OC/GEO 103 Lab 7

Lab 7
Coastal Geology

Things to Get Out of This Lab:

An understanding of beach deposition and the progressive growth and retreat of beaches throughout the seasons
Gain an appreciation for the need to monitor and protect our Oregon beaches

Materials: Graph paper.

Introduction

Some of the sand that you observed in Lab 3 came from eroded sediments that were transported by rivers that drain into the ocean. However, once sand is deposited into the ocean it may continue to travel distances far removed from the mouth of the river. This transport mechanism is called longshore drift. Longshore drift transports sediments along shorelines by way of longshore currents, which cause some of the changes that we see in the distribution of sands at a beach. Longshore shore currents travel parallel to the shoreline between the breaker zone and the shore. Along the Oregon and Northern California rocky coasts, the net direction of longshore transport is zero. Down in southern California, the net direction of longshore transport is to the south. Longshore currents are formed when incoming waves approach the shore at an oblique angle. The energy of the breakers has both a perpendicular and a parallel component to the shoreline. See the figure below.

Longshore currents are maintained by the wave energy component travelling parallel to the shoreline, which causes part of the water mass to be transported along the shoreline as well. The figure above shows that waves approaching from the right generate a longshore current that runs to the left. This is because there is a component of wave velocity to the left. If breakers are approaching perpendicular to the shoreline, longshore currents will not form because there is no horizontal component of wave energy parallel to the shoreline. Longshore transport of sand is caused by the longshore current in combination with the stirring up of sand by wave action.

The figure below shows how sand piles up behind obstructions in the longshore current. These obstructions interrupt the "downstream" flow of sand in the direction of the longshore current.

Some harbors need to be dredged on a regular basis to remove sand caught behind jetties, sand that would normally be carried down the beach if not for the existence of these man-made obstructions.

If you have visited an Oregon beach year-round you may have noticed that the amount of sand and the shape of beach change as the seasons do. Sand appears to disappear in the winter, sometimes leaving the beach quite rocky, and then reappears during the summer. You may also have noticed that the width of the beach varies depending on its location.

Much of the action at the beach takes place underwater, to a depth of about 10 m. Daily wave action moves sand up and down the beach. Seasonally, sand is moved onshore and offshore. The wide Oregon beaches that most of us think about and visit occur during the summer. The ideal summer beach has a gradual, smooth slope. The portion of the beach that is fairly flat and mostly dry is called the berm. At high tide, material may be deposited on top of the berm. During the storms of late fall the summer slope (berm) erodes and sand is transported offshore, perhaps forming a longshore bar. The winter beach has a narrower berm. The reason for this difference is the wave action. The large waves of winter erode the berm; the small waves of summer rebuild it. Actually, sand does not disappear during winter, but is stored offshore in the longshore bar. The figure below shows the major features of a beach profile.

Coastal geologists normally do beach profile measurements during the lowest daytime tide of a tidal cycle, or about once every two weeks.

Lab 8 Activities

The figure below is from a long term study of sand movement at beach near Carmel, California. It shows the progressive growth of the berm from the dates 4/14/46 to 9/4/46, and then its retreat on 12/10/46 and again on 4/21/47. Each line in the figure corresponds to the date when the beach profile was measured.

(1) Between April 21, 1946 and July 29, 1946, what happened to the berm? Where do you think the sand to build the berm might have come from?

(2) Between September 4, 1946 and February 21, 1947 what happened to the berm? What happened to the beach face?

(3) What causes the berm to retreat from the sea? What causes the berm to advance toward the sea?

(4) Circle the correct answers in the table below about the seasonal changes on the beach:

Season	Berm	Movement of Longshore Bar	Wave Conditions
Summer	large / small	onshore / offshore	large / small
Winter	large / small	onshore / offshore	large / small

(5) The table below is of seasonal beach profile data collected in southern California by Dr. William Prothero of UC-Santa Barbara Geological Sciences. These beach profile measurements were made at three different times during the year. Use these data to plot profiles of the beach during the different seasons.

Distance (m) along profile	3	6	9	12	15	18	21	24	27	30
SUMMER Distance (cm) between level line and beach	45	40	45	50	50	55	90	120	150	180
WINTER Distance (cm) between level line and beach	45	40	45	50	95	120	145	165	180	190
SPRING Distance (cm) between level line and beach	45	50	100	130	155	170	180	190	195	200

Plot these profiles on a piece of graph paper or with some graphing software. Label them appropriately (i.e., "summer," "winter," "spring" - use a different line style for each profile).

Read these plotting instructions carefully:

• At the top left-hand corner of the page, label the origin (the zero point).

• The vertical axis should go from 0 to 250 centimeters, with zero on top and 250 on the bottom. Note that higher values will plot lower on the graph.

• The horizontal axis should go from 0 to 40 meters.

Note: You will have to choose the correct intervals for the axes.

• Label the axes.

• Begin plotting the data. When you plot the other profiles, use a different color or line style for each one, and make sure they are properly labeled.

(6) The vertical and horizontal scales on your plot are different. Calculate the vertical exaggeration (V.E.) by dividing the horizontal distance per division on your graph paper by the vertical distance per division. Be sure to use the same units on the distances.

(7) Once all the profiles have been graphed, label the location of the features you can identify (the berm, the beach face, etc.).

Lab 7 Coastal Geology

Lab 7
Coastal Geology