We’ve designed more than 100 sanitary sewer systems for Portland-area developments, from single-lot residential infill to large multifamily and commercial sites. Every design is sized to the Portland Bureau of Environmental Services Sewer and Drainage Facilities Design Manual (BES SDFDM, March 2020), stamped by an Oregon PE, and built to get through BES plan check.
BES reviews sewer designs at the development review stage, and their engineers check the calculations. Here’s what that work involves and what they’re looking for.
What BES Plan Check Covers
Before we submit to BES, we confirm and document:
- – Wastewater generation estimates based on land use and projected occupancy
- – Peak flow calculations using a peaking factor applied to average daily flow, plus an I&I (infiltration and inflow) allowance for the receiving basin
- – Hydraulic pipe and grade calculations confirming capacity and self-cleansing velocity at design flow
- – PE-stamped plans showing plan and profile views, manhole locations, pipe materials, slopes, and cleanout spacing per code
- – Available capacity in the receiving public main to accept the project’s peak flow without surcharging
For larger projects, we run a capacity analysis on the downstream public system before submitting. That way there are no surprises from BES’s capacity review after the application is in.
Minimum Slopes and Pipe Sizing
The governing design criterion for a gravity sewer is self-cleansing velocity: a minimum of 2 feet per second at the design flow, so solids don’t settle and accumulate in the pipe. That velocity requirement sets the minimum slope for each pipe diameter.
Minimum slopes by diameter, based on the 2 ft/s self-cleansing criterion:
| Pipe diameter | Minimum slope |
|—|—|
| 4-inch | 2.08% |
| 6-inch | 1.04% |
| 8-inch | 0.40% |
| 12-inch | 0.22% |
| 18-inch | 0.12% |
Portland’s topography cuts both ways. Flat parcels in neighborhoods like Cully or Lents sometimes require careful grading to maintain adequate slope from building to main. Steep sites in the West Hills create the opposite problem: velocities can exceed 10 ft/s, and we use drop manholes or energy dissipators to manage the grade change without damaging the pipe or the receiving manhole structure.
We cover the full calculation method, including Manning’s equation and peaking factor selection, in our sanitary sewer slope and pipe sizing guide.
Connecting to Portland’s Public Sewer Main
New connections to Portland’s public sewer system require a BES connection permit. The process starts with locating the existing main. We pull BES sewer atlas records for the block and supplement with potholing where the records leave depth or alignment uncertain. That gives us accurate information before we commit to a lateral grade and connection detail on the plans.
Most residential and small commercial projects connect via a new private side sewer lateral running from the building to the public main at the property line or the street. The slope must meet the minimum requirements for the pipe diameter, and BES inspects the open-trench connection before backfill.
For larger projects, we coordinate directly with BES during design on system capacity. If the development triggers public main extension or upsizing requirements, those conversations happen before plan submittal. Finding a capacity constraint mid-review costs schedule; finding it during early coordination costs a conversation.
BES also requires a sewer scope inspection at property sale for parcels in certain basins with documented inflow and infiltration problems. If a project site falls in one of those basins, we flag it at the start of design so the client has accurate information before any purchase or permit decision.
When a Lift Station Is Needed
Not every Portland site can drain by gravity. Below-grade units in a basement, parcels at the low end of a block with no accessible downstream main, and sites where the public sewer runs significantly deeper than the building floor are candidates for a lift station.
We design duplex wet-well-and-pump systems to BES standards, including force main sizing, emergency overflow provisions, and controls. BES reviews lift station designs separately from the gravity system, and that review takes additional time. We identify lift station sites early so the added schedule and cost are in the project budget before anyone is surprised.
Our Portland Sewer Design Process
We start by pulling BES sewer atlas data and confirming the public main’s location, depth, and available capacity. From there, we size the lateral or on-site collection system to the project’s projected flows, run the hydraulic calculations, and coordinate with BES on capacity questions before the first submittal.
Our Portland civil engineering team handles BES development review from early scoping through plan check approval. Sewer design is one component of the complete site development package we produce for Portland multifamily, commercial, and institutional projects.
Start a Conversation
Call us at (503) 843-8211 or fill out the contact form below to tell us about your project. We’ll confirm the connection point, estimate the design scope, and let you know what BES will need from us to get through plan check.