The formation and removal of beaches strongly depends on the local ocean currents, the shape of the underground, such as reefs and headlands, and the pattern of impinging waves. Slight changes in currents, induced by either annual and weather changes or human construction, may alter a beach from depositional to erosional. Typically high energy waves fatten a steeper coastline, washing sands from the beaches offshore into a bar. Continuous action of smaller waves have enough energy to carry sand upon their break, which is then washed up the slope, where the water current loses kinetic energy and sand particles have time to settle and deposit, growing the beach.
At the present position, sediments have been deposited in ancient timescales in the bay shore and the delta, forming the flat land area between the onset of the mountains. The color and composition of the sand made out of predominantly igneous mineral rather than limestone and nacre tells that it originates from rock and land erosion rather than coral reefs. It is densely covered by rain forest and human settlement. Tree stems in the water, the urban sea walls further east and the depth of the water in front of them indicate that this beach is erosive since decades. Also sedimentation of the nearby western river mouth tends north-westwards, proving that the main currents of the bay go east to west.
Ample coconut palm tree vegetation hits the waterfront, growing on the sands, the bigger trees may reach their end of lifespan of 80 to 90 years. Dense roots of the palms form the shoreline exposing a step of 1m-2m height. At high tide, the waves directly push into this web of roots. The sand between the roots has been washed out, however, the web of natural fibers exposes enough mechanical strength and density to hold the soil behind in place. The elasticity of the fiber is needed to response to sudden forces under the shock of breaking waves. Toughness is given by the living organism, which may repair itself once over-torn, and new roots grow when space becomes available from dead or broken roots.
Without the protection of the vegetation, steps of the present height in sand would be washed away in one day, and the bay extending much further into land, either to a line of less aggressive exposure and slope, or even ultimately to the bare rock of the mountains. Here, the coconut trees retain the shoreline actively and probably much more efficiently than the nearby urban sea walls to the east, which expose impressive cracks and undermining by the waters. Fallen palm trees, though, remain in the water and dissipate energy of the waves. At dead tides, those obstacles bear shelters for new sedimentation, and allow new palm trees to grow. The reproduction process of coconut palms is well adopted to the extreme exposure at shore lines, namely through buoyancy of the fruits, which may drift over the water, wash up a beach like the present, seed and grow. Altogether, this example demonstrates that life on earth not only protects but also shapes the earth.
To log this earthcache as found, please answer the following questions:
Q1: What is the feature at GZ?
Q2: On the observations you make, explain why the beach is erosive.
Q3: What is the color of the sand. Is there more than one color? How does it segregate and why?
Q4: Photos welcome! (optional)
Image: Palm tree roots protecting the beach from erosion (false color)