Understanding different types of floods and the dangers they pose

Sometimes floods come in a flash; other times they build slowly. Some come from a coastline, others still from a river or lake.

There are numerous types and categorizations of floods, each coming with their own risks and levels of worry. According to AccuWeather Senior Meteorologist Alex Sosnowski, there are many different factors that can contribute to flooding and different elements can lead to different threats.

“Flooding is dependent on the amount of rain, terrain and soil type,” Sosnowski explained.

Here are some of the more commonly recognized flood categorizations and terms, along with different factors that can influence a flood’s severity.

Flash floods

Usually the most dangerous, flash floods occur following periods of heavy rainfall or release of water from lakes, rivers or dams. The National Weather Service defines a flash flood as any flood that develops in under six hours.

“Flash flooding generally refers to small streams and urban areas,” Sosnowski said. “Poor drainage locations in urban areas can flood in any downpour situation. It takes a bit more for small stream flooding and significantly more rainfall for river flooding to occur.”

It’s common for flash flood watches to be issued in areas that are favorable for flooding during periods of prolonged, heavy rain. Some qualities of an area that are more likely to get flash floods are densely populated areas, recent burn areas, low water crossings and paved surfaces.

River floods

As explained by its name, river floods occur when water levels rise above river banks due to excess rain. Sosnowski explained that a single heavy rainfall can turn a stream into a flood, but it is important to factor in the size of the river.

“Generally, the larger the stream, the longer it takes for the flooding to cycle through. A small stream in the mountains will flood exponentially faster than a wide, meandering river in the flatlands near the coast,” he explained. “Small stream flooding occurs in minutes to hours after a heavy rain. It may take several days to a week or more for a major river to flood near the coast. It has to do with the volume of water the stream can handle versus the slope of the terrain.”

Minor, moderate, major and record flooding

Minor, moderate, major and record flooding are the four different stages of stream and river flooding. Each stage is progressively more severe than its predecessor, according to Sosnowski. The National Weather Service considers minor flooding to be flooding that has minimal or no property damage but possibly some public threat. With minor flooding, a flood advisory is usually issued.

Moderate flooding is the stage when some evacuations begin and roads/structures near a stream are in danger of being impacted. With moderate flooding, water over the road is considered deep enough to make driving unsafe.

At the major flooding stage, significant evacuations are generally necessary. At this level, a flood warning is issued and several buildings are usually left with substantial damage or destruction. The National Weather Service writes that at this level, everyone in the area is considered to be threatened by the flooding and evacuation is mandatory. This is usually when the National Guard is called in to assist.

When a river is at record flood stage, it is the highest the water level has been since recording began. It's important to remember that this doesn't necessarily imply a major flood, but the flooding should still be taken seriously.

Saturated soil, other factors that influence flooding

“When the ground is saturated, it can no longer hold any water,” Sosnowski said. “In that case, the rain just runs off into streams and rivers. A sandy soil can take exponentially more rainfall than a heavy clay soil. Sand is permeable. Clay is practically impermeable.”

Areas that usually endure the most dangerous flooding are places that get numerous storms of heavy rain in a short time period. Even if the soil isn’t saturated from a recent rainfall, poor runoff can also lead to dangerously saturated soil, as the water can pool.

Flash flood situations are determined by a number of different factors, Sosnowski said, including soil type, saturation levels and drainage systems in the area.

Ice jams

Major flooding can also occur from non-rain events, such as a rapid thaw or an ice jam. The National Weather Service explains that an ice jam is when pieces of floating ice carried with a stream’s current obstruct the stream flow, causing water to be held back and cause flooding upstream.

Most likely to occur in early winter or early spring, the potential for ice jams is most dangerous when temperatures hover around the high 30s or low 40s.

With every categorization and measurement of flooding, Sosnowski said it’s important to remember that different areas of the flooding need to be gauged differently.

“While gauges may accurately measure the height of the river, the river bottom is in a constant state of flux,” he said. “So, the river may be deeper or shallower at that location, when the last flood occurred.”

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