Threats posed by heavy precipitation
The most immediate impact of heavy precipitation is the prospect of flooding. This risk can be heightened in urban areas where impervious pavements force water to quickly run off into sewer systems. Among recent examples:
In August 2017, Hurricane Harvey dumped 48 inches of rain on Houston, breaking the U.S. record for most rainfall from a single event and demonstrating that the biggest threat from tropical cyclones is not always coastal flooding and wind damage.
In July 2016, more than six inches of rain fell in two hours in historic Ellicott City, Maryland, causing more than $22 million in damages.
In September 2013, Boulder, Colorado, received almost a year’s worth of rainfall (17 inches) in four days. The resulting flooding destroyed homes, shut down thousands of oil and gas wells, and damaged crops.
In 2010, almost 20 inches of rain fell in the Nashville, Tennessee, area over three days. Losses in Nashville alone totaled over $1 billion.
In addition to flooding, heavy precipitation also increases the risk of landslides. When above-normal precipitation raises the water table and saturates the ground, slopes can lose their stability, causing a landslide. Washington state, which has a particularly high risk of landslides due to its terrain, is anticipating more frequent landslides as a result of climate change-induced increases in heavy downpours.
Excessive precipitation can also degrade water quality, harming human health and ecosystems. Stormwater runoff, which often includes pollutants like heavy metals, pesticides, nitrogen, and phosphorus, can end up in lakes, streams, and bays, damaging aquatic ecosystems and lowering water quality for human uses.
Many cities in the United States use a combined sewer system, where both stormwater and wastewater are mixed, treated, and released. Heavy rainfall can overwhelm such systems, sending excess stormwater, wastewater, and untreated sewage directly into the environment, risking public health and disrupting local fisheries.