Parking Lot Drainage in Lake Oswego, Oregon: Stop the Ponding

Why Lake Oswego Parking Lots Pond

A parking lot that holds water after every Lake Oswego rainstorm is more than a nuisance. Standing water freezes into hazards, accelerates asphalt failure, drives away customers, and can put a property owner on the wrong side of stormwater regulations. In Clackamas County's hilly terrain, lots face an extra burden the flat valley does not: runoff arriving from higher ground, on top of the rain falling directly on the pavement.

The root causes are usually a combination of inadequate slope, settled low spots, undersized or clogged drainage infrastructure, and incoming hillside runoff. A lot that was not graded precisely — or that has settled over the years — collects birdbaths and standing water that linger long after the storm passes, especially where clay subgrade slows how fast water can move away.

This guide covers why Lake Oswego lots pond and the drainage approaches that fix it. For the full engineering picture, see our commercial parking lot drainage design in Oregon guide, and for the broader context, our overview of property and site drainage in Oregon.

The Local Causes of Parking Lot Ponding

Runoff From Higher Ground

In Lake Oswego's hilly setting, many lots sit below adjacent slopes, streets, or properties and receive runoff flowing down onto them. A lot can pond not because of its own drainage failure, but because it is taking on more water than it was designed to handle. Intercepting that incoming flow is often the key.

Insufficient or Lost Slope

A parking lot needs consistent slope — generally a minimum of around 1 percent — to move water to its inlets. When a lot was under-graded or has settled, water has nowhere to flow and pools in the low areas.

Settled Low Spots and Birdbaths

Over time, the asphalt sub-base can settle unevenly, especially where it was poorly compacted or where heavy vehicles concentrate load. The result is a depression that holds water the surrounding pavement sheds. On clay subgrade, poor compaction shows up as ponding sooner.

Clogged or Undersized Catch Basins

Catch basins fill with leaves, sediment, and debris — and Lake Oswego's mature tree canopy drops plenty of both. A clogged inlet cannot drain the water reaching it, and on lots designed for lighter loads the inlets may simply be undersized.

Drainage Solutions for Lake Oswego Parking Lots

Interceptors for Incoming Runoff

Where a lot receives water from higher ground, a trench drain or interceptor along the uphill edge captures that flow before it spreads across the pavement. On hillside-adjacent lots, this is frequently the most impactful fix.

Regrading and Slope Correction

Restoring proper slope so water flows to the inlets is the most direct fix for ponding. This may mean milling and overlaying low areas or re-establishing the lot's grade. Getting the slope right is the foundation of any lasting solution.

Catch Basins and Inlet Spacing

Properly placed and sized catch basins capture surface water and carry it into the storm system. Adding inlets, upsizing them, or correcting spacing addresses lots that cannot move water fast enough. Routine cleaning — important under Lake Oswego's tree canopy — keeps them working through the wet season.

Oil-Water Separators and DEQ Compliance

Commercial parking lots in Oregon are subject to stormwater regulations, and many require oil-water separators or other treatment before runoff leaves the site. Bringing a lot into compliance with DEQ stormwater requirements is often part of a drainage upgrade, especially during redevelopment.

Why a Site Assessment Comes First

No two parking lots pond for exactly the same reason, and the wrong fix wastes money. A birdbath from a settled sub-base needs different treatment than a lot-wide slope problem, a clogged storm system, or runoff arriving from an uphill slope. A contractor who surveys the lot, shoots the grades, traces incoming runoff, and inspects the existing inlets can pinpoint why water is collecting and design a fix that actually solves it.

For commercial owners, the stakes go beyond standing water. A poorly draining lot fails faster — water that sits on asphalt works into cracks, undermines the base, and shortens the pavement's life. It can also create liability and compliance exposure. The detailed engineering considerations, from catch-basin spacing to oil-water separators and DEQ requirements, are covered in our commercial parking lot drainage design in Oregon guide.

Get Your Lake Oswego Parking Lot Drainage Quote

Cojo Excavation & Asphalt designs and builds parking lot drainage for Lake Oswego and Clackamas County commercial properties. We survey your lot, shoot the grades, trace incoming runoff, inspect the existing system, and deliver a clear, no-obligation quote for a fix built for local conditions.

Request a free drainage estimate and we will respond within 24 hours. Learn more about our excavation services and how we keep Lake Oswego commercial lots draining through the wet season.