Coastal Vulnerability in the Halifax Regional Municipality

Activity Rationale

Waves and storm surges superimposed on rising sea level cause flooding, threaten safety, and damage property and infrastructure in coastal urban centres. This activity, in partnership with planners in the Halifax Regional Municipality, provides the scientific basis for adaptation measures to be incorporated into the new Halifax Harbour Plan. It demonstrates how scientific knowledge can help to boost resilience in coastal communities.

Leader: Don Forbes

Members of the project team Dave Frobel (L) and Bob Taylor (R), both with the Geological Survey of Canada (GSC), surveying in June 2008 the level of a rail line that was undercut by waves in Hurricane Juan (September 2003). Halifax Harbour is at the right (low tide) and downtown Dartmouth in the background.

Members of the project team Dave Frobel (L) and Bob Taylor (R), both with the Geological Survey of Canada (GSC), surveying in June 2008 the level of a rail line that was undercut by waves in Hurricane Juan (September 2003). Halifax Harbour is at the right (low tide) and downtown Dartmouth in the background. Larger image

The Topic

Team member Gavin Manson (GSC) surveying seawall damaged in Post-Tropical Storm Noel (November 2007), Northwest Arm, Halifax Harbour (note fog in the distance in the harbour mouth), June 2008.
Team member Gavin Manson (GSC) surveying seawall damaged in Post-Tropical Storm Noel (November 2007), Northwest Arm, Halifax Harbour (note fog in the distance in the harbour mouth), June 2008. Larger image

The Halifax Regional Municipality (HRM) has been a leader among Canadian municipalities in climate-change planning. The HRM Regional Municipal Planning Strategy, adopted by Council in August 2006, explicitly included policies to address climate change. The planning strategy recognized a critical information gap related to the effects of climate change, including sea-level rise and storm surges, on the shores of Halifax Harbour and other coastal areas in HRM.  Halifax Harbour is a major seaport with billions of dollars in port, industrial, military, and municipal infrastructure, coastal and island parks, signature cultural heritage features, and ongoing commercial, residential, and recreational development.

A need was recognized to gather appropriate scientific data on the harbour shoreline, sea-level trends, flooding hazards and vulnerability to support incorporation of climate-change issues and adaptation measures into the new Halifax Harbour Plan. This activity fostered a partnership between Earth Sciences Sector (ESS) scientists, colleagues in other federal and provincial departments and academic institutions (Dalhousie University and Nova Scotia Community College), and planners in HRM to deliver the scientific knowledge required for effective adaptation planning.

Results

Present rates of sea-level rise and land subsidence

The Halifax tide gauge has been operating since 1895 with a near-complete record since 1920. The observations are collected and archived by the Department of Fisheries and Oceans and available on-line. These data show that mean sea level has been rising progressively relative to the gauge at an average rate of 3.20 ± 0.01 mm/yr (equivalent to 32 cm/century). This is the so-called ‘relative sea-level rise’, the rise in sea level relative to the shoreline - it includes both land motion (regional subsidence at Halifax) and sea-level rise, each representing about half of the observed trend.

Even without rising sea level, relative sea-level rise will be observed if the land is sinking. In the Maritimes, widespread regional subsidence is a continuing long-term legacy of the last ice age over 10 000 years ago. To obtain realistic estimates of future sea-level rise under climate change, it is necessary to know the magnitude of the subsidence. The rate of subsidence needs to be added to projections of sea-level rise from climate models in order to estimate the total relative sea-level rise to be expected at any locality.

Rates of crustal motion (subsidence or uplift) can be estimated from geological evidence of past relative sea levels or by direct measurement using fixed global navigation system (GNSS) receivers or time series of absolute gravity. As part of collaboration with the Geodetic Survey Division of Natural Resources Canada, we are measuring the rate of crustal subsidence in the Halifax region using a continuously operating global positioning system (GPS) receiver on a monument on bedrock overlooking the main entrance to the Bedford Institute of Oceanography (BIO) in Dartmouth.

