Beach Dynamics

The following section decribes beach dynamics and how they relate to sand levels and movement in the near-shore environment. Figure #1 is a summary of the seasonal changes which occur on the beaches along our coastline. The next section provides more specific details and explanations with additional resources for the subject.

Figure #1a: Winter Conditions on the Southern California Coast

v Highest overall coastal erosion

v High wave energy

v Higher astronomical tides (very significant)

v Northwest swell, south longshore current

v Sediment/sand migrates offshore (Lower overall sand levels)

v Coarser sediments ie: smaller rocky beach

v Upwelling, cooler, less-clear water

Figure #1b: Summer Conditions on the Southern California Coast

v Lower overall coastal erosion

v Lower wave energy

v Lower astronomical tides

v Southwest swell, slack to north longshore current

v Sediment/sand migrates onto beach (higher sand levels)

v Finer sediments ie: larger sandy beach

v Downwelling, warmer, clearer water

Explanation and Discussion

In the winter, we have larger surf, higher astronomical tides and lower overall sand levels. These factors combine to create favorable conditions for coastal erosion and are discussed in more detail below. High sand levels in place during these periods of high tides and surf are critical in preventing erosion along our coast.

Storm Centers and Longshore Transport

The storm centers which affect the surf in the San Diego area are different in the summer when compared with the winter. In the winter, strong storms in the Gulf of Alaska produce large surf with a northwest direction. In the summer, the storms are located near New Zealand and large surf is produced from a southwest direction. So in the winter we have a southern longshore current and in the summer a slack to weak northern longshore current. The predominant longshore flow is to the south. Figure #1 provides a summary of these conditions.

Seasonal Energy Levels

The energy levels of the waves and currents are different in the winter vs. the summer. This affects the beach shape and the amount of sand on the beach. Stronger winter currents leave coarser materials in the winter such as gravel and cobbles. Much of the sand-sized material is transported offshore. This results in significantly lower sand levels on the beach. While in contrast, the summer has smaller waves and weaker currents and the sand migrates back to the beach. This results in much higher sand levels. So the beach is narrower and rockier in the winter, and wider and sandier in the summer.

Winter beach in Encinitas, California. Note

gravel and cobbles on the beach and lack

of a sandy berm. Low sand levels in the winter

increase erosion of the sea cliff.

Sand Levels and the Formation of Sea Caves and Cliffs

When sand levels are low, waves break into a sea cliff at the highest tides.  Sea caves form and grow at the base of the cliff usually in areas where a zone of weakness exists.  Normally these zone of weakness are related to areas of fracturing or faulting. The sea caves grow to the point where they cannot support the rocks above them and the cliff will fail and retreat landward.  This failure type is called "rock fall". When higher levels of sand exist on the beach, waves will not break into the sea cliff at higher tides. It only takes a small increase in the height of sand on the beach to prevent this type of coastal erosion, usually ½ to 1 meter is sufficient. Click here to see a great interactive Beach Profile.

Early stages of sea cliff undercutting Rock-fall due to sea cliff failure.

and sea cave formation due to low

sand levels and wave erosion.

Tides

Tidal extremes are greater in the winter than in the summer in the Northern Hemisphere because of the elliptical orbits of the earth around the sun, and the moon around the earth. The moon’s elliptical orbit around the earth is on a 28 day cycle and it is closest to the earth during the “perigee”. The earths orbit around the sun is on a 1 year cycle and is closest to the sun during the winter (farther in the summer), this is called the ”perihelion”.

So, we get higher tidal extremes in the winter when we are at perigee and perihelion, because of the increase in gravitational force. This is very significant as only a small increase in a high tide maximum height can be enough to create a situation where waves are breaking directly into the sea cliff. When these high astronomical tides are combined with larger surf generally found in the winter, the potential for coastal erosion is maximized.