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 StandardA Coastal Perspective May 23, 2012

Mark Zito, GISP, CFMCDM Smith

Alex Sirotek, CFMCDM SmithRSC 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


Floodplain Boundary Standard Tolerances

Delineation Reliability of the floodplain per study methodology

Risk Class

Characteristics Detailed Approximate

A High population and densities within the floodplain, and/or high anticipated growth

+/- 1.0 ft/ 95% +/- 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


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


Riverine FBS


FBS Result

! FAIL

! PASS Horizontal

! PASS Vertical


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


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


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


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


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

Transect Based Surface

• Transect file used to develop water surface

– SWEL field used to determine elevation

– Additional Transects may be necessary


Ground = 11.8Water Surface = 12.2

Results of Transect based SWEL Method


~75% Passing

Runup Area

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


Analysis of Results

• Passing rates specified in PM 38 are achievable

• Combined Surface Test ~ 85% Passing


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

• Knowledge of Study Preferred

• Difficult to achieve passing rate

• Integer to Float Comparison

• Some results not verifiable– Erosion– PFD

• Reduction of zones due to width at mapping scale


13(12.5 – 13.4)

11.9 1.1

Static BFE Ground

FAIL!

MODEL AGREEMENT CHECKZone Designation and BFE

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


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


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


Validation Results


Pass on distance tolerance

Ignore BFE Check

All segments must pass

Aggregate Zone Check

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

outputCoastal 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



QUESTIONS

Documents

Floodplain Boundary Standard A Coastal Perspective May 23, 2012 Mark Zito, GISP, CFM CDM Smith Alex Sirotek, CFM CDM Smith RSC 1 Lead