Jetties and Groins
Jetties are built at entrances to rivers and harbors. Their purpose is to protect properties from storm and wave damage, and to keep sand out of channels (so that there is no beach). Jetties require high maintenance costs to manage because they impede longshore drift (which is continues relentlessly). Most the costs are for dredging sand from one side, and moving it down current to replenish sand to community beaches. Loss of the sand supply makes down current areas susceptible to beach loss and coastal erosion (a major problem for Southern California's coastal communities, Figures 12-46 and 12-47).
|Figure 12.46. Dana Point Harbor and Jetty
(Orange County, CA)
|Figure 12.47. Oceanside Harbor and Jetty (Northern San Diego County, CA)
Groins are built as barriers perpendicular to the beach in an attempt to stabilize shorelines. Their purpose is to trap sand migrating along the shore by longshore drift (Figure 12.48). Figure 12.9A is an aerial view of a wash-over fan created by a breach in Sandy Hook Spit (on the New Jersey side of New York City's Outer Harbor (see Figure 12.40).The inlet formed when coastal storm waves and currents cut an inlet across the spit. Note the sand trapped on the left side of the groins (longshore drift is moving left to right). Figure 12.49B shows an accretionary prism of sand building up at the end of Sandy Hook Spit. Figure 12.50 shows the growth of Rockaway Spit on the north east side of New York's Outer Harbor. It has grown nearly 2 miles (3 km)since the end of the Civil War (1866). The area has been heavily modified by construction of groins and a jetty to keep the inlet to Jamaica Bay accessible to boat navigation.
Other structures used to protect properties from the destruction by the sea
- Breakwaters are structures used to protect boats from large waves (jetties and groins are forms of breakwaters).
- Seawalls are walls built to protect land structures from large waves and coastal erosion (Figure 12.52 show an example of some seawalls used to stop cliff erosion).
- Rip Rap are piles of large boulders put on the beach or shoreline. They are cheap but take up beach space and are not as permanent as a seawall, and are unsightly and dangerous. However, they do create habitat for sea life that needs a hard substrate to live (Figure 12.52).
- Beach nourishment adds large amounts of sand to the beach to keep water away from land structures. Sand is dredged form harbor areas or mined from sand bars offshore and pumped onshore in slurries. The process is quite expensive.
|Figure 12.51. Seawalls built in Encinitas, CA are an attempt to stop sea cliff erosion.
|Figure 12.52. Rip-rap was used in the construction of the breakwater (Oceanside Harbor).