Drainage by Application · Transportation & Logistics
A wash rack is the only pavement on your property that gets a rainstorm on a schedule — thousands of gallons a shift at one slab, under axles that stop and steer instead of rolling past. Here is how to size the trench for the sluice instead of the sky, and where our scope ends and the interceptor contractor's begins.
A transit bus barn runs a drive-through lane where a 40-foot coach passes the arch every three minutes at shift change. A public works yard washes packers and plow trucks on an open pad, salt-caked all winter. A grain co-op power-washes combines in October and calls the same slab a biosecurity rinse pad in April; a construction fleet returns iron so caked in clay the first hundred gallons are archaeology.
What those sites share is not equipment. It is a slab taking a concentrated, chemically loaded volume of water on a human schedule rather than a meteorological one. Rain spreads over an hour across acres; wash water arrives in a burst, at one spot, moving sideways under pressure. And rain does not park an 80,000-pound tractor-trailer on the grate while it happens.
Two neighbors, named so we don't blur them. If you sell washes to the public — tunnels, self-serve wands, vacuum islands — the flows and economics differ, and that is our car wash drainage page. If your rack sits inside the fence line alongside motor pools, start at military base drainage.
An honest wash-pad plan has five jobs. We supply two; the other three are not ours.
Ours — intercept, collect, convey
A channel down the wash position to catch the sluice, a containment line at the threshold, catch basins at the low points to drop sediment, and drainage fittings to carry it downstream.
Ours — the storm side of the same pad
A wash pad carries two systems that must never meet: containment to sanitary through pretreatment, site drainage to storm. Cross them and you get a violation one way or, the other way, rain swamping an interceptor sized for a fraction of that flow. The perimeter line that keeps yard runoff out of containment causes more trouble by its absence than anything else.
Not ours, downstream by others — interceptors and separators
Grit and sand interceptors, oil-water separators, and permit-required pretreatment are engineered and supplied by others; our channels and basins feed them. Chase the inlet invert early — it is fixed before your trench depth is, and no channel drain does a separator's job.
Not ours — pumps, reclaim, treatment, and concrete washout
Below-invert collection means a pumped sump, and we sell neither pumps nor reclaim equipment. Ready-mix drum washout belongs in a lined pit, by others; slurry that reaches a channel sets inside it. Reclaim changes one thing here: recirculated water still crosses the slab at full arch flow, so size the trench for what crosses the concrete.
Not ours, contractor-installed — subsurface and french drains
Fleet yards use real subsurface drainage and it deserves a straight description. A french drain is perforated pipe bedded in washed stone and wrapped in geotextile; its job is groundwater. At a wash facility it belongs under the heavy-duty pavement base — a saturated subgrade is how a thick slab still fails under repeated 20,000-pound wheel loads — along a soggy yard edge, and behind the walls of a depressed rack. That is excavation-contractor work installed with the pavement section, and we do not sell it; none of our products is a french drain and we will never call one that. Once the base is dry, what lands on the surface is ours to collect.
Reefer and tanker fleets wash trailer interiors, so the scoping has to be honest. The bay floor, pad, apron, and yard are general-duty industrial and exterior drainage, sized for wheel loads and weather — our catalog. Hygienic and GMP interior drainage is a different discipline: crevice-free engineered sanitary stainless, the world USDA, FSMA, NSF, 3-A, and EHEDG expectations shape. We do not sell it, and nothing in our catalog meets those requirements.
Water on a rack does not fall; it is thrown. A wand returns it as sheet flow landing several feet out; an undercarriage blaster drives it down and outward in a ring. And the truck is a roof: water emerges from underneath in bands, dropping grit wherever it slows — on a flat bay, short of the trench.
Now get a flow number — the instantaneous rate, not gallons per vehicle. Add the nameplate flow of every nozzle, arch, and blaster that can run at once. Two hand wands at 4–6 gpm each is trivial; a rinse arch at 40 to 100 gpm is a 1½″ fire attack line inside ten feet of channel.
Then the daily total, because it explains the sediment. A pad running 30 gpm for a six-hour shift moves 10,800 gallons. On a 16′ × 60′ bay — 960 square feet — that is roughly 18 inches of rain every working day, against the 623 gallons per inch that 1,000 square feet of pavement sheds.
If the pad is open to the sky, add the weather: total the pad plus any roof or apron draining onto it, run it through our runoff calculator, and check the method in Sizing for Storms. The larger number sizes the run.
