French Drains & Drainage Systems in Sammamish, WA

Drainage & French drains in Sammamish: what to know

Interception starts at the top of the wet area. A curtain drain laid across the hillside catches flow before it reaches the lawn, and it follows the contour so it does not channel water into a neighbor’s yard. Sammamish’s drainage review starts at 2,000 square feet of new plus replaced impervious surface, or 7,000 square feet of land-disturbing activity, so the drain is planned alongside any new hard surface on the lot.

Downspout tightlines carry roof flow in solid pipe to a single outlet, apart from the perforated run. The outlet shapes the layout: it must land on ground that absorbs water or in a connection the city allows, clear of steep banks, the street, and the sidewalk. The city confirms the discharge point with staff before the trench is laid.

Permits and rules in Sammamish

Trenching on a slope can bring grading review, and an outlet that reaches the public right-of-way brings the Public Works Standards into play. Work within a critical area or its buffer cannot be exempted and needs a permit, except on publicly owned land, so the parcel’s critical area status is checked first. We check the parcel’s critical area status first, ask city staff which track applies, and confirm the governing review before excavation begins.

Sammamish’s clearing and grading page says a Clearing and Grading Permit or Site Development Permit is required before site work unless SMC 16.15.050 exempts the work. It caps cut and fill slopes at 2 horizontal to 1 vertical unless the Director approves, and it says work within critical areas or their buffers cannot be exempted, except on publicly owned land. Rules change, so we confirm the current requirements with the city before we start.

How we approach it

Find where slope water enters the yard before drawing the line
Lay the interception line along the contour above the wet ground
Keep downspout tightlines in solid pipe, apart from the perforated run
Settle the outlet and any right-of-way question before excavating

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

Questions from Sammamish homeowners

Is a steep bank an acceptable outlet for a Sammamish French drain?

No. Steep banks, the street, and neighboring yards are not used as outlets. The outlet goes to ground that can absorb the flow or to a storm connection the city allows, and the city confirms the point before the trench is laid.

Does a drain that brings Sammamish impervious surface to 2,000 square feet need review?

Yes, as part of that project. Drainage review starts at 2,000 square feet of new plus replaced impervious surface, and we check the critical area status and any right-of-way item before the trench is opened.

Will a drain help a wet Sammamish yard above a ravine?

It may. Interception can ease water arriving from higher ground, but a slope needs evaluation first, so we survey the grade and soils before proposing a route.