Tuesday, February 5, 2013

Do King Tides Matter on the Open Coast?


King Tides have received a lot of press lately and rightly so. Besides representing the oceanic manifestation of a relatively rare, but predictable, astronomical happenstance, King Tides represent a window into the future - a future where sea level rise makes today's extreme tides a more frequent occurrence. I won’t delve into the details of the gravitational forcings that produce King Tides – those are well described elsewhere. Suffice it to say, King Tides represent the highest naturally occurring (and totally predictable) tides of the year and occur when the sun, the moon, and the earth align in just the right way. Groups such as the California King Tides Initiative have promoted these naturally occurring tides as a proxy for what everyday high tides will look like in the future with, say, a foot of sea level rise (roughly equivalent to mid-21st century conditions). At San Francisco, the typical daily high tide is roughly 6 ft, while a King Tide is roughly 7 ft – a foot higher. When tides approach the 6.8 to 7.2 ft range at the Golden Gate, various shoreline assets, such as roads, parking lots (remember the Mill Valley park-and-ride?), bike trails, promenades, and so forth start experiencing nuisance flooding (shallow inundation). In addition, elevated tides present potential flooding problems for drainage outfalls to the Bay and creeks and rivers that discharge to tidal waters. The Marin County communities of Mill Valley and Sausalito, as well as San Francisco's Embarcadero, appear to be particularly vulnerable to King Tides.

January 2012 King Tides along San Francisco's Embarcadero

King Tide flooding is a relatively common occurrence in San Francisco Bay and has been more widely publicized recently, in part due to the efforts of the CA King Tides Initiative to educate citizens of the Bay Area. So clearly, King Tides are an important occurrence within San Francisco Bay (and other inland embayments, such as Humboldt/Arcata and Newport Bays) where relatively low-lying mildly sloping shorelines can experience noticeable flooding when high spring tides are just a few inches higher than normal.

But what about on the open coast? Do the beaches and bluffs (and more importantly, homeowners) of the California coastline care about unusually high tides? Well, the answer of course is – it depends. First, we have to consider what does "coastal flooding" mean along the open coast?  It’s more than just high tides. It’s the total water level – the maximum elevation achieved by waves running up on the beach, and is a combination of various surf zone processes, including the base tide level, storm surge, wave setup, and wave runup. I know what you're thinking... What is wave setup? Well, wave setup is a bit of a snoozer to explain, but the simplest definition is that it is an increase in the water level at the shoreline due to the presence of breaking waves. Basically, when you have large waves breaking across the surf zone, the average water level at the shoreline will be higher than it would be if you had no waves, due to water piling up at the shoreline. The last component – wave runup – is the process that you actually observe when you are standing on the beach. After waves break, they form bores which propagate landward and rush up onto the beach before retreating back into the ocean. So, all those components together – tides, storm surge, wave setup, and wave runup – combine to produce the ultimate maximum limit of inundation along the shoreline. For coastal property owners, understanding that maximum  wave runup elevation during a coastal flood event is the difference between having water in your living room or not.

Now, it turns out that wave setup and runup contribute up to two to three times as much to the total runup elevation as does the tide range. And wave processes are much more important along the California coast than is storm surge (the opposite is true along the Gulf and Atlantic coasts, where storm surge is dominant). An extreme wave runup elevation on an exposed beach in northern California might be on the order of 20-30 feet. Let's say 7 feet of that is due to tides and 2 ft is due to storm surge. That leaves roughly 10-20 feet due to wave processes - that's huge! So just how important are the tides? We'll get back to that in a minute.

Waves washing over the Rodeo Lagoon spit during high tide.