Last, check the outlet, because the outlet is the bottleneck — a 12″ channel on one 4″ outlet is a 4″ drain. Full-flow estimates at 1 percent slope, Manning n = 0.013:
| Outlet pipe | Approx. capacity | What that covers at a wash rack |
|---|---|---|
| 4″ | ~85 gpm | Covered hand-wash bay, two wands |
| 6″ | ~250 gpm | One bay or lane with a rinse arch |
| 8″ | ~540 gpm | Multi-bay rack; uncovered lane |
| 10″ | ~985 gpm | Drive-through lane plus yard |
| 12″ | ~1,600 gpm | Wash building plus site apron |
Two checks nobody runs. Pretreatment has a rated flow too — rate it for 50 gpm while the trench delivers 200 and the trench is not what floods. And keep velocity up: below roughly 2 feet per second grit packs, which is why Dura-Slope builds in 0.7 percent fall (8.4″ per 100 ft) and PowerDrain 0.5 percent.
The axles are not exotic: US federal limits put 20,000 pounds on a single axle, 34,000 on a tandem, 80,000 gross. A highway carries that all day. What makes a rack harder is how the load arrives.
How the classes map. EN 1433 rates a complete channel-and-grate system by proof load: A 15 (15 kN, about 3,400 lbf) pedestrian, B 125 (125 kN, about 28,000 lbf) light vehicular, C 250 (250 kN, about 56,000 lbf), D 400 (400 kN, about 90,000 lbf) roadway and commercial, E 600 (600 kN, about 135,000 lbf) industrial, F 900 (900 kN, about 202,000 lbf) extreme. In North American shorthand H-20 is read as a 16,000-pound wheel load and HS-25 as 20,000 — heuristics, not standards equivalence.
Our rule: Class D 400 is the floor on any pad a truck drives onto; Class E 600 wherever a bus, semi, packer, or mixer stops, idles, or steers over the grate line — on most racks, the primary bay trench; Class F 900 where the engineer wrote it. Full set in our Load Class Recommendation Guide.
Two riders. The system carries the rating, not the grate: a Class E channel in a thin pour is not a Class E installation. And an unlocked grate is rated for nothing — prefer boltless locking or stainless fasteners, because plated bolts in a bay wet 300 days a year seize and never come out.
Chemistry. Road-film detergents are alkaline, typically pH 11 to 13; aluminum brighteners are acidic at pH 1 to 2, and acids etch galvanized coatings, bare steel, and ordinary portland concrete from the surface inward. If brightener is in your bay weekly, galvanized grates and edge rails are a bad buy however good the load rating looks. Hold the manufacturer's chemical-resistance data against the SDS for what you spray.
Temperature and corrosion. Hot-water units commonly run near 180°F at the nozzle, and repeated hot-then-cold cycling is the clearest reason we move a spec off thin-wall plastic in a heavy bay. Ignore the rust bloom on ductile iron grates in a bay that never dries; that is surface oxide on a casting with real thickness. What fails is the thin stuff: fasteners, plated hardware, light-gauge frames, etched galvanizing.
Material, pattern, and heel-safe geometry live in the grates section, with ductile iron beside the other metal grate families and fitment under channel drain grates. Two wash-specific opinions: pick a slotted pattern that lets grit fall through where a bucket can catch it, because a perforated or mesh grate blinds over inside a week; and keep openings at or under 1/2″ where people walk the bay in wet boots.
The bay-length primary trench is the spine: a sloped channel running the full length of the wash position. A 20-foot trench in a 60-foot bay leaves 40 feet of slab looking for somewhere else to send water. Never let anyone value-engineer it.
The containment line at the threshold runs full width across the bay opening so wash water cannot escape onto the yard and yard water cannot walk into the interceptor. Covered, it can be shallower; on an open pad it is doing storm duty too.
Catch basins are sediment sumps first and drains second. An in-line basin mid-run is the underrated piece: a rodding point, a second outlet so a 100-foot run drains two directions instead of filling from the far end, and another place for grit to drop. Take a body with multiple openings so several runs share one structure; basin kits and universal outlets cover the tie-ins. What does not belong: atrium and decorative grates, which are landscape rather than wheel-path products, and plastic basin grates anywhere a tire lands.
Seven picks for seven real situations.
Vans, pickups, medium trucks · on the shelf
HDPE, 0.7 percent built-in fall, 48″ sections from 3.99″ deep past 6.35″. Spec it as a system: the ductile iron grate (DS-232) on the ductile iron frame (DS-200H), a trash bucket (DS-240) in the run, the in-line basin (DS-340) at the end. A loaded semi standing on the line every cycle wants a heavier body — have us confirm the class first.
