Stormwater Urbanism Site Design Strategy #1 – Not All Impervious Area is Created Equal

Strategy #1 – Not all impervious area is created equal: does the project’s scope, layout, and stormwater management strategy support compact development to reduce overall impervious area in the watershed?

Not all impervious area is the sworn enemy of watershed health. Compact development and accompanying preservation of contiguous natural resources can dramatically reduce total impervious area within the watershed when compared to conventional lower-density patterns (Protecting Water Resources with Higher Density Development, US EPA, 2006). In other words, for a given buildout, compact development typically requires far less total impervious area per capita – less streets, less parking surfaces, less building roof area – than the same buildout using typical Conventional Suburban Development (CSD) patterns. This leads to a decrease in stormwater runoff volume and pollutant loadings, not to mention infrastructure cost savings (Smart Growth & CSD: An Infrastructure Case Study, Morris Beacon Design, 2010).

per capita watershed impact decreases with density (chart: MBD)

To support implementation and preservation of compact neighborhoods, designers must understand the connection between their site design strategy and the rural-urban transect in the neighborhood they are working. Projects located within compact centers (the intensity of a center’s urbanism may vary – for example: urban core vs. a rural crossroads) should not be subject to extreme landscaping, open space, or stormwater BMP requirements, which may water down what should be small doses of higher-intensity development. Neither should they feature area-hogging structural BMPs. Urbanism must be allowed to be urban – this is where the real per capita watershed improvement is seen.

the rural-urban transect (credit: DPZ)

On the other hand, projects located towards a neighborhood edge or projects that don’t fit within a New Urbanist/Smart Growth land use framework will appropriately benefit from LID planning and design principles and strategic open space preservation to reduce their watershed impact. In fact, sometimes lower-intensity development should be asked to work harder, accommodating neighborhood or district scale flood control BMPs and increased performance standard requirements that aren’t appropriate or technically feasible within compact development centers.

Note: this is not to say that compact development patterns alone should carry the stormwater mitigation workload. Green structural BMPs should still be an integral part of any site design; however, BMP selection must be appropriate to urban context to support compact development and character of place – more on this topic in a future post.

Form-based codes that incentivize compact development and provide guidance for stormwater management design appropriate to land use context present an opportunity for communities to reduce their watershed impact AND encourage creation of livable, lovable places. Form based codes establish a guide for the location of centers, allocation of transect zones (the balance of urban/general/sub-urban/edge/rural/natural), and the scale and feel of each zone.

MBD is in the midst of developing a stormwater ordinance for Bluffton, South Carolina as part of a form-based code led by 180 Degrees Design Studio, where our strategy is to establish stormwater incentives for compact development (for example, reduced volume mitigation requirements for projects located within T-4, T-5, and T-6 zones) and match watershed impact to that of lower density LID buildout alternatives. However in the absence of a watershed-based zoning code and calibrated stormwater ordinance, it is the responsibility of the civil engineer to design to context.

Coming soon Strategy #2 – Plan with the land.