(KTXL) — A storm that started on New Year’s Eve and continued into the day after caused flooding near the Cosumnes River leading to road closers, including Highway 99 in Sacramento County.

At the time, Sacramento County officials confirmed to FOX40 News that it was at least three levees that were breached, which caused the flooding.

Sacramento County officials then proclaimed a “local state of emergency for winter storms,” due to the roadway impacts, rising creek and river levels, and flooding in Wilton, which is about 15 minutes east of Elk Grove.

What is a levee? 

According to National Geographic, a levee is a bank of a river, raised either naturally or constructed by people.

The City of Sacramento said on its website levees are designed to protect against a specific flood level and could be overtopped during severe weather events.

The National Flood Insurance Program (NFIP) said levees are meant to contain, control or divert the flow of water to reduce the risk of temporary flooding.

Levees reduce but don’t eliminate the risk to individuals and the structures behind them, according to the city. 

What causes a levee to fail? 

According to the California Water Resources Control Board, levee failures can be caused by overtopping and structural failure. 

Overtopping occurs when there are floods, tidal fluctuations, and wind-driven waves, the water control board said.

Structural failure can be caused by “inadequate foundations, subsidence, seepage, erosion, and burrowing animals,” according to the water control board.

Subsidence is the sinking of ground because of underground material movement, according to the National Oceanic and Atmospheric Administration.

Seepage is a movement of water that may take place through a dam, its foundation or abutments, according to the National Weather Service.

Earth movements such as earthquakes can also cause a levee failure.  

The City of Sacramento said a levee system failure or overtopping can create severe flooding and high water velocities.