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Vancouver Port Authority
Roberts Bank Container Expansion
Coastal Geomorphology Study
- 90 -
Northwest Hydraulic Consultants Ltd. /Triton Consultants Ltd.
File: 33863
November 2004
7.0
FUTURE CONDITIONS WITHOUT PROJECT
7.1
GENERAL CONSIDERATIONS
The following discussion provides a general scenario for the future physical site conditions on
Roberts Bank if the Deltaport Third Berth Project was not constructed. This assessment was
based primarily on the historical observations made over the last 75 years and our understanding
of the physical processes occurring on Roberts Bank. Following this, the potential additional
effects resulting from projected changes to climate and sea level are considered. Given the
uncertainties in these projections, as well as the complex response of ongoing morphological
changes on the Fraser Delta, this assessment is qualitative in nature.
7.2
FUTURE CHANGES ON THE TIDAL FLATS
The historical evidence and results from the numerical model investigations indicate that
sediment transport rates on most of Roberts Bank are relatively low. Furthermore, most longterm morphological changes were associated with tidal drainage channel formation in the intercauseway area or channel shifting near the outlet of Canoe Passage. Future morphologic changes
in the vicinity of Deltaport are expected to be minor. However, it is apparent that the network of
drainage channels that formed after the earlier port developments is still evolving and has not
reached a final state of equilibrium. In particular, the largest drainage channel (Channel 1 in
Figure 3-14) appears to be still in the process of extending both in the seaward and landward
directions. The large area of deposition near the head of the trunk channel also appears to be
extending further landward and accreting to a higher elevation. Field investigations and
analytical studies were carried out for seven tidal channels on Roberts Bank and in Boundary
Bay in order to assess the main factors controlling their characteristics (see Appendix C Section
6.5). Based on these studies, it appears the main trunk channel will continue to migrate landward
until reaching approximately +2 m Chart Datum. The zone of sand deposition at the head of the
channel will also shift landward. This should not impact native eelgrass (Zostera marina) since it
is confined to lower elevations. There is a chance that the smaller drainage channel (Channel 2)
to the east, may also continue to grow, particularly if the outlet can by-pass the crest protection
Vancouver Port Authority
Roberts Bank Container Expansion
Coastal Geomorphology Study
- 91 -
Northwest Hydraulic Consultants Ltd. /Triton Consultants Ltd.
File: 33863
November 2004
structure by joining with the larger channel along the creek. Only minor erosion is necessary to
cause this development.
7.3
POTENTIAL EFFECTS OF CLIMATE CHANGE
It is widely accepted in the scientific community that the increased production of greenhouse
gasses since the start of the Industrial Revolution in the early-1800s, has resulted in a general
increase in global temperatures. This measured increase to the present is small compared to the
increase of between 3°C and 6°C per century that is projected for the future. Of the changes that
are likely to arise from a general increase in global temperature, the two that are most likely to
affect the proposed Deltaport expansion at Roberts Bank are 1) sea level rise, and 2) increase in
extreme weather events.
7.3.1
Sea Level Rise
At the regional scale, sea level rise is measured relative to land and results from a combination of
isostatic, tectonic, and eustatic factors that are not independently measured (Clague et al., 1982).
Isostatic and tectonic factors relate to deformation of the earth’s crust due to glacial loading
(isostasy) and down warping relates to tectonic movements. Eustasy describes the change in
volume of the oceans as a result of the change of water stored in the earth’s glaciers. Overall
global warming is expected to tip the mass balance of ice caps, glaciers and ice fields in favour
of melting.
According to published reports, average global sea level has risen at around 2 mm/year during
the last century. Most recent studies have concluded that the sea level will rise at a faster rate
than in the last century due to the effects of global warming. The Inter-Governmental Panel on
Climate Change (IGPCC) issued predictions of changes to sea level in 1990, 1995 and 2001 for a
range of future scenarios. However, the range in projections has been very large. For example,
the 1990 forecast indicated sea level could rise by 11 to 77 cm by the year 2100. The updated
forecast in 2001 (IGPCC, 2001) was as follows:
Projections of global average sea-level rise from 1990 to 2100, using a
range of AOGCMs (Atmosphere-Ocean Global Climate Models) following
the IS92a scenario lie in the range 0.11 to 0.77 m. This range reflects the
Vancouver Port Authority
Roberts Bank Container Expansion
Coastal Geomorphology Study
- 92 -
Northwest Hydraulic Consultants Ltd. /Triton Consultants Ltd.
