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Does Snow Cause Flooding?

Many people don’t know that melting snow is part of the water cycle, but runoff from snowmelt is a key component. In fact, its importance depends on the weather: in a warmer climate, melting snow does not directly affect water availability, but in colder weather, much of the springtime runoff and streamflow in rivers is caused due to melting snow and ice.

After all, let’s not forget that snow is made up of frozen water. Generally, 6 inches of snowpack melt down to one inch of water, even though this number can vary from 3 to 15 inches of snow per inch of water.

Snowmelt and Flooding

During warmer weather, temperatures rise above freezing point, causing the snow to melt and the water to flow toward any river, stream, or body of water nearby. When this current of water reaches streams, rivers and drainage systems, it can immediately flood them, causing water to rise out of the banks, swamping any adjacent land.

Spring floods can occur anywhere downstream of large snowpacks. Low-lying ditches and streambeds are areas prone to flooding because water can be trapped. As this pool of stagnant water grows, it can make rivers to overflow, capable of flooding everything on their path.

The impact of snowmelt on potential flooding, largely during the springtime, is one of the main causes concern for many people living in low-lying areas. Apart from flooding, rapid snowmelt can bring about landslides and debris flows. In mountainous regions, snowmelt is an important component of runoff. Combined with certain weather conditions, like excessive rainfall on melting snow, it may even be a substantial cause of floods. In some parts of the world, snowmelt forecasting is used as a flood-warning tool to let residents know of snowmelt runoff and possible flooding.

Rain-On-Snow

In some parts of the world, annual springtime floods happen when rain falls on existing snowpacks, a phenomenon called “rain-on-snow.” Rainfall on snow is reasonably common during spring. Initially, the snow usually is able to absorb some of the rain and reduce the quantity of runoff at first. However, in the case of steady and heavy rain, or if the snowpack is already completely wet, additional melting can increase runoff.

Runoff during rain-on-snow events has been associated with mass-wasting of hill slopes, downstream flooding, and loss of life. Some studies indicate that the amount of forest cover can influence the magnitude of rain-on-snow events.

In 1996, a combination of circumstances contributed to massive flooding in the Northeast. Heavy snowfall followed by a sudden thaw and torrential rain affected rivers from New York through Pennsylvania. There were many casualties, and many residents had to evacuate their homes due to the raging rivers.  The damage done to buildings, bridges, dams, and other properties were exacerbated by ice blocks carried by the floodwaters.

Don’t let your guard down during winter and be wary of sudden changes in temperature changes.

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