Erosion of the Point Grey Cliffs
Project by Rens Harteveld
Project team includes Kees Lokman, Tugce Conger, Doug Doyle, Daan Rijks, Amir Taleghani, and David Gill
The Point Grey cliffs and its beaches have been eroding with an increased rate (average recession rate of 30.0 cm/year) due to changing environmental and climatic conditions and human-activities. Erosion of the cliffs poses increased risks to social and cultural assets such as the local species, natural features, and stories attached to specific locations, as well as to physical assets such as infrastructure and buildings.
In total, five different reasons for the cliff’s erosion were identified (Doyle, 2018, UBC/Pacific Spirit Regional Park Cliff Management, 2000):
- Wave longshore drift
- Ground water leakage from cliff aquifers
- Freeze and taw processes
- Wind and rain events
- Over land flows
From several reports (Golder Associates Ltd., 2015, Pool, 1975, UBC, 2004, UBC/Pacific Spirit Regional Park Cliff Management, 2000), it can be concluded that the problem of erosion hasn’t been resolved yet. The fact that the UBC cliffs are still retreating shows that our understanding on how the coastal processes contribute to the cliff’s erosion has been limited.
The project had two main objectives:
- to understand the coastal system and mechanisms that contribute to the erosion of the cliffs; and
- to develop a local wave model and analyze the system.
The coastal system of the area and mechanisms contributing to the Point Grey Cliffs’ erosion
Rens investigated the local and regional tidal characteristics, winds and waves, Fraser River discharge, sediment budgets, and human interventions (including the marine works and dredging activities) to understand the coastal system of the study area and the mechanisms contributing to the Point Grey Cliff’s erosion.
The mean tidal range per day over ten years show that during the winter and summer months the tidal range is generally higher than in the fall or spring months. Since the tidal range occasionally surpasses the 4.0 meter mark, the tidal environment may be considered macro-tidal.
Winds and waves
Mainly five fetch limited wave fields affect the cliffs. These are winds (and thus waves) from the South-West, North-West, and Western part (and two in between) of the Strait of Georgia. The significant storm wave heights in the Strait of Georgia are less than 2.1 meters and the maximum less than 3.3 meters (Barrie and Currie, 2000, Milliman, 1980).
Fraser River discharge
Fraser River is the largest river flowing into the Strait of Georgia. Fraser River annually discharges around of 17.3·109 kg/year sediments; the Main arm of the river takes 85% of the total discharge and 91-97% of the sediment load, where the North Arm receives the remaining 15% of the total discharge and 3-9% of the sediment load (Milliman, 1980, Thomson, 1981, Isfeld et al., 1996, Golder Associates Ltd., 2015). The Fraser River sediment load is distributed in the following way: 35% sand, 50% silt and 15% clay.
Most of the sediment input for the Point Grey cliffs and beaches come through the North Arm of the Fraser River (Golder Associates Ltd., 2015, Pool, 1975). Much of this sediment immediately travels westward into the Strait of Georgia and will never reach the Point Grey cliffs and beaches (Golder Associates Ltd., 2015).
Many human interventions have taken place in the Lower Mainland region over time that have altered the local and regional coastal system. These interventions can be grouped under marine works and dredging activities in Fraser River. The marine works include berm and groyne systems, jetties and breakwaters, and trifurcation works. Dredging has been conducted over the past decades to maintain the safe navigation of the Fraser River but the dredging in the North Arm has completely stopped (Hart, 2018).