Triplex & ADU with Basement Garage
Orange, CA
Overview
Moment prepared the LID stormwater compliance report for a development with three single-family homes, two ADUs, and a subterranean parking garage on a 0.29-acre lot (16,671 SF total building area). The project was classified as a Priority Project under the Orange County Model WQMP, requiring enhanced stormwater treatment.
The Challenge
The site sits within a seismically induced liquefaction hazard zone per the OC Stormwater Land Development Tool, which fundamentally changed the stormwater approach. In most Orange County projects, infiltration is the preferred LID strategy — but here, adding water to loose saturated soils would worsen seismic vulnerability. The basement garage added another layer of complexity to grading and drainage design.
Our Approach
Moment designed a lined bioretention basin on the east side of the site with underdrains that biotreats 100% of the Design Capture Volume (243.2 cu-ft) without any stormwater infiltration into the ground. The system captures and treats the 85th percentile, 24-hour storm (0.75 inches) while routing treated water to the storm drain system. Source control measures were specified per the OC Model WQMP, including storm drain stenciling (S-1/SD-13), hazardous material storage design (S-2/SD-34), trash enclosure design (S-3/SD-32), and efficient irrigation (S-4/SD-12).
Results
The lined bioretention system achieved full LID compliance on a site where the most common stormwater strategy — infiltration — was explicitly prohibited by geotechnical constraints. The design was submitted for City of Orange approval.
Technical Summary
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