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More monster waves will collide with the California coast after injuring onlookers and causing serious flooding

Storm debris fills the Rio Del Mar neighborhood of Aptos in Santa Cruz County. (Photo Provided/Nic Coury/AP)

(CNN) — A series of powerful storms in the Pacific Ocean are driving towering waves into the California coastline, triggering flooding and posing a significant risk to people and structures along the coast.

Monstrous, 20-foot-plus waves on Thursday crashed over seawalls and swept away and injured several people, forced rescues and sent a damaging surge of water through coastal California streets.

A brief dip in wave heights may occur in some locations along the California coast during the daytime Friday as one storm pushes ashore, but the danger will quickly ramp back up later Friday and Saturday as another system develops behind it.

“Overall, this is expected to be an exceptional high-surf and coastal flooding event that has not occurred in many years,” the weather service in Los Angeles warned.

Sea levels have risen along most of the California coastline over the past century, NOAA data shows, as global temperature climbs and melts glaciers and ice sheets. Higher sea levels are making coastal flooding events worse and will continue to do so in the future.

Waves as high as telephone poles – about 40 feet – could slam into San Francisco through Friday morning. Fifteen- to 20-foot waves are expected along the central and Southern California coasts through Saturday evening.

The first round of dangerous waves hit alongside high tide Thursday morning. Several people were injured by a huge wave that slammed into Ventura Beach.

The wave crashed over a barrier along the Southern California beach, tossing a wall of fast-moving water at a group of onlookers who rushed to escape the deluge, witness video shows. Seawater quickly knocked some people off their feet and pummeled cars as drivers tried to speed away.

Nearly 20 people were briefly swept away in the incident and eight people were taken to the hospital, Ventura officials said.

High water and dangerous rip currents will churn along some of California’s beaches through the weekend as much of the West coast – spanning from the US-Mexico border to southern Oregon – is under coastal flood and high-surf alerts, the National Weather Service said.

The dangerous conditions pose an “exceptional risk” of ocean drowning and damage to structures like piers and jetties, the National Weather Service said.

The dangerous wave risk will lessen on Saturday for Northern California, but continue in southern Oregon and rival Thursday’s impact in Southern California. The Ventura County coast, along with Hermosa, Manhattan and Palos Verdes beaches face the most extreme surf and are at risk for significant coastal flooding.

One of the storms helping to drive the waves will also bring rain and wind to California through Saturday. High wind alerts are in effect for California’s central coast and the Bay Area on Friday as wind gusts of up to 50 mph are expected. These strong wind gusts will churn up waters even further.

The spectacular waters have enthralled some surfers and onlookers, but weather officials are cautioning that the waves and strong rip tides can be perilous for those nearby.

“Large breaking waves can cause injury, wash people off beaches and rocks, and capsize small boats near shore,” the National Weather Service warned.

At least one fisherman fell overboard in a harbor off Oxnard Thursday and later came ashore uninjured, the Ventura County Fire Department said.

The conditions caused some road closures and evacuations in coastal California communities Thursday and prompted the rescue of five campers along the San Luis Obispo coast.

Crews in Ventura worked through the night to reinforce a mile-long sand barrier that was damaged by powerful waters on Thursday, the local fire department said. Standing about 7 feet tall, the raised wall helps protect beachside communities.

Hoping to discourage spectators, local officials closed a main access point to Ventura Pier, which saw a swell of about 20 feet on Thursday. Some coastal streets in the area have also been closed as a precaution.

In central California’s Santa Cruz, the sheriff’s office issued an evacuation warning for some areas on Thursday, which included areas of Rio Del Mar, where seawater filled beachside roadways and pushed against some homes, CNN affiliate KION reported. The warnings were lifted later in the day.

Massive waves attract surfers to Mavericks

Despite safety warnings, the massive waves are a welcome sight to surfers hungry to tackle the legendary swells at Mavericks Beach, about 25 miles south of San Francisco.

Pro surfers and spectators flocked to the beach on Thursday for a competition at the site known for having some of the biggest waves in the world, CNN affiliate KGO reported.

Local resident Ion Banner told the affiliate that surfers from Brazil, Tahiti and Hawaii were out in the water. “It’s pretty gnarly, it’s super big,” he said.

“The waves looked absolutely massive and it was everything that we expected,” said Miguel Blanco, who told KGO he flew in from Portugal to surf the waves. “It was really big, I’d say 40-60 foot waves.”

Mavericks’ jaw-dropping waves – caused by unique underwater rock formations – reach their biggest heights in the winter, when big wave surfers from all over the world make pilgrimages to face the swells.

The legendary waves and the surfers who brave them have been the subject of several films and documentaries, including the 2012 biopic “Chasing Mavericks” about American surfer Jay Moriarity’s journey to surf Mavericks as a teen.

On Thursday, the conditions were prime for surfers like Blanco.

“If it’s your turn, you just gotta go,” Blanco said. “When you see a big wave, you’re kinda scared but at the same time you’re feeling like you should go and you just go and enjoy the ride.”

CNN’s David Williams contributed to this report.