French Drain, Catch Basin, or Regrading: Which Problem Does Each Solve?
Each answers a different arrival, and the mistake that costs the most is choosing the device before identifying the water. A subsurface drain — what most people mean by a French drain — is for water already moving through the soil. A catch basin collects surface water at a low point and hands it to a pipe, so it is worth what its outlet is worth. Regrading changes only the path water takes over the ground.
None of them fixes roof water landing at the wall, and a water table that rises across the whole lot in winter is a constraint rather than a blockage. The diagnosis that comes first — what feeds the puddle, which way the ground falls, and whether the soil takes water at all — is its own subject in this cluster, under the drainage guide.
Four arrivals, three answers
Water reaches a low spot in one of four ways.
| Where the water is before it ponds | What you see | What addresses it | What does not |
|---|---|---|---|
| On the surface, from a roof outlet | A puddle under a downspout, a rill in the mulch | A longer outlet, a splash block, a dispersion path | A subsurface drain, which never meets surface flow |
| On the surface, from uphill or paving | A sheet crossing the yard; water against a slope or bed | Regrading, a berm, a surface channel | A French drain, which sits below the surface |
| In the soil, arriving slowly | Ground soft days after rain; a seep at a bank; water in a test hole | A subsurface drain across the flow | Regrading, which moves surface water only |
| In the ground across the lot, seasonally | Wet ground from December on; water in every hole | Possibly nothing built: the water table governs | Any drain whose outlet sits below the water it must collect |
The first row is the outlet decision.
The fourth ends projects. King County’s design manual makes infiltration conditional on depth to the maximum wet-season water table: at least 1 foot below a gravel-filled trench, 3 below a surface depression. Snohomish County’s infiltration sheet states the same limit — no shallow groundwater, hardpan or fill within 1 foot of the trench bottom — and sends the owner to a dispersion trench where those sit shallower. A device built to let water soak away cannot do so into ground already full.
One name, two jobs
The phrase covers two installations.
WSU Extension’s surface-water fact sheet — written, it states, for Western Washington — treats gravel trenches as an infiltration practice: stone-filled channels whose spaces hold stormwater while it soaks into the surrounding soil. It separates an open trench fed from a hard surface through a vegetated strip from a closed trench fed by a pipe through a sediment basin. Then it draws the line: some French and curtain drains are built to intercept shallow groundwater and move it away from foundations and septic drain fields, and those “are different, and are not discussed in this publication.”
So the words cover a surface-water infiltration trench and a groundwater interceptor. A trench drain is a third device: a grated channel set flush across a driveway, falling no less than 3/4 inch per 10 feet and not recommended on slopes over 5 percent. That is a surface channel, not a subsurface drain.
Regrading: the cheapest change, and the one with a rule attached
Where the water is on the surface and there is somewhere lawful for it to go, changing the grade usually means the least apparatus and the least maintenance. It is also the one with a rule attached: King County’s Surface Water Design Manual requires that runoff from a project “must be discharged at the natural location so as not to be diverted onto or away from downstream properties.” Its stated intent is the part to read twice: a diversion can do more damage than the development would have, and can affect properties relying on runoff for wells and ponds. A regrade that turns a wet corner into a neighbor’s yard is a diversion, not a solution.
The permit question is local, and the thresholds are lower than most owners assume. In unincorporated Snohomish County, grading is generally exempt from a land-disturbing-activity permit only where it is 2 feet from all property lines, does not obstruct or alter an existing drainage course or pattern, disturbs less than 7,000 square feet, creates or replaces less than 2,000 square feet of hard surface, collects stormwater from no more than 5,000 square feet, sits outside critical areas, and totals 100 cubic yards or less in 18 months. Your jurisdiction’s will differ.
The consequence: a regrade changes where water leaves your land, and the jurisdiction decides whether it may.
A catch basin is a collection point, not a drain
A catch basin — a yard drain — does one job: it takes water that has arrived at a low point and puts it into a pipe. It does not lower a water table and does not make soil drain.
