Hurricane Sandy, often called by its unofficial name ‘Superstorm Sandy’, impacted the northeastern United States in the fall of 2012. In its wake it left an estimated $75 billion in total damage. The long-term effects of the storm are not solely limited to the loss of life and property damage experienced; storms like this also tend to change people’s perception of risk. Sandy is often used as an impromptu measurement of a property’s likelihood of flooding. In our experience, it is often cited by the flood industry, real estate professionals, and the general public alike when expressing concerns related to flooding, i.e. “the house didn’t flood during Sandy, therefore it is impossible that I’m in a Special Flood Hazard Area”. This statement, while not necessarily 100% accurate, uses logic and personal experience to arrive at a conclusion based on common sense:

A historic storm event occurred + the structure didn’t flood= I must not be in a special flood hazard area.

This may be true for some structures but, unfortunately, it is not true for all. In order to be a better resource for our clients it was important for us here at WTG to help explain why that is. Where does the discrepancy lie and how can we better educate ourselves and, by extension, anyone who uses our services? To that end, one of our Certified Floodplain Managers conducted a detailed analysis of the data from Hurricane Sandy. The findings provide a clearer picture of the storm’s characteristics and shed some light on the variables that contributed to flooding and non-flooding that occurred.

Did Sandy meet the criteria to be classified as 100+ year flood event?

Simply put…yes & no. Storms can be very large in comparison to the of flooding events they cause, meaning that many more people experienced Hurricane Sandy than those who experienced a 1% annual chance flooding event (100 year flood). The majority of flooding that resulted from Hurricane Sandy was caused by its storm surge, which mainly impacted bay and coastal communities.

What did we find?

Hurricane Sandy Peak Flood Elevation Points were obtained from the USGS. Those points were then compared to the coastal Base Flood Elevation (BFE) that the point fell within. Much of the Jersey Shore, The Raritan Bay, Hudson River, and South Side of Long Island NY experienced large-scale flooding exceeding that of the 100-year flooding event elevation, conversely there were regions that did not.

We found a large difference between Hurricane Sandy’s peak flood elevations and the FEMA BFEs in Waackaak Creek and Thrones Creek in Monmouth County. Peak flood elevations for Hurricane Sandy in these areas were recorded at nearly six feet below the BFE. This does not mean that those creeks did not overflow their banks. They certainly did, causing homes to flood, but the flood elevations did not exceed FEMA’s 1% annual chance flooding (100-year flood) elevations probably in large part due to a levee protecting the two creeks from the storm surge in the bay. This levee would have been less helpful in a torrential rain event which is different from a storm surge.

Only a few miles away however, an area between Flat Creek and East Creek in Union Beach Borough did experience flood water elevations which exceeded the BFE. These differences are more than likely due to combinations of local differences in the storm itself, local flood protections, local geography, and local topography.

In the graphic below, Union Beach appears on the left with West Keansburg on the right. The numbers in red represent Sandy’s peak flood elevation above the BFE. In the case of the 0ft label, Sandy’s flood elevation and the BFE were equal. The river associated with this flooding was East Creek. The tan numbers represent Sandy’s flood elevations below the BFE. The rivers associated with these water elevations were Thrones Creek and Waackaack Creek. The flooding in these two river systems reached a height almost 6 feet below the BFE. All three of the above-mentioned stream systems connect into the Raritan Bay.

(The BFE’s for this region ranged from (11-12 feet) The Sandy Peak Flood Elevation Points ranged from 5.1 feet to 13 feet)

 

Below is a full view of the USGS Peak Flood Elevations for Hurricane Sandy compared to the FEMA Base Flood Elevations. Tan represents Sandy’s peak flood elevations that were less severe than the FEMA 1% annual chance flood event (100-year flood), and the red represents Sandy’s peak flood elevations more severe than the 1% annual chance flood event. This graphic helps display how localized the 1% annual chance flood event (100-year flood) can be within an impacted area.

When dealing with terms and concepts that are unfamiliar, it is helpful to be able to put things in proper context. It is our hope that this analysis will offer some clarification and will help people understand that previous storms aren’t always a bullet proof indication of a structure’s vulnerability to flooding. While the term “100-year storm” can seem like, and is used as, a blanket term it is actually a very localized event. Each storm is different and there are many variables to consider when trying to make sense of storms and flooding.

Our goal here at WTG is to provide accurate, up-to-date, and easy to understand information regarding flooding, flood hazard, and flood events. If you have questions regarding this blog, any of our other blogs, or any flood related questions in general, give us a call at 855-653-5663 and talk a one of the Certified Flood Plain Managers who staff our Flood Resource Center.