A decades-old stormwater solution that helps recharge groundwater in Modesto is also a major contributor to yearly street flooding and a potential source of contamination.
Modesto’s stormwater system is different from most other cities of its size in California. Instead of a traditional system using pipes that flow into rivers or out into the ocean, it heavily relies on thousands of rock wells — gravel-filled holes that drain untreated rainwater directly into the ground.
“It’s basically a hole that’s drilled straight into the ground with filler rock that creates voids around the rocks and gives a space for the water to go,” said Robert Englent, the Modesto wastewater collection system superintendent.
Rock wells work as a source of groundwater recharge, replenishing aquifers below. But they also are easily clogged by debris like leaves and trash, leading to major street flooding during heavy storms.
“The groundwater levels in the Central Valley have been sort of sinking everywhere, but the Modesto subbasin has some of the best groundwater levels around,” Englent said. “Now we don’t know for sure that it was a direct correlation to the rock wells, but we think that that’s a big part of it.”
Rock wells have to be thoroughly cleaned of debris because the grates that cover the holes leave room for refuse to settle in the cracks between the gravel. This prevents drainage and requires costly maintenance every two to three years.
Englent lifted a grate during one of the season’s first storm events in Modesto, revealing a pipe with a screen on top that was covered in small leaves.
“When the leaves get on the top like that, that’s really what causes a lot of problems,” he said.
Rock wells are listed in Modesto’s 2050 as a concern for flooding because they can take up to 40 hours to absorb stormwater and are incapable of withstanding major storms.
Brian Schardt, who lives in the Modesto Junior College area, said nearby streets can flood within a half hour to an hour during heavy downpours. “I wear winter waterproof boots when I go walk my dog when it rains, unfortunately,” Schardt said.
Every 10 years or so, maintenance will “rejuvenate” rock wells, digging out the sediment around the well and replacing it with the hope of extending its life, but eventually the wells become entirely defunct.
Of the many thousands of rock wells throughout Modesto, 325 are “abandoned,” meaning they can no longer absorb water. Over the years, the soil around the wells become solid, like cement, and prevents water from draining.
Allison Glasscock, 28, lived for years near Downey High School. As a teenager, she remembers walking through water that would reach her lower calf. She noticed the flooding would center around a drain close to Wylie Drive and Orchid Avenue, where the sidewalk dips.
“It would spread basically across that entire intersection and go up the block to the corner it was connected to,” Glasscock said. “Depending on the length of the storm, it could take a few days to a few weeks to fully drain out.”
She said it’s easy for parents and students of nearby Rose Elementary to misjudge how deep the water is, and the trek can be daunting for people using wheelchairs. “It wouldn’t look like it would be that deep of a puddle, but it’s very wide,” Glasscock said.
Some of the drains can be unclogged by what is called a “rake-off,” where city service workers literally use rakes to clear debris.
Andrew Wang, a 30-year-old electrical engineer, used that tactic to unclog a storm drain in Patterson on K and J streets along Route 33. “It was starting to become bad,” Wang said. “Using my rake, I started seeing a whirlpool, and that indicated to me it was getting unblocked.”
Other blockages require more drastic interventions and can take a lot longer to clear.
On the morning of Oct. 14, city workers responded to a call at Prescott Road near Sheldon Drive where water intruded into both lanes of traffic. The large trucks that suck up excess water can hold up to 2,500 gallons, but were expected to make two trips to remove pooling along the northbound lanes.
How did we get here?
The city of Modesto has over 9,000 rock wells installed as a more cost-effective answer to addressing the growing city’s storm infrastructure more than 50 years ago.
“The cities of Modesto, Turlock and Merced have very sandy soils, so the water actually just goes into the ground pretty readily, pretty easily,” Englent said. “So the thought was back in the ‘50s and ‘60s, ‘Why build all these pipes to get rid of the water when if it just goes into the ground it goes away pretty fast?’”
Since then, flooding has become more frequent, overwhelming the stopgap solution decided on by earlier city planners.
Because rock wells have direct access to underground aquifers, they are also a potential source for contamination of groundwater.
Untreated stormwater can carry harmful substances including gasoline and chemical runoff. As a result, the city created rules around and to be careful when cleaning or working on cars or using pesticides.
When gasoline does float to the top of rock wells during blockage, crews are discouraged from sucking up the water into their trucks because of the potential for explosion.
The city is required to monitor rock wells and conduct public education and outreach about the threat of surface contaminants getting into groundwater through the wells.
Stanislaus County also uses rock wells and of them as a stormwater solution, but it’s no longer the preferred option. The county now advocates for a better, more resilient solution: dry wells.
“The dry wells just handle solids so much better,” Englent said.
With dry wells, before stormwater reaches the drainage point, there is an intermediary basin that will catch heavy solids at the bottom and leaves and other light debris at the top. The water in the middle is then siphoned off and drained into the ground.
Each dry well can handle between two and four times the amount of water as one rock well and has the added benefit of a chamber to catch debris before it affects drainage. This does, however, mean that they require annual maintenance to clear them.
Pipe system plan abandoned over installation cost
The city once had a plan to build out a whole system of pipes that would drain into a catch basin or the river, but abandoned the idea once it became clear it would never be able to afford the cost of installing the system.
Instead, around 13 years ago, Modesto began the long process of replacing its aging rock wells with dry wells in areas that routinely flood. So far, it’s installed 91.
Marcella Avenue used to take two crews up to eight hours to address flooding, but now, with a new system in place, crew members can use that time to help unclog other areas.
“What you’re looking at here with this 91 is our attempt to address the worst flooding spots in the city, and once we deal with the worst, then we go to the next worst, and then the next worst and the next worst,” Englent said.
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