Giant surf pounded Hawai'i's north and west shores yesterday, bringing out big-wave riders and keeping lifeguards busy with warnings and rescues.

The largest waves, about 50 feet, broke on the outer reefs, and 25- to 35-foot wave faces were seen nearer the shoreline. Larger waves were expected last night as city lifeguards prepared to close Waimea Bay overnight.

During the day, O'ahu lifeguards issued 1,700 warnings, assisted 40 people and rescued 14 others.

Earl Dahlin, a longtime surfer, said he saw a wave with a 60-foot drop at Avalanche, off Hale'iwa Beach Park, and watched as buoys were sunk by crashing waves.

At Ali'i Beach Park, retired firefighter Sunny Archuleta said waves were washing up the shore about 60 feet. One wave caught a girl who was hunting for shells on the beach and swept her onto the boat jetty nearby, he said.

A lifeguard went to her aid, and Archuleta said the incident should be a reminder to all beachgoers.

"The cardinal rule for any kind of surfer, diver, fishermen is you don't turn your back on the waves," he said.

The 50-foot waves people saw yesterday are rare, coming about every two years, said Jonathan Hoag, a forecaster for the National Weather Service.

The northwest swell was generated by a powerful storm off Japan on Jan. 5 and gained strength last week, Hoag said.

The swell first hit Kaua'i, and as it works its way down the island chain, it gets smaller, he said.

"Kaua'i lifeguards (yesterday) morning said it had been 40 to 50 feet on the outer reef and caused some serious beach erosion," Hoag said. "I could see waves breaking across the entire length of Waimea Bay (O'ahu). That takes a 30- to 40-foot wave to do that."

Hoag said the waves will drop overnight but still have 20- to 30-foot faces today.

The giant surf kept lifeguards extremely busy, said Bryan Cheplic, spokesman for the city Emergency Services Department.

On the North Shore, lifeguards issued 931 warnings, assisted 20 people using water craft and rescued four people, including two tow-in surfers at a surf spot called Himalayas, Cheplic said. One of the surfers, a man in his 50s, was taken to a local hospital in serious condition, he said.

On the leeward side, lifeguards issued 802 warnings, assisted 20 and rescued 10 people, including a man in his 20s who had gone into the water to help a distressed swimmer, Cheplic said. He was taken to a local hospital in serious condition.

John Cummings, of the city Department of Emergency Management, said police notified the department about 4 yesterday morning about sand on the highway at Log Cabins and Laniākea Beach. The state Department of Transportation sent out a crew to clear the highway, Cummings said.

Last night, volunteers were at Waimea Bay to close the beach park overnight.

The surf last night was expected to peak at about 35 to 45 feet on the North Shore and drop overnight to 20 to 30 feet today.

West-facing shores were reported at 15 to 25 feet in the afternoon and dropping overnight to 10 to 15 feet today.

Ken Bradshaw, a longtime big-wave rider, said as many as a dozen tow-in surf teams were at Log Cabins yesterday to take advantage of the rare swell even though the conditions were "bumpy." He estimated wave faces at 40 to 50 feet.

"Waves this big are actually kind of rare," Bradshaw said."The last time we got to ride a wave this big was 2006."

Bradshaw, 57, said the big waves are an opportunity for surfers to challenge themselves in more ways than just riding the surf.

"It makes you highly motivated to stay in shape and to stay mentally and physically prepared for it," Bradshaw said before going back out to surf yesterday. "It keeps someone like myself young at heart and really enthusiastic about surfing."