Green Stormwater Infrastructure: Taming the Rapids that Storm our Local Waterways
On October 24th, 2021 I woke up to rain drumming against the roof of my house and saturating the soil in my garden. I ran outside, not only to get soaked by the first storm of the season, but also to check on the creek that runs through the bottom of my yard.
Easton Creek, which flows from the Burlingame Hills out to the San Francisco Bay, dried up at the end of the summer like a lot of California streams. On October 24th, however, there was a deluge of stormwater.
Brown, sediment-filled water raced around bends of the creek like a bobsled team and took all kinds of debris with it. Downstream the water gained speed and height as cement channels narrowed the stream, which flooded and threatened neighbors’ homes and driveways.
The October atmospheric river that combined with a “Bomb Cyclone” (1) poured 4.75 inches of rain in Woodside and more than two inches in Palo Alto (2). While this inundation of water would likely cause flooding in an undeveloped watershed, our urban and suburban infrastructure increased the severity of the flood event. Unpaved, vegetation-covered soil soaks up the rain and filters the water as it slowly percolates into local waterways. On October 24th, roofs and concrete streets met the rain and prevented the ground from absorbing the water. Instead, the rain slid down impervious surfaces while picking up pollutants and trash along the way, as it flowed into storm drains. The storm drains fed that runoff directly into waterways and caused a higher, flashier flood compared to rural, non-concrete environments.
This graph indicates that in urban environments, where stormwater meets impervious cement surfaces, there are higher stream flows (cfs) following a storm event compared to rural environments with more permeable or natural surfaces. This means urban environments are more susceptible to flooding.
As I witnessed water tearing through Easton Creek, I wondered how I might be able to help mitigate future flooding in my neighborhood. I thought about my recent projects with local green stormwater infrastructure (GSI). GSI are structures in the urban environment that capture rainwater from roofs and streets to store for later irrigation use, or to filter runoff and recharge groundwater (3). Grassroots Ecology has worked with the City of Palo Alto to install and maintain several rain barrels and rain gardens as ways to help prevent flooding of neighborhoods and “slow, sink, and spread” water into the ground.
If you are looking to help mitigate flooding in your local neighborhood and/or creeks here are three GSI solutions that you can implement at home or share with your local city council:
1. RAIN GARDENS
Rain gardens utilize fast draining soil, soil microbes, and plants to capture and filter stormwater. Rain gardens can come in a variety of shapes and forms including gardens that absorb water from your gutters to areas that collect urban runoff along roads.
Plants and soil microbes can help filter out pollutants such as car oil, E. coli, and heavy metals (4) before the water enters larger bodies of water like the ocean or San Francisco Bay. Plants can also increase the local biodiversity (5), attracting wildlife like birds, butterflies, and salamanders. Rain gardens use a sand and compost mixture on top of gravel to allow water to drain quickly and prevent flooding.
Over the last few years, Grassroots Ecology has created and tended to a rain garden at Bol Park (left). Currently, we are beautifying some rain gardens in the Southgate neighborhood of Palo Alto by replacing river rocks and struggling vegetation with mulch and native plants. Shown to the right is a comparison of a rain garden before and after Grassroots Ecology restored them. Before (top) the rain garden was filled with river rocks, which hindered the vegetation and compacted the soil. Grassroots Ecology replaced the rocks with compost, soil, and native plants (below).
2. PERMEABLE PAVEMENT
Permeable pavement is a porous urban surface that can absorb, filter, and slow urban runoff (6). Examples of porous surfaces include permeable concrete, permeable asphalt, and permeable pavers. These surfaces allow water to seep into the ground, which other forms of pavement would prevent. This decreases the peak flow after storms and mitigates flood risks.
Permeable pavers are a great alternative to concrete paths in your yard, as you can infill the area between pavers with grass, moss, or other forms of vegetation
3. RAIN BARRELS
Rain barrels collect rainwater that falls onto a roof and stores it for future use. Roofs can accumulate a lot of rainwater (0.6 gal/(ft^2*in)) that flow from the gutters to storm drains. Catching and storing this water not only reduces runoff flowing to our waterways, but the water can also be used to irrigate gardens later.
At Grassroots Ecology’s office and parks around Palo Alto, we harvest rainwater in barrels and use it to water native plant landscaping. The barrels can be hooked up to a hose or a drip irrigation system. This way we save water from running off into drains as well as beautify our neighborhoods. You can find water conservation rebates here (including rain barrels) in your area, and more information about how to place rain barrels on your property here.
Looking around my neighborhood I see storm drains that dump runoff from streets and gutters directly into Easton Creek. I see concrete that surrounds the creek bed and prevents water from infiltrating into the ground. I envision a future where green stormwater infrastructure is all around my neighborhood—mitigating peak storm flows, filtering pollutants from runoff, and creating healthier ecosystems for my plant, animal, and human neighbors.
SOURCES:
Cappucci, Matthew, et al. “An 'Extreme and Possible Historic Atmospheric River' Is Battering California.” The Washington Post, WP Company, 25 Oct. 2021, https://www.washingtonpost.com/weather/2021/10/24/california-atmospheric-river-soaked/.
Bay City News Service. “Storm Batters Midpeninsula with Wind, Heavy Rain.” Palo Alto Online, 25 Oct. 2021, https://paloaltoonline.com/news/2021/10/24/storm-soaks-the-bay-area.
EPA, Environmental Protection Agency, https://www.epa.gov/green-infrastructure/what-green-infrastructure.
Mahmoud, Ahmed, et al. “Evaluation of Field-Scale Stormwater Bioretention Structure Flow and Pollutant Load Reductions in a Semi-Arid Coastal Climate.” Ecological Engineering, Elsevier, 22 Apr. 2019, https://www.sciencedirect.com/science/article/pii/S2590290319300070.
Kazemi, Fatemeh, et al. “Streetscape Biodiversity and the Role of Bioretention Swales in an Australian Urban Environment.” Landscape and Urban Planning, Elsevier, 9 Mar. 2011, https://www.sciencedirect.com/science/article/abs/pii/S0169204611000648.
“Evaluating the Potential Benefits of Permeable Pavement on the Quantity and Quality of Stormwater Runoff Completed.” Evaluating the Potential Benefits of Permeable Pavement on the Quantity and Quality of Stormwater Runoff | U.S. Geological Survey, https://www.usgs.gov/centers/upper-midwest-water-science-center/science/evaluating-potential-benefits-permeable-pavement?qt-science_center_objects=0#qt-science_center_objects.
By Miles Brooks, Restoration Specialist
