What could be here
By “” we mean ground that soaks up rain instead of sending it all down a pipe: , planted strips, paving that water can pass through. What is a sponge? has the long version. This page imagines six spots in Marin City (the Gateway parking lot, the pond, MLK Academy, Golden Gate Village, Graham Park, and the Drake and Donahue corner) with more of their ground soaking up rain. None of it is proposed or funded; the real projects are on Proposals. To be upfront: the county has tested ideas like these here and found they do little for flooding, so each card says what it would do, and what it would not.
01 · The finding
Low Impact Development is the engineering term for most sponge measures: rain gardens, , planted swales. The Marin City Stormwater Plan modeled a set of it on Pacheco Street, and reported plainly:
“The results of the modeling effort showed little flood control benefit of the LID facilities. The LID facilities did not reduce peak flow rates and did minimal volume control during the 2-year storm event. These benefits were even less when the LID was run for the 10-year storm event.”
Worse in bigger storms, which is when it matters. That is not a small caveat, and we will not bury it. It is also what this site says elsewhere, from a different direction: infiltration does not open a gate, and a bowl below the tide line stays a bowl below the tide line.
But the plan's next sentence is not a dismissal: “LID is recommended as a complimentary feature to proposed improvement concepts to help treat water quality while also addressing flood control.” Complementary, meaning alongside the flood fix, not in place of it.
02 · What is actually coming
Any vision that skips past this is a daydream. The county has modeled and ranked these four projects, in this order.
01
First — the pump station and floodwall; assumed by everything below
A 50 cfs pump station at the pond and a permanent floodwall replacing the temporary muscle wall. [TODO: project team to confirm whether the plan's first project also includes an upsized pipe through the shopping center.]
Lets the pond drain against a high tide instead of waiting for the tide to drop. This is the lever the model calls the pump — the same capacity, for the same reason.
02
Second — funded; construction reported for 2027 (see Proposals)
Gives rainwater from the Phillips Drive hillside its own pipe to the bay, through a new culvert under Highway 101 north of the existing one.
The hillside drain stops sharing the pond's culvert, so the pond can empty faster. The pond keeps its one gate.
03
Third — ahead of the long-term bypass
Upsizes the pipe under Drake Avenue and increases inlet capacity. [TODO: project team to confirm the pipe length, sizes and inlet locations against the plan.]
Runoff currently cannot get into the Drake pipe fast enough, so it runs down the street and ponds at Drake and Donahue. This is about getting water underground at all.
04
Last — longer term, no date published
A high-flow bypass at Drake and Donahue routed to a new bay outfall, possibly combined with detention and pumping.
The one aimed at sea level rise and heavier rain rather than at today's storms.
If all four are built, the plan expects people to be able to get in and out of Marin City during a 10-year storm at mid-century climate, with ponding at the intersections under six inches: shallow enough to drive. That is the goal, stated by the people who will do the work, and nothing else on this page changes it.
03 · The picture
The same section as the mechanism page, with the county's drainage work below the line and sponge measures on the surface above it.
04 · Six places
Each of these says what it would not do as plainly as what it would. Most of this ground belongs to somebody, and the last line of every card says who.
Now A paved lot of several acres between Drake and Donahue and the pond, draining straight to the pond.
Could be Permeable paving in the outer bays, planted infiltration strips along the aisles, and trees in the islands.
Worth Several acres of pavement under one owner. A real gain for water quality, shade and cooler ground. Modest flood benefit, and only in small storms.
Privately owned. Nothing here happens without the owner.
Now A three-acre tidal marsh whose bottom has silted up by about two feet since it was built, holding roughly 14 acre-feet (about 4.5 million gallons) before it spills.
Could be Dredge back toward the original invert, then plant terraced wetland edges that clean water on its way through.
Worth The one intervention that would plausibly change flooding, because it restores lost storage. It is also the one that most needs an engineer.
Privately owned, per the county's 2025 stormwater plan. Coordination, not a proposal.
