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Shore Protection and Retreat: Elevate Land Surfaces

From Shore Protection and Retreat by James G. Titus and Michael Craghan (2009), which was chapter 6 of the Bush Administration's published sea level rise assessment, entitled Coastal Sensitivity to Sea Level Rise

Outline of the Chapter Elevating Land Surfaces

A second general approach to shore protection is to elevate land and structures. Tidal marshes have long adapted to sea-level rise by elevating their land surfaces to keep pace with the rising sea (Chapter 4). Elevating land and structures by the amount of sea-level rise can keep a community’s assets at the same elevation relative to the sea and thereby prevent them from becoming more vulnerable as sea level rises. These measures are sometimes collectively known as “soft” shore protection.

Beachfill, also known as beach nourishment or sand replenishment, involves the purposeful addition of the native beach material (usually sand but possibly gravel) to a beach to make it higher and wider. Sand from an offshore or inland source is added to a beach to provide a buffer against wave action and flooding (USACE, 2002; Dean and Dalrymple, 2002). Placing sand onto an eroding beach can offset the erosion that would otherwise occur over a limited time; but erosion processes continue, necessitating periodic re-nourishment.

Dunes are often part of a beach nourishment program. Although they also occur naturally, engineered dunes are designed to intercept wind-transported sand and keep it from being blown inland and off the beach. Planting dune grass and installing sand fencing increases the effectiveness and stability of dunes.

Elevating a cottage two feet
(a) FIgure 4-3 coastal wetland type.  (b) Figure 4-4:  Can wetlands keep pace with rising sea level?
Figure 12.5. Sequence of photos showing the elevation of a cottage at Long Beach Island, New Jersey. The first photo shows a front view of the house before the project. The second photo shows a side-rear view with the the bay in the background. The third shot shows workers placing flange beams under the chimney. The next 4 shots show the home gradually being elevated by a set of hydraulic jacks controlled from the blue truck. After additional blockwork is added, the house is lowered. The ground is then elevated about one foot. A final shot shows porch screens and a dog, but still no steps. [Photo source: © James G. Titus, used with permission]

Elevating land and structures is the equivalent of a beachfill operation in the area landward of the beach. In most cases, existing structures are temporarily elevated with hydraulic jacks and a new masonry wall is built up to the desired elevation, after which the house is lowered onto the wall (see Figure 12.5). In some cases the house is moved to the side, pilings are drilled, and the house is moved onto the pilings. Finally, sand, soil, or gravel are brought to the property to elevate the land surface. After a severe hurricane in 1900, most of Galveston, Texas was elevated by more than one meter (NRC, 1987). This form of shore protection can be implemented by individual property owners as needed, or as part of a comprehensive program. Several federal and state programs exist for elevating homes, which has become commonplace in some coastal areas, especially after a major flood (see also Chapters 9 and 10).

Dredge and fill was a very common approach until the 1970s, but it is rarely used today because of the resulting loss of tidal wetlands. Channels were dredged through the marsh, and the dredge material was used to elevate the remaining marsh to create dry land (e.g., Nordstrom, 1994). The overall effect was that tidal wetlands were converted to a combination of dry land suitable for home construction and navigable waterways to provide boat access to the new homes. The legacy of previous dredge-and-fill projects includes a large number of very low-lying communities along estuaries, including the bay sides of many developed barrier islands. Recently, some wetland restoration projects have used a similar approach to create wetlands, by using material from dredged navigation channels to elevate shallow water up to an elevation that sustains wetlands. (USFWS, 2008; see Section 11.2.2 in Chapter 11).

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