For thousands of years, humans have been trying to protect our shorelines from an encroaching sea – from the massive dikes of the Netherlands, to jetties, terminal groins and seawalls.
The most recent addition to mitigating the power of the ocean, beach nourishment, has been shown to be an effective means of shoreline management under the right conditions.
Beach nourishment is not a one size fits all solution. There are certain conditions that are needed for it to be effective, and there are some conditions where it will not work. Shorelines tend to be very dynamic, especially along barrier islands. A stretch of beach may retreat six or seven feet per year for ten years followed by a period of accretion. If that is the case, nourishment is probably not needed and may have a minimal effect, if any effect at all.
In describing what areas are most likely to need nourishment, experts in the field stress the necessity for long term studies to determine where nourishment will be most effective, or if it’s even needed.
One place that nourishment has been shown conclusively that it will not work is at an inlet. Inlets are so dynamic, and the current and drift of sand in and out of the channel are so great, that sand pumped onto the beach at the mouth of an inlet is quickly swept away. That is particularly true on the south side of an inlet along coastal North Carolina where the dominant drift of the nearshore current is north to south.
Jetties and Groins
Jetties and groins are hardened structures that are perpendicular to the shore and extend into the water. Jetties are usually constructed with longer, higher walls. Groins are typically used to stabilize a very specific area of beach; jetties are generally used for larger projects.
Jetties and groins can be useful in stabilizing the entrance to a harbor or channel. There is, as an example, a groin on the south side of Oregon Inlet that has stabilized the mouth of the inlet, keeping it open for navigation and for the use of the Bonner Bridge.
For beach protection, however, both are ineffective, and in some cases can cause damage.
The nearshore area of the ocean is constantly in motion with a current that moves parallel to the shore. That parallel current is termed “littoral drift” and it captures and transports large amounts of sand.
Along the Outer Banks, the littoral drift is usually (but not always) north to south; winds and climate conditions influence the direction of the drift. It’s important to understand littoral drift in order to understand why jetties can cause significant environmental damage.
On the updrift side of a jetty, sand is captured and the beach in the direct vicinity of the jetty will grow. Immediately next to the jetty on the downdrift side, there may also be accretion. The jetty in this case is acting much like a windbreak, and with no current to move the sand, it may accrete next to the jetty.
Farther downdrift it’s a very different story. Sand that would have been part of the littoral drift has been captured by the jetty. The result is significant loss of beach downdrift from the jetty. The possibility also exists that the jetty has disrupted the littoral drift, further disrupting the natural sand transport that replenishes beaches.
Temporary, But at least It’s Something
Rebuilding or replenishing a dune after a storm event will create a temporary barrier to the ocean—but it is truly temporary. Dunes exist on the landward side of the beach. With healthy vegetation as part of the system, they help to stabilize the shoreline, but do very little to mitigate the energy in waves.
If the ocean has reached the dunes, the wave action quickly creates a scarp or very steep face. Consisting of sand, there is little stability in the dune and it quickly begins to collapse. Evidence of how temporary dunes are can be seen in Kitty Hawk where owners are constantly bringing backhoes in to push sand back in front of their beachfront properties, building a new dune after a storm has destroyed it.
Dunes are an important part of a healthy shoreline, but it is the beach that absorbs the energy of the waves.