Changing Coastal Environments » Wave Types - Constructive and Destructive

What you'll learn this session

Study time: 30 minutes

The characteristics of constructive and destructive waves
How different wave types shape coastal landscapes
The processes of swash and backwash
How to identify wave types in the field
The impact of wave types on coastal erosion and deposition
Real-world examples of wave action on coastlines

Introduction to Wave Types

Waves are one of the most powerful forces shaping our coastlines. They transfer energy from the open ocean to the shore, constantly changing the shape of beaches and cliffs. Understanding different wave types is key to explaining why some beaches are sandy and wide while others are steep and pebbly.

Key Definitions:

Waves: Disturbances that transfer energy through water without moving the water itself very far.
Swash: The rush of water up the beach after a wave breaks.
Backwash: The return of water down the beach after the swash.
Fetch: The distance of open water over which wind blows to create waves.

How Waves Form

Most waves start with the wind. As wind blows across the water's surface, it creates friction, transferring energy to the water. The stronger the wind, the longer it blows and the greater the distance (fetch) over which it blows, the larger the waves will be.

Did You Know? 🌊

The largest wave ever surfed was approximately 24.4 metres (80 feet) high, ridden by Brazilian surfer Rodrigo Koxa in 2017 at Nazaré, Portugal. These massive waves form because of an underwater canyon that funnels wave energy toward the shore.

🌊 Constructive Waves

Constructive waves build up beaches by depositing more material than they remove. They're formed in calmer conditions and have a strong swash but weak backwash.

🌋 Destructive Waves

Destructive waves erode beaches by removing more material than they deposit. They're formed in stormy conditions and have a weak swash but strong backwash.

Constructive Waves: The Beach Builders

Constructive waves are typically low, frequent waves that occur around 6-8 times per minute. They gently roll onto the shore rather than crash dramatically.

Characteristics of Constructive Waves

Height: Usually less than 1 metre tall
Frequency: 6-8 waves per minute (low frequency)
Wavelength: Long wavelength (distance between wave crests)
Swash: Strong swash pushing material up the beach
Backwash: Weak backwash as much water percolates into the beach

The strong swash of constructive waves carries sediment up the beach. When the wave retreats, much of the water soaks into the sand, weakening the backwash. This means more material is deposited than removed, gradually building up the beach over time.

Beach Profile 🏖

Beaches shaped by constructive waves tend to be wide and flat with a gentle gradient. They often have berms (raised ridges of sand and shingle) and are typically made of fine sand, as the gentle wave action allows smaller particles to settle.

Destructive Waves: The Beach Eroders

Destructive waves are typically tall, powerful waves that occur less frequently, around 13-15 times per minute. They crash onto the shore with significant force.

Characteristics of Destructive Waves

Height: Usually more than 1 metre tall
Frequency: 13-15 waves per minute (high frequency)
Wavelength: Short wavelength
Swash: Weak swash as energy is expended in the wave breaking
Backwash: Strong backwash pulling material back to sea

The powerful breaking action of destructive waves means much of their energy is released when they hit the shore. This results in a relatively weak swash. However, the backwash is strong as water flows rapidly back down the beach, dragging beach material with it.

Beach Profile 🏄

Beaches shaped by destructive waves tend to be steep and narrow. They often have a ridge and runnel profile (alternating raised and lower sections) and are typically made of coarser materials like pebbles, as finer sediments are washed away.

Comparing Wave Impacts

🗺 Beach Shape

Constructive: Wide, flat beaches with gentle slopes
Destructive: Narrow, steep beaches with sharp drops

🟩 Sediment Size

Constructive: Fine sand and small particles
Destructive: Coarse sand, pebbles and shingle

💧 Coastal Features

Constructive: Spits, bars and wide beaches
Destructive: Wave-cut platforms, cliffs and narrow beaches

How to Identify Wave Types in the Field

When visiting a beach, you can identify the dominant wave type by observing several features:

Count the waves: Count how many waves break in one minute. Fewer than 10 suggests constructive waves; more than 10 suggests destructive waves.
Observe the breaking: Constructive waves surge up the beach; destructive waves plunge dramatically.
Beach gradient: A gentle slope suggests constructive waves; a steep slope suggests destructive waves.
Sediment type: Fine sand suggests constructive waves; pebbles and coarse material suggest destructive waves.

Case Study Focus: Chesil Beach, Dorset

Chesil Beach is an 18-mile long shingle barrier beach in Dorset, England. It's famous for its perfectly sorted pebbles, which increase in size from pea-sized at West Bay to potato-sized at Portland. This sorting is due to the longshore drift and the action of constructive and destructive waves working together. During storms, destructive waves dominate, moving larger pebbles further up the beach. In calmer conditions, constructive waves gradually sort the material by size.

Seasonal Changes in Wave Types

Many beaches experience different wave types throughout the year, creating a seasonal cycle of erosion and deposition:

☀ Summer Profile

During summer, calmer conditions lead to constructive waves dominating. This builds up a wide, gently sloping beach with a berm (ridge) at the high tide mark. This is sometimes called a "berm profile" or "summer profile".

🌨 Winter Profile

During winter, stormier conditions lead to destructive waves dominating. This erodes the beach, creating a steeper profile with a step at the high tide mark. Material is often moved offshore to form bars. This is sometimes called a "bar profile" or "winter profile".

Human Interaction with Wave Types

Understanding wave types is crucial for coastal management. Engineers design sea defences differently depending on the dominant wave type:

Groynes: Wooden or rock barriers built perpendicular to the shore to trap sediment moved by longshore drift, often used on beaches with constructive waves to maintain beach width.
Sea walls: Hard defences designed to reflect wave energy, particularly useful against destructive waves.
Beach nourishment: Adding sand or shingle to beaches to compensate for material removed by destructive waves.

Real-World Application 🌐

Surfers pay close attention to wave types! The perfect surfing wave is neither purely constructive nor destructive but has characteristics of both. Surfers look for waves that are steep enough to ride (like destructive waves) but that don't break too quickly, allowing them to ride along the wave face (more like constructive waves).

Summary: Wave Types and Coastal Environments

Waves are the primary sculptors of our coastlines, with constructive and destructive waves creating different coastal environments:

Constructive waves build up beaches through strong swash and weak backwash, creating wide, gently sloping beaches of fine sand.
Destructive waves erode beaches through weak swash and strong backwash, creating narrow, steep beaches of coarse material.
Most coastlines experience both wave types, with seasonal variations creating cycles of erosion and deposition.
Understanding wave types helps us predict coastal changes and design appropriate coastal management strategies.

Remember that while we categorise waves as either constructive or destructive, in reality, most waves fall somewhere on a spectrum between these two extremes and their impact depends on many factors including beach material, coastal shape and weather conditions.

🧠 Test Your Knowledge!