The next piece of the puzzle is the offshore wave conditions, since the waves combine with the tide to produce high wave runup elevations. Waves are typically characterized by their wave height (distance from trough to crest) and period (time between wave crests). Maximum wave heights along the California coast are on the order of 30-35 ft and maximum wave periods are in the 20-25 second range; however, those conditions very rarely, if ever, occur together. This got me wondering... historically, what combinations of tides, wave height, and wave period have resulted in the maximum observed wave runup elevations? We know that it is the wave runup elevation that produces flooding at the shoreline, not the tide or offshore wave height alone. If we were identify, say, the top 15 wave runup events for a particular beach, how important is the tide component? Are King Tides (or higher) a requirement for extreme coastal flooding along the California coast? Or can we get extreme coastal events during normal high tides, or even something lower - maybe mean sea level?

Whoa... that was a lot of questions there. Somebody needs to look at some data!

Let's start with some simulated data at a representative beach along central California's open coast. Below, I've listed the top 15 wave runup events during a 50-yr simulation period (in total, I examined the top 130 events). Right off the bat, we see that the top 5 events are quite close - within 1 foot of each other. So for argument's sake, let's just say they are equal - roughly 21 feet. The next thing we see is that there are many different combinations of tide, wave height, and wave period to arrive at the same runup elevation! The tide varies from 6.4 to 7.9 ft, wave height varies from 16.4 to 31 ft, and wave period varies from 15.9 to 21.2 seconds. The total wave runup elevation is a function of all three of these variables (beach slope is also important). It's also worth noting, that in all 15 cases, at least a high tide (5.8 ft) or greater is required to make the list. I'll also point out that the top 3 events are all from strong El Nino winters with extremely high tides, but that's a topic for another discussion.


Top 15 Wave Runup Events Along a Representative Central California Beach*

Rank
Date
Runup
Elevation (ft)
Tide (ft)
Wave Height (ft)
Wave Period (sec)
1
1/29/1998
20.9
7.4
16.4
21.2
2
1/27/1983
20.6
7.9
21.1
17.5
3
3/1/1983
20.4
7.8
21.0
17.5
4
2/9/1960
20.3
6.4
31.0
15.9
5
1/5/2008
20.1
6.5
24.2
17.5
6
2/14/2000
19.5
6.7
17.9
19.2
7
12/16/2002
19.5
7.4
23.6
15.9
8
10/28/1977
19.4
6.1
16.0
21.2
9
1/11/2001
19.1
7.7
20.8
15.9
10
12/27/2005
19.1
6.2
14.8
21.2
11
1/5/1995
19.0
5.8
15.7
21.2
12
2/8/1998
18.9
7.6
20.4
15.9
13
2/3/1998
18.9
8.2
18.3
15.9
14
1/8/1978
18.7
7.3
14.2
19.2
15
3/11/1986
18.5
6.3
23.6
15.9
 
I've highlighted a few interesting features in the table - namely the extreme tides in events 2-3, 9, and 12-13, the extreme wave height in event 4, and the very long period swell in events 1, 8, and 10-11. In general, each of these extreme runup events is due to one of two conditions: (1) an extreme occurrence of either tide, wave height, or wave period, coupled with moderate conditions in the other variables, or (2) strong (but not extreme) occurrences of tide, wave height, and wave period together. Case 1 is highlighted by event 4, which saw a moderate high tide (6.4 ft) and typical winter swell period (15.9 sec), but a monstrous wave height (31 ft). Case 2 is highlighted by event 6, which saw an above average, but not extreme, high tide (6.7 ft), and strong swell with a height of 24.2 ft and period of 19.2 sec. Neither of these conditions alone would be particularly remarkable, but when combined, they can produce extreme runup elevations.

Now back to the King Tides question. We see that an extreme high tide alone is not enough to get you on the top 15 list. Below, I've listed the top 15 tides out of the 130 wave runup events examined. There certainly are a few cases where an extreme tide was part of an extreme wave runup event, but in general, extreme tides do not equal extreme runup events. In other words, high tides can help, but you need big waves to get big runup elevations. That's a pretty obvious conclusion but it is interesting to look at the numbers. The most extreme tides shown here represent cases that occurred during El Nino winters when coastal water levels were elevated above predicted levels for the duration of the winter.

