Parking Lot Drainage in Tualatin, Oregon: Stop the Ponding

Why Tualatin Parking Lots Pond

Tualatin's flat valley ground and heavy clay make ponding one of the most common commercial-lot complaints in the area. A parking lot has to move every drop of rain to a safe outlet, and Tualatin gives it two handicaps: there's little natural fall to drive water toward inlets, and the clay subgrade holds water rather than absorbing any of it. Through the long Pacific Northwest wet season, and especially during atmospheric-river storms that drop inches in hours, a lot that's even slightly under-sloped or under-drained will pond — standing water across drive aisles and stalls that customers wade through and asphalt slowly fails beneath.

Ponding is rarely cosmetic. Standing water seeps into pavement cracks, undermines the base, and accelerates potholes and alligator cracking. On flat Tualatin lots, where water can sit for days because there's nowhere for it to drain, that damage adds up fast. A lot that ponds today needs resurfacing sooner than it should.

For a property owner, the usual question is why a lot ponds in the same spots winter after winter. The answer is almost always subgrade settling, inadequate slope on already-flat ground, or a drainage system that can't move water to a workable outlet.

Common Causes of Ponding in Tualatin

Subgrade settling. The compacted base under a section settles into a low spot — a "birdbath" — that traps water. The most common cause of localized ponding.

Inadequate slope on flat ground. Tualatin lots start with little fall, so any settling or under-design leaves water with no way to reach an inlet.

Outlet limitations. With flat surrounding terrain, getting collected water to a low-enough outlet can require a detention system or a tie-in that, if undersized, backs up.

Too few or poorly placed catch basins. Inlets spaced too far apart on flat ground let water pool before it ever reaches a drain.

Clogged or crushed lines. Leaves, sediment, and traffic over time fill or collapse the pipe carrying water off-site.

How Parking Lot Drainage Gets Fixed in Tualatin

Catch Basins and Inlets

The backbone of lot drainage is a network of catch basins at low points, connected by buried pipe to a storm system or approved outlet. On flat ground, correct inlet spacing and careful grade to each basin are what keep water moving. Our guide to commercial parking lot drainage design in Oregon covers slope, inlet spacing, and outlet design.

Regrading and Slope Correction

Where the lot has settled, the surface must be corrected — milling and overlaying to restore positive slope toward the inlets, or rebuilding the base under a failed area. On flat lots, establishing adequate fall is often the heart of the fix; adding a drain to a low spot without fixing grade only masks it.

Detention and Stormwater Treatment

Commercial sites in Oregon often must detain and treat runoff before it leaves the property. On Tualatin's flats, a detention pond or underground detention, paired with a water-quality swale or oil-water separator, is frequently part of the design under DEQ stormwater rules. Building this in from the start avoids costly retrofits — and on flat ground, detention is often the practical answer to a limited outlet.

Trench Drains

For wide expanses, entrances, and loading areas where water crosses the pavement, a load-rated trench drain intercepts sheet flow across its whole length before it can pool.

Why Ponding Is Worth Fixing Now

Every winter a lot ponds, the standing water shortens its pavement life — and on flat Tualatin lots that water sits longer, so the damage is worse. Correcting the drainage and affected pavement now almost always costs less than the full resurface that water damage forces, and a ponding lot is a slip-and-fall and liability concern for the owner. For how lot drainage fits the broader site-water picture, see our overview of property & site drainage in Oregon.

When to Call a Commercial Drainage Contractor

Lot drainage is rarely DIY, and on Tualatin's flat ground the design challenge — establishing slope and finding a workable outlet — makes professional judgment essential. Diagnosing whether the problem is grade, capacity, or a buried failure takes equipment and experience, and the fix usually means cutting pavement, setting structures, and tying into a storm or detention system under code. If your Tualatin lot ponds in the same spots each winter, if catch basins back up, or if you're planning a resurface, that's the time to bring in a contractor who knows flat-valley drainage.