• Structures built to protect coastlines may prevent erosion of part of the shoreline but can result in accelerated erosion elsewhere.
• Seawalls, groins, and breakwaters differ in their locations and orientations relative to the shoreline but all act to prevent erosion and/or encourage deposition.
• Artificial beach nourishment occurs when sand is dredged and pumped onto the beach from offshore.
• Lake Erie is the shallowest of the Great Lakes and is surrounded by the large population centers along U.S. and Canadian shores.
• Most of Lake Erie's shoreline is eroding and erosion rates are greatest where glacial deposits form the shoreline and are least where bedrock forms the coast.
• Presque Isle in the east basin of Lake Erie is eroding because of shoreline protection measures in the central and west basins.

echniques that attempt to prevent beach erosion revolve around methods to limit the removal of sediment along specific areas of the coast or involve adding material to areas undergoing erosion. Unfortunately, nearly all these methods have shortcomings. Most of these methods aim to prevent erosion but some recent regulations have recognized that erosion will inevitably occur and have instead focused on controlling construction adjacent to eroding coasts. Florida introduced strict regulations that required buildings constructed near the shoreline to meet rigorous standards to prevent destruction from storm surges or high winds. No buildings constructed to these standards failed when Hurricane Opel struck southern Florida in 1995. In contrast, 56% of all other habitable buildings in the storm's path were heavily damaged.

Seawalls
Seawalls are built to protect shoreline property owners from receding shorelines (Fig. 30). As such, they represent a barrier between waves and the shoreline. Waves are reflected back from the walls onto the adjoining beach and may promote beach erosion. Unfortunately, erosion is often exaggerated where the seawall ends, causing the shoreline to recede more rapidly on either side of the structure.

Figure 30. Seawall (left) at base of eroding cliff, north of Monterey, California. Note how erosion is exaggerated where the seawall ends. Image courtesy of USGS Center for Coastal Geology.

Groins
Groins are wall-like structures built along beaches to act as barriers to longshore currents (Fig. 31). A longshore current will lose velocity as it meets the groins, causing the current to deposit part of its sediment load on the upcurrent side of the groin, thus building up the adjacent beach. However, as the current passes the groin it picks up additional sediment on the downcurrent side of the structure causing local erosion.

Figure 31. Above: Simple animation of erosion and deposition associated with groins and longshore currents. Deposition occurs upcurrent from the groin, erosion occurs downcurrent. Longshore currents lose velocity when they meet an obstruction in their path such as a groin. Below: A groin adjacent to Cape Hatteras lighthouse, North Carolina, prior to relocation of lighthouse (1999). Image courtesy of USGS Center for Coastal Geology.

Breakwaters
Breakwaters are barriers built offshore to protect part of the shoreline (Figs. 32, 33). They act as obstacles to waves, preventing erosion and allowing the beach to grow behind the structure. However, the beach behind the breakwater often grows at the expense of the adjacent unprotected shoreline.

	Figure 32. Animation of changing shoreline behind a breakwater. Sediment is deposited in the slack water behind structure but is eroded from unprotected area of beach.

Figure 33. Breakwaters, south shore of Lake Erie, Maumee Bay State Park, Ohio. Image courtesy of the U.S. Army Corps of Engineers.

Artificial Beach Nourishment
Artificial beach nourishment occurs when sand is dredged and pumped onto the beach from offshore (Fig. 34). The beach will grow if material is added to the beach faster than natural processes remove it. This is a temporary fix because the sand is eroded again and must be replaced. Material added to many East Coast beaches remained for less than two years before the beach returned to its prenourishment state. One successful effort was for Miami Beach, Florida, which spent $64 million in the 1970s to stabilize and expand its beaches to meet the needs of the booming tourism industry. 

Figure 34. Beach nourishment project on Ocean City Beach, Maryland. Note wider beach near bottom of left image. Pipeline pumps sand collected offshore by a dredge ( right) onto beach. Click on left image to view larger version. Images courtesy of the U.S. Army Corps of Engineers Digital Images Library.

Figure 35. Map of Lake Erie. The map was modified from a bathymmetry map at the Great Lakes Forecasting System website.

Ninety-five percent of the lake's shoreline in Ohio is eroding (Fig. 36). Average erosion rates are 10 to 80 cm per year (0.4-2.7 feet per year) but rates of up to 33 meters (100 feet) per year have been recorded. Rates are largely controlled by the geology of the coastline. More resistant rocks such as sandstone erode slowly whereas glacial sediments and weaker rocks erode more rapidly. Economic losses from damages to structures are estimated to be millions of dollars per year.

Erosion of the shorelines of Lake Erie is evident in the image below that shows how the coastline receded southward, eroding the land along the northern edge of this subdivision. The white lines are roads. Notice how the east-west trending road near the center of the image is truncated by the cliff.

	Figure 36. Above: Erosion of the southern shore of Lake Erie at Painesville-on-the- Lake, Ohio. Note how roads end at top of receding cliff-line. Middle: The view from the lake (north) towards the shoreline (south). The cliff line is receding southward toward the houses in the picture. Image courtesy of Dr. Charles Carter. Below: Sequential images of cliff erosion along the southern shore of Lake Erie from April 1 to April 29, 1996. Click here to view weekly images from February 19-December 9, 1996. Images from USGS Center for Coastal Geology.

Further information on the costs and effects of coastal erosion in Ohio are provided at the Ohio Geological Survey website. Development around the Great Lakes has covered much of the land area, reducing sediment sources. Population around Lake Erie alone has climbed from 3 million to over 14 million people today.

Erosion control measures in the western half of the lake have reduced sediment supply and resulted in increased erosion rates along the shoreline in the east basin. Presque Isle is an unusual sand deposit that built outward from the Lake Erie shoreline near Erie, Pennsylvania (Fig. 37). The construction of coastal structures in Ohio to the west blocked the eastward flow of sediment needed to replenish the deposit. The narrow neck that connects the island to the mainland is eroding as fast as 2.5 meters per year. The U.S. Army Corps of Engineers has the responsibility of coming up with a plan to protect Presque Isle.

Figure 37. Left: View of Presque Isle, Pennsylvania, from the southwest looking along the shore of Lake Erie to the northeast. Erie, Pennsylvania, is to the right of the image. Current directions along the shoreline are from the bottom of the image toward the top. Right: Breakwaters and groins, Presque Isle. Deposition occurs behind breakwaters that form barriers to onshore currents. Deposition occurs upcurrent from the groins (right side of groins in these images) and erosion occurs downcurrent. Click on the lower right image to view a larger version. Images courtesy of the U.S. Army Corps of Engineers Digital Images Library.