In the readings on the previous page, much of the thinking was in response to a catastrophic event, such as Hurricane Sandy. On the Jersey Shore, the emphasis has been on building higher dunes to mitigate the problem of coastal hazards threatening human infrastructure. However, as the coastal scientists point out, dunes and other beach features have a tendency to migrate landward. As sea levels rise, this natural migration will accelerate. Options that involve working with these natural processes rather than trying to control them involve a longer-term approach and, as we have seen, can be controversial. We will look at some examples of places where these alternative methods have been employed using the principles of managed retreat or managed realignment, which are in contrast to the more reactionary approach of rebuilding structures in place and protecting them with dunes and other engineered lines of defense.
Managed retreat or managed realignment is a coastal management strategy that allows the shoreline to move inland, instead of attempting to hold the line with structural engineering. At the same time, natural coastal habitat is enhanced seaward of a new line of defense. This approach is relatively new but is gaining traction among coastal policy makers and managers in the face of increased coastal hazard risks. There is a growing recognition that attempting to “hold the line” in many places is a losing battle.
In response to sea level rise, intertidal habitats migrate landward. Human flood defenses such as sea walls prevent this migration. This leads to the narrowing of he intertidal area. This is sometimes refered to as "Coastal squeeze.
In many cases of managed retreat, human development is “moved” out of harm’s way and natural areas are restored to enhance their ecosystem services. Typically, flood defenses are set back from the shoreline, and flooding is allowed in the previously defended area. Usually, natural coastal habitat is preserved seaward of the man-made defense, and it provides extra protection or a buffer from flooding.
The concept is explained in the following quote from the "Climate Technology Centre and Network: Managed Realignment" Required Reading available on the Module 8 Roadmap. "Managed realignment is able to reduce both coastal flooding and erosion. It is the deliberate process of altering flood defences to allow flooding of a presently defended area. Managing this process helps to avoid uncertain outcomes and negative impacts. It also helps to maximise the potential benefits (Leggett et al., 2004).... The benefit of creating intertidal habitats lies in the fact that they are highly effective at attenuating wave energy. This helps to reduce offshore sediment transport and therefore erosion. Intertidal habitats also form dense root mats which increase the stability of intertidal sediments, helping to reduce erosion rates (USACE, 1989)."
Managed retreat can be complex and often contentious, as it can include delineating a new line to which structures can be built and home and business owners must be bought out.
Components of managed retreat may include:
- coastal planning;
- relocation and buy-back and buy-out programs;
- regulating types of development allowed;
- designating no-build areas;
- habitat restoration;
- replacement of built environment with green space.
For managed retreat or managed realignment to be successful, a number of criteria or conditions must be met, according to authors Gardiner et al, and Rupp and Nicholls. These are listed below. Think about some of the case studies we examined in Module 6. How many of these places met all of these conditions? Perhaps points 4-6 are the criteria that are lacking most often. As we will read in the following case studies, few managed retreat projects are accomplished without controversy and lengthy debate. In places where the level of development on the shoreline is high, managed realignment may not be an option at all, at least in the present conditions.
"Six of the most important conditions are given below (Gardiner et al., 2007; Rupp-Armstrong and Nicholls, forthcoming):
- presence of coastal defenses
- availability of low-lying land
- desire or need to improve flood or coastal defense systems
- presence of a sustainability-oriented coastal management attitude
- desire or need to create intertidal habitats
- societal awareness about the benefits of managed realignment"
In this module, we will explore examples of managed retreat in the U.S. and the U.K. to gain an understanding of the complexities of implementing these projects. We will also consider the discussions of managed retreat options in large cities that are particularly vulnerable to inundation.
In addition, we will look at the dilemma of whole communities facing decisions to relocate in the face of repeated flooding as well as other mitigation measures such as elevating homes and changing building codes.
Learning Check Point
Investigate alternative methods for non-structural shoreline hazard mitigation, including managed retreat and multi-layered defenses.
While this Learning Check Point is not for credit, you will be expected to know the material in Module 8 Quiz.
Read the information on the "Managed Realignment" page. (Required Reading on the Module 8 Roadmap) for a more complete explanation of Managed Retreat, then answer the questions below.
After reading, consider how you would answer the questions on the cards below. Click "Turn" to see the correct answer on the reverse side of each card.