Groynes are structures constructed perpendicular to the shoreline, extending from the beach into the sea. Their primary purpose is to interrupt and reduce the natural longshore drift of sediment along coastal areas. Longshore drift is a process driven by waves approaching the shore at an angle, which creates a zigzag movement of sediment along the coast. Groynes effectively disrupt this movement by trapping and accumulating sand and other sediments.

The principal effect of groynes on sediment dynamics is the reduction of longshore drift. By obstructing the flow of sediment, groynes lead to the accumulation of material on their updrift side. As a result, the beach on this side becomes wider and higher. This widening can be advantageous for beach nourishment and coastal protection, as a broader beach can absorb more wave energy, subsequently reducing the risks of coastal erosion and flooding.

However, while groynes enhance sediment accumulation on the updrift side, they can also cause sediment starvation on the downdrift side. This phenomenon may lead to increased erosion and a narrower beach on the downdrift side, a situation referred to as terminal groyne syndrome. Such effects can have significant environmental and socio-economic implications, including habitat loss and damage to coastal infrastructure.

The effectiveness of groynes is affected by several factors, including their size, spacing, and the characteristics of the local sediment and wave climate. For instance, larger groynes can capture more sediment but may also create greater disruption to longshore drift. In contrast, closely spaced groynes can offer more effective protection against coastal erosion, though they can exacerbate sediment starvation downdrift.

In summary, groynes play a crucial role in shaping coastal landscapes through their influence on sediment movement. Their impacts can be both beneficial and detrimental, necessitating careful consideration of design and management in relation to local conditions and potential consequences.