Seawalls significantly alter coastlines, primarily by preventing erosion of the land directly behind them while potentially exacerbating erosion in front of the structure and on adjacent beaches.
Understanding Seawall Impacts
Seawalls are structures built parallel to the shore to protect land from wave action. However, their effects on the coastline are complex and often not entirely beneficial.
Primary Effects of Seawalls:
- Protection of Hinterland: Seawalls, primarily, prevent erosion of the dunes and land areas directly behind them. This is their intended purpose, safeguarding buildings, infrastructure, and land from being washed away by the sea.
- Example: A seawall might protect a coastal town from a storm surge that would otherwise flood and damage homes.
- Increased Erosion in Front: Vertical seawalls often reflect wave energy instead of dissipating it. This redirected wave energy can increase the erosive force on the seabed directly in front of the wall, leading to a deepening of the water and a loss of beach.
- Example: A strong wave hits a vertical seawall and bounces back with greater force, scouring the sand at the base of the wall, and ultimately leading to the eventual loss of a beach.
- Lateral Erosion: Because the seawall is fixed, it prevents the natural movement of the shoreline. This can lead to more erosion on beaches adjacent to the seawall, as wave energy is diverted to those areas.
- Example: Sand that used to replenish a beach area is trapped by the seawall, resulting in the beach shrinking and more erosion to the sides of the protected area.
- Loss of Beach: Over time, continuous reflection of wave energy and the trapping of sand can lead to the loss of sandy beaches in front of or next to the seawall.
- Change in Sediment Transport: Seawalls disrupt natural patterns of sediment transport. Sand that would normally move along the shore, replenishing beaches, can be blocked or redirected, causing a deficit of sand in some places and an excess in others.
Modern Seawall Designs
Recognizing the limitations of vertical seawalls, newer designs have incorporated sloping structures, which aim to absorb wave energy rather than reflect it. This can help mitigate some of the negative impacts on shoreline erosion.
- Sloping Seawalls: These are designed to dissipate wave energy more effectively, reducing the amount of wave force reflected back into the sea. They are also less likely to accelerate erosion in front of the structure.
- Integrated Structures: Some modern seawalls integrate vegetation and other natural elements to help stabilize dunes and absorb wave energy.
Effects in Summary
| Aspect | Vertical Seawalls | Sloping Seawalls |
|---|---|---|
| Protection of Hinterland | Effective | Effective |
| Erosion in Front | Often increases due to reflected wave energy | Reduced due to wave energy dissipation |
| Lateral Erosion | Can increase due to deflected wave energy | Reduced due to better wave energy management |
| Beach Loss | Can lead to loss of beach in front or beside the wall | Less likely to cause beach loss |
| Sediment Transport | Disrupts natural patterns | Less disruptive with natural elements integration |
In conclusion, while seawalls can prevent erosion in some areas, they are not a perfect solution. They can have unintended negative consequences on the surrounding shoreline and should be carefully considered in coastal management plans.
