Thursday, August 1, 2013

Slowing the Erosion from Rising Sea Level and Storms

Breakwater at Westmoreland State Park
At the quarterly meeting of the Potomac Watershed Roundtable Scott Hardaway from the Virginia Institute of Marine Sciences at William and Mary and Mike Vanlandingham the last standing Shoreline Engineer from the Virginia Department of Environmental Quality (DEQ) spoke about shoreline erosion and stabilization in general and along the Potomac River, talking about the problem and potential solutions for slowing the natural forces that are eroding our shorelines. Since 1980 the DEQ has provided Shoreline Erosion Advisory Service to provide technical assistance in the form of an advisory report and plan reviews to landowners, state owned land, localities, and federal agencies experiencing tidal erosion along the 5,000 miles of tidal shoreline in Virginia.

Approximately 15,000 years ago the ocean coast was about 60 miles east of its present location, and sea level was about 300 feet lower. At that time there was no Chesapeake Bay. Instead there was a river that meandered out to sea. It is that ancient river that created the deep channel within the Bay and estuary waters. Sea level continues to rise in the Chesapeake Bay, it was estimated by Scott Hardaway to be rising at about a foot per century, and others have estimated that this rise will accelerate in the future. The rising sea level is one of two primary causes of shoreline erosion, the other is wave action. Storm events can cause powerful waves and change the shape of the shoreline as they erode and transport soil and sand from one part of the shore to another.

The factors that influences the way that the shore line will erode are; coastal geology, the amount of open water, existing shore conditions, storm surges, and rising sea level. While the erosion can be managed, it cannot be stopped. Rising sea level and storm waves are relentless forces. The erosion of shoreline in Virginia has been complicated by the rapid and extensive development in these areas in the past 25 years. The development changes the nature of the shore and creates difficulties in trying to implement an area strategy with multiple property owners who cannot or will not take the (decades) long term view. There are basically three strategies that can be implemented for fighting shoreline erosion: soft, hard and combination. There is little that can be done to permanently hold back the rise in sea level; however shoreline management strategies can be used to blunt the destruction of storm related wave action.

A soft strategy is utilizing wide fringing marshes, beaches and dunes to absorb the energy of waves and reduce the effects that storms will have on adjacent upland banks. With an adequate marsh fringe, beach or dune protection upland banks may only be impacted by the most severe events- at least for a while. Nonetheless, over time, marshes and beaches are eroded themselves and can no longer protect the shore and according to Mr. Vanlandingham, there are areas where a massive storm can erode 30 feet of shoreline in a single year though the shoreline overall averages a loss of 1 foot per year. As rising sea level and erosion narrow beaches and marshes over time, the upland banks are become impacted by storm surge which causes bank instability. Continual erosion can result in sudden collapse of an upland bank taking yards, decks, homes and roads.

Hard strategies to shoreline protection are riprap revetments, retaining walls with anchor systems and bulkheads. The combination strategies utilize groins in combination with the bulkheads and breakwaters. Bulkheads, revetments, and groins are the most common protection strategies currently employed to protect shorelines from erosion. Bulkhead and seawall are often used to describe the same thing, but there really is a difference: bulkheads are generally smaller and less expensive than seawalls. Bulkheads are usually made of wood. They are designed to retain upland soils and often provide minimal protection from severe storms. Seawalls are generally made of poured concrete and are designed to withstand the full force of waves.

In recent years, rock or riprap revetments became more widely used to protect shorelines. A properly designed and constructed rock revetment can last fifty years or more because it can be maintained by the addition of more stones. The revetments have sloped and rough stone faces that decrease wave reflection and bottom scour. Revetments need to be built high enough to withstand waves during extreme storms or they will not work. In addition, the banks need to be graded to create a stable slope.

