NWS Boston - Coastal Flood Threat and Inundation Mapping

Loading Inundation Overlays... Loading Tide Cycle Data...

What is...

...Total Water Level?
Astronomical Tide
+ Surge
= Total Water Level
Note: Surge is the departure from the astronomical tide and can be positive or negative.

Minor Coastal Flooding?
Flooding of the most vulnerable shore roads and/or basements due to height of storm tide or wave splashover. Majority of roads remain passable with only isolated closures due to nuisance flooding or rip rap. Spotty erosion of dunes or along the base of bluffs could occur due to waves on top of an elevated water level. There is no significant threat to life, and any impact on property is minimal.

Moderate Coastal Flooding?
Widespread flooding of vulnerable shore roads and/or basements due to height of storm tide and/or wave action. Numerous roads become impassable for a time, and closures are needed. Lives may be at risk for people who put themselves in harm’s way such as trying to drive through flooded roadways. A few neighborhoods may become isolated around the time of high tide. Isolated damage of very vulnerable structures such as docks or house decks/porches near the high tide line may be observed. Erosion of dunes and bluffs is likely due to waves on top of an elevated water level. Any stairs on the slopes of bluffs/dunes may be damaged.

Major Coastal Flooding
Coastal flooding severe enough to be life threatening along the immediate shore and to cause damage to at least some coastal homes/businesses along with widespread flooding of vulnerable shore roads and/or basements due to storm tide and/or wave action. Vulnerable homes or businesses may be severely damaged or destroyed. Numerous roads are impassable, some with washouts, and unsafe to cross on foot or by vehicle. A number of neighborhoods become isolated, possibly for an extended time. Response requires some evacuations and movement of vehicles and other property to beyond the reach of the storm tide and waves. Severe erosion of dunes and bluffs due to waves on top of abnormally high water levels should be expected. Erosion may be severe enough for some bluffs to slump or dunes to breach. Erosion of bluffs/dunes may threaten nearby structures or infrastructure. Major coastal flooding is covered by a Coastal Flood Warning with additional language as appropriate to indicate that the flooding will be major, severe, destructive, damaging, etc.

...A Velocity Zone?
Areas along coasts subject to inundation by the 1-percent-annual-chance flood event with additional hazards associated with storm-induced waves (>3 Feet).

...CCS PVC & Truro Overlays?
The ‘Inundation Extents’ layer is taken from the USGS 2013-2014 lidar data and are linked to local tidal datums.
The ‘Inundation Pathways’ layer was developed over several funded projects through the Coastal Resiliency grant program funded by the Massachusetts Office of Coastal Zone Management.

...MLLW?
Mean Lower Low Water (MLLW) is a tidal datum referenced by the National Weather Service for forecasting coastal flooding. MLLW represents the observed average of the lower low water height of each tidal day. For more information on tidal datums click here.

Coastal Inundation Mapping Scenarios Disclaimer

This product is based on the creation of inundation polygons and depth grids from static water surface elevations. The layers assume no components of near shore wave modeling. The depths associated with each layer were calculated using a 5 meter resolution digital elevation model for the pilot study area. Not to be used for navigation or in place of official National Weather Service flood warning and watch forecasts.

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NOTE: During times when the area is under a Tropical Storm/Hurricane Watch or Warning, the storm tide or total water level forecast will reflect a
plausible worst case scenario (versus the usual most likely scenario).

High Tide Cycle

- Inundation Overlays -
Total Water Level Height

(ft MLLW)

What is?