11.4: Beaches and Human Structures

The beachfront is a preferred location for constructing houses, condominiums, hotels, marinas, and other commercial establishments. Structures are built next to the beach despite the knowledge that beaches are constantly changing and subject to erosion and, hence, that the structures are vulnerable to damage by storm waves.

Most beaches migrate inland as coastal storm erosion transports sand offshore. The migration occurs more quickly in some places, particularly some barrier beaches, than in others. Nevertheless, coastal erosion continues inexorably on most coasts, carving away the base of coastal cliffs, removing sand by longshore drift, and pushing barrier islands back toward the mainland. In a few locations, beaches are built up, sometimes very rapidly, by coastal sand transport. Usually, however, beaches are built and extended seaward only where the land edge is uplifted by tectonic processes or where isostatic processes raise the land edge more quickly than erosion can occur. Accordingly, in most locations, maintenance of beachfront property requires a defense against the continual loss of beach due to erosion and the consequent increasing vulnerability of the property to storm waves.

When the beach in front of beachfront property loses a significant amount of sand, the traditional engineering response is to build seawalls or groins to restore or retain beach sand and protect the property from wave damage. These structures interfere with the normal movement of sand in the beach system and often have unintended consequences.

Seawalls 

When the beach in front of buildings or highways has eroded or retreated so far that winter storm waves can reach and damage the structures, a seawall is often built to break up or reflect storm waves. The seawall is usually constructed parallel to the beach and behind the area of beach used for summer recreation (Fig. 11-23). It is made of large boulders, concrete, or steel, and either replaces or covers beach sand that normally would be mobile and eroded by the strongest winter storm waves. The wall usually works well for some years, but beach erosion continues on either end of the walled section. As the adjacent coastline retreats, the wall protrudes progressively farther seaward on the beach (Fig. 11-23) and is increasingly exposed to waves. Eventually the beach in front of the wall becomes very narrow, and the wall itself is undermined and eroded by waves. At this point, a continual cycle of rebuilding and destruction of the wall begins, and the beach essentially disappears.

A seawall has another unintended effect. Because it is built within the area of wave action, it prevents waves from eroding sand from what was previously the upper part of the beach. Consequently, the amount of sand carried offshore to form longshore bars is reduced. The seawall also prevents the beach from migrating landward and the waves from progressively eroding the coast to provide new sand. Thus, it reduces the amount of sand available to replace the sand lost through longshore drift. Because less sand is available, the beach becomes narrower and longshore bars are further depleted. The narrower beach and smaller longshore bars absorb less wave energy, increasing the wave energy reaching and eroding the seawall, even though storms are no more intense.

Groins and Jetties

Beaches progressively lose sand and become narrower when structures prevent the normal erosional retreat of the coastline or when the sand supply to the longshore drift system is reduced by other means, such as the damming of rivers. When the beach erodes and narrows in front of a valuable piece of real estate, groins are often built to try to restore it. A groin is a wall built perpendicular to the beach from the backshore out to beyond the surf zone (Fig. 11-24a). The groin’s purpose is to block the longshore drift so that sand accumulates on the upcurrent side of the groin, widening the beach at that point. The problem with groins is that they further deplete sand supply to the beach on their downcurrent side, where severe erosion may ensue. A common solution to this problem is to build another groin downcurrent from the first. Eventually a long series of groins may be built along the entire length of the beach (Fig. 11-24b). The net effect is to alter the beach so that wave erosion is enhanced on the downcurrent sides of the groins while segments of “normal”-width beach are maintained on the upcurrent sides (Fig. 11-24a). In many areas, groins are a temporary solution at best because they do not stop shoreline retreat, but merely redistribute a declining sand supply.

   

Figure 11-24. Groins designed to stop beach erosion modify the longshore sand transport and, consequently, the beach. (a) Sand accumulates on the upcurrent (longshore drift current) side of the groin and is eroded from the downcurrent side. (b) Often a line of groins is constructed along the length of a beach, which creates a saw-toothed beach shoreline like this one at Cape May, New Jersey. The groins do slow the recession of the barrier island but will not provide effective protection as sea level rises. (c) You can see that the barrier island continues to recede beyond the location where the line of groins ends in this image of Beach Haven, New Jersey where the barrier island has already retreated in the undeveloped and unprotected area just south of the line of groins. Within perhaps fifty years, all the structures that you see on this barrier island will be gone or will require massive seawalls entirely around the perimeter of the island to protect them.

Other structures built on the coast also cause problems by interfering with the longshore drift system. For example, rock jetties are built to protect inlets between barrier beaches (Fig. 11-25). Such inlets are dynamic features that normally appear, disappear, and move up and down the coast as sand is moved in the longshore drift system. Jetties stabilize an inlet by blocking waves that otherwise would erode its banks and by blocking longshore drift into the inlet that would cause shoaling. Beaches often become wider on the upcurrent sides of inlet jetties. In contrast, downcurrent beaches can be eroded and severely depleted of sand, and the inlet behind the island can be silted up as the island retreats inland.

