Coastal Geology: Part I

Forces Affecting Shoreline Development

Nick Warner

Objective To analyze the dynamic nature of

shorelines….

Relating to the forces modify our shorelines

How do shoreline processes affect us?

How do we affect shoreline processes?

Introduction

Coastal areas vary greatly in character. Most are dynamic (constantly changing). They change due to erosional forces. Erosional forces? water and wind They change due to tidal forces. They change due to tectonic forces. Tectonic forces? uplift and subsidence

Forces that Shape our Coastline: Erosional Forces

Erosional forces

What is the main agent of erosion at a coastline?

Water, specifically wave movement.

What causes a wave to form?

Wind

Waves Waves you see are just the surface

expression of actual wave motion.

The Waveform

a. b.

Wave Terminology

Wave Crest: Top of the wave

Wave Trough: Low part between two crests

Wavelength: Horizontal distance from top of one crest to the top of another crest

Wave Height (amplitude): Vertical distance from trough to crest

Waves Continued

How do breaker waves form?

Do you ever see a breaker far offshore?

Breakers form when waves approach the shore.

Why?

Breaker Waves

Friction between wave bottom and seafloor causes the top of the wave to topple over.

Shoreline erosion and sediment movement

Moving water (waves) is the primary agent of shoreline erosion and sediment movement.

Energy from the waves picks sediment up from one area and deposits it in another area along a shoreline.

When waves break at the coast they move sediment along the beach front.

Beach Profile

Swash and Backwash

This process creates a gentle beach profile

Beach Erosion

Beach erosion cuts into vegetated area

Vegetation reduces rate of beach erosion

Shoreline Erosion Points or

protrusions along a coast are particularly susceptible to erosion.

Wave Refraction: Bending of waves as they interact with the coastline

Wave Refraction

Longshore Currents Do waves always hit perpendicular

to the shore?

Waves often approach the shore at oblique angles.

Longshore Currents Swash and backwash

moves sediment along the shore.

Littoral Drift: The movement of sediment along the shore, parallel to the shoreface.

Longshore Current: Current of water parallel to the beach front. Created when waves hit the shore at an oblique angle

Shoreline Features Created by Sediment Movement

Spit: Continuation of a beach created by movement of sand parallel to the shore.

Spit

Shoreline Features

Spit

Cape Cod, Mass.

Shoreline Features

Baymouth Bar: Thin beach that blocks the entrance to a bay.

Shoreline Features Barrier Island: long, narrow, offshore

deposits of sand or sediments that parallel the coast line. Some barrier islands can extend for 100 miles (160 km) or more.

Example: The Outer Banks: NC

Barrier Island Profile

Shoreline Features How do barrier islands form?

Current Theory: Barrier islands formed right after the last ice age. (~18000 ya)

As glaciers melted ocean levels rose submerging the regions behind the higher beach front.

Today, rivers that carry sediment to the ocean have built up the barrier islands.

Affect of Humans on Shorelines

Why do we care about the processes that shape the coast?

The coast is dynamic, houses and hotels and human structures are for the most part static.

How do we attempt to control the coast?

Man-Made Coastal Structures

Man-Made Structures

Shoreline reinforcement (Jersey Shore)

Man-Made Structures

Jetty

Man-Made Structures

Sand Fences

Next Time: Other Forces?? Tidal Forces & Tectonic

High Tide and Low Tide

Passive Continental Margins (Atlantic Coast)

Active Continental Margins (Pacific Coast)