King County’s simplified drainage requirements show the role. Runoff from roadways, driveways and parking areas must pass through a yard drain or catch basin with a down-turned elbow before entering an infiltration trench, the elbow trapping spilled material in the sump where a homeowner can clean it up. The basin is a collection and sediment point on the way to somewhere else, and that somewhere else is the design: one with no outlet, a blocked outlet, or an outlet that falls flat is a settling pit that fills with silt and then with water. King County requires sediment removed yearly, or more often.
A subsurface drain depends on its outlet and on its soil
A subsurface drain is a perforated pipe in a trench below the surface, usually set in gravel, placed to intercept water in the ground and lead it away. The standard a Washington city publishes is blunt about its dependencies: it intercepts, collects and conveys groundwater to a satisfactory outlet; relief drains lower the water table across a flat area while interceptor drains cut across a slope to keep the ground below from saturating; depth is set by how far the water table must fall, to a practical maximum of 6 feet; the soil must be deep and permeable enough for a drain at 2 to 6 feet to work; and the outlet must exist, by gravity or pumping.
What makes one fail is a shorter list: too small a pipe, too little fall to keep it from silting, no filter material, a reverse grade, or roots from a tree planted too near. A line that has already stopped accepting water is its own diagnosis.
Colorado State University Extension states the limit no detail removes: a drain tile works only to the extent the soil lets water reach it, and only where the water can fall to an outlet. Its answer for a genuinely high water table is not a drain at all — it may be to raise the soil in raised beds or berms, because there is nowhere for the water to go.
The two tests, and the season that governs one
Two observations decide the choice.
The first is a site map. WSU Extension’s version diagrams how water moves on the parcel: property lines, structures, paving, slopes, drainage pathways, buried utilities, the septic system and well, and where water ponds and leaves. The same fact sheet warns that federal soils maps are not reliable at site scale, so the map records what you see; it does not replace a test.
The second is a hole, and the season decides what it proves. Snohomish County’s RainScaping guide asks for a 12-inch hole in the area of concern, a note of whether water stands at the bottom, and a measurement down to it if so. Groundwater is usually closest to the surface from December through April, and in dry months you may not observe the characteristic at all. For an infiltration design the county is stricter: a 3-foot hole observed for 24 hours, standing water at the end meaning shallow groundwater.
Water in the hole in February points to the ground; water crossing the surface while the hole stays dry points to the grade.
Before anything is dug
Washington’s dig law applies to a homeowner with a shovel, and to a test hole. The Utilities and Transportation Commission’s sequence: mark the area with white lining, give notice through 811 between two and ten full business days before the work-to-begin date, hold work until every known facility is marked or accounted for, and keep the marks for up to 45 days. Since 1 January 2026 work may not begin until a positive response has come back from every operator in the area.
The gap is private plant. Snohomish County states that the public locate usually does not mark private utilities between the street and the house — power, irrigation, septic and water among them — and advises a private locator where their positions are unknown.
Two questions belong in the same call: what may be discharged and where, which the authority owning the receiving water decides; and, where groundwater is drinking water, the county’s rule that runoff from driveways, parking areas and yards must not enter an infiltration system at all.
What each one asks of you afterwards
- A regrade settles, and beds and paving get built over it; the fall can be lost unnoticed.
- A catch basin needs its sediment out yearly or more often.
- A subsurface drain needs an annual check that it runs free, a clear outlet, no roots, no sediment; prolonged ponding may mean it is plugged, and a plugged device is replaced rather than cleaned.
- A surface channel needs its grate cleared before and during storms, and its outlet protected from erosion.
Next step
Before commissioning anything, get five answers in writing: where the water comes from, where the device will discharge, who permits that discharge, what happens when the outlet fails, and what maintenance it needs each year. A household can answer the first with a site map and one wet-season hole; the second and third belong to the local planning department or surface-water program, which can also say whether infiltration is allowed on the lot at all.
A slope, a retaining wall, ground that has moved, or water arriving inside is a geotechnical question rather than a drainage-device one, and the county sheet names the threshold at which it wants an engineer involved.