Parking Lot Drainage in Hillsboro, Oregon: Stop the Ponding

Why Hillsboro Parking Lots Pond

Hillsboro's growth along the Sunset Corridor and through its tech and commercial districts has put a lot of asphalt on the flat Tualatin Valley floor — and flat valley ground is exactly where parking lots struggle to drain. Washington County's heavy clay sub-base holds water, the seasonal water table runs high, and the level terrain gives runoff almost no slope to follow. Add months of steady Pacific Northwest rain and a Hillsboro lot can pond badly without an engineered drainage system doing the work the land will not.

A ponding parking lot is more than an eyesore for a property manager. Standing water accelerates asphalt failure, undermines the base, breeds potholes, washes out striping, and creates a liability hazard. On Hillsboro's flat clay, the problem sets in fast. The causes are finite and the fixes are known — but they have to account for the flat ground, the clay, and the high water table. This guide walks through all of it.

For the wider picture, see our guide to property and site drainage in Oregon. For lot grading engineering, see parking lot drainage design in Oregon.

What Causes Lot Ponding in Washington County

Flat Ground and Marginal Slope

A lot needs roughly a 1 to 2 percent slope to drain, and on the flat Tualatin Valley floor that slope is hard to build and easy to lose. As Hillsboro asphalt settles under traffic, even a well-graded lot can flatten into low spots that hold water, because there is little natural fall to carry it off.

Saturated Clay Sub-base

When the clay base under the asphalt stays saturated through the wet season, it loses load-bearing strength. The surface sinks, new low spots form, and the lot becomes more prone to ponding and potholing — a defining Hillsboro problem.

A High Water Table

The valley's groundwater rises in winter, keeping the sub-base charged with water and reducing how much additional rain the ground can take. That pushes more water to the surface and into the drainage system.

Clogged or Undersized Catch Basins

Months of rain wash silt and debris into the inlets and storm lines. A basin that worked in summer backs up once half-full, and undersized systems cannot move Hillsboro's peak winter flows.

How Parking Lot Drainage Gets Fixed in Hillsboro

The repair depends on the cause, so an assessment comes first. Common solutions include:

• Regrading and an asphalt overlay — Milling and overlaying to re-establish positive slope is the most durable fix for widespread ponding on flat, settled lots.
• Adding or relocating catch basins — A new inlet tied into the storm system gives water somewhere to go where it currently collects, important where flat ground offers no surface escape.
• Trench drains — Across entrances and drive aisles, linear trench drains capture sheet flow that flat ground will not move on its own.
• Storm line repair or replacement — Cracked or undersized pipe gets excavated and replaced, often with larger line to handle wet-season flows.
• Sub-base repair — Where saturated clay has failed the base, the affected area is excavated and rebuilt before resurfacing.

Our excavation services cover the grading, trenching, and catch-basin work involved.

What Parking Lot Drainage Costs in Hillsboro

Drainage corrections are priced by the foot, by the structure, and by the volume of excavation — never a flat rate. Industry baseline ranges commonly referenced include:

Work	Industry Baseline Range
Catch basin installation (each)	$1,200–$3,500
Trench drain (per linear foot)	$50–$150
Storm line run (per linear foot)	$25–$60
Asphalt overlay for regrading (per sq ft)	$2–$5

In Hillsboro, cost drivers are excavation depth, whether the clay sub-base needs rebuilding, the distance to a storm tie-in, and how much asphalt has to be removed. Flat ground can mean more catch basins or trench drains to compensate for the lack of slope. A new catch basin and short pipe run sits at the low end; a full regrade with sub-base repair runs well above it. A site assessment is the only way to budget accurately.

Why a Site Assessment Matters in Hillsboro

Two Hillsboro lots with identical ponding can need completely different repairs. One may have a flat slope correctable with an overlay; the next may have a saturated, failing clay sub-base or a high water table that changes the whole approach. A proper assessment measures the slope, inspects and flushes the catch basins and lines, evaluates the sub-base, and accounts for the water table. Skipping it is how property owners pay twice — first for a cosmetic patch, then for the real repair after it ponds again next winter.

Get Your Hillsboro Parking Lot Drainage Quote

Cojo Excavation & Asphalt helps Hillsboro and Washington County property managers stop ponding for good, with solutions designed for flat valley clay and a high winter water table. We assess your slope, inspect your drainage, and recommend the most cost-effective fix for your actual conditions.

Request a free drainage assessment and we will respond within 24 hours. Learn more about our excavation services for Hillsboro-area commercial properties.