A property's Flood Factor considers the risk from four major types of flooding: high intensity rainfall, overflowing rivers, high tides, and coastal storm surge.
Flood Factor® scores are derived from the First Street Foundation Flood Model. The Flood Model allows for the understanding of risk from multiple types of flood events. Because a property can be impacted by more than one flood type, the model accounts for risk from multiple sources. A home located near a river and in an area where high intensity rainfall is expected to increase due to climate change, would be at risk from both high intensity rainfall and overflowing rivers and streams. Because these different flood types are often interconnected, they are first analyzed independently, then “coupled” together where appropriate.
What causes a flood?
There are four major types of flooding: 1) high intensity rainfall, 2) overflowing rivers and streams, 3) high tides, and 4) storm surge. The Flood Model considers risk of flooding from all four sources and forecasts how risk may change over time due to changes in the environment.
High intensity rainfall causes flooding when an area’s sewage system and draining canals lack the necessary capacity to drain away the amount of rain that falls. Urban areas are particularly susceptible because there is little open soil that can store water. Flooding from extreme rain events are directly incorporated into the Flood Model through an analysis of historic rainfall levels and changing environmental factors leading to increases, and potentially decreases, in extreme rain events.
The impact of high intensity rainfall
Within a 22 month period between 2016 and 2018, Ellicott City Maryland saw two extreme rainfall events that caused flash floods, as the grounds were unable to absorb the sudden, high-volume of rain that fell within hours. Each event had a 0.1-0.2% chance of occurring in a given year.
Extreme rain events are increasing
As the atmosphere warms, there is more evaporation and more water available for rain, which contributes to changing weather patterns and flood risks. Extreme rain events are increasing in duration, intensity, and frequency, which causes more urban and flash floods.
River flooding, also known as fluvial flooding, occurs when a river overspills its banks after there has been an increase of water volume in the river channel. Overbank flooding of rivers and streams happens when there is an increase of water volume in a river channel, causing it to spill onto and flood the adjacent floodplain. River flooding is incorporated directly into the First Street Foundation Flood Model through an analysis of historical water level readings from river gauges and their interaction with the surrounding elevation levels.
Ongoing riverine floods can intensify and become flash floods when heavy rainfall results in a rapid surge of rising flood waters. Increases and decreases in the frequency and severity of flood events near rivers generally correspond with increases and decreases in the frequency of heavy rain events. Warmer temperatures cause more water to evaporate from the land and oceans, which in turn changes the severity and frequency of heavy precipitation affecting the severity and frequency of river flooding.
The impact of river flooding
The Midwestern Floods of 2019 affected nearly 14 million people across 13 states, causing $6.2 billion dollars in damages. The floods occurred during the wettest 12-month period ever recorded, and were exacerbated by heavy and frequent rains that caused the Missouri River and Mississippi River to set new record levels in 42 different locations.
Rescue personnel walk along Main Street in Ellicott City, Md. Roaring flash floods in 2018 turned the Main Street in Ellicott City, Md into a raging river, the same area that been wracked by similar devastation two years ago. (Libby Solomon/The Baltimore Sun via AP)
Tidal flooding, sometimes known as sunny day flooding, king tide flooding, or nuisance flooding, is the temporary flooding of low-lying areas near the coast. It usually occurs during exceptionally high tide events, like full and new moons, and some areas experience this flooding multiple times a year. Tidal flooding is incorporated into the First Street Foundation Flood Model through the addition of coastal tide gauge station readings to adjust local water levels for integration into our modeled flood risk layers.
Damage caused by high tides
Many coastal areas are seeing floods more often even when there are no storms, as local sea levels increase the number of “sunny” day floods caused by high tides. There are now over 40 locations in the U.S. with accelerating high tide flooding trends, including Washington D.C. which saw 22 days with high tide floods in 2018.
Sea level rise is increasing risk
Most of America’s coast is experiencing rising seas caused by global ice melt, thermal expansion, changes in ocean circulation, and local land sinkage. Sea level rise makes high tides higher, which puts more pressure on coastal drainage systems.
Storm surge refers to the rise of water generated by a storm, over and above predicted water levels. The more severe the hurricane or storm, the greater the storm surge, and the further water can travel. This flooding is often exacerbated by the rainfall that occurs along with the storm. Storm surge flooding is incorporated directly into the Flood Model through the simulation of storm tracks based on a historical analysis of coastal water levels and changing environmental factors.
Warming oceans and sea level rise exacerbate risk
Sea level rise exacerbates the risk and damage of tidal and storm surge flooding. Homes that were once safe from this type of flooding may be at risk now or in the near future due to the sea level rising. As the sea levels rise, there is more water available to contribute to hurricane storm surge. This means that water can travel further inland, or flood in greater depths. A warmer atmosphere also means warmer oceans. The sea’s surface temperature is 1.5˚F warmer than it was in 1950 and will rise another 0.5˚F by 2050. Higher ocean surface temperatures fuel hurricanes and offshore storms with more water and power, so these systems reach further inland and further north, are more intense, and last longer.
A road covered by floodwater left in the wake of Hurricane and Tropical Storm Harvey near Houston, Texas. (Scott Olsen / Getty Images)
The impact of storm surge
Sea level rise exacerbates the risk and damage of tidal and storm surge flooding. Homes that were once safe from this type of flooding may be at risk now or in the near future due to the sea level rising. As the sea levels rise, there is more water available to contribute to hurricane storm surge. This means that water can travel further inland, or flood in greater depths.
Personalized flood risk
A property’s estimated cost to repair flood damage provides a powerful way to understand flood risk in terms of the financial cost to property owners. Flood damage estimates on Flood Factor considers the location of a building and source of flooding when determining the subsequent cost of its flood risk. For example, storm surge from a hurricane causes a different type of damage than structural damage from heavy rain.