Sustainable stormwater management aims to reconnect the natural water cycle. Green infrastructure intercepts stormwater runoff at its source and does one or more of the following, depending on project goals:
DETENTION
Collect and hold runoff in temporary storage facilities or vegetated systems before slowly releasing the water into the downstream system.
RETENTION
Capture and hold stormwater on-site to reduce runoff to sewer systems. Water is then evaporated, transpired, or infiltrated through the soil.
(BIO)FILTRATION
Remove particulate matter and other pollutants by filtering stormwater runoff through porous media such as sand, soil, or other filter.
INFILTRATION
Absorb stormwater through the ground surface and into the soil.
Green infrastructure systems are sensitive to regional weather, climate, and ecology. These overarching conditions inform stormwater management needs and goals at the programmatic and project levels. Designers must consider the regional climate conditions, today and in the future.
At a most basic level, it is important to consider the amount of precipitation that falls, and the rate at which water evaporates from the surface or infiltrates back into the ground. Local weather and climate determine precipitation patterns and temperature, and soil composition affects how quickly water is absorbed into the ground. These factors determine regional sizing and modeling coefficients for runoff and infiltration.
Evapotranspiration Rate
Evapotranspiration rate, or the amount of water that can be evaporated or transpired from a surface over a specified duration, varies by latitude, topography, altitude, wind conditions, and time of year, as well as surface characteristics such as permeability, albedo, and types of vegetation.
In some climates, evapotranspiration varies widely through the seasons; green infrastructure should be designed with consideration for high and low expected conditions. Irrigation may be necessary during the very dry season, while fast infiltration is desirable during the very rainy seasons. In cold climates, snow storage is critical during winter.
Infiltration Rate
Green infrastructure elements and systems may be designed to infiltrate a specified amount of stormwater; for example, a system goal may be to infiltrate the first inch of precipitation of each storm event in a tributary area, or to match pre-development runoff conditions.
Native underlying soils affect the infiltration rate. Regions with naturally fast-draining soils, especially gravel or sandy soils, may have dramatic infiltration capacity, but require accounting for water seepage around built structures. Potential effects on local drinking water supplies may also need to be considered in areas with high infiltration capacity. In areas with shallow bedrock or clay, infiltration rates may be naturally low. A high groundwater table may also affect a city’s ability to implement green stormwater infrastructure.
Plant Health
Plants and trees absorb stormwater runoff and stabilize the soil against erosion. GSI projects should use native, non-invasive, drought-tolerant vegetation. Select plants that can tolerate the inundation of stormwater runoff but also be adaptable to local climate conditions such as the dry season or severe cold conditions. In snowy climates, select plants that can tolerate salt.
Regional Climates
Low Temperature / Low Precipitation
Julianne Sabula
Recharging groundwater is necessary all year; snow pack often feeds groundwater and surface waterways through the rest of the year.
Snow clearance and storage are considerations, as is the space for operating snow plows and removal equipment.
Frequent freeze-thaw cycles make materials selection an important considerationâconcrete, pavement, and markings may wear or fade more quickly.
Low Temperature / High Precipitation
Eric Wheeler
In some regions, green infrastructure must be designed to withstand heavy snowfall. Consider winter conditions when selecting construction materials and plantings.
Snow and ice clearance is a major factor; consider the space required for operating snow plows or snow removal equipment, as well as the space required to store snow while safely maintaining mobility.
In cities with cold temperatures and frequent ice events, consider how the use of road salts to de-ice roadways will affect water quality and plant health in stormwater infrastructure.
High Temperature / Low Precipitation
TUSCON, AZ
Cities in the North American West and Southwest are
arid throughout the year, and may be fighting persistent drought conditions. Water conservation and reuse is a major consideration. Any stormwater that can be captured and returned to groundwater must be to maintain healthy vegetation.
Plants that can tolerate very little water are incorporated into street vegetation for their ability to withstand dry seasons. However, vegetation should be chosen to withstand flooding conditions, too.
High Temperature / High Precipitation
Infrogmation, Flickr
Cities located along coastlines or riverfronts, especially in the North American South and East Coast, typically have very rainy seasons where flooding risk increases.
These cities may rely upon surrounding wetland or deltas to attenuate flooding from nearby water bodies.
Green infrastructure systems are typically focused on reducing runoff flow and subsequent erosion and contamination that negatively impact fish and wildlife habitat.