File: 33863
November 2004
systematic uncertainty of modelling. The main contributions to this sealevel rise are:
•
a thermal expansion of 0.11 to 0.43 m, accelerating through the 21st
century;
•
a glacier contribution of 0.01 to 0.23 m;
•
a Greenland contribution of -0.02 to 0.09 m; and
•
an Antarctic contribution of -0.17 to 0.02 m.
The US Environmental Protection Agency (Titus and Narayanan, 1995) has provided
probability-based estimates for various future scenarios. For the median case, sea level was
estimated to rise 0.15 m by the year 2050 and 0.34 m by 2100. There was estimated to be a 1%
chance that global warming will raise sea level by 1 m by the year 2100. These figures represent
only eustatic changes in sea level and do not include effects of local or relative sea-level change
induced by factors such as ground subsidence or settlement. In deltaic areas, ground subsidence
may significantly affect local or relative sea-level differences.
Estimates of local sea-level rise in the region of Vancouver based on analysis of long-term tidal
gauge records (ie. Point Atkinson) are in the order of 1 to 2 mm/yr (Clague et al., 1982). The
recent tectonic history of the delta area was briefly described by Mathews et al. (1970). They
suggested that a combination of compaction and local down-warping has resulted in an average
subsidence of approximately 1.2 mm/year. Kellerhals and Murray (1969) reported rates of
subsidence in Boundary Bay of between 0.4 to 1.7 mm/year. These rates are relatively small
compared to the projected eustatic changes that may occur.
Based on these studies, it seems reasonable to assume that relative sea-level could rise by
approximately 0.5 m by the year 2100. However, the uncertainty of the model projections limits
the usefulness of making quantitative assessments of expected impacts.
7.3.2
Extreme Weather Events
A rise in global temperature is expected to produce a complex response in the global circulation
of weather, which may result in greater temperature increases in the central portions of the North
Vancouver Port Authority
Roberts Bank Container Expansion
Coastal Geomorphology Study
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Northwest Hydraulic Consultants Ltd. /Triton Consultants Ltd.
File: 33863
November 2004
American continent, while coastal and southern regions may experience a more moderate rise in
temperature. In addition, many researchers point to an increase in severe weather events as a
likely outcome of global climate change (for examples see McBean, 2004). Powerful storms with
high winds are responsible for the largest wind generated waves at Roberts Bank. An increase in
extreme weather events is expected to generate a greater frequency of large storms as well as an
overall increase in the magnitude of storm intensity.
7.3.3
Effect of Climate Change
Climate change in the short-term is expected to have a minor impact on the physical environment
of Roberts Bank. However, in the long-term, the physical conditions experienced at Roberts
Bank will likely be modified as a result of sea-level change and a possible increase in the
incidence of severe storm activity. The following discussion is based on an assumed net sea level
rise of 0.5 m by the year 2100, which is near the median of published projections.
Since eelgrass is sensitive to light exposure as well as the duration of wetting and drying, the
increased water depths on the tidal flats would be expected to result in a landward expansion in
its extent and density. Further expansion landward would also modify the tidal drainage paths
and tidal flow distribution on the upper portions of the flats. Flow concentration from the
eelgrass, as well as the increased tidal prism would be expected to promote further landward
growth of the existing tidal drainage network. This would also promote continued shoreward
migration of the large sandy bars that have formed at the head of the main trunk channels.
At the seaward end of the tidal flats, a 0.5 m rise in tide level would reduce the number of times
a year that the tide drops below the edge of the crest protection. The higher water levels would
significantly reduce the velocities (and sediment movement) at the seaward ends of the existing
drainage channels and would probably end further seaward extension.
Generalized response models have been proposed to describe the effects of sea level rise on the
overall equilibrium shoreline profile (Bruun, 1998). The basic concept is that with a rise in sea
level, the equilibrium profile of the beach and shallow offshore will move upwards and
landward. The upper beach will erode, because of a landward translation of the profile. Sediment
that is eroded from the upper beach will be deposited offshore. It is not clear that such a model is
Vancouver Port Authority
Roberts Bank Container Expansion
Coastal Geomorphology Study
- 94 -
Northwest Hydraulic Consultants Ltd. /Triton Consultants Ltd.
File: 33863
November 2004
applicable to the case at Roberts Bank that has such a high tidal range, low sediment supply and
low wave energy.
The effect of an increase in the frequency and magnitude of severe storms would be compounded
by the expected increase in relative sea level. Larger and more frequent waves could cause some
increased erosion on the upper tidal flats. This could result in further re-suspension of fine
sediment that may have accumulated in the relatively sheltered areas near the upper slopes,
resulting in a change of substrate to more sandy deposits.