22 - Low Impact Development (LID)

Background / Introduction

Note: The City has a Customer Service Request (CSR) Portal (located in the Customer Portal) to allow members of the public to make contact with City Staff for questions and/or assistance on LID related matter.  

Urban runoff discharged from municipal storm drain systems has been identified by local, regional, and national research programs as one of the principal causes of water quality impacts in most urban areas. Urban runoff potentially contains a host of pollutants such as trash and debris, bacteria and viruses, oil and grease, sediments, nutrients, metals, and toxic chemicals.

These contaminants can adversely affect receiving and coastal waters, associated biota, and public health. An epidemiological study by the Santa Monica Bay Restoration Project was conducted to investigate possible health effects of swimming in Santa Monica Bay. Study results indicated that individuals swimming near flowing storm drain outlets have a greater risk of developing various symptoms of illnesses compared to those swimming 400 yards away from the same drains. In addition, oil and grease from parking lots, leaking petroleum or other hydrocarbon products, leachate from storage tanks, pesticides, cleaning solvents, and other toxic chemicals can contaminate stormwater and be transported downstream into water bodies, such as ponds, lakes, reservoirs, and receiving waters such as rivers, creeks, channels, and oceans. Fertilizer constituents from lawns and golf courses or leaking septic tanks can cause algal blooms. Disturbances of the soil from construction can allow silt to wash into storm channels and receiving waters, making them muddy, cloudy, and inhospitable to natural aquatic organisms. Heavy metals are toxic to aquatic organisms and many artificial surfaces of the urban environment such as galvanized metal, paint, or preserved wood containing metals contribute to stormwater pollution as the surfaces corrode, flake, dissolve, or decay.  

Land development and construction activities significantly alter drainage patterns and contribute pollutants to urban runoff primarily through erosion and removal or change of existing natural vegetation. When homes, shops, work places, recreational areas, roads, parking lots, and structures are built, increased flows are discharged into local waterways. As the amount of impervious surface increases, water that once percolated into the soil now flows over the land surface. Accordingly, increases in impervious surfaces can increase the frequency and intensity of stormwater flows through a watershed. Flow from rainstorms and other water uses wash rapidly across the impervious landscape, scouring the surface of various kinds of urban pollutants such as automotive fluids, cleaning solvents, toxic or hazardous chemicals, detergents, sediment, metals, bacteria, pesticides, oil and grease, and food wastes. These pollutants, unfiltered and unfettered, flow through stormwater infrastructure and ultimately contaminate receiving waters.

The purpose of this section is to assist developers in complying with the requirements of the Development Planning Program regulations of the City’s Stormwater Program. This section summarizes the City’s project review and permitting process, identifies stormwater mitigation measures, and references source and treatment control BMP information. It provides guidance for individuals involved in new development and redevelopment projects. The target audience for this section includes developers, designers, contractors, homeowners, and City staff who are engaged in plan-checking, permitting, and inspections related to land development activities. This section also documents the necessary forms and worksheets required to be completed by the developer for approval.

Laws, Codes and Regulations

• City of Los Angeles Stormwater LID Ordinance: Current LID Ordinance 188125 (Council File 22-0600-S54) dated February 20, 2024 and effective April 2, 2024 (previous Stormwater LID Ordinance 181899 was November 2011 and updated September 2015), with the stated purpose of:
  • Requiring the use of LID standards and practices in future developments and redevelopments to encourage the beneficial use of rainwater and urban runoff
  • Reducing stormwater/urban runoff while improving water quality
  • Promoting evapotranspiration from green roofs and rainwater harvesting
  • Reducing offsite runoff and providing increased groundwater recharge
  • Reducing erosion and hydrologic impacts downstream
  • Enhancing the recreational and aesthetic values in our communities
• State of California General Plan Law (CGPL) for Municipalities: Municipalities are required to develop policies and regulations that guide development within the municipality.  Each development project is reviewed for conformance with these policies.
• California Environmental Quality Act (CEQA): Projects are subject to review and comment for potential adverse environmental impacts, including impacts from stormwater discharges

