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The Longshore Current at Crystal Cove State Park

by Eric B. Fisher, Tracy Overfield, Shinobu Kikuchi, Corina Kesler, and Mike Odera

 Introduction

It may not seem like it, but waves are basically a commotion created by the passage of energy through a medium. This disturbance has an immense affect on our coastlines and for this and many other reasons scientists study waves. There are many ways to examine ocean waves. Now that we know what a wave is, let’s explore another one of its components. The highest peak of a progressive wave above the average water level is known as the crest. The wave crest is an important identifier of the wave period.  Wave period is simply the time it takes for two consecutive wave crests to pass a fixed point; the use of a large rock in the ocean is a good way to determine wave period. Another aspect of waves which stirs up great interest is the wavelength. This is the horizontal distance between two consecutive wave crests in a progressive wave. The understanding of all of these concepts is vital in understanding the longshore current.

As progressive waves enter the surf zone, or shallow water, they begin to slow down and bend; this phenomenon is referred to as wave refraction. When a wave’s refraction is incomplete as it approaches the shore this current approaches the surf zone at an angle, resulting in the longshore current. As stated by Garrison in Figure 10.6: “Speed is equal to wavelength divided by period”. The relationship between speed, wavelength and period, in ocean waves, can be found by measuring the wavelength (look at “methods” for the exact way to measure wavelength) and dividing it by time (wave period in seconds). Scientists have devised a formula to illustrate this relationship:

S=L/T

where S=Speed, L=Wavelength, and T=Wave Period

The Flounders group hypothesis is that the longer the wave period of a given wave, the faster will be the longshore current speed. To explore this hypothesis further, The Flounders set off on an exciting mission. Their 4-hour research expedition began at Crystal Cove State Park on the 7th of September at 8:00 AM.  The Flounders conducted a fascinating experiment that related the longshore current with wave period.

Methods

On Tuesday, September 7, 1999, our group went to Crystal Cove State Park at 8:00 AM. We started measuring the longshore current by throwing a half-pound grapefruit into the water at Point A (Figure 2), in the middle of the beach. We then measured the distance it traveled using a 25-foot long Stanley measuring tape. We measured the time it took to travel that distance using a Timex watch. We took these measurements every 15 minutes. Later on we discovered that an easier way to take the measurements was to premeasure 50 feet along the beach in two measurements of 25 feet each. Once we did this, we continued measuring the time, but the distance remained constant. Any time the grapefruit hit the beach before traveling the 50 feet (or at least 25 feet), we threw out the measurement and immediately remeasured. By knowing these two measurements, distance and time, we were able to mathematically calculate the speed of the longshore current by using the equation S = L/T, where S = speed, L = length traveled, and T = time.  

As we were measuring the longshore current speed, another member of our group was at Point B (Figure 2) measuring the wave period. Point B is located on a rocky point at Crystal Cove State Park, as illustrated.  To measure the wave period, she used a different Timex watch and measured the time it took two successive wave crests to pass a stationary rock in the ocean, approximately 50 yards straight out from her at Point C (Figure 2).

Results

According to Table 1, the range of wave period from 8am to 12pm on September 7th at Crystal Cove State Park varied from 10 seconds to 18 seconds.  The average of wave period for these 17 measurements is 13 seconds.  The trend of the wave period generally increases with the passing of time.

Table 2 shows the long-shore current speed ranges from 0.19 to 1.67 feet per second with the average of 0.89 feet per second.  There is no ostensible trend for the pattern of the long-shore current.

The scatter graph reflects our study; it shows no apparent correlation between the wave period and the longshore current speed.

Discussion

According to our data, there is no apparent relationship between the longshore current and wave period.

The first reason supporting the above conclusion is the use of insufficient measurement. The measurements were gathered in a duration of four hours. This is only one sixth of a 24hr day. To improve the results on a similar experiment, it would be advisable to study the relationship over a longer period of time (2-3 days). Also, more measurements than every 15 minutes could be made to obtain a more accurate depiction of the relationship between wave period and longshore current speed.

Second, the tide level at Crystal Cove on September 7, 1999, was inconstant. In fact, as the day wore on, the tide level was generally decreasing, from a low high tide of ~4˝ feet at 8:55 AM to a high low tide of ~2 feet at 2:15 PM. As a result of varying tide levels, the wave speed may have changed with the tides. To obtain more prolific results, it is expedient to carry out such an experiment at a time when the tide is comparably steady.

Finally, wave interaction on the above mentioned date was substantial to muddle the true results. The swash and backwash had significant difference in energy. When contrary, the contrast was too sudden and short-lived. Crystal Cove is neither a cobble beach nor a wide beach. For more accurate results, one of the two kinds of beaches would be consummate for such an experiment.

 

Date

Time

Wave Period

(Seconds)

Longshore Current Speed

(feet/second)

September 7, 1999

8:00 AM

12

0.81

September 7, 1999

8:15 AM

10

0.84

September 7, 1999

8:30 AM

11

0.63

September 7, 1999

8:45 AM

14

0.69

September 7, 1999

9:00 AM

13

0.71

September 7, 1999

9:15 AM

12

0.57

September 7, 1999

9:30 AM

10

1.25

September 7, 1999

9:45 AM

10

1.67

September 7, 1999

10:00 AM

12

0.61

September 7, 1999

10:15 AM

14

1.39

September 7, 1999

10:30 AM

14

0.81

September 7, 1999

10:45 AM

14

1.39

September 7, 1999

11:00 AM

15

1.00

September 7, 1999

11:15 AM

14

0.78

September 7, 1999

11:30 AM

14

0.19

September 7, 1999

11:45 AM

14

0.60

September 7, 1999

12:00 PM

18

1.19

Average

 

13

0.89

Table 1:           Wave Periods and Longshore Current Speeds at Crystal Cove State Park

                        on September 7, 1999 between 8 AM and 12 Noon.


Figure 1: Plot of Wave Period versus Longshore Current Speed                 

 


Figure 2:         Author’s rendition of the relative locations of each measurement at Crystal Cove State Park on September 7, 1999.