Put catch basins at the low points where water already collects, and understand that an inlet placed anywhere else on a slope will only catch part of what passes it.
That second half is the part most people never hear. A grate sitting on a grade intercepts the flow travelling within the width of the grate and lets everything wider than that run straight past. The water spreading across a driveway is almost always wider than the grate you are putting in it, so most of it keeps going.
Design practice compounds this. Highway drainage guidance directs engineers to assume grates are 30 to 50 percent clogged when sizing them, because debris collects at inlets as a matter of routine. So the effective capacity of any grate is roughly half the number on the catalogue page.
Get placement right and both problems mostly disappear. Get it wrong and you can install three basins and still have standing water.
The First Question Is Sag or Grade
Every inlet sits in one of two hydraulic situations, and they behave completely differently.
A sag inlet sits at a low point, where water has nowhere else to go. It captures effectively everything reaching it, until it fills and water backs up around it.
An inlet on grade sits on a slope with water running past it. It captures a portion and passes the remainder downstream, which drainage guidance calls bypass flow.
| Property | Sag Inlet | Inlet on Grade |
|---|---|---|
| Capture | Effectively all approaching flow | A fraction, the rest bypasses |
| Failure mode | Backs up and ponds | Water continues downhill unnoticed |
| Clogging consequence | Immediate ponding | Silent, more bypass |
| Best used for | The main collection point | Reducing flow along a route |
Almost every residential drainage complaint involves water at a low point, which means most residential inlets should be sag inlets. Placing one halfway down a slope because that is where the trench is convenient produces an inlet that intercepts a modest share of the water and leaves the rest to arrive where it always did. Sorting that out on the ground is the first step in most drainage solutions involving inlets.
Why an Inlet on a Slope Misses Most of the Water
Highway drainage engineering has precise language for this, and it explains the residential version exactly.
Flow arriving at an inlet is split into two parts. Frontal flow is the water travelling within the width of the grate, and at low velocities essentially all of it is captured. Side flow is the water travelling outside that width, and only a small portion of it gets intercepted along the length of the grate.
The practical consequence is stated plainly in drainage guidance: capture depends on the upstream spread width, which is typically substantially wider than the grate, and the portion of the spread exceeding the grate width simply passes by without the grate having a chance to catch it.
Picture a driveway where water spreads six feet wide as it runs down. A two foot grate in the middle of that flow catches the two feet in front of it and a sliver of what passes alongside. Two thirds of the water carries on.
Inlet efficiency depends on cross slope, longitudinal slope, total flow, and surface roughness. All of those work in the same direction: steeper and faster means less capture.
Three responses that actually work:
- Move the inlet to the low point where the water has to stop.
- Widen the collection, using a trench drain across the flow path rather than a point inlet in the middle of it.
- Concentrate the flow first, with grading or a shallow swale, so the water arrives narrow enough for the grate to take it.
Sag Inlets Catch Everything, Until They Do Not
A sag inlet has the opposite problem. It works completely right up until it stops, and when it stops the water has nowhere to go but up.
Highway practice handles this with flanking inlets. Guidance describes placing additional inlets on the approaches either side of the low point, positioned to limit spread on the low gradient approach and to act in relief of the sag inlet if it becomes clogged.
The residential translation is straightforward. If a single inlet at a low point is the only thing standing between a storm and a garage floor, it is a single point of failure. A second inlet slightly upslope, or a secondary overflow route out of the low area, converts a blocked grate from a flood into an inconvenience.
Ask one question of any sag inlet: when this grate is covered in leaves during a storm, where does the water go? If the answer is into a building, the design is incomplete.
Design Assumes Half the Grate Is Blocked
This is the most useful number in the article.
State highway hydraulics guidance based on federal methodology instructs designers to assume inlets are 30 to 50 percent clogged, with a 50 percent clogging factor applied as standard to slotted drains, trench drains, and scuppers. Adjusted grate widths are tabulated specifically to account for it.
That is not pessimism. Debris floating in the gutter tends to collect at inlets and plug part or all of the grate, and any inlet in a yard, driveway, or farm lane is in a debris environment.
Consequences for placement and selection:
- Oversize the grate relative to the calculated need. Half of it is notionally unavailable.
- Grate style matters. Small-opening grates designed for pedestrian and bicycle safety clog more easily and intercept less flow.
- Combination inlets resist clogging. Where a curb exists, guidance considers curb-opening and grate combinations preferable at sag points, because the curb opening keeps working when the grate blinds over.
- Do not place inlets under trees if the alternative exists.
