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!