County of Los Angeles

Low Impact Development Standards Manual

CountyCaliforniahighNo

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Other manuals in California: California, California Department of Transportation (Caltrans), City of San Diego

Document Metadata

Jurisdiction level

county

“guidance for the implementation of stormwater quality control measures in new development and redevelopment projects in unincorporated areas of the County”
Section 1.1, Page 1-1

State

California

“watersheds of Los Angeles and Ventura Counties (CAS004004, Order No. R4-2021-0105)”
Section 1.1, Page 1-1

Version / edition

2025

“This 2025 LID Standards Manual is an update of the 2014 LID Standards Manual”
Section 1.1, Page 1-1

Adoption / effective date

Not found in document

No evidence cited

Last revised date

Not found in document

No evidence cited

Relationship to state manual

independent

“The 2025 LID Standards Manual supersedes the 2014 LID Standards Manual and addresses the following objectives and goals”
Section 1.1, Page 1-1

Design Criteria

Design storm return periods (years)

2-year, 5-year, 10-year, 25-year, 50-year

“project applicants must conduct hydrology and hydraulic frequency analyses for LID, 2-, 5-, 10-, 25-, and 50-year storm events per the LACPW Hydraulic and Hydrology manuals”
Section 8.3, Page 8-1

Water quality volume method

Stormwater Quality Design Volume (SWQDv) from the greater of the 0.75-inch, 24-hour rain event or the 85th percentile, 24-hour rain event (LA County 85th percentile isohyetal map); Priority Development Projects must retain 100% of SWQDv on-site via infiltration, bioretention, and/or rainfall harvest and use. Flow-based alternative sizing: 0.2 in/hr intensity or 85th percentile hourly intensity × 2.

“The design storm, from which the SWQDv is calculated, is defined as the greater of the following: The 0.75-inch, 24-hour rain event; The 85th percentile, 24-hour rain event as determined from the Los Angeles County 85th percentile precipitation isohyetal map”
Section 6.2, Page 6-1

Peak flow calculation method(s)

MODRAT (Modified Rational Method), Rational equation (Q = Cd × I × A)

“The volume of stormwater runoff that must be retained at a project site is calculated using MODRAT”
Section 6.2, Page 6-1

Required hydrologic/hydraulic software

HydroCalc, HEC-RAS

“LACPW developed a hydrologic calculator (HydroCalc)... Use of this program is encouraged”
Section 6.3, Page 6-3

Approved BMP categories

Bioretention (RET-1), Infiltration Basin (RET-2), Infiltration Trench (RET-3), Dry Well (RET-4), Permeable Pavement without an Underdrain (RET-5), Rain Barrel/Cistern (RET-6), Biofiltration (BIO-1), Stormwater Planter (VEG-1), Tree-well Filter (VEG-2), Vegetated Swales (VEG-3), Vegetated Filter Strip (VEG-4), Sand Filters (T-1), Constructed Wetlands (T-2), Extended Detention Basin (T-3), Wet Pond (T-4), Permeable Pavement with an Underdrain (T-5), Proprietary Treatment Control Measures (T-6)

“Retention-based stormwater quality control measures (RET-1 to RET-6); Biofiltration (BIO-1); Vegetation-based stormwater quality control measures (VEG-1 to VEG-4); Treatment based-stormwater quality control measures (T-1 to T-6)”
Section 7.2, Page 7-1