By Jenny Neyman
Credence Clearwater Revival’s John Fogerty might talk about a bad moon rising last week. But luckily for Kenai, though a particularly close new moon caused extra-high tidal ranges, there were no hurricanes ablowin’ at the time, thus avoiding any rivers overlowin’, much less any rage and ruin to the unstable bluffs along the river mouth.
Cook Inlet was awash in its highest tides and widest tidal ranges of the year last week. Anchorage on Wednesday saw a high tide of 33.1 feet at 7:48 p.m., followed by a low tide of minus 5.2 feet at 2:55 a.m. Thursday, for a tidal range of 38.3 feet. At Homer, a high tide of 22.3 feet at 2:55 p.m. Wednesday was followed by a low of minus 4.9 at 9:16 p.m., for a range of 27.2 feet. And at the mouth of the Kenai River, a high tide of 24.9 feet at 4:42 p.m. Wednesday was followed by a low of minus 4.5 at 11:31 p.m., for a rage of 29.4 feet.
The high highs and low lows were created by a conjunction of factors having to do with the position of the moon. A little Earth science 101 refresher, here — tides are essentially long-period waves rolling around the planet as the ocean is affected by the gravitational pull of the moon and sun. Twice each month, tidal ranges are a little larger than average as the Earth, sun and moon are nearly in alignment. As seen from Earth, that’s either a full moon — when Earth is between the moon and the sun — or a new moon — when the moon is between Earth and the sun. In both cases the gravitational pull from the sun and moon combine and tug a little harder at Earth’s oceans, making high tides a littler higher, and low tides a little lower.
These are confusingly named “spring tides,” which have nothing to do with the season of spring, but come from the idea of “springing forth.” Spring tides happen twice every 28-day lunar month, all year-round.
In addition to spring tides, the moon also exerts a little extra pull on our oceans when it is nearest to the Earth. That’s called perigee, or a “super moon,” and it also results in slightly higher tidal ranges. Three or four times a year, a new or full moon will coincide with a super moon. This month’s new moon was Jan. 20, and the lunar (pear-eh-gee) happened about a day and seven hours later, on Jan. 21.
All together, that’s called a perigean spring tide, and that combination of factors causes even higher tidal ranges than either factor alone.
That’s what happened last week. Along most coasts around the globe, the effect to tidal ranges is only a couple of inches. But since Cook Inlet already sees some of the highest tidal ranges on the planet, the effect of a perigean spring tide can be 6 inches or more beyond average.
An area’s tidal range has to do with its several factors, including its topography, water depth, shoreline configuration and size of the ocean basin. The Pacific Ocean, due to its large size, has larger tides because there’s more water for gravitational forces to affect. The southern coast of Alaska and British Columbia have even larger tides than elsewhere in the Pacific because that coast is shaped somewhat like a funnel, with the long-period wave starting south and increasing as the funnel narrows. Cook Inlet is the narrowest end of that funnel, so the wave effect is accentuated even more, resulting in average tidal ranges of 30-plus feet.
So what does this mean for the coast of Cook Inlet? Luckily, the new, super moon wasn’t really a bad one, in the parlance of Credence Clearwater Revival, though it had potential to be. High tides can be highly destructive to the unstable bluffs along Cook Inlet, especially in stormy conditions, when wind-driven waves lash the base of the bluff, eroding away the toe and causing the loose material above to come crashing down.
This month’s perigean spring tide came in calm weather, so the high water merely lapped at the base of the bluffs, but even that can cause problems, said Orson Smith, a retired professor with the University of Alaska Anchorage College of Engineering and the Army Corps of Engineers, specializing in coastal and harbor projects in Alaska, many in Cook Inlet.
“Those bluffs usually erode fastest when you have high water and waves,” Smith said. “But their nature is such that when they get wet, when they get saturated, at the toe, at the bottom of the bluff, the soil doesn’t really have much strength. It’s liable to carve away at a weak place at the toe, which is the foundation. Everything above it can come sliding down. So getting wet at all — saturated, filling the pores of the soil with water — weakens the bluff, and at the toe, it’s serious.”
The city of Kenai has been working for decades to secure funding to construct a seawall to protect the bluffs along the river mouth from further erosion, which is eating away at the property above at a rate of about 3 feet per year. According to the “On the Coast” publication which Smith helped author for the Kenai Peninsula Borough Coastal Management Program, the projected loss of residential and commercial structures, diminished land values and damage to city utilities in the mile-long erosion zone is estimated at $15 million through 2058. The $5 million Kenai Senior Citizens Center is of particular concern, as the building is currently less than 125 feet from the edge of the eroding bluff.
Without some sort of reinforcement, bluff erosion will inevitable, inexorably continue. It doesn’t take much for material to shake loose of the bluff face and slough onto the shore below. Strong wave action is particularly destructive, but even high tides without major waves can cause damage, and so can other, seemingly minor causes, such as wind, rain, concentrations of runoff, drainage through the bluff face and even foot traffic.
“It’s just the nature of those bluffs, they’re going to erode. They erode with wind and rain. Small earthquakes shake loose that gravel, they’re just not strong, geologically speaking, so they’re prone to it,” Smith said.