The table below shows the number of smaller diameter culverts equal in water carrying capacity to that of one larger sized culvert. It is based on culverts laid on the same slope. 2.5 2.5 5.0 10.9 Erosion Water exiting the culvert can erode the land at the outlet. The greater the velocity of flow, the greater the erosion. Erosion at the outlet of an upstream culvert will loosen sediment and debris, which can build up in ditches and inside a culvert downstream.
Buildup slows the flow.
Ultimately sediment is carried into streams, ponds, or lakes. A solution for low velocities is to plant vegetation. For higher velocities, crews should construct a stone splash pad or plunge pool at the outlet. Typically, for culverts with a diameter of 30 inches or less, the depth of the plunge pool should equal the diameter of the culvert. Inspection determines whether culvert cleaning, repairs, or replacement are needed.