How high was that tide? October 28, 2015 Edition

On October 28, 2015, southern California experienced unusually high tides combined with a swell event that caused coastal flooding in a variety of locations. Many locations experienced tides in excess of 7 ft NAVD88. So, how severe were these tide levels?

I queried the NOAA Tides and Currents website and found the maximum observed water level at each tide station from Point Reyes to San Diego for this date. I then compared that elevation to the extreme water level estimates at each location to estimate the return period of the observed water level. The results indicate that from Santa Barbara to San Diego, today's high tide ranged from a 3 to 7 year event. North of Point Conception, tides corresponded to approximately an annual (1-yr) event. The results are shown below in Table 1. Note that these water levels do not include wave effects, which can further elevate flood levels at the shoreline.

Table 1. Observed high tide levels on October 28, 2015 and estimated return periods.

Figure 1. Predicted and observed water levels at the La Jolla Tide Station. October 26-29, 2015.

The primary reason for these elevated tide levels is the combination of spring tides and El Niño. Spring tides occur twice each lunar month and result from the alignment of the moon, earth, and sun (at full moon and new moon). El Niño influences coastal water levels in California through two primary mechanisms: thermal expansion and Kelvin waves. Thermal expansion is caused by above average sea surface temperatures in the equatorial Pacific and along the California coast as a result of El Niño. Kelvin waves are a complex oceanographic phenomenon that help transfer warm waters and elevated water levels across the equatorial Pacific and northward along the coast of North America.

We are already seeing the effects of El Niño at tide stations along the California coast. Figure 2 shows the average non-tidal residual (NTR) at each active tide station from September 15 to October 15, 2015. NTR represents the difference between the observed water level and the predicted astronomical tide:

NTR = observed water level - predicted astronomical tide

NTR is typically referred to as "storm surge" but actually includes all factors which increase water levels at the coast, including thermal expansion and Kelvin wave effects due to El Niño and storm surge effects such as atmospheric pressure and wind setup.

The red curve shows the average NTR and the black bars show the 10% and 90% exceedance levels over this month-long time period. As can be seen, southern California is currently experiencing water levels that are on average 0.6 to 0.8 ft above normal, with some NTR values approaching 1 ft. The effect of the elevated NTRs decreases northward up to Arena Cove. The spike at Humboldt deviates from that trend but I haven't investigated the reasons for that in detail yet.

Figure 2. Average non-tidal residual at California tide stations from September 15-October 15, 2015. Black bars indicate the 10% and 90% exceedance levels.

As winter progresses, coastal storms will contribute to the total magnitude of the NTR and will increase the likelihood of experiencing even higher tide levels. This is especially the case in December through February, when we typically experience our highest tides of the year.

Stay tuned for future updates!

West vs. East Coast Beaches: The best of both worlds

Today's blog post was written by guest blogger, Donna Fitzgerald, of Wilmington, NC. Donna has lived in Wilmington for 7 years and enjoys taking visits to the beach with her two daughters. Donna is an avid reader and writer- what better place to compose than on the sandy shores!

Thanks, Donna, for researching this topic and sharing with the Beach Blog!

The beaches throughout the United States all offer some varying levels of beauty. Many will argue which area of the country contains the best beaches. For some, it's the surfing conditions that make a beach the best. For others, the beach terrain and weather conditions may make one coast more favorable than another.

There are many differences between the coastal beaches of California and the shoreline of North Carolina. Some differences are notable and evident to the average beach-goer. Other aspects are less obvious and require research and understanding of the varying geographic areas.

Sandy beaches and dunes are typical along the Outer Banks of North Carolina.

Physical Differences

The coast of North Carolina is mostly made up of barrier island chains that allow visitors to travel over many bridges to get from one island to the next. One of the most famous areas, the Outer Banks, is completely made up of thin island chains and sandy beaches separated from the mainland by large shallow bays. The coast of western beaches - California in particular - contains mostly beaches that are immediately adjacent to the mainland areas. Barrier islands and inlets are few and far between.

As we all know, the coast of California is under constant watch for tectonic activity. The east coast is what is referred to as a passive margin, while the California coast is on an active margin. With this being said, California is much more prone to earthquakes and unpredictable tectonic shifts.

Climate

The eastern shores of the United States are much more prone to experiencing tropical cyclones that form off the Atlantic. With these storms comes the risk of tropical depressions, tropical storms, and hurricanes that can impact the coast. With this being said, the climate in each area varies greatly. In general, the beaches of the Pacific Northwest (northern California, Oregon, and Washington) receive the highest amounts of precipitation. East coast beaches receive moderate precipitation and central and southern California beaches receive the least. El Niño cycles can greatly affect the distribution of precipitation along the west coast.

The waves roll in at Pacific State Beach, about 15 miles south of San Francisco.

Other Differences

Sandy beaches mostly make up the shorelines of east coast beaches. To the north, you may find a few New England beaches that feature rocky, jagged shorelines. The east coast primarily features sandy shores that make beach time more enjoyable and relaxing for visitors.

The west coast beaches, like those along the coast of California often feature rocky terrain and steep slopes. The California shoreline often contains many more steep hills and cliffs, like those along Big Sur in central California. These can pose threats to visitors and special cautions should be exercised.

One of the biggest advantages of west coast beaches in comparison to the east coast beaches is the consistent wave climate. California beaches are known for having higher, longer period waves due to the offshore wave climate and narrower continental shelf along the west coast. Surfing conditions are more ideal on the west coast compared to the east coast, where waves are less consistent. This being said, great east coast surfing spots can be found.

Steep beaches with waves make for fun skimboarding at Montara State Beach, about 30 miles south of San Francisco.

Visitors

If you are visiting either coast, be sure to take note of these unique differences. Some tend to favor visiting the east coast beaches because of their sandy shorelines and enjoyable temperatures. Others believe California and west coast beaches are better to visit because of their geographic diversity and great surfing conditions.

Which coast do you prefer?

Sunset near Cape Hatteras, on the Outer Banks of North Carolina.