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 CharacteristicsDetailedApproximate AHigh population and densities within the floodplain, and/or high anticipated growth +/- 1.0 ft/ 95%+/- ½ contour 95% BMedium population and densities within the floodplain, and/or modest anticipated growth +/- 1.0 ft/ 90%+/- ½ contour 90% CLow population and densities within the floodplain, small or no anticipated growth +/- 1.0 ft/ 85%+/- ½ contour 85% DUndetermined Risk, likely subject to flooding NA EMinimal risk of flooding; area not studiedNA Coastal Floodplain Boundary Standard

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

Riverine FBS Coastal Floodplain Boundary Standard FBS Result ! 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 Coastal Floodplain Boundary Standard 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 Coastal Floodplain Boundary Standard Ground = 11.8 Water Surface = 12.2 Ground = 11.8 Water Surface = 12.2

Results of Transect based SWEL Method Coastal Floodplain Boundary Standard ~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 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 Test75% BFE to Ground Test60% Within 30 Feet Structure10% Defined by PFD7% Horizontal Tolerance15% 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 Coastal Floodplain Boundary Standard Static BFE Ground FAIL!

MODEL AGREEMENT CHECK Zone 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 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 Coastal Floodplain Boundary Standard 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 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