GPS antenna on geodetic monument (brass cap on concrete pillar) set on glacially-scoured bedrock outcrop overlooking the main entrance at the Bedford Institute of Oceanography.

GPS antenna on geodetic monument (brass cap on concrete pillar) set on glacially-scoured bedrock outcrop overlooking the main entrance at the Bedford Institute of Oceanography. Larger image

Debris deposited on undercut CN rail line at Dartmouth Point on the morning after Hurricane Juan, 29 September 2003. Run-up at this location was 1.64 m above the highest water level recorded at the Halifax tide gauge across the harbour. Note that water level remains unusually high at the time of photography.

Debris deposited on undercut CN rail line at Dartmouth Point on the morning after Hurricane Juan, 29 September 2003. Run-up at this location was 1.64 m above the highest water level recorded at the Halifax tide gauge across the harbour. Note that water level remains unusually high at the time of photography. Larger image



Documenting storm impacts

Run-up levels (in centimetres) above maximum recorded water level in Hurricane Juan, from real-time kinematic (RTK) differential GPS surveys of observed run-up levels or debris lines. Background image is post-Juan aerial photography (courtesy Nova Scotia Department of Natural Resources).
Run-up levels (in centimetres) above maximum recorded water level in Hurricane Juan, from real-time kinematic (RTK) differential GPS surveys of observed run-up levels or debris lines. Background image is post-Juan aerial photography (courtesy Nova Scotia Department of Natural Resources). Larger image

Effects of climate change at the coast are often first encountered during extreme events whose impact may be worsened by rising sea level or other climate factors. Hurricane Juan was an exceptional storm that caused a record water level (2.91 m above Chart Datum) at the Halifax tide-gauge just before midnight on 28-29 September 2003. The maximum storm surge during this event (the difference between the observed water level and predicted tide, including any seiche effects) was 1.63 m at midnight. Off the mouth of Halifax Harbour, deepwater significant wave height (average of the highest third of the waves) was measured at 9.0 m with a maximum wave height of 19.9 m. Due to some instrument wander, the significant wave height may have been higher. These waves propagated northward up the axis of the harbour parallel to the eastern eyewall, riding on record high water levels near the peak of the storm.

Shoreline sites facing south and projecting into the harbour, such as Point Pleasant and Dartmouth Point, recorded high ‘wave run-up’ (runup combined with any seiche) >1 m with a maximum observed value of 1.7 m. Run-up as high as 1.1 m was measured along the Halifax waterfront at Bishop’s Landing. This event caused record flooding in the downtown core, along the Dartmouth waterfront, and at numerous other places around the harbour, as well as severe damage to boats and small vessels, transportation infrastructure, and waterfront structures.

Projecting future water levels in Halifax Harbour

Future extreme water levels resulting from climate change can be estimated by combining projections of sea-level rise with statistical information on storm water levels. For the Halifax Harbour study, a generalized extreme values distribution of annual extreme water levels at Halifax from 1920 to 2007 was developed in partnership with Dalhousie University. This indicates that the record water level in Hurricane Juan is not anomalous but lies close to the curve at a return period of approximately 100 years. The previous record water level was reached in a winter storm on 23 February 1967.

Accelerated rates of sea-level rise are anticipated over coming decades in response to ocean warming and the melting of mountain glaciers, ice caps, and the ice sheets in Greenland and Antarctica. The projections vary as a function of emission scenarios, as reported in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). This report projects global mean sea level rise (from 1980-1999 to 2090-2099) ranging from 0.18-0.38 m for the lowest emission scenario to 0.26-0.59 m for the highest emission scenario. Since the completion of the IPCC 2007 report, new evidence has been published suggesting that sea-level rise is tracking at or above the A1FI projections and may be increased substantially by accelerating ice loss from Greenland and Antarctica.

With these reports in mind and adopting a precautionary approach, a number of potential design water levels were selected in collaboration with planners at HRM. These combined sea-level rise with storms of varying intensity (storm water level return periods). We used three scenarios of sea-level rise: (1) a continuation of the historical rate (0.18 m by 2100); (2) the upper limit of the highest projection from the 2007 IPCC report (0.59 m by 2100); and (3) an extreme projection from the recent literature (1.3 m by 2100). All of the above were combined with the measured rate of subsidence to give local rates of relative sea-level rise. These results are presented in an ESS Open File report.

