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Land use, riparian buffers and the effects of urbanization on stormwater runoff in the Hoy Creek Watershed, Coquitlam, B.C. Houston, Jayna

Abstract

The Lower Fraser Valley in British Columbia is undergoing rapid urbanization and due to the agricultural land reserve much o f this urbanization is occurring on sloping land. Concerns have been expressed that the increased climatic variability and the generation of impervious surfaces as a result of urbanization will have serious impacts and consequences on stormwater management in the region. The objectives of this research were to determine how the land use has changed in the Hoy Creek Watershed in Coquitlam and document how this change impacts the stormwater hydrology. Land use and land cover GIS maps were produced for 1979 and 1999 and the emphasis was placed on determining differences in impervious surfaces in the urban areas and different land cover in all the non-urban areas. The results showed that the urban area expanded by 29% over the 20-year period mostly at the expense o f forest cover, which declined by 38%. The total impervious area for the watershed increased from 7% to 25% and considering the new areas slated for development in 1999 this is expected to increase to 36% by mid 2000. The stream buffer zone integrity was examined for a 10, 30, 50, and 100 m wide zone and the results showed that the vegetated portion o f the buffer zone declined from a very effective vegetation cover of 94% within the 10 m zone to a mere 58% within the 100 m buffer zone, and at the same time the total impervious surface area within the buffer zone increased from 4.5% in the 10 m buffer to 18% in the 100 m buffer zone. In terms o f buffer zone integrity it was found that the Lower Hoy Creek and part o f West Hoy had the lowest riparian zone integrity and these areas were identified as key areas for rehabilitation o f the riparian buffer. Soil penetrometer resistance was measured in the main combined soil and vegetation categories and the results were then converted into percolation rates using regression analysis. The watershed was then divided into three percolation categories with 35% of the watershed falling into the high percolation class (0.89 cm/min) and 45% into the moderate percolation rate (0.4 cm/min). The impact of the impervious surfaces and the partially pervious areas on stormwater runoff was then estimated for the watershed using an average storm (2 year return period-33 mm) and the maximum storm (112 mm). The results showed that the average storm produced 6.2 mm o f runoff in 1999 and this is expected to reach 7.9 mm once development of forest that is currently designated as urban area is completed. For the maximum storm this resulted in 64.4 mm runoff reaching 74.6 mm once the forest development is completed. This represents a 16% increase in maximum storm runoff based on the past climatic record. Given the growing evidence o f increasing climatic extremes it is possible that these values may increase even further. To verify these calculations the Soil Conservation Service curve number method was used to determine runoff values for the same watershed. Using this approach it was possible to show that the average storm runoff increased by more than 100% between 1979 and 1999 and the maximum storm runoff increased by 36% over the same time period. The results between the two methods were comparable but the SCS curve method produced somewhat lower values than the regression approach using percolation rates inferred from cone penetrometer readings. The results showed that more attention needs to be given to designing urban development with low impact design that niinimizes imperviousness. At the same time larger buffer zones are needed in the more urban areas to facilitate the development o f stormwater detention systems within the buffer zone.

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