General Conditions

• The Federal Clean Water Act: Principal vehicle for control of stormwater pollution. Under the Federal Clean Water Act, each municipality throughout the nation is issued a stormwater permit through the National Pollutant Discharge Elimination System (NPDES) program. The primary goal of each permit is to stop polluted discharges from entering the storm drain system, local receiving water body and coastal water.  In California, the NPDES stormwater permitting program is administered by the State Water Resources Control Board (SWRCB) through its nine Regional Boards.  
• Los Angeles Regional Water Quality Control Board (Regional Board or RWQCB):  Board adopted Order related to the NPDES Stormwater Permit (Permit) for the County of Los Angeles and cities within. The Permit was issued to Los Angeles County Flood Control District, the county of Los Angeles, and 85 incorporated cities within the coastal watersheds of Los Angeles County, Ventura County Watershed Protection District, the County of Ventura, and 10 incorporated cities within Ventura County to reduce pollutants discharged from their Municipal Separate Storm Sewer Systems (MS4) to the Maximum Extent Practicable (MEP) statutory standard. 
  
  The requirement to implement the Permit is based on federal and state statutes, including Section 402(p) of the Federal Clean Water Act, Section 6217 of the Coastal Zone Act Reauthorization Amendments (CZARA) of 1990, and the California Water Code. The Federal Clean Water Act amendments of 1987 established a framework for regulating stormwater discharges from municipal, industrial, and construction activities under the NPDES program. The primary objectives of the stormwater program requirements are to reduce the discharge of pollutants from stormwater conveyance systems, prohibit non-stormwater and trash discharges, and to meet MEP statutory standard while complying with applicable water quality-based effluent limitations. Orders related to the City of Los Angeles include:
  • CURRENT - July 23, 2021 Order No RA 2021-0105 (NPDES No.  CAS004004) - Effective September 11, 2021
  • November 8, 2012 Order No RA 2012-0175(NPDES No.  CAS004001) - Effective December 28, 2012

Based on the Permit issued by the Regional Board, each permittee is required to develop and implement a number of stormwater management programs designed to reduce pollutants in stormwater and urban runoff including: 

• Public Information and Participation Program
• Industrial/Commercial Facilities Program
• Illicit Connections and Illicit Discharges Elimination Program
• Planning and Land Development Program
  • Focuses on preventing pollutants that could be generated from new development and redevelopment projects from reaching stormwater conveyance systems and receiving waters. Under this program, the RWQCB developed requirements for the Standard Urban Stormwater Mitigation Plan (SUSMP) which requires specific development and redevelopment categories to manage stormwater runoff.  In 2002, the City of Los Angeles implemented the SUSMP program requiring all the affected land development projects to capture or treat stormwater runoff.
    
    A relatively recent stormwater management approach aimed at achieving this goal is the use of Low Impact Development (LID). Recent actions by the RWQCB, SWRCB, and US EPA have prioritized the use of LID as the preferred approach to stormwater management, including for the purpose of water quality compliance.  
• Development Construction Program
• Public Agency Activities Program
• Monitoring and Reporting Program

Definitions

Please review the Low Impact Development Definitions page to understand any definitions that may be referenced in this section.

• Low Impact Development (LID): A stormwater management strategy that seeks to mitigate the impacts of increases in runoff and stormwater pollution as close to its source as possible. LID comprises a set of site design approaches and Best Management Practices (BMPs) that promote the use of natural systems for infiltration, evapotranspiration, and use of stormwater. These LID practices can effectively remove nutrients, bacteria, and metals from stormwater while reducing the volume and intensity of stormwater flows. With respect to urban development and redevelopment projects, it can be applied onsite to mimic the site’s predevelopment drainage characteristics. Through the use of various infiltration techniques, LID is geared towards minimizing surface area that produces large amounts of runoff and does not allow water to infiltrate into the ground. Where infiltration is infeasible, the use of bioretention, rain gardens, vegetated rooftops, and rain barrels that will store, evaporate, detain, and/or treat runoff can be used. (Different South Coast Regional Water Quality Control Board)

Project Review and Permitting

Plan Approval

The requirement to incorporate stormwater pollution control measures into the design plans of new development and redevelopment projects is implemented through the City’s plan review and approval process in order to mitigate stormwater quality impacts. During the review process, the plans will be reviewed for compliance with the City’s General Plans, zoning ordinances, and other applicable local ordinances and codes, including stormwater requirements. Plans and specifications will be reviewed to ensure that the appropriate BMPs are incorporated to address stormwater pollution prevention goals. The reviewer will also determine if project designs need to be modified to address stormwater pollution prevention objectives.

