The sentence "These common acts of nature are what leave beaches with less sand." best supports the idea that beach erosion is a natural process. It refers to the natural influences causing the gradual depletion and reshaping of beaches due to wave and wind action.
The sentence that supports the idea that beach erosion is a natural act is: "These common acts of nature are what leave beaches with less sand." (paragraph 2).
This statement clearly defines beach erosion as a natural process resulting from various natural events such as wave action, winds, and storms. As waves crash against the shore, they transport sand and sediment, which leads to the gradual reshaping of beaches.
Other natural processes described in the provided materials, like overwash during storms and the dredging projects' impact on marine ecosystems, also relate to the dynamics of coastal environments but emphasize human intervention or the consequences thereof.