French Drains & Drainage Systems in Mill Creek, WA

Drainage & French drains in Mill Creek: what to know

The city says a Stormwater Site Plan shows how runoff is handled during and after construction, and that its requirements vary with project size and proximity to stormwater features or water bodies. Mill Creek uses the 2019 Ecology manual, Volume I, to decide which requirements apply. The city also amended its low impact development code in 2017 under Ordinance 2017-814.

The trench falls toward the outlet, and its stone and pipe are set to carry the flow rather than hold it. Roof runoff gets a dedicated solid line, kept separate from the perforated pipe, and discharges at a spot picked for ground able to absorb it. A run that reaches the public right-of-way or ties into a city line is a separate question, and we put it to the city before the trench opens.

Permits and rules in Mill Creek

We check whether the project needs a Stormwater Site Plan, and how its size and location change the requirements. The Ecology construction stormwater general permit may apply to new builds and redevelopments, so we confirm that answer before the crew mobilizes. A run that crosses the right-of-way or links to a city line goes to the counter before any work there begins.

Rules change and vary by parcel, so we confirm the current requirements with the city before we start.

How we approach it

Find the lowest exit the parcel can offer before layout
Fall the trench toward that exit from the wet zone
Run roof water in solid line to its own discharge
Check whether a Stormwater Site Plan applies before cutting

For the full process, see our Drainage Systems & French Drains page.

Questions from Mill Creek homeowners

Which Mill Creek drain outlet reaches the right-of-way review?

Any run reaching into the public right-of-way goes to the city for a decision. Utility connections appear among the right-of-way examples on the city’s page, so the counter is consulted before any outlet lands on public ground.

When does a Mill Creek drain need a Stormwater Site Plan?

The city says Site Plan requirements vary with project size and proximity to stormwater features or water bodies. We check both before the trench opens, and we confirm whether the Ecology construction stormwater general permit applies to the job.

Which site features are measured before a rear-yard drain goes in Mill Creek?

The lowest exit on the parcel, the grade along the trench, and the soil the water must pass through. A drain goes in only where a workable exit exists, and where one does not, the estimate says so rather than proposing a trench that cannot drain.