The slope of the land surface ranges from nearly horizontal on floodplains to more than 20o through much of the Coast and Cascade Mountains and on escarpments bordering uplands in the Fraser Valley. Why is slope important? First, it affects surface drainage -- in a general sense, drainage improves as the land surface steepens. Second, slope is an important factor in the stability of the land surface -- most landslides in the Vancouver area occur on slopes that are steeper than 20o(red areas on this map). Locations of many of the landslides that have occurred in this century in the Fraser Valley are plotted on the map (landslides in the Coast and Cascade Mountains are not included). Most landslides in the Fraser Valley involve Ice Age sediments and are triggered by intense rainstorms. In contrast, many of the landslides in the Coast and Cascade Mountains are in bedrock (rockfalls and rockslides). A common type of landslide in both regions is rapid flows of water-saturated debris (debris flows).

<10o 10 - 20o > 20o

landslide

Locations of landslides from Armstrong and Hicock 1979, 1980 (see ADDITIONAL INFORMATION) and G.H. Eisbacher and J.J. Clague, 1981, Urban landslides in the vicinity of Vancouver, British Columbia, with special reference to the December 1979 rainstorm, Canadian Geotechnical Journal, v.18, pp. 205-216. Slope data derived from British Columbia government Terrain Resource Information Management (TRIM) data.