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Geomorphic constraints to urban residential development in the Seymour area, District of North Vancouver, B.C.

University of British Columbia

The opportunity to apply preventive planning is available in the Seymour area of North Vancouver, B.C., the largest portion of Greater Vancouver potentially open, to urban expansion. Natural areas which have certain geomorphic limitations to residential use can be identified and excluded from development, preserving them as open space integrated within an urban residential setting. Intrusive bedrock of the Coast Mountains largely determines the overall landscape of the Seymour area, however, below about 650 feet (200 m.) the terrain is subdued and underlain by thick surficial deposits related to at least two major glaciations separated by a nonglacial interval. Most of the sediments exposed at the surface were deposited either during the Fraser Glaciation (Vashon Drift and Capilano Sediments) or in post-glacial time (Holocene Sediments). Residential development of the land is dependent on surface and subsurface distribution of the geological materials, on the physical characteristics of the materials (which determine their suitability for urban use), and on the geological processes acting on the landscape. Topographic conditions that affect residential development include elevation and degree of slope. Urbanization at higher elevations will encounter increased precipitation, more snow, and cooler temperatures, all of which add to the costs of designing and maintaining houses, roads, and drainage systems. Slopes less than 20%, not a limiting factor, are mainly underlain by surficial sediments. Steep slopes (greater than 27½ %), such as those underlain by bedrock at high elevations and those along deeply incised valleys and ravines, severely restrict development. Urbanization interacts with surface hydrology in two ways. Structures constructed along the drainage channels may constrict or block flow, increasing the flood hazard. Development of subdivisions can alter surface and subsurface flow, leading to greater surface runoff, higher peak flows, and increased sediment yield. As perennial streams with their associated wetlands and flood-susceptible areas are the most significant hydrological features they should be restricted from development. A moderately strong earthquake (average intensity VII) is a distinct possibility in the Seymour area. In such an event, higher intensities would be felt in areas underlain by fill or saturated Capilano cohesive sediments. Liquefaction of the Maplewood mudflats and slope failures are possible secondary effects of an earthquake. In order to evaluate the geomorphic capability of land for urban residential use, limiting physical factors must be identified and defined. Characteristics which can be restrictive include slope or landscape position, depth to bedrock or impervious layer, depth to water table, texture (and stoniness), drainage (including stream channels), consolidation history, flood frequency, stability of surficial material, and seismic conditions. Areas of similar physical characteristics are delineated and rated according to the number and severity of their limiting factors. The largest tracts of land which have low constraint to development are gentle, till-covered slopes or gentle to moderate slopes underlain by Capilano cohesionless sediments. Limitations in these areas are minor and can be easily overcome at minimum economic and environmental cost. The main areas with medium constraint are those where depth to bedrock and slope are limiting and where steeply and moderately sloping land is underlain by impermeable till. Parts of the delta of Lynn Creek and Seymour River are also rated medium. Moderate restrictions need to be recognized but can be overcome by special planning, design, and maintenance. Areas of high constraint include steep bedrock slopes, steep erodable or unstable slopes, wetlands and flood-susceptible areas, ravine drainage channels, tidal mudflats, and gently sloping land underlain by Capilano cohesive sediments. Development of these areas will require special design, major reclamation, and intensive maintenance which may pose severe economic and enivronmental limitations.

Text

2010-02-24

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http://hdl.handle.net/2429/20828

Geological Sciences

University of British Columbia

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