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Green infrastructure in urban streets complements traditional piped water drainage systems. Vegetation, soils, and natural processes capture and infiltrate or evaporate water before it enters the piped system. Green infrastructure can help reduce flooding and water pollution by absorbing and filtering stormwater. It simultaneously provides a natural relief to the built environment, improves the street aesthetic, and delivers
benefits to the community. Green infrastructure must be carefully coordinated to avoid conflicts with utility placement, high water table levels, and subterranean conditions such as the location of bedrock. Considering the soil conditions is critical when planning green infrastructure strategies. While the components and processes involved in green infrastructure are vast, some of the major components are listed below.
Swales carry water like pipes and are designed as shallow, open, planted channels to convey runoff and remove pollutants. They are an alternative to a piped drainage system where space and grade is available. Water moves horizontally along the surface or in subsurface layers. Swales slow water flow and trap sediments to improve the water quality.
Rain gardens have a special soil filter media that can remove pollutants from road runoff. Configure plant and soil filtration systems as garden beds or street tree pits such that they are designed to treat stormwater runoff. Rain gardens are also called bioretention systems, flat bioswales, flow-through planters, or pervious strips. Some are designed to allow water to infiltrate underlying soils while others are designed to collect the treated water and convey the clean water downstream.
Permeable paving allows rainfall to move through the pavement to the soil beneath and provide water to landscape areas nearby. Alternate surfaces with permeable pavement to reduce stormwater runoff and recharge the water table. These may be in the form of block pavers with infiltration gaps between pavers, or porous material with infiltration gaps within the material.
Street Trees and Planting
Trees provide shade and cool the air, benefitting the comfort of people using the street. While designing, find opportunities for plants and trees within the streetscape to reduce the amount of hard, impermeable surface. Plan and set aside adequate space for trees in the early stages of development to achieve better outcomes. Plant trees in green strips, parking bays, and rain gardens. High-quality plant stock and appropriate planting techniques are vital to the success of any planting.
Tree Pit and Soil Volume
Continuous tree pits increase the plantable surface area and provide more space for tree roots. Ensure sufficient growth area, soil volumes, and hydration for trees. Coordinate tree planting with other infrastructure elements to avoid conflict, particularly with transportation and utilities. Where space is constrained, consider employing suspended and permeable pavements, strata cells, structural soils, and passive irrigation to improve soil conditions and overall health.
Directing stormwater to the surface of landscaped areas and tree pits provides irrigation to the plants and reduces stormwater runoff into local drains and citywide systems. Passive irrigation is one of the simpler and easier ways to incorporate water-sensitive design.
Adapted by Global Street Design Guide published by Island Press.