New development and redevelopment projects are mainly processed through Department of City Planning (DCP) and LADBS.  Entitlement approvals are processed by DCP and these projects require discretionary action.  Building/Grading Permit approvals are processed by LADBS. Applicants should refer to the Department of City Planning and Department of Building and Safety for their requirements. 

Responsibilities of City Counter Staff

To verify compliance with all City Codes, it is recommended that the architect, civil engineer, plumbing engineer, and/or landscape architect coordinate at the early stage of the project design.  Also City plan-checking staff is available for consultation regarding the applicable requirements based on the project concept or constraints.

1. Step One - Identify appropriate BMPs
   1. Identify, evaluate, and incorporate into the plan documents the appropriate BMPs for the project categories listed in the Stormwater Management section of this page.
      
      Approval for development projects and building/grading permits will not be granted/issued until appropriate and applicable stormwater BMPs are incorporated into the project design plans. Also, a plumbing permit from LADBS will be required for certain treatment control BMPs such as grease traps, sump pumps, and clarifiers. If an infiltration BMP is chosen for treatment control, a soils report to address the feasibility of infiltration will be required to be submitted with the plan for review and approval.
2. Step Two– Submit LID plans to WPD for review
   1. For first review, submit the following files to the LID portal at https://lid.lacitysan.org under “Plan Check Review” Application:
      1. One (1) full set of plans (plot/site, architectural building elevations, utility, mechanical, plumbing, grading and landscape plans).
      2. LID Report which at a minimum include, but not limited to, at least the following:
         • Existing site conditions
         • Scope of work (Proposed site conditions)
         • Discussion on feasibility based on the "BMP Prioritization and Selection" of this page (infiltration, capture & use, and high efficiency biofiltration)
         • Volume calculation (by tributary area)
           • If certain areas will not be treated, quantify and explain how it will be compensated in a different tributary area.
      3. Plans must include, but not limited to, at least the following:
         • Location of all BMPs on plans, including elevations and drainage patterns.
         • Detailed drawings of all BMPs, including model, size, and capacity
         • “No Dumping Drains to Ocean” Stenciling note and/or detail
         • Trash enclosure location and details
         • Landscaping areas
         • Stormwater Observation Report Form
         • Manufacturer’s product specifications to verify that the selected BMP model can adequately handle the design volume.
         • Operation & Maintenance Plan as discussed in the "BMP Maintenance" section of this document
         • 8.5” x 11” Plot Plan clearly showing BMP size and location(s)
         • Final plans must be stamped and signed by an engineer and/or architect
3. Step Three – WPD Approval
   1. City plan-checking staff will review the submitted documents and identify corrections. Once all LID requirements have been met, City staff will digitally stamp/approve the final plans, sign the applicant’s clearance worksheet, and clear the project in the LADBS plan check tracking system (Plan Check and Inspection System [PCIS]).

Inspection

Please refer to 05 - ACOS - Stormwater section under Building & Safety Clearances, Technical Procedures, ACOS Clearances.

BMP Maintenance

Maintenance is crucial for proper and continuous operation, effectiveness, and efficiency of a structural or treatment control BMP. The cost of long-term maintenance should be evaluated during the BMP selection process.

A Covenant and Agreement (C&A) document must be signed and notarized by the legal owner or authorized agent of the property and shall be recorded with the County Recorder’s Office. By signing and notarizing the Covenant and Agreement maintenance form, the legal property owner affirms they will perform regular and long-term maintenance of all BMPs installed onsite. 

