This is a historial post from Hiking the California Trail, a 1998/2002 book set by Bob Lorentz and Richard Nichols. Where possible an update has been provided.
The waves we see dashing against the coast start hundreds of miles offshore. As winds blow across the ocean’s surface, they create waves of various sizes.A wave’s size depends on wind velocity, duration and the distance the wind blows across the open ocean.
Waves break, showing a churning crest of foam along their leading edge when they become oversteep in deep water or when they enter shallow water. Waves break when they reach a water depth around 125% of their height. So we see endless sets of breakers as we explore the shore, each wave different than the wave preceding it, surf varying greatly from one day to the next, sometimes even within a day. “Never turn your back on the ocean” is a cardinal rule of the coast because each wave can be so different from the last. The constant attack of waves upon the shore fascinates us with its ever changing motion, but it also holds great danger. This danger increases the closer you get to the water and the larger the waves.
Two kinds of waves hold particular danger, sleeper waves and tsunamis. Sleeper waves, also known as rogue or killer waves, can occur any time although they’re infrequent and unpredictable. They are most common on days of particularly big surf, when storms are active far at sea or in winter, but you never know when they’ll strike. As the nickname killer implies, they present grave danger, sometimes being twice the size of the prevailing surf. They strike with powerful force, running far up the beach.
Large sleeper waves can sweep shoreline visitors into the sea. These rogues have claimed the lives of many an unwary seaside visitor. When you walk the coast, you must be aware that sleeper waves can strike at any time. Always be ready to retreat up the beach at the first hint of oversize waves. If you get swamped, be ready to abandon your gear or backpack. It’s better to lose your possessions than your life.
The second dangerous wave, the tsunami or tidal wave, results from earthquakes, slides or volcanic eruptions at sea. Tsunamis offer extreme and widespread danger. You cannot outrun a tsunami. If you see one from the beach it’s probably too late. Unlike sleeper waves, tsunamis are predicted by seismographic readings. Radio and TV stations broadcast tsunami warnings whenever seismological activity indicates they might occur. If you hear such a warning, get away from the coast.
Tsunamis 75 feet high have been seen. Tsunamis travel 400 miles an hour and are hardly visible on the open ocean. As they approach shore they rise up suddenly. The shallow depth of California ‘s continental shelf lessens the chance of 75-foot-high tsunamis, but smaller tsunamis can cause serious damage. The April 1964 Alaska quake launched tsunamis at Crescent City, California. A series of 12 foot waves smashed ashore, running a third of a mile inland, destroying 29 blocks of the business district and causing $27 million in damage. Crescent City was rebuilt, its harbor today protected by 1600 25ton concrete tetrapods shaped like children’s jacks.
Be aware of the dangers of oversize waves wherever you explore California’s outer coast.
—Originally Published in Hiking the California Coastal Trail: Guide to Walking the Golden State's Beaches and Bluff from Border to Border - Volume One: Oregon to Monterey (2nd Edition) by Bob Lorentzen and Richard Nichols