New PRELIMINARY Coastal Flood Maps

Summary of FEMA RiskMap Review for Monroe County

Woods Hole Group completed a review of FEMA’s December 2019 RiskMap study for Monroe County. The review identified eight RiskMap study components that are of concern because of (a) the use of non-standard approaches in certain study areas, (b) inconsistencies in methodology with other FEMA Coastal RiskMap studies, (c) discrepancies between the RiskMap documentation and the analyses, or (d) possible errors in the analysis. A summary of the study components of concern is provided below. Errors or inconsistencies with the first five study components could result in regional impacts to the FEMA Flood Insurance Rate Maps (FIRMs), while issues with the last three study components could result in more localized impacts to the FIRMs.

1. Storm Climatology & Selection

FEMA use of a single Coastal Reference Point (CRP) in Miami-Dade County to establish a set of synthetic storms for modeling is not representative of the Keys (see picture below). The storms defined using this single CRP are not realistic for Monroe County in terms of storm rate and intensity. The adjacent Southwest Florida RiskMap study used spatially varied storm rates and storm parameters; this type of analysis would be more appropriate for the Keys. It also appears that the synthetic storms did not include attenuation caused by landmasses.

2. Statistical Analysis of Storm Sets

A non-standard analysis that combined storm sets from the east and west was used to develop the 1 percent water levels  (see picture below). The Combined Probability Analysis (CPS) assumed the storm sets were independent even though both sets have Atlantic and Caribbean genesis storms, potentially leading to inflated 1 percent water levels on the ocean side of the Keys. Additionally, the study did not include measurements errors in the statistical analysis, which is inconsistent with other FEMA RiskMap studies.

3. Wave Model Validation

Documentation for the South Florida RiskMap study indicates the wave model was not validated against measured data, citing the lack of available data as the reason. However, a review of wave data collected by the NOAA National Data Buoy Center shows measured data at three buoys for storms impacting the Keys (see picture below). A comparison of model results against data collected during Hurricane Andrew shows an overprediction of wave height and period during the peak of the hurricane.

4. Hydrodynamic/Wave Model Mesh Resolution

Errors in the development of the hydrodynamic/wave model mesh were identified at seven-channel locations throughout the Keys. The mesh resolution was not fine enough to allow the conveyance of storm surge and waves during all simulated storms (see picture below).

5. Hydrodynamic/Wave Modeling of Reefs

When considering reefs in the hydrodynamic/wave model, FEMA used friction values lower than recommended by the scientific literature (Figure 5). Since friction created by the reefs acts to dissipate the amount of wave energy reaching the shoreline, this issue could have an impact on the overland wave modeling and flood zone mapping.

6. Overland Modeling of Mangroves

FEMA’s characterization of mangrove height and density differs from data available from the 2014-2016 South Florida Water Management District for four distinct areas of the Keys (Figure 6). Since mangroves act to dissipate the amount of wave energy reaching the shoreline, this issue could have an impact on the overland wave modeling and flood zone mapping.

7. Transect Spacing & Location for Overland Modeling

Transect spacing in Monroe County is 2.2 to 2.7 times greater than the transect spacing in the neighboring counties of Palm Beach and Broward. Given the irregular nature of the shoreline in the Keys, as well as variations in development and vegetation, the transect spacing in Monroe County should be reduced. This will help to characterize irregularities in the shoreline and improve the accuracy of flood zone mapping between transects.

8. Topography Used for Modeling and Mapping

Topography used by FEMA for modeling and mapping differs from Monroe County mobile LiDAR data collected post-hurricane Irma in seven areas throughout Monroe County. In these specific areas, the FEMA topography is lower than the mobile LiDAR data by 0.5 to 1 ft (see picture below). Since the ground elevations influence the extent of flooding and wave action along the coast, this issue could have an impact on the overland wave modeling and flood zone mapping.

Woods Hole Group is currently working to further investigate the influence of these areas of concern on the FIRMs, and to develop the technical information necessary for an appeal of the December 2019 maps.

