Development changes the water cycle
When farmland, forest or meadow is built over, rain that once soaked into the ground and moved slowly to streams instead runs off roofs and pavement quickly. The result can be higher peak flows, more frequent flooding downstream, erosion of stream banks and more pollutants washed into waterways. Redevelopment of already paved land is less dramatic but still presents chances to improve on existing conditions.
For this reason, many cities and regions now set stormwater standards for new development. Green infrastructure is a common way to meet them.
How standards typically work
Rules vary, but common approaches include:
- Retention standards. Requiring a site to keep a certain depth of rainfall on site, through soaking in, evaporation or reuse, for example the amount from small, frequent storms.
- Peak flow limits. Requiring that the rate of runoff after development not exceed the rate before.
- Water quality treatment. Requiring that runoff from the first part of a storm, which tends to carry more pollutants, be treated.
- Credits. Reducing the required size of drainage facilities or fees when green features are used.
Green infrastructure allows developers to meet these with planted basins, permeable paving, green roofs, tree trenches and similar features, often spread across the site instead of concentrated in one large pond.
What the features do
Permeable paving lets water seep through to a stone reservoir below. Bioretention areas receive runoff and filter it through soil. Green roofs hold back some rain. Trees intercept rainfall and take up water. Together they reduce the volume that leaves the site.
Features are usually sized to capture the frequent small storms, which account for most annual runoff and carry much of the pollution. For very large storms, sites still need overflow routes and often conventional detention.
Heat
Large paved areas and dark roofs make new developments hotter. Street trees, shaded parking, light-colored surfaces, planted areas and green roofs reduce surface temperatures. Designers can plan tree canopy as part of the site layout, not as a late addition. Roots need space; setting aside enough soil volume early prevents later conflicts with utilities and paving.
Cost trade-offs
For developers, green features can sometimes replace some underground pipes and ponds, saving money, particularly on large, flat sites with permeable soils. In other cases, they add cost because of specialized materials, design effort and unfamiliar procedures. Costs vary by region, soils and local contractor experience, and early claims of savings should be tested against actual quotes.
A common concern is that extra requirements raise housing costs. The effect is typically a small part of total construction cost on a project, but it adds to a stack of other costs and can matter at the margins for affordable housing. Cities can offset this by offering fee credits, shared regional facilities or flexible compliance options.
Maintenance and responsibility
Who maintains the features after the developer leaves is a decisive question. Options include:
- A homeowners’ association or building owner under a recorded maintenance agreement.
- The city, funded by stormwater fees or special assessments.
- A stormwater utility that takes over features in public rights-of-way.
Inspection programs matter. Studies and local experience repeatedly show that unmaintained features lose function, and cities that do not inspect find many failing. Planning simple designs that require less frequent work reduces the burden. A system that looks good in the plan but is unrealistic to maintain is a poor investment.
Fitting with other needs
Land is limited in development, and green infrastructure competes with parking, buildings and utilities. Multi-purpose designs, such as a plaza that doubles as a shallow basin in storms or a park with a flood storage area, help. Features near buildings need protective design to keep water from foundations. Soil conditions, groundwater levels and contamination can all restrict infiltration, and some sites need lined features or alternative methods.
Why it matters
Handled well, green infrastructure in new development reduces downstream impacts, supports healthier waterways, cools streets and makes places more pleasant, often at comparable cost to conventional methods. Handled poorly, it creates maintenance headaches. The difference is usually in the planning of soils, responsibilities and funding, not in the features themselves. Results vary with climate and site, so local performance data are worth seeking.