Is a Giant Polar Umbrella the Solution to the Melting Polar Caps?
March 21, 2013Private: Sea-Level Rise Planning: A Local Focus
April 2, 2013by Rachel Gruzen, M.E.M., LEED AP
Superstorm Sandy impacted our waterfront communities and reminded us that the coastline is a continually changing environment. Ecological solutions are key to improved coastal resiliency, as the first blog in this series explored. While some look to fight coastal change with hardening structures such as seawalls and groins, these approaches protect targeted resources but at a cost to neighboring properties and natural resources. Natural solutions, such as habitat restoration, comprehensively increase resiliency to a system, but to ensure their effectiveness, we must first understand how certain habitat restoration typologies function. As part two of the series, this blog examines the benefits and limitations of dunes and assesses how and where they can be effective coastal protections.
The Shoreline is Continually Reshaped by Storm Energy
Storm energy is a critical factor in shaping the shoreline, given its power to mold beaches, shift sediments, and alter hydrology in a short time. Storm frequency in the Northeast has increased in recent decades, and sea level rise is causing the intertidal zone and its associated habitat typologies to migrate inland.[1] Together, these forces are making significant and rapid changes to the interface of land and sea. Those who have built homes and businesses at the water’s edge have an acute awareness of the quickening coastal metamorphosis and the associated vulnerability of their structures and investments.
Dunes Are Created by Littoral Processes
Dunes naturally form in sand-rich environments as sand migrates alongshore, shifted by wind and waves, encounters structures and landforms, and molds into masses including dunes, barrier islands, and sand bars. Dunes are even connected to the lifecycle of barrier island marshes, as dune sand blows over the barrier island into wetlands, and the diffused particles contribute to marsh accretion. These alongshore processes are called littoral processes. Although dunes form naturally, they can also be artificially created. Most commonly, they are sculpted from dredged sand pulled from the ocean floor or relocated sand that has been transported by a storm to an undesirable location.
Dunes Provide Protection as Well as Ecological and Economic Benefits
The volume and mass of dunes make them effective barricades to wave energy in storms. They prevent surging seawater from moving inland and they block winds. In addition, they have significant ecological value, providing habitat for shorebirds that nest on the seaward side and protection for coastal vegetation that grows on the landward side in the lee of the elements. On Long Island where double dunes exist, swales between the two dunes even support rare orchids.[2] In addition to storm protection, dunes and their related morphologies of barrier beaches and sand bars provide economic value for many including beach goers, surfers, and naturalists.
Are Dunes the Solution Everywhere?
Dunes are effective coastal barriers with enormous ecological and economic benefits. But we also must recognize they are a changing landscape. They require sand inputs and are not static masses. Because of their transformative nature, they may not be the best solution everywhere. Dune restoration planning requires looking at the full coastal dynamics of a site to understand the energy regime and littoral processes and whether they will nourish the formation of dunes. Two critical components are the fetch (the distance the wind and waves have traveled, which contributes to their power) and the aspect (angle) of winds and waves hitting the shoreline.
We must also consider the site’s geology to understand the sediment characteristics and availability of sand deposits, specifically the potential for sand inputs when winds and waves extract sand out during the next storm. Even if there is sand today, there may not be enough sand reserve in the geologic layers to support dune processes in the future. In addition, we must consider coastal structures that interface with coastal processes. For example, hardening structures such as groins and seawalls can block the drift of sand and starve a restoration site of new inputs.
Another consideration in dune restoration planning is climate change and its associated increase in storm activity and sea level rise, both of which are causing dunes and barrier islands to migrate landward. According to one study, during Hurricane Sandy, Fire Island may have migrated inland by 65 to 85 feet. But if the dune system cannot migrate, for example if it hits a hardening structure or homes, the material may be lost—in some cases diffused inland or pulled offshore and dumped below the wave base. This eliminates the opportunity for dune accretion and its associated benefits. Sand can be artificially sustained in lieu of natural growth, but the solution is not permanent if the underlying ecological and geological processes are not there to support it.
Dune restoration is an excellent solution for creating a storm-resilient coastline; however, to be an effective solution, we must understand that dunes are part of highly active shoreline processes. The best solutions for coastal resiliency emerge from both an understanding that the coastline is dynamic and changing, and knowledge of site-specific, local ecological processes.
References:
[1] Ralph Lewis, Geologist, Long Island Sound Resource Center, University of Connecticut. March 12, 2013.
[2] U.S. Fish & Wildlife Service, Amagansett National Wildlife Refuge. New York Times. “In Island’s Shifted Sands, Signs of Hurricane’s Power”. December 17, 2012.
Images:
Hurricane Sandy:Pre-Storm and Post-Storm 3D Lidar Topography: Fire Island, NY. U.S. Geological Survey.
Hurricane Sandy: Pre-and Post-Storm Photo Comparisons – New York. U.S. Geological Survey.