UNBC Geog300 Project

Impermeable surface areas in the urban setting pose problems for water quality and aquatic environments. Runoff water from these locations generally contains higher levels of pollutants and is warmer in temperature. Furthermore, insufficient storm water management infrastructure can over-extend municipal waste water treatment technology, resulting with discharges into aquatic environments. Impermeable surface areas often correspond with intensive land use, such as commercial and industrial locations. A coverage of impermeable surfaces in the Prince George "Bowl" area highlighted the congruency of impermeable area and land use. Moreover, the Prince George flood plain extends over much of this impermeable surface, creating problems for water quality management and flood control. Future work is recommended to better understand the existing storm water infrastructure and to identify possible gaps where improvement could be effected. In particular, the possibility of installing oil/grit separators or reducing impermeable area is suggested.

Introduction

Impermeable surface areas in the urban setting pose problems for water quality and aquatic environments. Many municipalities do not have separate storm and sewage water pipe infrastructure, so storm events overload treatment facilities. Polluted water with effluent is then discharged into aquatic environments to maintain storage capacities. Furthermore, stormwater runoff is polluted with gasoline, sediment and other contaminants. This water can then run directly into aquatic environments, seriously affecting habitat and water quality. Also, runoff water is generally warmer than that in aquatic environments, causing temperature fluctuations which affect organisms.

Proper stormwater retention facilities range from vegetated landscapes to detention ponds. Ideally, areas bordering watercourses remain vegetated to protect water quality from runoff as well to decrease erosion along banks.

Historically, municipalities have not explicitly planned for storm water management. Areas with insufficient water detention/treatment facilities can thus arise, creating problems for land holders and government. Targeting under-serviced areas permits municipalities to avoid liability due to flooding while enhancing water quality protection.

Determining where the highest impermeable surface area is allows municipalities to designate locations of potential concern. Often such areas correspond to intensive built-up land uses, such as commercial and industrial zones. The latter zones pose particular concerns for water management due to extra chemical contamination that may be present in the soil and waste water.

The "Bowl" area of Prince George was studied to determine the extent of impermeable surfaces. Types of land use and hydrographic information were overlaid upon these surfaces to permit analysis of convergence.

Data Source

Impermeable surface area data in this project was developed from a 2002 IKONOS satellite image, georeferenced to the Prince George rivers.shp shapefile. The latter shape file was projected as NAD 1983 Zone 10 UTM.

The floodplain.shp, heavy_industry.shp, light_industry.shp and commercial.shp shapefiles were digitised using a screen capture taken from the City of Prince George PGMap site. The screen capture consisted of the flood plain extent and land use designations from the Prince George Official Community Plan, as adopted in 2001. These shape files were also georeferenced to the Prince George rivers.shp file.

Data Manipulation

The "Bowl" area of Prince George was selected as the study location. This location was delimited by the Nechako and Fraser Rivers, and by the base of Cranbrook Hill.

Bands in the Ikonos satellite image were set to display vegetation in infrared, with vegetation appearing red. The image was then saved as a .tif for editing.
Adobe Illustrator was used to select all red pixels using tolerance levels of 25-30 with the Magic Wand tool. These selections were copied to a new layer, which was then saved as a new .tif file.
The new, isolated vegetation coverage image was then imported into ArcMap and georeferenced to the Prince George rivers.shp file. What were transparent areas in Adobe Illustrator now appear black.
To enable raster analysis, multiple image bands needed to be collapsed into one. Spatial Analyst was used to reclassify the data. Results were saved as an ESRI Grid File.
After reclassification, the lowest cohort option represented impermeable surface area on land. Raster Calculator was used to select all values within this cohort in order to simplify layer creation.
The resulting calculation displayed impermeable surface area and areas of water with a value of '1'. The raster was then converted to polygon feature shape file.
A clip was created of the "Bowl" area, as encompassed by the Nechako and Fraser Rivers as well as the base of Cranbrook Hill. Areas corresponding to attribute data ‘1’ (impermeable surfaces) were selected and copied into a new polygon shapefile.
The areas of the clip and of the impermeable surfaces could then be calculated using the Calculate Statistics tool to determine the percent impermeable surface of the "Bowl" (formula as below).

