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Skip Navigation LinksDNREC : Climate Change : Climate Change Shoreline Erosion

 
Climate Change

 

Shoreline erosion and migration

Shoreline erosion is the temporary or permanent loss of sand from the beach or dune systems. Although sea level rise is not thought to cause significant beach erosion in itself, higher water levels in combination with wind-driven waves can cause severe erosion.

 

Storms are major causes of shoreline erosion, though storm-induced erosion is rather temporary or local in nature.  Man-made structures like groins, jetties, and sea walls also can contribute to erosion by altering the movement of sediment along the coast.

 

Sea level rise is linked to shoreline migration, which refers to the gradual movement of shorelines and dune features in a landward direction, though not necessarily with any decrease in size or perceptible erosion. The illustration at right shows the typical evolution of a marsh as the sea level rises, and how hard structural stabilization of coastal property (via bulkheads or sea walls) could lead to complete wetland loss. These hard structures also deflect wave energy downward, often leading to more severe erosion at the site. Some states have banned hard-structural stabilization projects along the coastline because of their associated erosion problems.

 

Shorelines are one of the most dynamic areas of the ocean environment, and scientists have long known that sea level has been higher and lower in times past. By taking core samples and conducting sediment analysis, for example, geologists can determine which areas were above or below sea level at different points in the earth’s geologic history.

 

Visual evidence of past shoreline migration can be seen by the untrained eye as well.

 

In the photograph at right, exposed roots and tree stumps along the shoreline are remnants of Delaware forests that grew several hundred years ago when the sea level was lower and the shoreline extended further out. 

 

Coastal Flooding

Sea level rise also has the potential to exacerbate the damaging effects of coastal storms by increasing the severity of flooding in coastal communities. The graph below, based on tidal gauge data at Breakwater Harbor in Lewes shows the trend in sea level rise for this area during the 20th century. While the average daily tidal range at Breakwater Harbor is over four feet, there is a measurable rise in sea level of about one foot per 100 years. This sea level rise itself will not cause a rapid flooding event because it occurs gradually over a long period of time. However, one foot of sea level rise, when combined with a storm causing storm surge, will measurably increase flood risk and flood damage.

 

 

 

Protecting coastal property and mitigating shoreline erosion

When combined with wind-driven waves, sea level rise can cause shoreline erosion that leaves boardwalks, hotels and houses along the coastline vulnerable to storm damage. Typically, Delaware's beaches include a berm and dune system. The berm and dune system naturally migrates landward, but infrastructure built on areas along the coast block that process. Instead, material is eroded and carried offshore. This decreases beach width and can eliminate or damage the dune systems, exposing coastal properties to storm damage.

 

The most effective way to protect against coastal erosion and shoreline migration is through setbacks. Setting new development and construction sufficiently back from the shoreline allows erosion and shoreline migration to occur without narrowing of beach width or enabling damage to structures. When managed properly through setbacks, natural dunes can migrate landward while maintaining sufficient volume to provide protection and habitat.

 

Another management strategy when setbacks prove inadequate for sufficient beach and dune width to mitigate coastal erosion and restore coastal protection is beach nourishment. Sand and other material is trucked in, or dredged, from an off-site location and placed on the eroding coast to construct a new beach and dune system. Grasses are planted within the dune areas to reduce loss of sand by wind erosion and to increase its stability. Delaware has implemented many of these projects, including the federally-shared Rehoboth/Dewey (2005) and Bethany/South Bethany storm damage reduction projects (2007-08).

 

Protecting property from flooding and and the effects of sea-level rise

Recommended practices for protecting vulnerable property from the threat of sea level rise are similar to those already used to protect coastal property from severe flood events. These practices include:

  • Adding “freeboard” above the 100-year floodplain requirements to new construction activities;
  • Requiring coastal development setbacks from beaches to allow for shoreline migration;
  • Creating sturdier foundations to account for the landward expansion of wave hazard areas;
  • Designing landscaping and plantings with attention to increased water levels and soil moisture.

The photograph at right shows a manufactured home being placed in a tidal floodplain in Sussex County. Current statutory requirements would allow the floor of the home to be placed as low as the mapped 100-year flood elevation.  Meeting only this minimum requirement would result in the home being vulnerable to flood damage in storms that exceed the 100-year flood.  By adding 2 feet of freeboard, the property is more likely to be protected from the 100-year flood during the expected lifetime of the structure, even accounting for predicted sea level rise. This practice also will significantly reduce the cost of flood insurance.

 

FEMA’s 100-year floodplain maps for tidal areas are a good reference for determining which land areas may be at increased flood risk in the future due to higher flood heights associated with sea level rise.  The DNREC’s flood mitigation page provides links to the New Castle County, Kent County, and Sussex County mapping sites, which allow users to view floodplain information as an overlay, such as in the map below.

 

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