726
overestimates but not grossly so (the maximum value measured in the cell was
96 cm after 5 years). Overall, mean deposition (calculated from the point data) for
the reference marsh was 3.5 cm (0.7 cm per year; Table 21.2); this value compares
well to work done in the upper Bay of Fundy in the past (Chmura et al. 2001;
Ollerhead et al. 2003) and to the 2012, 2013, and 2014 values of 0.8, 0.6, and 0.7 cm
per year, respectively, for this reference marsh. Mean deposition in the restoration
cells in 2015 (since 2010) was 35.1 cm or 7.0 cm per year. This compares to 10.0,
9.0, 8.8 and 7.7 cm per year for measurements done in 2011, 2012, 2013, and 2014,
respectively (Fig. 21.13). Thus, there is decreasing mean annual deposition, which
makes sense given that there is less frequent tidal flooding of the restoration marsh
surface as elevation increases with deposition. In other words, the rate of deposition
should be slowing down and it is. It is also likely that slight compaction of the sediments is contributing to the apparent decrease in mean annual deposition rate, but
this cannot be resolved with the RTK DGPS measurements.
Thus, the data collected demonstrate that the mean amount of sediment deposition in the restoration cells is still an order of magnitude greater than in the adjacent
reference salt marsh. This outcome is on the order of what was expected based on
empirical work measuring sediment deposition over single tidal cycles and longer
temporal scales on a nearby salt marsh (van Proosdij et al. 2000, 2006a, b). The
deposition in the cells also remains spatially variable (Fig. 21.12). There is now
more than 90 cm of deposition in some low areas and essentially none at some high
areas. There has been little deposition near the breaches due to higher water flows
out of the cells on the ebb tide (i.e., no opportunity for deposition). Furthermore, the
rate of deposition in the SE cell remains close to double that in the NW cell. This
makes sense given some of the lowest topographic areas that existed in the SE cell
prior to restoration (i.e., the greatest depth of water can pool in the lowest areas,
providing the greatest amount of suspended sediment to settle out of the water
column).
21.4.3 What Happened to the Dikes?
The impetus for the restoration project was erosion of the “old” (seaward) dike, and
it has continued to erode. In 2010 and 2011, the three breaches in the old dike were
mapped using ground-based laser scanning (LiDAR). Details of this work are not
included here, but the key conclusion from that work was that significant erosion on
the seaward face of the old dike was occurring beside the openings and that it was
causing armor stone on the sides of the breaches to shift and collapse.
The change in shape of the breaches and the ongoing erosion of the whole old
dike was studied in 2012. Three digital elevation models (DEMs) were derived from
airborne LiDAR data from 2006, 2009/2010, and 2012 using ESRI’s ArcGIS software. The 2006 LiDAR mission was flown before the dikes were breached, the 2010
LiDAR mission about 2 months post breach, and the 2012 LiDAR mission about 1.5
years post breach. Thus, these data offered an opportunity to assess changes from
L.K. Boone et al.
overestimates but not grossly so (the maximum value measured in the cell was
96 cm after 5 years). Overall, mean deposition (calculated from the point data) for
the reference marsh was 3.5 cm (0.7 cm per year; Table 21.2); this value compares
well to work done in the upper Bay of Fundy in the past (Chmura et al. 2001;
Ollerhead et al. 2003) and to the 2012, 2013, and 2014 values of 0.8, 0.6, and 0.7 cm
per year, respectively, for this reference marsh. Mean deposition in the restoration
cells in 2015 (since 2010) was 35.1 cm or 7.0 cm per year. This compares to 10.0,
9.0, 8.8 and 7.7 cm per year for measurements done in 2011, 2012, 2013, and 2014,
respectively (Fig. 21.13). Thus, there is decreasing mean annual deposition, which
makes sense given that there is less frequent tidal flooding of the restoration marsh
surface as elevation increases with deposition. In other words, the rate of deposition
should be slowing down and it is. It is also likely that slight compaction of the sediments is contributing to the apparent decrease in mean annual deposition rate, but
this cannot be resolved with the RTK DGPS measurements.
Thus, the data collected demonstrate that the mean amount of sediment deposition in the restoration cells is still an order of magnitude greater than in the adjacent
reference salt marsh. This outcome is on the order of what was expected based on
empirical work measuring sediment deposition over single tidal cycles and longer
temporal scales on a nearby salt marsh (van Proosdij et al. 2000, 2006a, b). The
deposition in the cells also remains spatially variable (Fig. 21.12). There is now
more than 90 cm of deposition in some low areas and essentially none at some high
areas. There has been little deposition near the breaches due to higher water flows
out of the cells on the ebb tide (i.e., no opportunity for deposition). Furthermore, the
rate of deposition in the SE cell remains close to double that in the NW cell. This
makes sense given some of the lowest topographic areas that existed in the SE cell
prior to restoration (i.e., the greatest depth of water can pool in the lowest areas,
providing the greatest amount of suspended sediment to settle out of the water
column).
21.4.3 What Happened to the Dikes?
The impetus for the restoration project was erosion of the “old” (seaward) dike, and
it has continued to erode. In 2010 and 2011, the three breaches in the old dike were
mapped using ground-based laser scanning (LiDAR). Details of this work are not
included here, but the key conclusion from that work was that significant erosion on
the seaward face of the old dike was occurring beside the openings and that it was
causing armor stone on the sides of the breaches to shift and collapse.
The change in shape of the breaches and the ongoing erosion of the whole old
dike was studied in 2012. Three digital elevation models (DEMs) were derived from
airborne LiDAR data from 2006, 2009/2010, and 2012 using ESRI’s ArcGIS software. The 2006 LiDAR mission was flown before the dikes were breached, the 2010
LiDAR mission about 2 months post breach, and the 2012 LiDAR mission about 1.5
years post breach. Thus, these data offered an opportunity to assess changes from
L.K. Boone et al.