Siting and layout of development

The site layout is determined in part by the opportunities and constraints of the natural heritage system. The location and configuration of elements, such as streets, sidewalks, driveways, and buildings, within the framework of the natural heritage system provides many opportunities to reduce stormwater runoff. The goals of the site layout are to provide a functional and livable urban form while minimizing environmental impact. The techniques below highlight some of the ways in which site layouts can minimize their hydrologic impacts and preserve natural drainage patterns.

Fit the design to the terrain

Using the terrain and natural drainage as a design element is an integral part to creating a hydrologically functional landscape (Prince George’s County, 1999). Fitting development to the terrain will reduce the amount of clearing and grading required and the extent of necessary underground drainage infrastructure. This helps to preserve predevelopment drainage boundaries which helps to maintain distribution of flows. Generally, siting development in upland areas will take advantage of lowland areas for conveyance, storage, and treatment (Figure 3.2.2).

Open space and clustered development

Clustering development increases the development density in less sensitive areas of the site while leaving the rest of the site as protected community open space. The open space can be undisturbed natural area or actively used recreational space. Features that often characterize open space or clustered development are smaller lots, higher density of structures in one area of a site, shared driveways, and shared parking. From a stormwater perspective, clustered development reduces the amount of impervious surface, reduces pressure on buffer areas, reduces the construction footprint, and provides more area and options for stormwater controls including LID practices (CWP, 1998).

Street network designs

Certain roadway network designs create less impervious area than others. Figure 3.2.3 from the Canadian Mortgage and Housing Corporation (2002) demonstrates that loop and cul-de-sac street patterns require less area for streets. These layouts by themselves may not achieve the many goals of urban design. However, used in a hybrid form together or with other street patterns, they can meet multiple urban design objectives and reduce the necessary street area (CMHC, 2002). A study comparing different road network designs for a hypothetical community showed a fused grid pattern can reduce impervious cover by 4.3% compared to a traditional neighbourhood design (CMHC, 2007).

Reduce roadway setbacks and lot frontages

The lengths of setbacks and frontages are a determinant for the area of pavement, street, driveways, and walkways, needed to service a development. Municipal zoning regulations for setbacks and frontages have been found to be a significant influence on the production of stormwater runoff. A study of residential parcels in Madison, Wisconsin found that reducing setbacks by 3 m and frontages by 5.5 m resulted in a 14% reduction of stormwater runoff (Stone and Bullen, 2006).

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Movement of water from one location to another.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A hybrid neighbourhood and district layout model that combines the geometries of inner city grid road patterns with the loop and cul-de-sac road patterns of conventional suburbs.