The second process involved creating secondary layers to compare land use and hydrographic features. The floodplain.shp, heavy_industry.shp, light_industry.shp and commercial.shp shapefiles were digitised "heads up" using a screen capture taken from the City of Prince George PGMap site. The screen capture consisted of the flood plain extent and land use designations from the Prince George Official Community Plan, as adopted in 2001. The image was georeferenced to the Prince George rivers.shp polygon shape file before digitising.

Spatial Analysis Methods

The percentage of impermeable area was calculated using the formula identified in Data Manipulation.

Analysis Results

The area of the "Bowl" extent under study was 2650 ha, while the impermeable surface area was 1227 ha. Using the formula established in the Data Manipulation section:

Therefore, approximately 46% of the surface area in the Prince George "Bowl" is impermeable.

Visual analysis presented a strong correlation with particular land uses and impermeable surface area. Residential areas were generally more permeable than their commercial and industrial counterparts. The Prince George downtown has very few permeable surfaces.

The northern extent of the flood plain coverage overlies industrial and commercial impermeable surfaces. The southern extent of the flood plain coverage generally follows reserved vegetated areas. The north-western flood plain coverage is over vegetated areas as well.

Conclusions

There is a strong correlation between land use and impermeable surface area in Prince George. Commercial and industrial land uses result in more impermeable surfaces than residential areas. The Prince George 200-year flood plain covers industrial and commercial areas, which could be problematic given the extent of land imperviousness. Industrial areas affected include the rail yards, where a history of land contamination creates issues if the land is flooded for a period of time. Furthermore, non-point sources of pollution from traffic and commercial operations can also readily find their way into the nearby watercourses. Careful attention should be paid to water management dynamics in these areas in order to protect aquatic resources.

Approximately 46% of the surface area in Prince George is impermeable. Schueler (1994) defines potential stream degradation based on extent of impervious surface. He classifies stream degradation in three categories:

Given Prince George has approximately 46 % impermeable surface area, streams in the Prince George "Bowl" would be generally be classed as degraded under this scheme. These streams discharge into the Fraser and Nechako Rivers, transporting any pollution contained. Depending on stream location, the extent of degradation may be more or less pronounced due to variations in nearby impermeable surfaces.

The degree of impermeable surface area will create negative issues if climate change increases flood risk in northern British Columbia. Extended periods of inundation may overwhelm urban stormwater management systems. The lack of land for water infiltration will create flood hazards as water cannot escape through the soil profile or vegetation usage. Increased bank erosion is also possible, particularly bordering the rail yards. Hence, considering how future hydrological regimes are managed is as important for Prince George as current water management. Attention to the largest impermeable areas should include an assessment of storm water management infrastructure and any possible improvements. These areas are distinctly observable in the developed impervious surface area shape file. These locations correspond to commercial and industrial land uses which could result with increased aquatic pollution due to related activities. Hence, adequate storm water management is essential to protect aquatic resources and habitat in Prince Georg

Future Work and Project Limitations

Future work should involve superimposing sewer and related storm water management GIS data for enhanced analysis. Such data was not available for this project. Permeable areas for flood water retention could be identified and protected/enhanced in future Official Community Plans and municipal policies. Options for reducing impermeable area should be explored, or technology to lessen the impact of impermeable areas investigated. An example of new technology that can be installed in previously developed areas is the oil/grit separator. Also, determining alternatives to impermeable surfaces during new development or reducing current impermeablity is recommended.

More detailed data could give a better understanding of the degree of impervious surface. Not all surfaces that are impervious are equal in impermeability, nor are all permeable surfaces equal. Different types of vegetation and their underlying geology influence permeability. Furthermore, many urban street trees are surrounded by impermeable surfaces, so they would not serve as effective water retention areas compared to more naturalised locations.

Project limitations include a minor offset in the impermeable surface area layer and original .tif file, likely due to differences in "heads up" georeferencing. Furthermore, the Magic Wand tool used to generate the original raster coverage of impermeable surfaces may have over or under selected pixels. The proportion of vegetation to impermeable surface would thus be altered. However, this error would have minimal impact on calculations given visual inspection results in a high degree of converging overlay. Hence the impermeable surface calculation is an acceptable estimation of impermeable surfaces in Prince George.

References

Schueler, T. (1994) The Importance of Imperviousness. Watershed Protection Techniques 1(3).

GEOG 300 Term Project

Calculating Impermeable Surface Area in The Prince George "Bowl"

Abstract