How were historic floods recreated for Flood Factor?

Learn about which historic floods were recreated for Flood Factor.

Types of historic floods 

The First Street Foundation Flood Model has recreated 55 major historic floods, including both riverine floods as well as coastal floods associated with hurricanes, tropical storms, and nor’easters. The floods are modeled simulations recreated and calibrated using recorded flood data and validated against known government flood claims. 

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Rain flooding

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Riverine flooding

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Tidal flooding

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Surge flooding 

While heavy rains are often associated with both types of flood events, they are not included as part of the recreated flood models.  The modeled historic floods only include storm surge from coastal events and only river overflows from inland events. The current methodology used does not allow for the recreation of flood events from heavy rains, and as such, events that contained a lot of rain may not look exactly as documented from other sources.

Historical Analyses

The First Street Foundation Flood Model has developed a method in order to recreate historic flooding events. The model has been used to recreate both riverine flooding events as well as the flooding associated with coastal storms like hurricanes, tropical storms, and nor’easters. To create the catalog of events, First Street relied on hydrologic models and public data sources detailing the impacts of flooding.  Many of these methods were first tested and validated against remotely sensed satellite imagery of existing water after flooding events in order to validate methods for accuracy.

River flooding events are recreated by analyzing historic USGS stream gauge readings during the time periods associated with identified flood events. The historic simulations are then evaluated for correlation recorded high water marks and insurance claims of flooding made during the event.  Major coastal flooding events are recreated through a combination of AD-CIRC+SWAN modeling and geospatial analyses based on specific storm characteristics, such as Hurricane Katrina and then compared to the terrain of the area using a digital elevation model. These recreations are then compared to recorded high water marks and calibrated to align with those observed water depths.  In the recreation of both riverine and coastal surge events, comparisons against the First Street adaptation database are performed to see if there are any flood control structures in the extents of the model in order to accurately account for known protections against historic flooding events.

Historic floods on Flood Factor

Currently, locations on Flood Factor only show information for the recreated historic floods that likely impacted them. Properties are considered to have been impacted by a historic flood if its modeled recreation indicates that flood water reached the largest building on the property or, if there are no buildings, the center of the property lot. Neighborhoods, zip codes, cities, and counties are considered to have been impacted by a historic flood if any of the properties within their boundaries were impacted.

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Provide past flooding information

The historic flood information shown on Flood Factor is not comprehensive, and does not include heavy rains. However, individuals are encouraged to share information if they know a location has flooded in the past, as this information will help improve the historic flood models and may be added to the database and shown on Flood Factor in the future.

 

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