713
(Terzaghi et al. 1996). Given that the fill materials comprising the dikes themselves
were more erodible, the decision was made to armor the sides of the dike openings
with rock repurposed from the sites.
A variety of breach design possibilities were explored to find the best way to
minimize wave impact on the new dike. The openings had to prevent waves from the
Bay impinging on the new dike to the extent possible. The new dike needed to survive with minimal protection until a “healthy” marsh established in front of it. Some
other design elements were considered, such as grading the site, excavating creeks
and/or pools, and creating mounds within the cells to possibly speed up the establishment of vegetation; however, these ideas were abandoned when it became evident that the project budget could not possibly accommodate such features.
Site design began with a review of a previously conducted regional survey of
potential restoration sites (Millard 2008). This work provided a solid foundation for
planning the project and was an invaluable starting point. Millard (2008) considered
the viability of restoring salt marshes in several locations near Aulac, NB, including
the site restored in this project. The possibility of raising the ground elevation in the
potential restoration sites via sediment deposition by tiding (i.e., a breach in the dike
that could be opened and closed) and by creating a full breach was analyzed. Millard
concluded that the only way to get adequate sediment deposition (to raise the ground
to a level where vegetation could establish) was through tiding, which was not possible due to budgetary constraints. This conclusion assumed that sediment settling
speed could be estimated using Stokes’ Law (which is about the frictional force
exerted on small spherical objects in a viscous fluid). However, silt and clay particles are often deposited in flocculated forms, which have a higher settling speed
than the individual constituent particles (Christiansen et al. 2000).
Given that our task was to develop a design for this site that could not involve
tiding or adding fill (too expensive), Millard (2008)’s conclusions were carefully
considered. We ultimately disagreed with some of her conclusions for two reason:
(i) we had concerns that the volume to be filled by sedimentation had been overestimated and, most crucially, (ii) we are convinced, based on data from field experiments, that flocculation plays a significant role in sediment settling in this
environment and that Stokes’ Law is not a good predictor of actual sediment settling
rate.
Other cautions offered in Millard’s assessment we fully shared. For example, our
site “experiences high relative exposure [to waves] and seaward dykes are currently
being eroded. Even if the elevation of the land was raised by sediment loading, it
may not be possible to ever completely remove the seaward dykes without further
eroding the land behind, including the newly deposited material” (Millard 2008,
p. 133). As discussed in this chapter, attempting to design a project whereby viable
salt marsh would develop before the seaward dike eroded completely, leaving the
whole site vulnerable to erosion by waves, was, and remains, a major challenge.
21 Returning the Tide to Dikelands in a Macrotidal and Ice-Influenced Environment…
(Terzaghi et al. 1996). Given that the fill materials comprising the dikes themselves
were more erodible, the decision was made to armor the sides of the dike openings
with rock repurposed from the sites.
A variety of breach design possibilities were explored to find the best way to
minimize wave impact on the new dike. The openings had to prevent waves from the
Bay impinging on the new dike to the extent possible. The new dike needed to survive with minimal protection until a “healthy” marsh established in front of it. Some
other design elements were considered, such as grading the site, excavating creeks
and/or pools, and creating mounds within the cells to possibly speed up the establishment of vegetation; however, these ideas were abandoned when it became evident that the project budget could not possibly accommodate such features.
Site design began with a review of a previously conducted regional survey of
potential restoration sites (Millard 2008). This work provided a solid foundation for
planning the project and was an invaluable starting point. Millard (2008) considered
the viability of restoring salt marshes in several locations near Aulac, NB, including
the site restored in this project. The possibility of raising the ground elevation in the
potential restoration sites via sediment deposition by tiding (i.e., a breach in the dike
that could be opened and closed) and by creating a full breach was analyzed. Millard
concluded that the only way to get adequate sediment deposition (to raise the ground
to a level where vegetation could establish) was through tiding, which was not possible due to budgetary constraints. This conclusion assumed that sediment settling
speed could be estimated using Stokes’ Law (which is about the frictional force
exerted on small spherical objects in a viscous fluid). However, silt and clay particles are often deposited in flocculated forms, which have a higher settling speed
than the individual constituent particles (Christiansen et al. 2000).
Given that our task was to develop a design for this site that could not involve
tiding or adding fill (too expensive), Millard (2008)’s conclusions were carefully
considered. We ultimately disagreed with some of her conclusions for two reason:
(i) we had concerns that the volume to be filled by sedimentation had been overestimated and, most crucially, (ii) we are convinced, based on data from field experiments, that flocculation plays a significant role in sediment settling in this
environment and that Stokes’ Law is not a good predictor of actual sediment settling
rate.
Other cautions offered in Millard’s assessment we fully shared. For example, our
site “experiences high relative exposure [to waves] and seaward dykes are currently
being eroded. Even if the elevation of the land was raised by sediment loading, it
may not be possible to ever completely remove the seaward dykes without further
eroding the land behind, including the newly deposited material” (Millard 2008,
p. 133). As discussed in this chapter, attempting to design a project whereby viable
salt marsh would develop before the seaward dike eroded completely, leaving the
whole site vulnerable to erosion by waves, was, and remains, a major challenge.
21 Returning the Tide to Dikelands in a Macrotidal and Ice-Influenced Environment…