Carolina Environmental Diversity Explorations
Small sand volume barrier islands · By Dirk Frankenberg
Figure 3 shows the mid-tide beach with evidence of recent accretion of sand to the upper beach. Look closely at the beach profile and you will see that the surface is slightly higher and more covered with shells both above and below the relatively shell-free area on the lower middle foreground. This topography is a stranded feature called a ridge and runnel. When waves carry sand up onto a beach, a feature like the low ridge on the seaward side of this image develops as sand drops out of suspension in the slowing water near the end of the wave uprush. Once formed, this ridge creates an impediment to subsequent waves washing as far up the beach. Only those that are bigger or occur at a higher tide level can carry water and sand over the ridge top. When they do, though, the water washes down the ridge slope, carrying sand with it.
Both sand and water are moving landward on this landward-facing slope of the beach ridge. The water drains away laterally until it finds a trough through which it can flow back to the ocean. If you look really closely at Figure 3 you can see such a trough just beyond the second large white shell on the ridge in the right foreground. The lower surface over which water flows laterally along the beach is called a runnel. These ridge and runnel systems are responsible for the slow but steady accretion of sand back onto storm-eroded beaches. This one is doing its work in the aftermath of Hurricanes Dennis and Floyd in 1999.