Is Denser Greener? an Evaluation of Higher Density Development as an Urban Stormwater-quality Best Management Practice

A simple spreadsheet model was used to evaluate potential water quality benefits of high-density development. The question was whether the reduced land consumed by higher density development (vs. standard suburban developments) would offset the worse water quality generated by a greater amount of impervious surface in the smaller area. Total runoff volume and per acre loadings of total phosphorous, total nitrogen, and total suspended solids increased with density as expected, but per capita loadings and runoff decreased markedly with density. For a constant or given population, then, higher density can result in dramatically lower total loadings than more diffuse suburban densities. The model showed that a simple doubling of standard suburban densities [to 8 dwelling units per acre (DUA) from about 3 to 5 DUA] in most cases could do more to reduce contaminant loadings associated with urban growth than many traditional stormwater best management practices (BMPs), and that higher densities such as those associated with transit-oriented development could outperform almost all traditional BMPs, in terms of reduced loadings per a constant population. Because higher density is associated with vibrant urban life, building a better city may be the best BMP to mitigate the water quality damage that will accompany the massive urban growth expected for the next several decades. (KEY TERMS: urban areas; watersheds; urbanization; water policy; modeling; best management practices; stormwater management; watershed management.) Jacob, John S. and Ricardo Lopez, 2009. Is Denser Greener? An Evaluation of Higher Density Development as an Urban Stormwater-Quality Best Management Practice. Journal of the American Water Resources Association (JAWRA) 45(3):687-701. DOI: 10.1111/j.1752-1688.2009.00316.x A substantial increase in the United States (U.S.) population, mainly in urban areas, is forecast for the next 30 years. Urbanization is already a major contributor of the runoff pollution that degrades many of our waterways. This new urban growth will very likely contribute to further degradation, but because more than half of the urban built environment for 2030 has yet to be built (Nelson, 2006), we have an opportunity to substantially mitigate the effects of the new construction. Most prescriptions for mitigating contaminated urban runoff involve practices that attempt to restore, to some degree, the predevelopment hydrology, usually through some form of infiltration and ⁄or detention (Brabec et al., 2002; USEPA, 2002; Clar et al., 2004). Relatively low density development, Paper No. JAWRA-08-0105-P of the Journal of the American Water Resources Association (JAWRA). Received June 4, 2008; accepted November 11, 2008. a 2009 American Water Resources Association. Discussions are open until six months from print publication. Respectively, Director of the Texas Coastal Watershed Program (Jacob), Department of Recreation, Parks, and Tourism Science, Texas A&M University, College Station, Texas [Jacob also holds a joint appointment with the Texas A&M Sea Grant Program and with the Texas AgriLife Extension Service]; and Faculty – Geospatial Modeler and Instructor (Lopez), Department of Medicine, Environmental Health Section – EHS, Chronic Disease Prevention and Control Research Center, Baylor College of Medicine, Houston, Texas (E-Mail ⁄ Jacob: jjacob@