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The Imperial Beach Field Trip

Address: 1680 Seacoast Dr, Imperial Beach, CA 91932

Supplies: Acid, hand lenses, stopwatches

Dr. Sasha Carter created parts of this field trip packet.

This is a part of the state that has in some sense seen more than its share of human impact. While the international border cuts through our field area, and as a result of evolving trade and immigration policy along with the general mess of human development there has been quite a bit of pollution and damage to the beach and the salt marsh behind it. There is no fewer than 5 owners of the land in the marsh, not all of whom have the same intentions and often are at odds with one another. But do not lose hope. There have also been some substantial efforts toward conservation. And as a result of informed efforts the metaphorical barbaloots and their barbaloot suits are returning.

On this trip, our focus is on sediment erosion, deposition, and transport.

Beaches get their sand from 4 places:

1. Rivers that transport the weathering products (small pieces of original rock, clays, and ions)
2. Erosion of cliffs (smaller pieces of the original cliff)
3. Biology (shells, diatoms, radiolarians etc . . .)
4. Chemical precipitation from water (ooids) (less common)

In this part of CA, # 1 & 2 have historically been the most important. Sand is moved along the beach via a process known as longshore drift. Sand is pushed diagonally up the beach and then retreats perpendicular to shore.

Eventually, the sand either gets deposited on the shore and blown inland, or it goes down a submarine canyon. If a beach is in equilibrium sand in = sand out the beach is stable. If an input is reduced then the beach will erode away unless a replacement sediment source is found

This replacement source is usually sand from a drowned beach offshore. We call it beach Nourishment.

STOP 1: Imperial Beach

a) Are the boulders here naturally found in this environment?

b) What types of rock are they?

c) What wildlife do we see?

d) From what direction are the waves traveling towards the shore?

e) What direction is longshore drift?

f) How big are the sand grains?

STOP 2: Tijuana Slough National Wildlife Refuge

This is your chance to gaze upon all that was before you in admiration. It’s also the best spot to see some of the soil eroding off local hillsides into the ponds and marsh,

a) You are looking at an estuary. Describe what you see.

b) Ideally what process would dominate this coastline? Emergence, submergence or Deposition?

c) Is your answer for (b) really happening now?

d) What direction is water flowing at the mouth of the river?

e) Does this look like a high energy or low energy environment?

f) Is the sediment different here from Stop 1?

g) How big are the sand grains here?

STOP 3: Beach

a) Are the grains below you sharp or rounded?

b) Does this look like a high energy or low energy environment?

c) What’s the interval between waves?

d) What’s their wave height?

e) Sketch how the wave looks in a profile view as they break?

f) Does it look closer to high tide or low tide?

g) What evidence can you see of the last high tide?

h) Was it higher or lower than other high tides?