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Floods, Pt. 2: What Are the 3 Types of Floods?

Last month, we opened our discussion of flood insurance with the story of the devastating Johnstown flood of 1889. That flood was caused by a number of factors, not the least of which was the short-sighted and poor management of the South Fork Dam. But at the end of the day, the immediate cause was a massive rainstorm that pounded the area for days leading up to the catastrophe. The lake and its impounding dam simply couldn’t contain the weight of the additional water, and more than 2,000 people died as a result.

That’s an extreme example of a flood. Fortunately, most of us will never encounter a 60-foot wall of water bearing down on us from a broken dam. That type of flood, called a flash flood, isn’t the only common flooding situation home and business owners need to be aware of, however.

In this article, we’ll look at the three main types of floods and what causes them.

Note: In this article, we’ll only be looking at “natural” flooding. Floods caused by plumbing damage, sewage backups, or water from any other artificial source are not discussed here.

Fluvial Floods: Overflowing Rivers, Streams, Lakes, and Ponds

These are the floods with which most people are most familiar. Footage from flooded cities along the banks of the Ohio, Mississippi, and other major rivers is a common enough sight on the evening news, and our rivers in Pennsylvania are notorious for trying to escape their banks and make their way into our homes and businesses.

A fluvial flood happens when the water level in a naturally occurring body of water rises to the point that the body can overflow and water escapes into the surrounding terrain. The excess water can come from an expected source (springtime thaw) or an unexpected source (significant rainfall).

The flood’s severity largely depends on the shape of the land surrounding the body of water and the amount of water involved in the flood. In flatter areas, water tends to spread out shallowly over a large area, leading to less significant damage to a large number of homes and businesses. In hilly terrain with rivers running in valleys, flooding occurs faster, water levels are deeper, and the amount of debris carried in the water is typically much higher.

While water levels can rise with shocking speed, residents in an area susceptible to pluvial flooding generally have at least a few hours of warning before water levels reach the flood stage.

Unless measures are taken to divert flood waters, a fluvial flood will tend to travel downstream and affect several communities before dispersing.

Pluvial Floods: Flash Floods and Surface Water Floods

While fluvial floods are primarily a concern for those living near streams, rivers, and lakes, pluvial floods can happen just about anywhere. Even in arid locations like the southwestern US, sudden bursts of intense rain can cause flash flooding in areas that are hundreds of miles from the nearest large river or lake.

Most pluvial floods fall into one of two categories:

A surface water flood is what happens when surface drainage systems – storm sewers, catchment ponds, canals, and drainage ditches – are overwhelmed by a sudden, intense fall of rain. Surface water floods tend to develop slowly, giving residents plenty of time to evacuate, but can reach almost three feet in depth and cause significant property damage.

Flash floods are the hazardous and hard-to-predict rapid movement of a large volume of water. Typically, a flash flood occurs when heavy rainfall occurs uphill from a particular location or when a dam or levee fails and releases the waters it was holding back. Flash floods are responsible for most flood-related deaths, as the water is typically very fast-moving, unexpected, and hard to escape.

Alluvial Flooding: Coastal Storm Surge

Thanks to the geography of our state, alluvial floods are the only type from which Pennsylvania residents are largely immune. While those in the Lake Erie region could experience alluvial floods from time to time, these flooding events are mostly constrained to areas on the seacoast.

An alluvial flood, more commonly referred to as a storm surge, is what happens when high winds literally push a mass of water onto shore and into a coastal region. They’re a common occurrence along the Atlantic and Gulf coasts of the southeastern US. Storm surges are usually accompanied by tropical storms that dump millions of gallons of rain into the already-flooded area, exacerbating the situation and making storm surges some of the most expensive floods.

A flooded roadway in the Chicago area on a cloudy day.

All Together, Now

All three common natural flooding types can occur in conjunction, making for a perilous and costly situation. When Hurricane Katrina struck New Orleans in 2005, a river flood caused by the storm’s earliest rain bands swelled water levels in the Mississippi and Atchafalaya Rivers, Lake Pontchartrain, and the hundreds of miles of bayou between New Orleans and the Gulf of Mexico. When combined with the alluvial flood from the storm surge, the incredible volume of water burst several protective levees, leading to flash flooding throughout the city’s low-lying areas.

If a Flood Has Caused Water Damage in Your Home, Contact AfterCare Restoration Right Away to Begin Mitigation and Restoration. We Work With NFIP and Private Carrier Flood Insurers: 215.515.1000

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