• For residential properties where the structural or treatment control BMPs are located within a common area and will be maintained by a homeowner’s association, language regarding the responsibility for maintenance must be included in the project’s conditions, covenants and restrictions (CC&Rs).  The C&A is bound to the property and transfers to the new owner with any subsequent sale of the property.  If the property owner is not signing the C&A, the applicant will be required to submit an original Letter of Authority designating the individuals signing the C&A form.  
• Attached to the C&A will be an Operation and Maintenance (O&M) Plan that shall describe the bulleted items below. In addition, a maintenance log shall be maintained at the facility to document all of the bulleted items below. These documents may be inspected by the City of Los Angeles at any time and shall be made available upon request.
  • BMP operation and maintenance procedures
  • Employee training program and duties
  • Operating schedule
  • Maintenance frequency
  • Routine service schedule
  • Other activities

Public Agency Projects

Stormwater mitigation measures are required for all projects subject to the LID Plan. City projects that will be processed through DCP and/or LADBS will be subject to the review and approval process described in the above "Plan Approval" section.  For other City projects that don't undergo plan review and approval with DCP and/or LADBS, the public agency must use this section to incorporate the required stormwater mitigation measures into their projects.

Public agency projects other than the City of Los Angeles, such as State of California, County of Los Angeles, the Metropolitan Transit Authority that require a permit from LADBS are required to prepare a LID Plan and implement stormwater mitigation measures as well as meet the thresholds for development or redevelopment projects as follows.  

1. New Development projects of the following categories:
   1. Projects equal to 1 acre or greater of disturbed area and adding 10,000 SF or more of impervious surface area (collectively over the entire project site)
   2. Industrial parks of 10,000 SF or more of surface area.
   3. Commercial malls of 10,000 SF or more of surface area
2. Redevelopment projects that create and/ or replace 5,000 SF or more of impervious surface area (collectively over the entire project site) on any of the following:
   1. Existing sites of 10,000 SF or more of impervious surface area
   2. Industrial parks of 10,000 SF or more of surface area
   3. Commercial malls of 10,000 SF or more of surface area
3. New development and redevelopment projects that create and/ or replace 5000 SF or more of impervious surface area (correctively over the entire project site) and support one or more of the following uses:
   1. Parking lots
   2. Automotive service facilities (SIC 5013, 5014, 5511, 5541, 7532-7534 or 7536-7539)
   3. Gasoline islands
4. New development and redevelopment projects that create and/ or replaces 2,500 SF or more of impervious surface area, and are located partly or wholly within an Environmentally Sensitive Area as defined in the MS4 Permit.

In addition, non-roadway transportation projects that meet the following threshold for LID categories are also required to implement stormwater mitigation measures.  

1. Street and road construction of 10,000 SF or more of impervious surface area shall follow U.S. EPA guidance regarding Managing Wet Weather with Green Infrastructure; Green Streets (December 2008 EPA-833-F08-009) or as may be amended, to the maximum extent practicable. Street and road construction applies to standalone streets, roads, highways, and freeway projects. Temporary access roads are not subject to this requirement. Projects under this category are otherwise exempt from the requirements of this article and are not considered Priority Development and Redevelopment Projects.
   
   Examples of such projects include the rail lines and stations, airport runways, and busways. Such projects must incorporate stormwater BMPs into their design plans and specifications, which must be submitted to WPD for review and approval. 

Stormwater Mitigation Measures

General Requirements

Applicants for all new developments and redevelopments projects will be required to incorporate stormwater mitigation measures into their design plans and submit the plans to the City for review and approval as indicated in the "Project Review and Permitting" section of this page. Projects must incorporate the following performance measures and practices into their design plans.

