New Jersey: Coastal Flood Inundation Mapping

The New Jersey Department of Environmental Protection (NJDEP) and New Jersey Office of Emergency Management (NJOEM) partnered with the United States Geological Survey (USGS) to improve the effectiveness of the existing tide gage early warning network by developing a library of flood inundation maps for 25 tide gages covering coastal communities from Greenwich in Cumberland County through New Milford in Bergen County. Tide elevation readings at each tide gage are now associated with flooding in the surrounding community during two simulated storm scenarios.

Flood inundation maps are visualizations of flood water. They display where water may exist and how deep the water could be relative to the local land surface. The flood maps from this partnership are available from the Interagency Flood Risk Management (InFRM)  Flood Decision Support Toolbox (FDST)  and real-time data for each of the 25 stations can be accessed directly from the FDST or from the  National Water Dashboard (NWD) .

On January 9th and 10th, 2024, a winter storm brought heavy rain and coastal flooding to New Jersey. USGS tide gage 01408168 Barnegat Bay at Mantoloking, located on the Mantoloking bridge connecting Brick Township and Mantoloking Borough, recorded a provisional high tide elevation of 3.5 feet late on January 9th. Drone footage captured the extent of flooding in Bay Head, NJ on the morning of January 10th. Compare the image to the flood map underneath which represents flooding in the Bay Head, NJ area while 01408168 is reading 3.0 feet during a simulated tropical storm.

The connection between flood maps and readings at USGS tide gages gives emergency management personnel and residents the ability to use current, historic, or forecasted tide gage elevation data in hazard mitigation planning. In the Bay Head example, the estimated flood extents can reveal vulnerable areas and give some indication of how early during a storm event those areas become inundated.

As of 2024, some available flood mapping products rely upon the “bathtub” method of displaying inundation, where water surfaces are increased in an even, incremental fashion across a spatial extent of interest. In the absence of a calibrated hydrodynamic model, this is one way to visualize estimated flooding within communities. However, the bathtub method does not consider the effects of wind and wave forces upon the coastal environment which can produce significant, localized differences in tidal water-surface elevations.

The hydrodynamic model used for the USGS flood maps is a product of the  Federal Emergency Management Agency (FEMA) Region 2 storm surge study  initiated to produce updated flood insurance rate maps (FIRMs). Coastal storms were modeled using the ADvanced CIRCulation (ADCIRC) hydrodynamic model with a loose coupling with the Simulating Waves Nearshore (SWAN) model. ADCIRC computes wind and wave driven storm surge within the region of interest and SWAN computes wind-driven wave fields.

The utility of flood mapping using storm-forced inundation comes from modeling different storms which could have varying impacts depending on factors such as wind speed and landfall location. Compare the flooding shown in the two sliders below. Scenario 1 shows a dramatic example where the simulated storm makes landfall directly in Ocean City, NJ as a Category 3 on the  Saffir-Simpson Scale  with max wind speeds of 125 mph (108.95 knots). In Scenario 2, the simulated storm makes landfall further south in Wildwood, NJ as a Category 2 with max wind speeds of 102 mph (88.83 knots), and initial impacts to Ocean City are not as severe. The storm tracks can be viewed in the next section.

For consistent comparison, each storm scenario is shown with bathtub flooding in the left panel and all maps show USGS tide gage 01411320 Great Egg Harbor Bay at Ocean City measuring 5.0 feet NAVD88.

For the FEMA Region 2 Coastal Storm Surge Study, over 200 storms were simulated in ADCIRC. Some of these were simulations of real storms, but the majority were synthetic tropical storms created by varying a suite of storm parameters such as landfall location, maximum wind speed, and storm radius among others. To generate informative flood maps that are useful to both emergency management personnel and the public, the USGS narrowed the storm scenarios down for display. Storms were ranked based on greatest potential risk for moderate to severe flooding in New Jersey coastal communities. Six storms were selected among the 25 gages.

Use the map below to compare the selected storms described above. Change the visibility of data groups in the left panel to compare storm scenarios. Each group contains the storm track, the USGS tide gages that have flood map libraries generated from the storm, and points that represent where the storm had the greatest impact compared to all other simulated storms.

Scroll through the map below to see the process of creating a storm-forced flood map at 01410600 Absecon Channel at Atlantic City, NJ. The simulated storm in this scenario is NJA0705 and the gage is reading 5.0 feet NAVD88.

The area each USGS tide gage could cover was determined based on predicted surge extents, with the intent to provide flood mapping for most of the coastline from Cumberland to Bergen County. The coastline of New Jersey is not a straight uniform feature and tide gages are not evenly spaced which resulted in map extents that varied by tide gage. Additionally, some map extents were flagged with areas of increased uncertainty because of the distance from the USGS tide gage or complex physical features such as marsh impoundments, infrastructure, and intricate tidal waterways. Extending the mapped areas to include areas of increased uncertainty was considered a more conservative approach compared to leaving a void in the mapping where users and management officials would need to approximate their own interpretation of flooding.

High tide or nuisance flooding is not represented. This type of flooding occurs in low-lying areas during higher-than-average tides without any storm forces. While the maps from this study may expose areas prone to flooding, the tide gage readings and associated flooding are simulated only during a storm event. On days with these "king" tides, tide gages may record elevations available in the FDST, but the flooding visualized in the tool would not be an accurate representation. This is explained in disclaimers but can be misleading when viewing the symbology and notifications online.

Click through the slideshow below to see the Flood Decision Support Toolbox in action.