The Base Flood Elevation (BFE) is how high floodwater is likely to rise during a 1%-annual-chance flood event. It is one way to measure and indicate flood risk. However, the study that established the BFE is only a snapshot in time. There are many factors that can cause floodwaters to rise above the BFE. These factors include debris-blocked bridge and culvert openings; blocked city storm sewer drains; higher-intensity rain events; storm tracks causing coinciding peak flows of flooding sources; high backwater conditions; and heavy rains on frozen ground with considerable snow depths. There is also always the potential for an event more severe than the 1%-annual-chance event. To communicate and reduce flood risk in areas beyond FEMA’s regulatory flood zones, communities need more information (especially spatial information) about flooding that exceeds the 1%-annual-chance event.
Freeboard is the additional height above the BFE to which a building is protected from flooding; it is used as a safety factor in floodplain management. The Design Flood Elevation (DFE) is the sum of the BFE and freeboard; it is the elevation used for regulatory purposes in communities with higher standards. Freeboard standards are typically only applied vertically within the limits of a mapped floodplain.
Per Maryland Coast Smart regulations (effective Sept. 1, 2020), state construction projects and projects in coastal areas that receive more than $500,000 in state funding must apply the horizontal limits of a floodplain corresponding to a DFE with 3 feet of freeboard. This shows wider inundation areas. To show the higher freeboard depths for buildings within the FEMA 100-year floodplain, as well as the wider inundation areas corresponding to the DFE, the Maryland Department of the Environment, the Maryland Department of Natural Resources, and other state agencies developed the Coast Smart - Climate Ready Action Boundary (CS-CRAB) for the Maryland Coast Smart Council. This feature is similar to the Federal Flood Risk Management Standard (FFRMS).
Showing this wider inundation area helps show how far flooding can extend beyond the Special Flood Hazard Area (SFHA) when flood heights are higher than the BFE. Using the information on these public facing maps, communities can mitigate risk in areas beyond the high at-risk areas known as Special Flood Hazard Areas. For example, communities can use the freeboard delineation on the map to determine how much a structure’s lowest floor should be elevated or floodproofed. By adopting these higher standards, communities can help reduce flood risk in, around and beyond high-risk flood areas. This will further protect lives and property.
- We can use advanced geospatial data to see risk more clearly.
- State agencies will use this product to apply more resilient siting, design and planning criteria in Maryland’s coastal areas.
- Users will see the impact of Maryland Coast Smart regulations on state projects.
- People can use the data to assess risk and resiliency both inside and outside of the 100-year floodplain and the state Coast Smart Climate Ready Action Boundary (CS-CRAB) limits.
- Communities can use these data for higher standards, Community Rating System (CRS) credits, improved bond ratings and resiliency.
- Floodplain administrators can regulate areas outside the Special Flood Hazard Area to the Design Flood Elevation, even though they are outside of the 1%-annual-chance mapped floodplain.
- Mapping and managing flood risk in areas beyond the 1%-annual-chance flood zone boosts public safety and reduces recovery costs after floods. The freeboard approach enhances risk awareness communication and visualization. The data help communities prepare for impacts that future development may have on current floodplains.
- A freeboard requirement gives an extra margin of protection against flooding. This also protects electrical, heating, ventilation, plumbing, air conditioning units, and other utility service facilities (including ductwork) that flooding could affect. This reduces the costs associated with FEMA’s Individual Assistance, Hazard Mitigation Grant Program, and Public Assistance program.
- The FEMA Community Rating System provides lower flood insurance premiums in eligible communities with higher standards such as freeboard requirements. Areas with higher freeboard ordinances score CRS points; this may help reduce flood insurance costs for National Flood Insurance Program policyholders.
The Maryland Department of Environment requested funds through the Cooperating Technical Partner (CTP) grant program to create 2-foot and 3-foot depth grids for countywide risk products in Maryland’s coastal communities and riverine counties. The Coast Smart Council, which consists of state agencies, governor appointees, and the Department of Natural Resources, addressed how state-funded projects would boost resiliency for future events such as sea level rise and climate change.
State agencies wanted to define the possibility of enhanced flood risks by using familiar, easily understood, and verifiable data that the public could review. As the CTP depth grids were being developed, the state’s National Flood Insurance Program (NFIP) office showed the Coast Smart Council that these data could help show the risks inside and beyond the limits of the floodplain. The NFIP office could also show the relationship between these risks and each community’s floodplain data already being used for siting and design considerations.
The Coast Smart Council revised guidelines to incorporate floodplain and Federal Flood Risk Management Standards (FFRMS) principles in the FEMA coastal floodplains (now 3 feet higher) and the newly inundated CS-CRAB areas. The data for the coastal counties have been updated to include the nontidal or riverine floodplains in these counties; the same process is now underway for the other Maryland counties that are completely nontidal or riverine.
Nontidal (riverine) floodplains are currently not part of the state’s regulatory product. Since the Riverine CRAB (R-CRAB) is only advisory, the data are posted on the state’s floodplain outreach website to show the 2-foot and 3-foot depth grids from the initial CTP funding request. Maryland’s state NFIP office will also upgrade the state’s Model Floodplain Ordinance to include the Coastal and/or R-CRAB as a higher standard.