Mapping future flood levels

Flood extents associated with each of the water-level options were determined by flood simulation in a GIS, using a high-resolution digital elevation model derived from airborne topographic LiDAR (light detection and ranging) (also known as scanning airborne laser altimetry). This involves an infrared laser beam which scans back and forth beneath the aircraft, providing a swath of laser hits for which the horizontal and vertical coordinates (position and elevation) can be determined with a horizontal precision of about ±0.3 m and vertical precision of ±0.15 m or better. Similar surveys have been undertaken elsewhere in the Maritimes over the past decade for the purpose of mapping coastal inundation under storm surges superimposed on rising sea levels and these provided the model for the Halifax Harbour work (e.g. Webster and Forbes, 2006; Webster et al., 2006).

A LiDAR survey was undertaken in the spring of 2007 after snowmelt and prior to full leaf-on to develop a bare-earth digital elevation model (DEM) as a basis for flood-hazard mapping around Halifax Harbour. The data acquisition was funded and managed by HRM with contributions from the Nova Scotia Department of Energy and the Halifax Port Authority and in-kind contributions from other partners. The bare-earth model was developed from the initial surface elevation model (which includes vegetation and buildings) by classifying individual laser hits as on-ground or off-ground and removing the latter from the model.

Project team members Paul Fraser (L) and Dustin Whalen (R), both with the GSC, working with LiDAR digital elevation model for the Halifax Harbour area, March 2009.
Project team members Paul Fraser (L) and Dustin Whalen (R), both with the GSC, working with LiDAR digital elevation model for the Halifax Harbour area, March 2009. Larger image

The combined models have numerous potential uses, including modelling of surface water drainage, mapping of surficial geology, quantification of urban tree canopy, determination of cut and fill for development projects, or view-plane analysis, to name but a few that have been applied in HRM. The survey coverage acquired in 2007 included the entire drainage basin of Halifax Harbour and additional coverage along the Eastern Shore, covering a total area of about 1393 km2.

The LiDAR digital elevation model (seen in figure above, showing Halifax Harbour and surrounding basin on the left-hand screen and a detail of the urban core on the right-hand screen) is being used to evaluate the extent of flooding for various scenarios of sea-level rise and combined tide and storm-surge extreme water levels.

Links

Publications

Please note that subscriptions may be required to access some articles. To request a copy of publications, or for any more information, please contact Don Forbes.

Forbes, D.L., Craymer, M., Daigle, R., Manson, G.K., Mazzotti, S., O’Reilly, C., Parkes, G.S., Taylor, R.B., Thompson, K. and Webster, T. 2008. Creeping higher: Preparing for higher sea levels in Atlantic Canada. BIO 2007 in Review. Bedford Institute of Oceanography, Dartmouth, NS, p. 14-17.

Forbes, D.L., Manson, G.K., Taylor, R.B., Thompson, K.R. and Charles, J. 2009. Halifax Harbour extreme water levels in the context of climate change: Scenarios for a 100-year planning horizon. GSC Open File (in review).

Webster, T.L. and Forbes, D.L. 2006. Airborne laser altimetry for predictive modeling of coastal storm-surge flooding. In Remote Sensing of Aquatic Coastal Ecosystem Processes: Science and Management Applications (Richardson, L.L. and LeDrew, E.F., editors). Springer, Dordrecht, p. 157-182.

Webster, T.L., Forbes, D.L., MacKinnon, E. and Roberts, D. 2006. Flood-risk mapping for storm-surge events and sea-level rise using LiDAR for southeast New Brunswick. Canadian Journal of Remote Sensing, 32, 194-211.

Additional publications

Coastal Impacts of Climate Change and Sea-Level Rise on Prince Edward Island / Prince Edward Island (McCulloch et al. PEI report available for download)