July 5, 2008 Beach Monitoring with Jim Johannesen · 6 July 2008, 09:47 by Julie Loyd
Beach geologist Jim Johannsen visited us to show us how to measure beaches.
While geologists have studied beaches in general, there usually isn’t funding to study a single beach long-term. We study local beaches not only for our own interest but to contribute to basic geologic knowledge. In an era of sea level and climate changes, there is rich opportunity for new research. Data which people collect from our island should be sent to Julie who will archive them at the WCL and send them on to Jim.
It is not precisely known how beaches change seasonally, or how they will change due to climate change. Because of the ice caps and Greenland melting, there will be more water in the oceans, and because warm water takes up more space, the level will rise even further. NOAA has been tracking a gradual rise through 1960, more recently it gets steeper. In our area, sea level will be l to 5 feet higher by 2100. Our water temperature has a 20 to 30 year cycle, as well as the El/La Nino cycle which is shorter. The long-term trend is warming. Around here, the El Nino warm water smashes into Peru and Ecuador, works its way up the coast into the Strait of Georgia. In the 97/98 El Nino winter, the Sound had 1.1 foot of water higher than average. With more storms, that was a big erosion year.
There are about six things that would be useful to measure, which could be done in 5 minutes up to half a day. You don’t learn much from single measurements. With good protocols, a group of people could do a useful long term study of a beach.
The geometry of local shores: The bluffs have vegetation at top, then glacial till, then sand. Starting from the toe of the bluff, there is the high beach with vegetation and drift logs, then a sand slope, cobbles, and the waterline.
Some ways to monitor a shore starting with the easiest:
1. Photo Points. Find a fixed point to stand at that you can locate again and again over the years. Using the highest resolution possible, take pictures at low tide in four directions: alongshore, bluffwards, towards the water. Ideally, you’d do this monthly and right after storms, but two to four times a year would be good, too. Expect to see the most erosion around March, and to see a high beach around August. Date and label photos. Julie has a pin drive and can collect photos electronically.
2. Beach Profiles: Using two poles and a rope, measure the fall of the beach at 3 meter intervals. Record the beach composition and any eelgrass or critters at the appropriate spot. Barry, Bill T, Laurie G, Julie L, and others can train you. We measured Glenn’s beach, and including explaining and setup, it took 30 minutes total. We walked from a fir tree out to low tide.
3. Quadrats: A quadrat is a wood or PVC square 1/2 meter to the side), which you set at the minus one foot mark (and, if you like, also at the minus three foot mark). Depending on how detailed you want to get, you either take a picture of the quadrat, count the creatures in it by family, or identify them all the way to species. You can find four very nice identification guides here
4. Beach Change:
a. Wave height: Set a marked stake in the water or measure a rock or piling beforehand. During storms, measure the troughs and crests of waves.
b. Longshore current speed: Lots of beach change is driven by suspended fine sand pushed by waves. Pace out 50 meters, throw something like a crab buoy or driftwood in the water and time how long it takes to go that 50 meters. Measure on calm and stormy days, at differing tides. You can get tide predictions here. I have yet to figure out how to get to the NOAA current predictions.
c. gravel transport rate: spray paint the gravel green or pink, measure motion and direction with GPS or a tape measure. Scrub, dry, prime, and paint the rock. Do it right. Things might move down or up the beach depending on size or time of season. After a few months you might find 20% of them. In Britain they created aluminum pebbles with fish tags in them, each pebble has a unique electronic signal.
d. Drift log mobility: Tag individual logs and follow them around.
e. Vegetative change: From a fixed point, map the edges.
f. Sediment scour: The idea is to see how much the sand level changes over a season. Do this at the +8.5 and at the +5 tide mark. Digging down a foot or two below where you think the sand level has been at its lowest (March?), bury a measured column of crushed bricks or shells, pinpointing the spot exactly by triangulating from known spots. As the sand accumulates above the column, you can dig down to the column and measure how much has accumulated.
Questions:
Bill: Do we get material from the Fraser River?
Jim: On an island where you’re separated by deep channels, the source of beach sediment is what you see on the bluffs and that’s it. River contributions are clay and silt which ends up in deep water. The beach sand that vanishes in winter gets pulled offshore and then pushed back up maybe 300 meters down the beach in summer. You get bluff erosion from Hammond, moving towards Sandy Point; it’s a one-way conveyor. If everybody bulkheaded the bluffs by Hammond, you’d see beach erosion march down the beach. In the future, more and more houses will have to be moved back.
Glen: What vegetation would be good to hold a cliff once it’s slid? Scotch broom will colonize but it’s an invasive.
Jim; Dune grass, if there’s room at the toe. Firs and cedars, snowberry, willow, ocean spray, salal, Oregon-grape; plants with strong roots, can retard erosion. A 20 foot native plant buffer at the top of the bluff would be good. The County encourages that. You can limb the firs to keep the view window.
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