French Drains & Drainage Systems in Mercer Island, WA

Drainage & French drains in Mercer Island: what to know

The first drainage question is one the city has already mapped: can water soak in on this lot? Its infiltration feasibility map includes areas where infiltrating drainage is not permitted, and drainage reports for two island lots treated roof-water infiltration and permeable pavement as not permitted on that basis. Where infiltration is closed, a drain needs a pipe to somewhere else.

Somewhere else is limited. In one report, a stub-out connection was infeasible because no gravity connection point existed, and the city system that did take the runoff outfalls to Lake Washington. In an erosion hazard area, development may not create a net increase in geological instability, so concentrated water cannot simply be released at a slope edge.

Permits and rules in Mercer Island

Drainage designs on the island apply Ecology’s 2019 stormwater manual together with the City of Mercer Island Drainage Requirements, and land-disturbing activity is subject to MICC 15.09. A pipe into the street is right-of-way work with its own permit. Inside critical areas the standard exemptions do not apply. Before quoting, we look up the infiltration map, hazard layers, and legal outlet for your parcel.

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

How we approach it

Check the infiltration map first; where infiltration is closed, plan a piped outlet
Locate the storm main and its depth before promising a gravity connection
Keep outfalls away from slope edges so erosion hazard areas are not made worse
Trench and pipe in the dry months where the grading limit applies

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

Questions from Mercer Island homeowners

Can I use a dry well or infiltration trench on my Mercer Island lot?

Only if the city’s map allows it. The infiltration feasibility map includes areas where infiltrating drainage is not permitted, and island drainage reports applied that to roof-water infiltration and permeable pavement. We check your parcel’s map status before designing around infiltration.

Where can a Mercer Island French drain discharge?

Where infiltration is closed, island drainage reports use connections to city storm pipes, which lead to Lake Washington. A pipe into the street is right-of-way work with its own permit, and on a slope the outlet must not add erosion. We settle the outlet before digging.

Does a French drain in my Mercer Island yard need a permit?

It can. Land-disturbing activity is subject to the city’s stormwater chapter, MICC 15.09, and in critical areas the standard exemptions do not apply, so most work in the ground or vegetation needs a permit. A shallow yard drain on an unmapped lot is a smaller question. We ask the city before digging.