Preliminary Coastal Maps

In December 2019, the Federal Emergency Management Agency (FEMA) released new PRELIMINARY coastal flood maps after a multi-year study of Monroe County’s coastal flood risks. After an extensive vetting process, these maps will replace the current flood maps. The current maps are based on 30-plus-year-old studies.

Monroe County wants to ensure all residents and business owners are aware of the potential for a change in flood risk for their property so that they can be more proactive in reducing their flood risk. 

View Preliminary Coastal Maps

• FEMA FIRM PRELIMINARY Mapping tool  (NOTE: Use in Chrome Browser for optimum user experience)  Click in the FIRM Panel section where the property is located to view the popup containing a link to the Draft Map (PDF) or search by property address in the top left-hand corner.
• FEMA PRELIMINARY Map Service Center - Select State (FL), County (Monroe County) to get to the webpage listing various area maps. Warning maps on the site are very large and may take several minutes to open/download.

IMPORTANT: These maps don’t show an additional difference in elevation due to a change in mapping standards between the old and new proposed flood maps (datum NGVD29-> NAVD88). This means that all NGVD29 elevations (e.g., BFE, Lowest Floor Elevation) need to be converted to NAVD88. While there is no set conversion factor, as it varies throughout Monroe County, on average there is -1.5 foot conversion. To account for this change, ON AVERAGE you should add +1.5 foot to any apparent increase. For example, if your building was in an AE-8 flood zone and is still shown in an AE-8 zone, then it actually experienced an increase of 1.5 feet. Another example, if your building was in an AE-6 flood zone and is proposed to be in an AE-9 zone, the increase appears to be three-feet.  However, the actual increase would be 4.5 feet.

To convert one vertical datum to another, visit the ArcGIS SFWMD Elevation Conversion: NGVD29 to NAVD88 mapping tool.

Previous X-Zone Buildings

Structures removed from the X-zones didn’t previously have a flood level associated with them, so this mapping standard difference wouldn’t be relevant.

View Current Flood Maps

Visit the FEMA Flood Map Service Center and search by property address.  

Flood Risk and New Building

To emphasize that flood risks are changing and to make sure property owners understand this is coming, Monroe County Building Department will be requiring owner’s to sign a form with permits that acknowledge the issuance of these new PRELIMINARY coastal flood maps and the fact that these maps may indicate a future change to the required elevation of a building currently being permitted. 

Building Department Permit Application with new signature page 3 -1

This is intended to help homeowners understand that what they might be proposing to build today, under the existing flood maps, could become non-conforming after the maps are changed, thereby making their flood risk and insurance costs greater. Owners should think about designing their improvements to meet the proposed PRELIMINARY maps to assure they are addressing potential future risks.

FEMA Map Review Process

During the week of January 27, 2020, FEMA held Community meetings throughout the Keys, offering the public an opportunity to view and comment in person on the proposed PRELIMINARY Flood Insurance Rate Maps. Following this, Monroe County, through its Consultant, examined the maps and the accompanying Flood Insurance Studies provided by FEMA.

Monroe County’s 90-Day Appeal and Comment period concluded on June 23rd, 2021.  While no private appeals were submitted, Monroe County, through its consultant, has appealed the Preliminary FIRMS.  FEMA is currently reviewing the County’s appeal.

Once FEMA reviews and processes all appeals, they will publish the final FIRMS. The Final FIRMs will most likely become effective sometime in 2021-2022.

When that happens, the county will formally adopt the maps by ordinance and the maps will be used when reviewing permits; and the final maps will establish what a finished floor elevation needs to be and determine building and site design requirements to reduce future risk of flooding. New lender requirements may go into effect along with flood insurance requirements, as well as changes in flood insurance rates as a result of map changes.

For More Information

For more information about the FEMA mapping process, email FloodMaps@MonroeCounty-FL.gov or call 305-453-8759. To learn more about flood insurance, talk to your insurance agent, or visit FloodSmart.gov.