Now Roofs and paved ground on the slope above the flat.
Could be Green roof on part of the building, rain gardens at the downspouts, and signage explaining where the water goes.
Worth Small hydrologically. Large educationally — this is the site where drainage can be taught rather than buried.
School district and the Community Services District.
Now A leveled bench with paved walkways between the buildings.
Could be Permeable walkways, planted swales along the paths, and shade trees.
Worth Comfort, shade and water quality more than flood control. It is uphill of the problem, not in it.
Residents first. This is housing, and the people who live there decide what happens to where they live.
Now A public park where the hill meets the flat.
Could be A shallow detention basin that is a lawn most of the year and a pond for a day after a storm.
Worth Catches hillside runoff before it reaches the flat. Floodable public space is the technique Shenzhen Bay Park demonstrates at scale.
Public land, and a park people already use — so the bar is that it stays a good park.
Now The low corner, by the Gateway at the way into town. The county's plan puts a bigger pipe and more inlets here; it is where the water stands.
Could be Planted curb strips and tree wells between the new inlets, sized to take the first rush of dirty water off the street before it reaches them.
Worth Would not change whether the street floods — the pipe does that. Would change what the street is like the rest of the year.
County right-of-way, alongside work in the county's plan.
05 · The case that survives
Not the flood. The county's plan takes that on with pipes, a pump and an outfall. Anyone arguing that rain gardens alone will keep Donahue Street open will lose to a hydraulic model.
What is left is not small:
06 · Downstream
That last point deserves more than a bullet. The county said sponge measures do little for flooding here and recommended them anyway, “to help treat water quality”. On a page about flooding that reads like a consolation prize. It is not, and what makes it real is on the other side of the embankment.
Richardson Bay holds one of the last healthy beds in San Francisco Bay — by reported area the second largest, and the most genetically diverse.
Eelgrass is a flowering plant, not a seaweed. It is nursery ground for juvenile leopard sharks, bat rays and silversides; Pacific herring spawn on its blades; and the birds of the Pacific Flyway feed over it on the way down the coast. It also does what this site keeps asking of Marin City's own ground: it slows water, holds sediment in place, and clears what passes through.
Eelgrass needs light, and turbidity is among the most significant drivers of seagrass decline worldwide. Sediment in the water column is what takes the light away.
Marin City's carries a lot of it. The People's Plan describes runoff that “carries high sediment or mud which heavily impacts local businesses and homeowners”, and traces it to erosion gullies in the hills above the neighborhood. The county's own plan records the pond silting up by about two feet since it was built. What the pond does not keep goes out through the culvert into the bay.
The seasons line up too. The Richardson Bay Audubon Sanctuary closes from October 1 to March 31 for herring spawning and the migration, the same months the storms come. The bay is at its most sensitive just when the bowl is emptying into it.
Nobody has measured this chain, and neither have we. That hillside erosion produces sediment is documented. That turbidity harms eelgrass is documented. That Marin City's outflow measurably affects Richardson Bay's eelgrass is not. Nobody has sampled what leaves the culvert, so we will not claim it. It is a plausible chain with a measurable middle, and measuring it would be real work for somebody. [TODO: turbidity and sediment load at the culvert outlet, through a storm, against the bay outside it.]
It does not make Marin City the polluter. The sediment starts on hillsides, much of which is Golden Gate National Recreation Area rather than anybody's back yard. The People's Plan made that point in 2018, proposing erosion work above Golden Gate Village as a place to begin co-managing the with the GGNRA. A community that floods should not also be handed the bill for what washes through it.
And it does not make the bay the reason to do this. Marin City is the reason. Still, the water-quality case has a named beneficiary and a funded restoration next door (fifteen acres over four years under a $2.8 million EPA grant), so there are funders who care about it. Measures that do little for a 10-year storm can still be fundable on other grounds.
Sources for this section. Everything above is somebody else's reporting or research.