Finding the Right Locations on Your Property
Plans are useful. Observation is better.
- Walk the property during rain, not after. Where water collects, where it runs, and how wide it spreads are all visible only while it is happening.
- Mark the low points with a stake as you find them.
- Check them with an instrument afterward, because the eye is unreliable on shallow slopes.
- Trace the route from each low point to a possible outlet, and confirm the fall exists.
- Work backward from the outlet elevation to see how deep each inlet can be.
- Look for existing evidence between storms: silt fans, flattened grass, bare soil, moss, and the persistent green stripe that marks where water sits.
The most common placement error is putting an inlet where the wet spot appears rather than where the water gathers. Those are frequently a few feet apart, and a few feet is enough. Establishing the actual elevations is excavation and grading survey work, and it takes an hour.
Where Not to Put One
| Location | Why Not |
|---|---|
| Directly against a foundation | Concentrates water at the wall and adds a failure point beside it |
| In a driveway wheel path, without load-rated components | Grates and basins crack under vehicle loads |
| At the property line | Concentrated discharge onto a neighbour creates liability |
| Mid-slope, as the only inlet | Most of the flow bypasses it |
| Under a tree canopy | Guaranteed leaf clogging every autumn |
| At a low point with no outlet | An inlet with nowhere to drain is a hole that fills |
| In a mowing path, set proud of grade | Damaged by equipment, and a trip hazard |
That penultimate row causes more wasted money than any other. An inlet is a collection point, not a disposal method, and installing one in a low area on a flat lot with no fall available achieves nothing. Where a property genuinely has no outlet elevation to work with, our article on draining a yard with no slope covers the alternatives.
Sump or No Sump
Two similar-looking structures with different jobs.
A catch basin has a sump, a chamber below the outlet pipe where sediment settles out before water leaves. It needs periodic cleaning, and that cleaning is what protects the pipe network downstream.
A yard inlet has no sump. The outlet is at the bottom, so sediment passes straight into the pipe.
Where the drainage area delivers sediment, and in this region most do, a sump is worth having. It concentrates the maintenance at one accessible point instead of distributing it along a buried line where nobody can reach it.
The trade-off is that a sump holds standing water between storms, which some owners dislike. Choosing between them is part of designing a drainage system rather than a detail to leave to whatever the supplier has in stock.
How Many, and How Far Apart
On a route, spacing is a function of how much flow accumulates between inlets and how much each one lets past. Highway design works down a slope inlet by inlet, carrying the bypass from each into the calculation for the next.
The residential version:
- One inlet per distinct low point, at minimum.
- A second inlet upslope of any critical sag, as relief.
- On a long paved slope, several inlets rather than one big one, because each captures a share and bypass accumulates.
- Consider a trench drain instead of point inlets where flow crosses a route in a wide sheet, such as at the top of a driveway apron.
Bigger is not usually the answer on a slope. Wider is.
For larger properties and commercial sites, inlet networks become part of a designed conveyance system with capacity calculations behind them, which is stormwater management work rather than yard drainage.
What Central Pennsylvania Adds
Leaves. Autumn leaf fall is the single most reliable cause of inlet blockage here, and it arrives just before the season when nobody wants to be outside checking grates.
Freeze. A grate iced over is a fully clogged grate, and the February thaw arrives on frozen ground that absorbs nothing. Inlets that work all summer fail in the one week that matters.
Clay soils. Water does not soak in, so more of it reaches the inlet, and the fines it carries settle in the sump rather than passing through.
Limited fall. Many properties in the region have modest relief, which makes the outlet elevation the binding constraint on how deep any inlet can be set.
Conditions differ noticeably between parcels across Adams County and neighbouring counties, so the layout that works on one lot may be wrong next door.
Final Thoughts
Catch basin placement comes down to a single distinction. An inlet at a low point captures what reaches it. An inlet on a slope captures the flow within its own width and passes the rest along.
Add to that the design assumption that grates run 30 to 50 percent clogged, and the practical rules follow: put inlets where water actually stops, make them wider rather than deeper where flow spreads out, add relief at any sag that protects a building, and never install one without confirming there is an outlet for it to reach.
The cheapest thing you can do before any of it is stand outside during a storm and watch. Fifteen minutes in the rain tells you more about where an inlet belongs than an afternoon looking at the same yard dry.
JDI Site Solutions designs and installs catch basins, inlets, and complete drainage systems for homeowners, farms, and businesses across Adams County and Central Pennsylvania. Call JDI Site Solutions to have your low points and outlet elevations surveyed.