Abstract This paper examines the relationship between urban form and impervious surface. Smart growth development (compact, mixed-use, pedestrian friendly,

Abstract This paper examines the relationship between urban form and impervious

surface. Smart growth development (compact, mixed-use, pedestrian friendly, etc) has been held up as a solution to the negative consequences of urban sprawl. For example, smart growth has been hailed as a solution to traffic congestions, the loss of open space, the consumption of open space and other environmental impacts attributed to sprawl. This analysis explores the relationship of sprawl/smart growth to one very widely used indicator of water quality, impervious surface. The study first grades development in Gloucester County, NJ on a smart growth/sprawl scale utilizing a housing-unit density (Hasse 2004). The analysis evaluates impervious surface at a sub-watershed level as derived from NJ DEP land use/land cover data. A correlation evaluation is then made between sprawl/smart growth and gross as well as percentage amounts of impervious surface. The results indicate that while sprawl development actually has less intense impervious surface coverage per acre due to its dispersed nature, the total impervious surface contributed by sprawl is substantially higher than contributed by smart growth when calculated on a per-capita basis. The study concludes that smart growth is locally more impacting to water quality at the site-level due to its compact nature but overall less impacting on the regional-level due to its smaller total footprint compared to sprawl patterns of development.

Sprawl has been claimed to impose greater impacts to land resources (Hasse and Lathrop, 2003) than compact development

Urbanization results in the creation of impervious surfaceParking lots, sidewalks, buildings, driveways, etc.

Impervious Surface Coverage and Water Quality

Arnold, Chester L Jr and Gibbons, C James

Research Questions

1. Does Sprawl create more or less intense impervious surface than high-density growth?

2. At what scale?

3. Does sprawl create more or less total impervious surface per capita?

4. At what scale?

Hypotheses

1) Sprawl creates a lower total amount of impervious surface within a sub-watershed

(Sprawl is good for water quality at the sub-watershed level)

2) Sprawl creates a greater total amount of impervious surface per capita

(Sprawl creates a greater overall impervious surface footprint)

Methods – (1)

• Create a sprawl index related to density at housing unit level– Identify housing units as point layer– Assign the average housing population by

census tract– Created Sprawl Index

• Create Density Surface in Spatial Analyst (660 ft radius)

• Assign the density value back to the housing points

Housing Units Graded for Sprawl (Density)

• Sprawl index represents density of population within ¼ mile of any given housing unit

Methods – (1 continued)

• Evaluate correlation of sprawl index with percent impervious surface per watershed

• Summarize the amount of Impervious Surface per watershed at different scales– HUC 11– HUC 14– Sub Watershed

Gloucester County Impervious Surface

Results - Basin

Percent Impervious Versus DensityHUC11 (Basin)

R2 = 0.7004

0.00

0.05

0.10

0.15

0.20

0.25

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Impervious Surface Per Person Versus DensityHUC 11 (Basin)

R2 = 0.0528

0.00

0.05

0.10

0.15

0.20

0.25

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Results - Watershed

Percent Impervious Versus DensityHUC 14 (Watershed)

R2 = 0.6716

0.00

0.05

0.10

0.15

0.20

0.25

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00

Impervious Acres Per Person Versus DensityHUC 14 (Watershed)

R2 = 0.3518

0.00

0.05

0.10

0.15

0.20

0.25

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

Results – Sub-watershed

Impervious Surface Per Person Versus DensitySub-Watershed

R2 = 0.4398

0.00

0.05

0.10

0.15

0.20

0.25

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

Percent Impervious Surface Versus DensitySub-Watershed

R2 = 0.5852

0.00

0.05

0.10

0.15

0.20

0.25

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0

Discussion

• Sprawl does create a lower total amount of impervious surface within a sub-watershed as compared to non-sprawling development– Relationship is strongest at the basin level (largest

scale)– Many of the watersheds high sprawl values (i.e. low

density) will receive more future development and can expect to increase I.S.

Discussion – cont.

• Sprawl creates more total impervious surface per person housed– Relationship is strongest at the sub-watershed level– Many of the watersheds have impervious surface not

related to residential development (commercial and industrial).

Conclusion

• Sprawl does have a relationship to impervious surface although it is a complex one.

• Scale is an important factor

• More research needed to determine the most appropriate scale of analysis

• More sophisticated measure of sprawl would make a stronger conclusion