What is accretion?
Accretion is the natural process by which wind or water deposits sand, clay or silt against a structure, dune or riverbank. Through accretion, soil builds up — which can be both beneficial and problematic in construction and water management.
How does accretion occur?
Accretion happens when the flow speed of wind or water decreases, causing carried particles to settle:
- Wind — sand is transported and accumulates against obstacles such as walls, fences or dunes
- Water — silt and sand settle where flow velocity drops, for example on the inside of river bends or behind groynes
The opposite of accretion is erosion: the wearing away of soil by wind or water.
Application
Accretion plays a role in:
- Coastal protection — beach nourishment and groynes encourage accretion to widen the coastline
- River banks and levees — controlled accretion strengthens banks naturally
- Foundations — unwanted accretion against a foundation wall can increase soil pressure
- Drainage — accretion can block drainage pipes with sand or silt
- Site management — around buildings, accretion can raise the ground level, directing water toward the facade
Preventing unwanted accretion
- Install drainage — prevents water from washing soil toward the structure
- Gravel fill — a gravel layer around the foundation prevents build-up of fine material
- Ensure gradient — make sure the ground slopes away from the building
Related terms
- Slope
- Foundation
- Drainage
- Water barrier
- Erosion
Learn more about building terms in our knowledge base at fredsdiyplans.com.