French Drains & Drainage Systems in Kent, WA

Drainage & French drains in Kent: what to know

Kent’s surface water manual starts from a simple principle: runoff leaves a site at its natural location. Where a small concentrated flow has no pipe downstream, the manual allows a rock pad or dispersal trench if there is no significant harm. Larger flows need a conveyance system and recorded easements.

The city’s stormwater basin maps name the Big Soos, Mill, Garrison, Springbrook, and Des Moines Creek basins, so a yard may drain toward a creek the city regulates. The same map set marks the drainage areas that feed landslide hazard areas, so where a pipe discharges matters there. On the valley floor, with little fall to work with, finding an outlet can be the hardest part.

Permits and rules in Kent

Two numbers start Kent’s drainage review of a permitted project: 2,000 square feet of hard surface, new plus replaced, and 7,000 square feet of disturbed ground. Building or changing a drainage pipe or ditch 12 inches or larger starts it at any size, as does working near a flood hazard area, stream, lake, or wetland. We look up the basin, the hazard mapping, and the outlet for your lot with Kent Public Works before we quote.

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

How we approach it

Identify which creek basin your lot drains toward before choosing an outlet
Probe the uphill side for silt layers that hold water
Keep concentrated flow off slopes the city maps as landslide hazards
Add cleanouts and a rock pad or dispersal trench sized to the flow

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

Questions from Kent homeowners

Can I tie my Kent French drain into the street storm system?

Sometimes. A connection to the public system is a city matter and can bring drainage review, and a drain should never go to the sanitary sewer. Where no storm line is reachable, a rock pad or dispersal trench on your lot may take small flows. We ask Public Works first.

My Kent backyard stays soggy in spring. Is that a drainage problem or a creek problem?

Check the map first. Kent regulates streams, wetlands, and their buffers as critical areas, and drainage work near them draws extra review. A soggy yard near a regulated stream may be groundwater or a drainage course, not a failing gutter, so we look before we trench.

Will a drain in my Kent yard make a slope less stable?

It can if the outlet is wrong. Concentrated water released onto a slope is one of the main risks, and Kent’s stormwater maps mark landslide hazard drainage areas. We aim outlets away from slope edges and ask the city when a lot drains toward a mapped area.