Yard Drainage & Runoff Control on San Juan Island

Let me be exact about what this page is, because the words overlap in an unhelpful way. This is stormwater and site work: surface water, roof water, driveway water, and the low spot in the yard that is a pond from November to March. It is trenching, grading, gravel, perforated pipe, catch basins, and outlets. It is not plumbing. I do not do supply piping or waste piping of any kind — no water lines, no drain or sewer lines, no fixtures. Plumbing is a licensed trade in Washington and it stays with a licensed plumber. If what you actually have is a sewer or septic line problem, you will know because the water is not rainwater, and that call goes to a plumber or a septic professional the same day, and I will make it with you.

The diagnosis is the whole job and it happens in the rain. Almost every drainage complaint I get described as “the yard floods” turns out to be a roof problem wearing a yard costume. A single downspout dumping at the base of the wall is delivering the entire roof plane into one square foot of soil, and no amount of trenching downhill fixes that as cheaply as a proper extension does. So I start at the top: gutters and downspouts first, then the driveway and any hard surface that concentrates flow, then the natural path the water was taking across the lot before the house was built, and only then do I start talking about pipe. Half the drainage jobs I look at get solved in the first two steps, and I will tell you that even though the small fix is the smaller invoice.

Island lots make this specific. A lot of San Juan Island is a thin mantle of glacial soil over rock, sometimes a foot or two, and the difference between a lot that infiltrates and a lot that does not is often measured in inches. On a shallow site, water arriving on the surface travels sideways on top of the rock, which is why houses cut into a hillside get a persistent seep on the uphill wall while the yard fifteen feet away is dry. Dig a test hole before designing anything. I dig them, watch how fast they drain, and design to what the hole tells me instead of to a diagram, because a dry well in soil that will not accept water is an expensive hole full of gravel.

The toolkit, and when each piece is actually the right answer. Regrading a low spot and building positive fall away from the building is the first and cheapest move, and it is underused. A vegetated or gravel swale is a shallow shaped channel that carries surface flow across a property to somewhere it can spread out, and on a sloped island lot it often beats buried pipe because there is nothing to clog and you can see whether it is working. A French drain — perforated pipe in a fabric-wrapped gravel trench, laid at a half to one percent fall, which is roughly six inches to a foot of drop per hundred feet — is for intercepting subsurface flow moving down a slope toward the house. A trench drain across a driveway or garage apron catches sheet flow before it runs into a building. A catch basin gives a low point somewhere to go and gives me a place to clean out sediment later. A dry well stores and infiltrates, and only works where the test hole says it will.

Every one of these has to end somewhere, and the outlet is where amateur drainage work goes wrong. Pipe has to daylight at a lower elevation, spread out, and slow down. A four-inch solid pipe firing at full velocity onto a neighbor’s lower lot or into a bank is not drainage, it is erosion with a schedule. I finish outlets with a dispersion detail — a tee, a gravel spreader, or a rock apron — so the water leaves as sheet flow rather than a jet, and I keep discharge on your property or into the natural drainage course it was already using. San Juan County’s own grading standards are explicit that development must control erosion and sedimentation to prevent damage to adjoining property and downstream channels, and that you cannot discharge drainage into county roads or ditches within the right of way without the county engineer’s approval. That is not a technicality I get to skip because the trench is short.

On permits, here is the honest version. Small residential drainage work on an existing developed lot is usually not its own permit, but that is a sentence with real edges on it. San Juan County ties stormwater requirements to land-disturbing area and new impervious area, and grading of one hundred cubic yards or more triggers state environmental review. The county also regulates clearing, grading, and fill inside shoreline jurisdiction, which reaches two hundred feet landward of the ordinary high water mark, and inside critical areas: wetlands, geologically hazardous slopes, and habitat areas. The whole county is treated as a critical aquifer recharge area. What that means in practice is that a short trench and a regraded low spot behind a house on a flat interior lot is maintenance, while trenching near a bluff, a wetland, a steep slope, or the shoreline is a call to the Department of Community Development before a shovel moves. I make that call, in your name, in writing, and I do not start work on that kind of site without an answer.

The septic rule is absolute and I want it in plain language: I do not dig on, near, or uphill of a drainfield, and I do not route water toward one. Washington Department of Health guidance is direct about keeping surface runoff away from the tank, the drainfield, and the reserve area, and about keeping vehicles and equipment off all three. A drainfield that is taking your roof water is a drainfield being asked to treat rainwater, and that shortens its life in a place where a replacement system is a very large number and a scarce approved area. Before I dig anywhere on a rural lot I want the as-built septic drawing, which the county health department usually has on file. If the work is anywhere near the system, the design of it is the septic professional’s call and not mine, and I will say so and stop.

There is one thing I will not sell you, which is a big drainage system for a problem that is seasonal and cosmetic. A yard that puddles for two days after a December atmospheric river and then drains is a yard, not a defect. What deserves money is water against a foundation, water entering a crawlspace, standing water over a septic area, a saturated slope above a building, and a walkway that ices or grows algae because it never dries. Those I will chase hard. A soft patch of lawn in February I will usually tell you to live with, and spend your budget on the downspouts instead.