High-Density Urban Apartment — Civil Package
San Diego, CA
Overview
Moment prepared the complete civil stormwater package — hydrology report, LID planter design, and Water Pollution Control Plan (WPCP) — for an 8-story urban apartment (53,773 SF) replacing two existing office buildings on a 0.35-acre site. The design achieved zero net increase in peak stormwater runoff while meeting MS4 Permit and City of San Diego stormwater requirements.
The Challenge
Replacing two low-rise office buildings with an 8-story apartment on a 0.35-acre lot dramatically increased building coverage and potential runoff intensity. The civil design had to demonstrate zero net increase in 50-year peak discharge to comply with City stormwater policy, while the construction phase required a full WPCP for a 7-month build window spanning both wet and dry seasons.
Our Approach
Moment used the Rational Method with a 50-year storm event (3.2-inch isopluvial) and a runoff coefficient (C) of 0.80 to calculate peak discharge. The stormwater management system uses seven planter boxes with underdrains along the east and north building faces, with overflow daylighting to the adjacent street gutter. The WPCP addressed a March–October 2026 construction schedule with phased erosion/sediment controls, daily wet-season and weekly dry-season BMP inspections, and weather-triggered 24-hour response actions.
Results
The civil package achieved zero net increase in 50-year peak runoff (Q₅₀ = 1.41 cfs pre- and post-development), demonstrating that high-density development can meet stringent stormwater requirements. The WPCP was classified as Low Priority, requiring quarterly wet-season and as-needed dry-season inspections.
Technical Summary
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