BEACH FIELD TRIP TO HALF MOON BAY
FIELD TRIP OBJECTIVES:
The objectives of this field trip are to test the hypotheses made in class the previous
week-how do the waves change along the Half Moon Bay coast, and what is the direction of the
longshore current. Primarily, you should observe that the coast is a dynamic, ever-changing
environment. As waves break, the coast responds in ways that are sometimes barely perceptible
over the time span of a day but that can be quite dramatic over the time span of many years.
FIELD TRIP ACTIVITIES:
1. Meet at the parking area by Miramar Beach. To reach the site, turn west from Highway 1 (right
if you're heading south) onto Mirada Dr. and proceed to the end of the road by the beach cliffs.
Parking is around the cul de sac at the road's end. Mirada Dr. is just south of El Granada Beach.
Figure 1 is a location map of the study sites along Half Moon Bay; notice that the map also includes
the offshore bathymetry.
What geologic feature is responsible for the offshore ridge that extends southeast from Pillar
Point (look at the geologic map of San Mateo County to find the answer)? This feature produces
a scarp along the east side of Pillar Point and a low spot where the Half Moon Bay airport is located.
At Miramar Beach we will measure the wave characteristics and longshore current speed and
direction. Miramar Beach is located at the north end of Half Moon Bay. We will then travel south
to Redondo Beach (by Miramontes Point on Figure 1), located at the south end of the bay. We will
measure the waves and currents there to compare with measurements from Miramar Beach. Figure
2 is the map we used in class last week to analyze wave energy along Half Moon Bay. We want to
see how our observations agree with our predictions from last week's lab.
Remember that the predictions we made last week were for swell waves approaching the coast from
the northwest. Our conditions on this day could be quite different. Make your interpretations based
on your observations. Don't try to fit your observations into a preconceived hypothesis.
Take a few minutes to just observe the beach, recording its general shape, what the waves look
like, what the wind is like, etc.
Measure the waves (height and period) and the longshore current at Miramar Beach.
Equipment: hand level, surf markers, stopwatch or watch second hand, marker poles, tape measure.
Measuring wave height: Wave height can be measured from the shore. Whenever you stand close
to the swash zone, periodically check the wave action so you are not caught unaware by a large
breaking wave. Better yet, have a friend "stand watch" while you proceed. Place marker pole at the
water line and align the breaker crest with the horizon or use the hand level to obtain a horizontal
line. Where the horizon cuts the staff read off the wave height (see Figure 3 on wave height data
sheet). Measure the heights of several waves and then average them once you have completed
your observations. Record on Figure 3 data sheet.
Measuring wave period: Measure the wave period by counting the number of breaking waves
observed at a given point during 60 seconds. Divide 60 by the number of waves (seconds/wave) to
obtain an average period. The wave measurements work best if one person makes the observations
while another records. Record on Figure 3 data sheet.
Measuring the longshore current: Measure the direction and speed of the longshore current flow
by following and timing the movement of brightly colored surf markers or a biodegradable object
such as a grapefruit. Place a marker pole in the beach, then measure off 50 feet (or 20 meters)
parallel to the beach and place the second marker pole in the sand. Try to estimate the probable
direction of the longshore current from observing how the waves approach the coastline. Release
the markers into the surf from the "upcurrent" marker pole. Time the movement of the surf markers
until they clear the second marker pole. Also note the type of path they make in the surf zone. To
calculate the average speed of the current, divide the known distance between the marker poles by
the time it took for the markers to move between them: SPEED=DISTANCE/TIME. Convert the
measurements to a feet/second or meters/second speed. Use Figure 4 to record data and draw the
path of the path of the surf markers.
Before leaving Miramar Beach, observe the coastal cliff and estimate its height. This cliff is
the seaward edge of a flat terrace (called the Half Moon Bay terrace) that the town of Half Moon
Bay is built on. As we drive south along Highway 1, observe the width of this flat terrace, which
is ideal for growing vegetables such as artichokes and brussel sprouts!
2. Drive to Redondo Beach, about 1.5 km (1 mile) south of the town of Half Moon Bay. To reach
the site, turn west from Highway 1 (right if you're heading south) onto Redondo Beach Dr. What
differences can you observe in the height and width of the marine terrace here compared to
Miramar Beach? For help with this question, look at Figure 7, which is a map of Half Moon Bay,
and Figure 8, which shows profiles across the coast at locations A, B, and C (marked on Figure 7).
The Half Moon Bay terrace is about 100,000 years old and was formed at sea level. It has since
been lifted up above the water level by tectonic activity along the coast-for example, movements
on faults such as the Seal Cove fault that runs along the western edge of Half Moon Bay.
Again take a few minutes to just observe the beach, recording its general shape, what the
waves look like, what the wind is like, etc.
Measure the height and period of the waves and the direction and speed of the longshore
current at Redondo Beach, as you did at Miramar Beach. Record your measurements on Figures
5 and 6 data sheets.
Questions to Answer:
1. How do your measurements from Miramar Beach compare to those from Redondo Beach?
Compare wave height, wave period, and longshore speed and direction.
2. Explain the reasons for the differences or the similarities. For example, are the waves at the
north and south ends of Half Moon Bay affected by refraction around the Pillar Point
headland? Remember that refraction is greatest when the waves approach the coast from the
northwest. From which direction were the waves approaching the coast on the day of the
field trip (instructor will provide offshore weather data)?
3. Refer to last week's lab. How did your actual observations compare to the predictions you
made last week? Account for similarities and differences.
REPORT GUIDELINES:
The report is due in class the week after the field trip. The report should consist of about 2 pages
of TYPED, DOUBLE SPACED text and associated figures. Refer to the figures in your report to
make them an integral part of the text. Also give your report a title. Use your notes as a guide for
your report, but please organize it in narrative form rather than as a list of answers to the questions.
The report should include answers to the questions posed in the handout (but organized in narrative
form) and the following figures:
1. Any of the maps that you refer to in the text (for example, you should include a location
map [Figure 1], and possibly the wave orthogonals map [Figure 2]).
2. Figures 3, 4, 5, 6: data sheets for wave and current measurements.
Be sure to carefully record all of your observations during the course of the half-day excursion.
You may feel at the time that you will remember what you have seen, but usually you will not recall
without some written and visual aids. Remember that a picture is worth a thousand words.