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Phase 2 · Design · Step 2.4

Stormwater management plan (BES SIM)

Any new impervious area triggers BES's Stormwater Management Manual. Most DADUs use the Simplified Approach — a drywell, infiltration planter, or rain garden does the job.

Who
Designer, Civil eng., Bureau of Environmental Services
How long
2-3 weeks
Cost
$1,500-$4,000
You end up with
Stormwater plan + sizing calcs

If you skip this: BES requires on-site stormwater management for any new impervious area. A submittal without an SWMM-compliant plan and sizing calcs gets bounced for completeness.

What governs

Bureau of Environmental Services (BES) requires every project that adds impervious surface (roof + paving) to manage that stormwater on-site. The rules live in the Stormwater Management Manual (SWMM). For a typical DADU you'll use the Simplified Approach, which skips a full hydraulic calculation.

The three common DADU solutions

  1. Drywell. Perforated underground chamber that infiltrates roof runoff. Cheapest. Works in most Portland soils. ~$2,500-$4,500 installed.
  2. Infiltration planter. Above-grade box with engineered soil that takes downspout runoff and infiltrates. Needs a sliver of yard space. ~$3,000-$5,000.
  3. Rain garden. Depressed planted area. Most landscape value. Takes the most space. ~$2,500-$5,000.

You may need a geotechnical infiltration test to size the system. Some east-side and west-hill soils don't infiltrate well; in those cases a flow-through planter discharging to the combined sewer (with BES approval) is the fallback.

What the deliverable contains

  • A scaled site plan showing the stormwater facility
  • Sizing calculations per the SWMM
  • Notes that key the construction details

This usually lives on the architectural sheet set. Some designers do the calcs themselves; some bring in a civil engineer.

Where this information came from

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