17.5: Human Interface with Shorelines
Human Interface with Shorelines
There are various modifications that we make to influence shoreline processes for our own purposes. Sometimes these changes are effective and appear to be beneficial, although in most cases there are unintended negative consequences that we do not recognize until much later.
Occasionally beaches are armored with riprap Links to an external site. and concrete blocks to limit the natural erosion that threatens the properties at the shore (Figure 17.13). The unintended effect of this installation will be to deprive the area of sediment. If the armor remains in place for several decades, there is a significant risk that the depositional landforms will start to erode. This could also affect the biology of the area, including many of the organisms that use that area as their habitat, and people who use the area for recreation.
Figure 17.13: In February 2017, large boulders were installed as rip-rap to armor the shore against further erosion at Goleta Beach in Southern California. (Public Domain, Daniel Hoover/USGS Links to an external site.)
Seawalls Links to an external site. also limit erosion and can be useful amenities for the public; however, they too have geological and ecological costs. Seawalls affect the behavior of waves and longshore currents, sometimes with negative results. When a shoreline is “hardened” in this way, important marine habitat is lost and sediment production is reduced, which can affect beaches elsewhere.
Breakwaters Links to an external site. are structures that run parallel to the shore (Figure 17.14). Typically, these have acted as islands and sand is deposited in the low-energy water behind them, similar to how a tombolo may form. Negative impacts are not yet well understood but likely involve loss of marine animal habitat.
Groins Links to an external site. are structures constructed perpendicular to the beach (Figure 17.14). They have an effect that is similar to breakwaters, while trapping sediment by slowing the longshore current.
Figure 17.14: Ventura Harbor has both breakwaters and groins present. (CC-BY 4.0, Chloe Branciforte via Google Earth)
Most of the sediment that forms beaches along our coasts comes from rivers, so if we want to take care of beaches, we must take care of rivers. When a river is dammed, its sediment load is deposited in the resulting reservoir, and while the reservoir is filling up that sediment cannot get to the sea. During this time, beaches and other depositional landforms within miles of the river’s mouth are at risk of erosion as longshore currents displace sediment that is not replenished by the river.
For more on our nation's changing coastlines, read this USGS Fact Sheet - Shoreline Change Research Links to an external site..