Coastal Processes with Jim Johannesen II · 10 March 2008, 07:58 by Julie Loyd
The second half of the lecture. 
Please note that words that are underlined are links. Click on them to go to a website related to that word.
Tides are caused by the gravitational effects of the moon (stronger) and sun (46% weaker). Water is pulled towards the moon on the side facing the moon, causing a bulge of water on the moon side of the planet. The earth is also swinging around the moon, causing a bulge of water opposite the moon side of the planet. Strong spring tides happen when the sun and moon are lined up, weaker neap tides when the moon seems to sit 90º to the sun in the sky. We have one or the other every two weeks. Puget Sound neap tides s are around 8’, spring tides around 13’. This makes us mesotidal. Macrotidal zones would get tides as greater than 4 meters, microtidal zones get lower than 2 meters.
In addition, the moon does not usually hang directly over the equator. When the moon is near our latitudes, the tides are higher than 12 hours later, when it is closer to southern latitudes. So, in general, higher latitudes have a bigger difference in daily high tides, though this can be modified by sounds and even embayments. For example, in the Puget Sound, the momentum of the tide rushing down to Olympia means that while in Friday Harbor the tidal exchange is around 8 feet, in Olympia it is around 13 or 14 feet.
The relative motion of the moon to the Pacific means that the tidal high is rushing around the Pacific rim, going north from California to us, up, and then around down the China side. If you imagine the Pacific as swirling tea in a teacup, you could predict that Hawaii would have a relatively milder tidal exchange than we do.
Tidal erosion is greatest in microtidal areas because water transfers energy to the same spot all day long. Beaches also erode and accrete seasonally. Maurice Schwartz, speaking about a Canadian beach, found a 1 to 2 meter erosion from winter to summer, and a 1 to 2 meter accretion from summer to winter.
Wind
The most common are prevailing winds, and the strongest are predominant winds.
In winter, we’ll get strong winds from the SE, switching to S with rain, then swinging SW. In summer, Northwesterlies come down the Strait of Georgia bringing sand.
It is the storms that change beach shapes the most. A high tide combined with a high wind causes storm surges. For example, in a Jan. 2003 storm, the predicted high in Tacoma was 13.1, but the recorded high was 14.26 feet. Here is our local predicted vs. actual tide table.
Building setbacks are often based on zoning, but they should be based on erosion rates and predicted life of the building. For our area, we don’t really have erosion rates. From wind data you can hind cast wave data sometimes, but often it’s from an area that doesn’t apply here. For the “Coastal Zone Atlas” 1977, 78, they took the wind record from Bellingham Airport, and used it to predict sand movement in San Juan County. They were wrong 50% of the time. Jim re-did it based on actual data.
Sediments
Bluffs typically provide 90% of beach sediment, the rest comes from streams and rivers. We don’t understand deepwater sand contributions.
Drift logs typically slow erosion, particularly with a root wad locked in to the sediment. Wind blown sand drops behind the logs. They encourage deposition of wind blown sand and vegetation and buffer wave attack. If they are cut logs, they can act as battering rams.
Historically, there were miles of wood locked up in the lower rivers. They used to pay Indians a dollar a day to cut and burn so they could get their steamboats out. There would have been more beach stability. Those truly big pieces would have been almost never mobile.
In lakes, fish like deadfall habitat. We don’t have that anymore on our shorelines. It’s thought to be important for hiding places, as a source of nutrients, and as a source of terrestrial insects, but it’s not studied. See Brennan’s Marine Riparian Vegetation Communities of Puget Sound. and Johannesen’s Beaches and Bluffs of the Puget Sound
They used to make loggers clear wood out of streams to help the fish. Now, they make them leave some wood in the streams.
Rain-on-snow events are the biggest wood-moving storms.
Coastal Erosion
A Florida hurricane can remove 20 feet of shoreline in one night, but here, erosion’s at about the same rate as fingernail growth, 1 to 3 inches a year.
Littoral or net shore drift is the direction that material moves along a beach. That’s the key to understanding a beach.
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Big Picture Highlights Coastal Processes with Jim Johannesen Part I