When constructed they were believed to help the beaches, but are now known to be a factor in increasing the erosion rate. As waves hit the seawalls they create forse which will take more sand with it, causing more erosion. The picture shows an example of a seawall, which have been shown to increase beach erosion. This picture shows also has a seawall. This seawall is protecting the house from the waters, but as you can see the area in front of the house there is no beach left, where on the side of the house there is much sand.
Groins are structures that extend out to sea.
They were built to protect the beaches in between. Their purpose was to trap and retain the sand in order to renourish the beach. However they interrupt longshore transport and as the picture shows it causes erosion to the areas that don't receive the sand they would have through the natural flow. The diagram to the left shows how groins work. This diagram shows a groin up close.