   

Figure 11-25. Jetties. (a) Rock jetties are often constructed on either side of an inlet between barrier islands to prevent silting of navigation channels and erosion of the sides of the inlet by waves from boats’ wakes. These jetties obstruct the longshore sand transport, leading to the accumulation of sand and widening of the beach on the upcurrent side, and erosion and loss of the beach on the downstream side. (b) Three Mile Harbor, East Hampton, Long Island, New York.

Harbors

On coasts with few natural inlets, jetties or breakwaters are commonly built to provide a safe harbor for small boats. Two such harbor designs are shown in Figures 11-26 and Figure 11-27. The dogleg jetty on the upcurrent side of the Santa Barbara harbor (Fig. 11-26) interrupts the longshore drift so that it carries sand around the jetty but not across the deep harbor entrance, which is too deep for waves to sustain the sand movement. Sand flows around the jetty and accumulates in the harbor, making parts of the harbor too shallow for navigation. The beach that is downcurrent of the harbor is depleted of sand and threatened by severe erosion. These problems are addressed by a very expensive (millions of dollars per year), energy-intensive annual dredging project. Sand is normally dredged from the harbor, pumped through a pipeline, and discharged downcurrent, where it re-joins the longshore drift.

   

Figure 11-26. The jetties of the Santa Barbara harbor breakwater have interfered with the longshore drift transport system. (a) When the breakwater was first built, sand accumulated and progressively widened the beach on the north side of the main jetty (the direction from which waves most often come, the left in this photo) until the sand began to be transported south again. However, sand transported around this breakwater cannot be transported across the deep harbor mouth and accumulates in a spit inside the harbor mouth. The beach on the south side of the harbor has had its supply of sand from longshore drift cut off. Periodic but continuous dredging is needed to keep the harbor from silting up. The dredged sand is deposited on the south-side beach to prevent this beach from eroding further. (b) Since the breakwater was first constructed this satellite image shows how the deeper channel in the harbor mouth provides a break in the wave pattern that interrupts the longshore drift. The sand spit that developed has now been armoured with rocks, and dredging continues from the area at the end of the spit where sand still accumulates. This sand is still dredged annually, pumped through a pipeline, and discharged on the beach farther down the coast than the original discharge location. Note the sports stadium, roads, parking lots and other buildings now built on the area of sand accumulation on the left of the breakwater that was previously ocean where longshore drift carried sand accumulated to extend the beach within the first few years after the breakwater was built.

   

Figure 11-27. (a) Some boat harbors consist of a breakwater or jetty built parallel to the shore. The breakwater protects boats but also interferes with the longshore drift by blocking the waves from reaching the beach behind the breakwater. (b) In this image the submerged remains of the Santa Monica harbor wall (the dark line almost parallel to the shore) are parallel to the beach line. When it was first built, before it sank, the wall blocked enough wave energy to allow sand to build up on the beach under the pier so the harbor became too shallow and unusable. The wall has now sunk deep enough that its effect on the longshore drift is much reduced The pier is elevated on pilings, which allows the wave energy to pass through. As a result, the pier does not block the longshore drift. (c) Offshore breakwaters are used not only to create harbors but also to control coastal erosion as in this satellite image of an area near Norfolk, Virginia. The built up spits of sand behind each of the segments of the offshore breakwater are caused by the interruption of longshore drift. This type of segmented breakwater temporarily slows overall erosion but many would consider that it also makes the beach unnatural and unsightly.

In the 1930s, in Santa Monica, California a harbor was created by constructing a detached breakwater offshore from the end of the Santa Monica Pier. The breakwater was a simple structure built parallel to the beach beyond the surf zone, and was designed to shelter boats. Because this breakwater did not encroach on the surf zone, it seemingly should not have interfered with the long-shore drift. Unfortunately, this was not the case, because this type of offshore breakwater prevents waves from reaching a segment of the beach behind it. Since only low-energy waves break on this segment, the longshore drift is reduced just as effectively as it would be by a groin passing through the surf zone. Soon after the Santa Barbara harbor was constructed, the beach behind the breakwater became greatly enlarged, reducing the area of safe anchorage for boats. The only solution would have been an expensive dredging program. However, the breakwater was constructed of rock piles on a sandy seafloor and quickly sank into the seabed. You can see the submerged structure in (Fig. 11-27b). Multiple segmented detached breakwaters are now used along many coastlines like this one in Norfolk, Virginia and especially in Europe to deliberately slow down the longshore drift and attempt to maintain sand on the beach (Fig. 11-27c). As with other structures that attempt to slow down loss of beach sand these segmented breakwaters are, at best, a temporary solution.