagation polygon, but this information is not explicitly
included in the submittals and would have to be manually created
– Compare transect based SWEL surface and the static BFEs to the
ground surface
• Test points only have to pass one of these two checks, and it is easier to
standardize or automate
• Model Agreement Check
– Compare CHAMP data to floodplain along transect
• Based on Appendix M Schema
Coastal Floodplain Boundary Standard
Pre-processing Steps
• Due to changes in the schema,
the process described in the
guidance doc is no longer valid
• Flood Hazard Area used instead
of Flood Hazard Line
– Query AE, VE, AH and Static_BFE
>0
– Dissolve on Fld_Zone and
Static_BFE
– Join Flood Hazard Line to
dissolved Area
– Convert to points every 100 feet
Coastal Floodplain Boundary Standard
Static BFE Based Comparison
• BFE value is applied to points from Flood Hazard Area and
compared to the Ground Surface elevation
– Integer compared to Float, essentially comparing a rounded number
to a precise one
• This is essentially the same
as the Version 3.0 static
BFE guidance
• Resulted in a ~60%
passing rate
• Many exceptions points
would be required if this
were the only method
used
Coastal Floodplain Boundary Standard
Transect Based Surface
• Transect file used to develop water surface
– SWEL field used to determine elevation
– Additional Transects may be necessary
Ground = 11.8
Water Surface = 12.2
Coastal Floodplain Boundary Standard
Results of Transect based SWEL Method
Runup Area
~75% Passing
Coastal Floodplain Boundary Standard
Hierarchy of Testing Points
Compare SWEL Surface to Ground, pass on +/1 foot
Compare Static BFE to Ground, pass on +/- 1
foot
Check within 30 feet of structure for splash
zone
Check for match with PFD (38 feet horizontal)
Check 38 foot horizontal for SWEL to Ground
and BFE to Ground
Coastal Floodplain Boundary Standard
Analysis of Results
• Passing rates specified in PM 38 are
achievable
• Combined Surface Test ~ 85% Passing
Coastal Floodplain Boundary Standard
SWEL to Ground Test
75%
BFE to Ground Test
60%
Within 30 Feet Structure
10%
Defined by PFD
7%
Horizontal Tolerance
15%
Total Pass Rate (not a sum)
96%
Limitations
• Does not evaluate internal
zones
• Some results not verifiable
– Erosion
– PFD
• Knowledge of Study Preferred • Reduction of zones due to
width at mapping scale
• Difficult to achieve passing rate
Static BFE
Ground
• Integer to Float Comparison
FAIL!
Coastal Floodplain Boundary Standard
Zone Designation and BFE
MODEL AGREEMENT CHECK
Internal Zone Check
• Method is not a boundary check, but an agreement check
between the models and information within the Flood Hazard
Area
• Compares Zone designation and BFE value in model to DFIRM
database along Transect
• Limitations
– Runup cannot easily be compared to mapping
– Only evaluates transects
– Assumes average of merged zones used
Coastal Floodplain Boundary Standard
Process Overview
• Dynamic Segmentation used to locate
WHAFIS 6 results along transect
– Transects converted to route
– WHAFIS 6 Table events located along
route
– Events intersected with Flood Hazard
Area
Coastal Floodplain Boundary Standard
Validation Process
• Validate both zone designation and BFE
• If zone designations match, segment passes
– Exception - 0.2% will match X in model
– Exception - AO cannot be validated
• If BFE values match, segment passes
– Ignore Zone X or 0.2%
– Exception – differences between map scale and model accuracy
Coastal Floodplain Boundary Standard
Validation Results
Pass on distance
tolerance
Ignore BFE Check
Aggregate Zone Check
All segments must pass
Coastal Floodplain Boundary Standard
Limitations
• Multiple models used to develop floodplain, manual exceptions
required
– Overland Wave Propagation
• Intact Structure
• Failed Structure
– Runup
•
•
•
•
ACES
TAW Method
Shore Protection Method
Runup 2.0
– Primary Frontal Dune
• Difficult to account for differences between map scale and model
output
Coastal Floodplain Boundary Standard
Conclusion
• The Coastal Floodplain Boundary Standard is achievable using the
data from the Coastal Deliverable
• Tolerances can be maintained from PM 38
– Landward boundary of coastal SFHA can be checked in a similar
fashion to the Riverine procedures
– Assessing ‘Passing’ status requires several methods
– Some exceptions are manual
• Model agreement check validates internal zones
– Tolerances and methods need to be formalized, but provides a
method to procedurally assess flood elevations and zones
Coastal Floodplain Boundary Standard
Coastal Floodplain Boundary Standard