Services
Stormwater & Erosion Control
Drainage calculations, erosion control plans, stormwater pollution prevention plans (SWPPPs) and site inspections that carry a California project from grading permit to final stabilization.
What is stormwater and erosion control?
Stormwater and erosion control is the engineering and compliance work that manages rain falling on a development site: it sizes permanent drainage, keeps soil on site during construction, and prepares the plans and records that state and local stormwater permits require.
The State Water Board's Construction General Permit covers sites that disturb one acre or more of soil, or less than an acre within a larger common plan that totals an acre or more. Municipal stormwater permits and local ordinances add grading, erosion control and post-construction standards, such as Low Impact Development (LID) features that capture and treat runoff, so requirements vary by city and county.
Most stormwater work starts from a topographic survey and a grading plan, so drainage areas, slopes and discharge points stay consistent from design through construction.
Stormwater & Erosion Control services
Each service can be ordered on its own; the FAQ below explains which a project needs.
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Stormwater Calculations
Signed runoff, LID, leach trench, basin and pipe sizing calculations for grading, building and improvement permits, prepared to your agency's drainage criteria.
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Erosion & Sediment Control Plans
Site-specific sheets, phasing and details for grading and building permits that keep soil out of storm drains during construction.
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Stormwater Pollution Prevention Plans (SWPPP)
A site-specific plan written by a Qualified SWPPP Developer (QSD), plus the filing package for Construction General Permit coverage.
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SWPPP Inspections
Weekly, monthly and storm-event inspections with photos, rain records, corrective actions and required sampling under the 2022 Construction General Permit.
Process
How stormwater services fit into a project
The four services follow a project from design to close-out.
Size permanent drainage
During design, stormwater calculations size the trenches, basins, LID facilities and pipes shown on the grading and improvement plans.
Draw the erosion controls
An erosion and sediment control plan goes in with the grading or building permit and shows controls for each construction phase.
Obtain permit coverage
Sites disturbing an acre or more, counting any larger common plan, need a SWPPP and Construction General Permit coverage before work starts.
Inspect and close out
SWPPP inspections run weekly, monthly and around forecast storms until the site reaches final stabilization and the Notice of Termination is approved, ending coverage.
Stormwater & Erosion Control FAQs
Which stormwater services does my project need?
The stormwater services a project needs depend on its size, its permits and how it changes drainage. A grading permit, or a building permit that disturbs soil, usually requires an erosion and sediment control plan. Adding hard surfaces or changing how water leaves the site usually calls for stormwater calculations. Disturbing one acre or more, or less than an acre within a larger common plan that totals an acre or more, adds a SWPPP and inspections.
How do construction and post-construction stormwater rules differ?
Construction stormwater rules control sediment and other pollutants while soil is exposed, through the erosion control plan, SWPPP and inspections. Post-construction rules cover the finished site: permanent LID and drainage facilities. Under the State Water Board's Small MS4 (municipal separate storm sewer system) permit for smaller cities and counties, in effect as of September 2026, most projects creating or replacing 5,000 square feet or more of impervious surface need LID. Larger Phase I cities and counties set their own rules.
Who can prepare stormwater plans in California?
Stormwater calculations and drainage designs in California are civil engineering (Business and Professions Code §6731), prepared by or under the responsible charge of a licensed civil engineer who signs and seals them (§6735); architects practicing architecture are exempt (§6737). A SWPPP must be written by a Qualified SWPPP Developer (QSD), and a Qualified SWPPP Practitioner (QSP) carries it out. Each city or county decides who may prepare its erosion control plans.
Do stormwater rules apply to projects under one acre?
Stormwater rules still apply to projects under one acre, mostly through the city or county. A site under one acre needs Construction General Permit coverage and a SWPPP only if it is part of a larger common plan totaling an acre or more. Small MS4 cities and counties must enforce a construction runoff ordinance for smaller sites, and California's green building code (CALGreen) §4.106.2 requires new homes under one acre to control construction runoff.
What affects the cost and schedule of stormwater services?
The cost and schedule of stormwater services depend on site size and disturbed acreage, slopes and soils, the number of drainage areas, which permits apply, the construction schedule and risk level, and plan-check rounds. Share the site address, current plans, disturbed acreage and any agency comments; the proposal then sets scope and timing.
Further reading
- SWPPP requirements for California construction sites: a guide to the 2022 Construction General Permit
- Rainy-season SWPPP inspections in California: weekly, pre-storm, during-storm and post-storm requirements
- Post-construction stormwater and LID in California: what the draft Small MS4 permit means for new development
Sources & references
- Construction Stormwater General Permit, Order WQ 2022-0057-DWQ (State Water Resources Control Board)
- Phase II Small MS4 General Permit program (State Water Resources Control Board)
- Business and Professions Code §6731, civil engineering defined (California Legislative Information)
- Business and Professions Code §6735, civil engineering documents (California Legislative Information)
- 2025 California Green Building Standards Code (CALGreen), Chapter 4, §4.106.2 (ICC Digital Codes, free public viewer)
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