NJ Weather 101: Coastal flooding categories, explained
Any time there is a significant storm system approaching the Jersey Shore, residents collectively hold their breath. Coastal flooding happens several times every year, as strong winds push ocean water toward our coastline. It's called storm surge. And it often leads to challenging travel conditions, property damage, and other water-induced headaches.
I want to share two useful resources I reference for coastal flood guidance. Both basically produce the same tide forecast information, but displayed in a slightly different form. (The first link here clearly delineates flood stages and statistics for each tide gauge, while the second breaks down each surge component over time.)
It is important to note that coastal flooding and inundation forecasts are highly dynamic and volatile. That is, they are very sensitive to the exact track of a given storm, and the precise direction and speed of the prevailing winds. Therefore, everything is subject to change as a situation evolves. And the effects of a flooding event can be very different from county to county, town to town, and even street to street.
When coastal flooding is in the forecast, we use very specific vocabulary to describe the potential impacts and risks. The purpose of this article is not to discuss the science, history, or societal impacts of flooding. Instead, I want to provide some insight into the meaning of our five categories of coastal flooding.
(Technical Note: The numbers includes below reference the Mean Lower Low Water datum.)
Localized Minor Flooding (Surge up to 1 foot)
Some low-lying areas along tidal waterways are so vulnerable that even a few inches of water rise can cause big puddles to form. Sometimes this level of flooding is caused by a distant hurricane or weak coastal storm. Sometimes an unusually high astronomical tide can force tidal waterways to splash over docks, bridges, and bulkheads. Such water rise rarely causes problems, nor does it last very long.
Minor Flooding (Surge 1 to 2 feet)
Flood Stages: 6.7 feet at Sandy Hook, 6 feet at Atlantic City, 6.2 feet at Cape May
I often call this level of coastal flooding "the usual spots". Any time a storm (such as a nor'easter or tropical storm) passes between New Jersey and Bermuda, rough surf and at least a slight water rise are expected. This happens several times a year, and can necessitate local road closures and detours.
Moderate Flooding (Surge 2 to 3 feet)
Moderate Flood Stages: 7.7 feet at Sandy Hook, 7 feet at Atlantic City, 7.2 feet at Cape May
Now we're getting serious. It takes a pretty powerful coastal storm system to push storm surge levels close to 3 feet. Side streets that are adjacent to tidal waterways may flood out. Even major highways may have lane closures during high tide. Property damage may occur — especially to vehicles and non-raised homes. I'd estimate this category of flooding now happens in New Jersey about once or twice a year.
Major Flooding (Surge 3 to 4+ feet)
Major Flood Stages: 8.7 feet at Sandy Hook, 8 feet at Atlantic City, 8.2 feet at Cape May
Well, it's called "major" for a reason. Widespread water inundation issues at the times of high tide. The magnitude of water rise is so great in this scenario that back bays and tidal waterways often don't have time to drain completely. That leads to several perilous high tide cycles in a row, a constant battering of waves and water. Road shutdowns are likely. Property damage is likely. Evacuations are possible.
Record Flooding (Surge 4+ feet)
Current Record Crests: 14.40 feet at Sandy Hook (10/29/12), 8.98 feet at Atlantic City (12/11/92), 9.36 feet at Cape May (1/23/16)
Truly historic flood levels, which only come from a truly historic storm. This is obviously a designation that we never want to see in our forecasts. Such dramatic flooding would be accompanied by dangerous rain (tropical storm), snow and ice (blizzard), and wind. Evacuations are likely. If your area generally floods under any of the previous categories and conditions, you are probably going to end up underwater at record (or even near-record) flood levels.