ACO's own vehicle-wash line · quote-only, never through the cart
The spec when the thing standing on your grate line weighs forty thousand pounds and does it forty times a shift. Polymer concrete with an integrally cast ductile iron edge rail — no joint to work loose under steering scrub — a partial-radius invert that keeps velocity up on a grit load, and 0.5 percent fall across 1-meter sections. Grate intake makes width a real decision: S100K 4″ 25.70 in², S200K 8″ 70.10 in², S300K 12″ 122.60 in². Slotted or longitudinal, boltless PowerLok or tamper-resistant 4-bolt — settle that in the RFQ. Installed class follows grate and detail, which is why the range is quote-only: send us the run.
Aggressive chemistry · stainless edge rail · quote-only
Where acid brightener runs weekly the edge rail dies first; the KS variants answer that with a stainless steel edge rail instead of galvanized. Pick width off the flow — KS100 4″, KS200 8″, KS300 12″ — then change grate spec along the run as it passes the wash position, the walkway, and the door.
Ag, quarry, construction iron · Class E grates · orderable directly
What we reach for when the payload is clay rather than road film: fiber-reinforced high-performance concrete with the edge stability of a cast product. Class E ductile iron slotted grates on 8″ (Tec V200), a Class E Boltmax on 12″ (Tec V300), Class D ADA bar grates for the walking stretch, plus locking devices and silt buckets — orderable directly.
Chemical exposure at the top of the load range · quoted on request
Pro-Plus 100 is an SMC/GRP composite body with a 4-3/4″ grate opening, and its grates run from a stainless steel bar grate at Load Class E to ductile iron slotted grates published to Load Class F, including ADA heel-proof patterns; Pro-Plus 200 adds a stainless slot grate at Class E. Heel-safe geometry normally costs load rating; here it does not. Quote the run.
Storm side of the pad · diversion and perimeter · on the shelf
The diversion line around an open pad is a storm job, not containment. The 8″ Pro Series offers a heavy-traffic grate and a ductile iron grate; the 12″ steps up intake with a cast iron heavy-traffic grate and a security clip. Size it from the runoff calculator — and if trucks turn here, this line is Class D too.
Cheapest insurance on the pad · on the shelf
The last structure before pretreatment should be sized to hold solids, not merely pass water: 18″ and 24″ square bodies with deep sumps, two- and four-opening configurations, cast iron grates matched to the pavement class, risers to chase finished elevation. The 12″ body that serves a lawn is full in a week on a mud fleet. But a grit trap is not an interceptor.
Size it for the worst ten seconds, not the shift total. Add the nameplate flow of every nozzle that can run at once — hand wands are commonly 4 to 6 gpm, a rinse arch 40 to 100. Open to the sky, add the storm: 1,000 square feet of paving sheds about 623 gallons per inch of rain. Then check the outlet, which caps everything — at 1 percent slope, Manning n = 0.013, roughly 250 gpm through 6-inch pipe, 540 through 8-inch, 985 through 10-inch.
Class D 400 (400 kN, about a 90,000 lbf proof load) is our floor for any pad a truck drives onto. Class E 600 (600 kN, about 135,000 lbf) wherever a bus, semi, packer, or mixer stops, idles, or steers over the grate line — on most fleet racks, the primary bay trench. Class F 900 (900 kN, about 202,000 lbf) where the engineer wrote it. H-20 is read as a 16,000-pound wheel load and HS-25 as 20,000, but those are trade heuristics, not standards equivalence.
Most jurisdictions require wash-bay discharge to reach sanitary through a grit or sand interceptor and an oil-water separator, sized and permitted by your engineer. We sell none of that — no separators, interceptors, reclaim equipment, or pumps; our channels and basins only feed it. Fix the interceptor inlet invert before anyone orders channel, because it decides how deep your trench can be.
They can. Road-film detergents are alkaline, typically pH 11 to 13; aluminum brighteners are acidic at pH 1 to 2, which is hard on galvanized coatings, bare steel, and ordinary portland concrete. Hot-water units near 180 degrees F add thermal cycling. Where brightener is in weekly use we go to ductile iron or stainless grates on a polymer concrete or fiber-reinforced concrete body.
Not for the water you are washing with. A french drain is perforated pipe bedded in washed stone and wrapped in geotextile, and its job is groundwater — under the pavement base, along a soggy yard edge, behind the walls of a depressed rack. That is excavation-contractor scope and we do not sell it. Wash water has to be caught at the surface instead.
Send the bay dimensions, the wand or arch flow, the heaviest vehicle that stops on the grate line, and the interceptor invert — we'll take off the system and quote it within one business day.
The ACO lines are quote-only; Dura-Slope, Filcoten, Pro Series, and catch basins ship from stock — shop channel & trench drains. Prefer to talk? Call 803-324-3225, email [email protected], or use the contact form.
