Mill Valley & Sausalito Drainage for Canyon Lots, Hillside Streets & Bayside Flats

Three drainage problems in one corridor

Southern Marin's drainage changes block by block. A house in Cascade Canyon deals with hillside seepage and shade that never lets the soil dry out. A house in Sycamore Park, a mile downhill, sits on old tidal flat where the water table is only a few feet down. A house on the hillside streets of Sausalito has water coming from the house above it and has to send its own water safely to the house below. The right fix depends on which of these problems you have, and many properties have two.

Canyon lots: Cascade Canyon, Blithedale Canyon & Mount Tam's lower slopes

The canyons that run up toward Mount Tamalpais from downtown Mill Valley are the wettest residential ground in Marin. Homes along Cascade Drive and up through Blithedale Canyon sit on narrow benches between steep side slopes and the creek. Atmospheric-river storms soak the hillside above, and that water moves downslope through the colluvium on top of the Franciscan bedrock and clay, surfacing at the first cut it reaches: your foundation, your driveway cut, or the back of your retaining wall.

Redwood shade makes it worse. Lots that get two hours of winter sun stay saturated from November to April, so crawlspaces that would dry out between storms elsewhere hold standing water for months. The usual fix is a deep interceptor curtain drain cut across the slope uphill of the house, down to the clay or rock contact. Where the house is built right into the hill, that is paired with an interior crawlspace perimeter drain and sump pump.

Redwood duff and surface catch basins

Redwood needles, bark, and duff are the single biggest cause of failed surface drainage in these canyons. Duff mats flat over grates, packs into the sump of a catch basin, and gets into downspout tightlines through open leaders. When a basin clogs, the runoff it was meant to capture sheets across the patio toward the house. That runoff is often what floods a crawlspace.

• Larger catch basins with a deep sediment sump below the outlet pipe, so debris settles instead of entering the line
• Raised or domed grates in planting areas, which stay open when flat grates mat over
• Gutter guards and leaf-separating downspout filters ahead of every tightline
• Cleanouts at every bend, so a plumber's snake or jetter can clear the line without digging
• A fall cleaning before the first storm, and a check after every major windstorm

Homestead Valley & Tam Valley

Homestead Valley and Tamalpais Valley are unincorporated, built up the side drainages that feed Coyote Creek and Richardson Bay. Lots are steep, many homes are mid-century or older with minimal original drainage, and terraced gardens are held up by aging wood and rock walls. If a wall is leaning or weeping, the problem is almost always water behind it. See retaining wall drainage for how back drains and relief outlets are retrofitted. Because these areas are outside Mill Valley city limits, curb cores and street work go through Marin County.

Narrow access means hand-trenching

Mill Valley's network of public steps, lanes, and paths is one of the town's best features. It also means many homes have no vehicle access at all. A mini-excavator can't reach a house at the top of a stairway, so curtain drains and footing drains are hand-dug. Drain rock arrives by the bag or by bucket, and spoils go back down the same way. Sausalito's hillside streets are similar, with steep driveways, tight switchbacks, and little or no parking for a material drop.

Hand-trenching costs more in labor than machine work, but it is also more careful. Crews can follow the clay contact by feel, work around redwood roots and foundation piers, and avoid tearing up established landscaping. A good estimate for a step-access lot spells out staging, haul-off, and street-parking or encroachment requirements up front.

Sausalito hillside streets & Hurricane Gulch

Sausalito's hillsides rise straight from the bay. Homes on the steep streets above Bridgeway, including the slopes around Hurricane Gulch, are stacked so that each lot's runoff becomes the next lot's problem. Sausalito has a long, well-documented history of winter slope movement, so water management here is about slope stability as well as dry crawlspaces.

• Never daylight onto the slope. Concentrated discharge on a saturated Sausalito hillside can start erosion or worse. Collected water should be piped in closed, solid lines to the street or an approved storm connection.
• Tall walls need drainage behind them. Multi-level homes rely on tall retaining walls and stem walls that hold back saturated soil. Weep holes and back drains are critical.
• Geotechnical input is common. Deep excavation on a steep or previously moved slope may need a geotechnical engineer's sign-off before work starts.

Bayside flats: Sycamore Park, Almonte & the Richardson Bay shoreline

At the bottom of the corridor, the problem flips. Sycamore Park, the Almonte and Manzanita flats, and the low ground along Richardson Bay were built on or near former tidal marsh. The water table is shallow, it rises with heavy rain, and it is partly influenced by the tides. King tides that flood the shoreline paths and low streets can coincide with winter storms.

Here, deep curtain drains are not the answer, because there is no hillside water to intercept and digging only reaches groundwater sooner. The fix is crawlspace dewatering: shallow interior collection drains, a sealed sump with a cast-iron pump and battery backup, and a reinforced vapor barrier once the soil is controlled. The outlet design matters just as much. A gravity outlet into a gutter that is backed up by a high tide does nothing, so pumped discharge with a check valve is the norm.

Water under the house or coming off the hill? Call (415) 555-0100 or request an estimate from a Mill Valley and Sausalito drainage specialist.