Green Stormwater Infrastructure

In a natural environment, rain is absorbed and filtered by plants and soils. But in densely populated urban areas like Palo Alto, impervious surfaces (that make up our buildings, streets, and parking lots) disrupt this natural water cycle.

Stormwater collects pollutants when it washes over impervious surfaces, which then flows into our local creeks and the San Francisco Bay, causing negative impacts on water quality and wildlife. This stormwater may also cause flooding and erosion, harming properties and wildlife habitat.

Green Stormwater Infrastructure (GSI) is infrastructure built into our urban environment to collect, slow, and clean stormwater runoff through the use of natural processes.

Green Stormwater Infrastructure Benefits

  • Improves water and air quality
  • Aids with climate change resilience and mitigation
  • Reduces ponding in our neighborhoods and commercial areas
  • Supports wildlife habitat
  • Enhances multi-modal travel and safety
  • Saves money
  • Increases property values

The Stages of Development

This slideshow shows the stages of development and the impacts to our water cycle.

Pre-Urban Development

Pre-Urban Development; in natural environment rain is absorbed and filtered by plants/soil leaving little surface flow.

Urban Development

Urban Development with impervious surfaces disrupt the natural water cycle. These surfaces increase surface flow to our drainage system.

Balanced Development With GSI

Balanced Development with GSI help collect, slow, and clean stormwater runoff and rebalance the natural water cycle by allowing water to evaporate and absorb into the ground

Pre-Urban Development: In a natural environment, rain is absorbed and filtered by plants and soils leaving very little surface flow while the majority of the water evaporated into the air or soaked into the ground. Image source: San Mateo Countywide Pollution Prevention Program (SMCWPPP).

Urban Development: In densely populated urban areas like Palo Alto, impervious surfaces (that make up our buildings, streets, and parking lots) disrupt this natural water cycle. These surfaces great increase surface flow to our drainage system. Image source: SMCWPPP.

Balanced Development With GSI: Implementing GSI in Palo Alto  will help to collect, slow, and clean stormwater runoff and rebalance the natural water cycle by allowing water to evaporate and absorb into the ground. Image source: SMCWPPP.

Types of GSI

These slideshows describe the six main types of GSI: 

Bioretention Areas

Bioretention areas filter runoff collected from hardscapes through drought-tolerant plants and well-draining soils.

Biorention area with drought tolerant plants.

Bioretention Areas

They can also provide traffic-calming features and pedestrian safety measures when placed by bike lanes or at intersections.

Biorention area by a bicycle lane and crosswalk.

Bioretention Areas

Examples of these areas can be found in the form of rain gardens, stormwater planters, and bioretention curb extensions.

Biorention area example in the form of rain gardens, stormwater planters, and bioretention curb extensions.

Pervious Pavement

Pervious pavement is installed in parking stalls, walkways, and paved areas throughout the City.

Pervious pavement on crosswalk


Rainwater Harvesting

Rainwater harvesting is the process of collecting rainwater from impervious surfaces (like a roof) and storing it for later use.

Rainwater harvesting via rain barrel

Rainwater Harvesting

Storage facilities that can be used to harvest rainwater include rain barrels and above-ground or below-ground cisterns.

Rainwater harvesting via above-ground cistern.

How Can Residents and Businesses Implement GSI on Their Properties?

  • Build a rain garden to divert rainwater from a roof into a garden and reduce ponding while beautifying property and providing habitat for birds, butterflies, and other animals.
  • Install pervious pavement (such as porous asphalt and permeable interlocking concrete pavers) that allows rainfall to infiltrate into the soil below and reduces runoff and ponding.
  • Install a rain barrel or cistern to help capture and reuse rainwater for your irrigation needs and lower your demand for potable water—especially during the dry summer months.

Learn about the rebates that the City of Palo Alto provides for rain gardens, pervious pavement, rain barrels, and cisterns!

How Is Palo Alto Implementing GSI Citywide?

  • City of Palo Alto GSI Plan: As part of regional permit requirements, the City of Palo Alto developed a GSI Plan(PDF, 23MB). This Plan focuses on GSI as an integral part of project design and describes how the City will gradually integrate GSI features.
  • Local Partnerships: The City partners with local community-based organizations such as Grassroot Ecology to help maintain current GSI projects, as well as Valley Water to help implement and leverage funds available for the City’s stormwater rebates.
  • Pilot Projects: The City and Grassroots Ecology are updating bioretention areas in the Southgate neighborhood to improve their performance. As part of this pilot project, improvements include adding compost to the soil, replacing rocks with mulch, and exchanging current plants with native and drought-tolerant species.
  • Tracking Tools: City staff are developing an online database to improve GSI inspections, track both public and private GSI measures, and capture the increase of GSI throughout the City over time.
  • Resources: The City is creating documents and guidelines to standardize design and maintenance of GSI measures throughout the City. These standard will be piloted on City property before being used to direct GSI design on private property.
  • Identifying Additional Funding Resources: Palo Alto property owners approved the Stormwater Management Fee in 2017 to fund projects that improve the City’s storm drain system and reduce pollution to our local creeks and the Bay. Staff are identifying funding sources to leverage the Fee to continue installing and maintaining GSI throughout the City.