Between the 1950s and 1980s, groins were a popular way to trap sand and build a modest beach area and are widely seen in beach communities. A groin is a wood structure perpendicular to the shoreline designed to “catch” sand and prevent erosion of the beach. On a relatively wide sand beach the sand will accumulate on the up drift side of a groin. If enough sand were available, the shoreline banks would gain some degree of protection from erosion. However, the sand capture by the groin will prevent the sand from reaching down drift areas increasing erosion there and can create difficulties and lawsuits amongst property owners. Breakwaters can work as a better strategy if used along a long span of shoreline. Breakwaters are built offshore to control shoreline erosion by maintaining a wide, protective beach.

The breakwater, sitting out perpendicular to the shore, “breaks” the force of the waves and dissipates the energy so the waves do not erode the beach or upland banks. Unlike groins that merely capture sand. Breakwater systems are designed to create stable beaches and allow various species of marsh grasses to be established at the site.
from Hardaway
In the past decade or so, coastal engineers use combinations of hard and soft structures in storm damage reduction design. A rock seawall buried within a dune was constructed in 2000 in Virginia Beach, Virginia by the Army Corps of Engineers to protect critical naval infrastructure. Such approaches have been adopted because they proved to be both cost-effective and environmentally friendly alternatives to more classical coastal structure design. Yet, because of the rarity of extreme flood and wave events, these multi-level designs have not been demonstrated to be effective as clearly as they were in New Jersey during Hurricane Sandy in October 2012 when the fate of two adjacent communities demonstrated the effectiveness of a rock seawall buried within a dune.

Hurricane Sandy devastated the Jersey shoreline destroying many coastal communities, caused widespread erosion of the sand dunes as well as having the Barrier Island breached in some locations. Along the hardest-hit stretch of the New Jersey shore are two adjacent coastal communities: the Boroughs of Bay Head and Mantoloking. Before Hurricane Sandy, these adjacent boroughs featured similar topography and residential development. Yet, while similar surges and large waves arrived at their shores, the communities experienced vastly different levels of destruction. A team of scientists lead by Jennifer L. Irish, associate professor of civil and environmental engineering in the College of Engineering at Virginia Tech investigated the shoreline immediately after the storm, and recently published their findings.
From J.L. Irish Buried Seawall

The cause of difference in damage between the two communities turned out to be a long forgotten sea wall originally built in 1882 that had formed the core of the Bay Head dune very much like the structure installed at Virginia Beach. The stone seawall had been covered over with fine dune sand by Aeolian transport and beach nourishment during the twentieth century and forgotten. While similar surges and large waves arrived at both towns the amount of damage and erosion was vastly different between the two. In Mantoloking the dune structure was entirely sand and their entire sand dune was destroyed by the storm. Water washed over the barrier spit and opened three breaches hundreds of feet wide and the sand was swept away by the waves. In Bay Head, only the portion of the dune located seaward of the seawall was eroded and the section of dune behind the seawall received only minor local scouring. The dune remained in place and the sand remained on the beach. In addition in Bay Head only one oceanfront home was destroyed. In Mantoloking, more than half of the oceanfront homes were classified as damaged or destroyed.

The discovery of the relic seawall came as a surprise to many of the residents, generations of families do not stay in communities and there is little realistic long term planning for future storms and rising sea levels. This relic seawall and the deposited dune sand combined to form a combination soft and hard structure that is now in use to protect the shoreline. This design was discovered as the effective protector of the Bay Head shoreline and demonstrated to work during the “Superstorm.”

Shoreline protection strategies continue to evolve. In many locations, elevated shoreline stabilization structures are combined with beach nourishment for shoreline protection. Nontraditional technologies (beach drains, geotextile bags, artificial breakwater structures, wetlands, etc.) are also being investigated in field experiments. Nonetheless, man cannot hold off the rising seas forever. First we protect the shore with engineered barriers (of all types), then we rebuild the beaches by adding sand and marshes. Ultimately we will have to accommodate the rising sea level by raising structures and retreating from the shore.

No comments:

Post a Comment