Top 15 Tide-Ranked Wave Runup Events Along a Representative Central California Beach*

Tide Rank
Overall
Rank
Date
Runup
Elevation (ft)
Tide (ft)
Wave
Height (ft)
Wave
Period (sec)
1
13
2/3/1998
18.9
8.2
18.3
15.9
2
2
1/27/1983
20.6
7.9
21.1
17.5
3
3
3/1/1983
20.4
7.8
21.0
17.5
4
26
1/17/1973
17.7
7.8
15.7
15.9
5
86
11/30/1982
16.2
7.8
20.4
11.9
6
111
2/15/1969
15.9
7.8
12.8
14.4
7
90
12/31/2005
16.1
7.7
20.0
11.9
8
93
12/11/1993
16.1
7.7
13.6
14.4
9
9
1/11/2001
19.1
7.7
20.8
15.9
10
58
2/6/1978
16.8
7.6
9.3
19.2
11
12
2/8/1998
18.9
7.6
20.4
15.9
12
36
1/24/1983
17.4
7.5
15.8
15.9
13
28
11/14/1997
17.6
7.5
16.4
15.9
14
128
1/8/2005
15.7
7.4
13.4
14.4
15
7
12/16/2002
19.5
7.4
23.6
15.9

So now we've established that while at least a high tide is a required component of an extreme wave runup event (in fact, out of the top 130 wave runup events, the lowest coincident tide was a 4.5 ft tide - essentially a lower high tide), extreme high tides are not a necessary requirement.

So where does that leave us with King Tides? Assuming a King Tide at San Francisco is defined as a tide that exceeds about 6.8 ft, then roughly half of the top 15 wave runup events occurred during a King Tide or higher. It just so happens that our King Tides window of interest - roughly December-February, also coincides with our Pacific Ocean winter storm season!  How crazy is that? Due to the highly predictable astronomical motions of the sun, moon, and earth, we know precisely when and how high the tides will be during the winter storm months (excluding storm surge effects). What we don't know, however, is the timing of winter storm waves relative to these predetermined-potentially-coastal-flood-causing King Tide phenomena. So basically, every winter, we role the dice and the randomness of the Pacific Ocean basin storm machine kicks out swell events that may or may not wreak havoc on our coastline, depending on their timing with the tides. Throw in the additional variable of storm surge, which can further elevate tides above their predicted levels (especially during El Nino winters) and the picture becomes even more complex.

Despite all this randomness and complexity, one thing is clear. Winter season King Tides set the stage for extreme open coast flood events - but the big swells may or may not show up at the right time. Every winter, the King Tides come and go. Those of us along California's embayment shorelines see the effects of King Tides in the form of nuisance flooding. For those of us along the coastline, it takes a special joint occurrence of high tides and big waves to get noticed, and when that happens - like during the El Nino winters of 1997-98 and 1982-83 - we get slammed.

So are King Tides important along the open coast? Well, it's not so much the "King Tide" phenomenon by itself as much as the need for high tides that occur coincident with large storm waves. That combination is certainly much more likely during times of King Tides than during any other time during the winter months. Couple that with the potential for sea level rise to further elevate the high tides we experience, and suddenly that tide component starts becoming even more important. So yeah, King Tides, here's a nod to you from the open coast...






*Disclaimer: Wave runup data are simulated and are shown for illustrative purposes only. Results should not be used for any engineering or planning purposes. Site specific evaluations of coastal flood hazards by a qualified professional are recommended for any project along the coastline. A good place to start is FEMA's Flood Insurance Rate Map for your community.


1 comment:

  1. Do King Tides matter on the east coast? Any correlation with road over wash at the s-curves on Hattteras Island? Why is the tidal range so low north of Cape Lookout?

    ReplyDelete