1. Lessen the water quality impacts of development by using smart growth practices such as compact development, directing development towards existing communities via infill or redevelopment, and safeguarding of environmentally sensitive areas.
2. Minimize the adverse impacts from stormwater runoff on the biological integrity of Natural Drainage Systems and the beneficial uses of water bodies in accordance with requirements under CEQA (Cal. Pub. Resources Code § 21000 et seq.).
3. Minimize the percentage of impervious surfaces on land developments by minimizing soil compaction during construction, designing projects to minimize the impervious area footprint, and employing Low Impact Development (LID) design principles to mimic predevelopment hydrology through infiltration, evapotranspiration and rainfall harvest and use.
4. Maintain existing riparian buffers and enhance riparian buffers when possible.
5. Minimize pollutant loadings from impervious surfaces such as roof tops, parking lots, and roadways through the use of properly designed, technically appropriate BMPs (including Source Control BMPs such as good housekeeping practices), LID Strategies, and Treatment Control BMPs.
6. Properly select, design and maintain LID and Hydromodification Control BMPs to address pollutants that are likely to be generated, reduce changes to pre-development hydrology, assure long-term function, and avoid the breeding of vectors.
7. Prioritize the selection of BMPs to remove storm water pollutants, reduce stormwater runoff volume, and beneficially use storm water to support an integrated approach to protecting water quality and managing water resources in the following order of preference:
   1. On-site infiltration, bioretention, evapotranspiration from green roof, and/or rainfall harvest and use;
   2. If the above (7.a.) is infeasible, then on-site biofiltration, off-site groundwater replenishment, and/or off-site retrofit; and/or   
   3. If the above (7.b.) is infeasible, on-site treatment shall be proposed 

Priority Development Projects

The following Priority Development Projects shall comply with the standard and requirements of updated LID Ordinance 188125 (Council File 22-0600-S54) dated February 20, 2024:

1. New Development projects of the following categories:
   1. Projects equal to 1 acre or greater of disturbed area and adding 10,000 SF or more of impervious surface area (collectively over the entire project site)
   2. Industrial parks of 10,000 SF or more of surface area.
   3. Commercial malls of 10,000 SF or more of surface area
2. Redevelopment projects that create and/ or replace 5,000 SF or more of impervious surface area (collectively over the entire project site) on any of the following:
   1. Existing sites of 10,000 SF or more of impervious surface area
   2. Industrial parks of 10,000 SF or more of surface area
   3. Commercial malls of 10,000 SF or more of surface area
3. New development and redevelopment projects that create and/ or replace 5000 SF or more of impervious surface area (correctively over the entire project site) and support one or more of the following uses:
   1. Restaurants
   2. Parking lots
   3. Automotive service facilities s (SIC 5013, 5014, 5511, 5541, 7532-7534 or 7536-7539)
   4. Retail Gasoline outlets
4. New development and redevelopment projects that create and/ or replaces 2,500 SF or more of impervious surface area, and are located partly or wholly within an Environmentally Sensitive Area as defined in the MS4 Permit
5. Street and road construction of 10,000 SF or more of impervious surface area shall follow U.S. EPA guidance regarding Managing Wet Weather with Green Infrastructure; Green Streets (December 2008 EPA-833-F08-009) or as may be amended, to the maximum extent practicable. Street and road construction applies to standalone streets, roads, highways, and freeway projects. Temporary access roads are not subject to this requirement. Projects under this category are otherwise exempt from the requirements of this article and are not considered Priority Development and Redevelopment Projects.

Priority Development Projects are required to implement structural BMPs to comply with the following:

1. Stormwater runoff will be infiltrated, evapotranspired, captured and used, and/or treated through high removal efficiency Best Management Practices onsite, through stormwater management techniques as identified in the "Prioritization of BMP Selection: section of this handbook. The onsite stormwater management techniques must be properly sized, at a minimum, to infiltrate, evapotranspire, store for use, and/or treat through a high removal efficiency biofiltration/biotreatment system, without any stormwater runoff leaving the site to the maximum extent feasible, for at least the volume of water produced by the stormwater quality design storm event that results from:
   1. The 0.75-inch, 24-hour rain event, or
   2. The 85th percentile 24-hour runoff event determined from the Los Angeles County 85th percentile precipitation isohyetal map, whichever is greater.

Refer to Los Angeles County website to determine the depth of the 85th percentile, 24-hour runoff event and the Sample Design Calculations attachment for additional information. 

Requirements

Developments shall adhere to the following requirements:

1. For new developments or redevelopments resulting in an alteration of at least fifty percent or more of the impervious surfaces of an existing developed site, the entire site shall comply with the standards and requirements of the "Priority Development Projects" section.
2. Where the redevelopment results in an alteration of less than fifty percent of the impervious surfaces of an existing developed site, only such incremental development shall comply with the standards and requirements of the "Priority Development Projects" section.
   
   If partial or complete onsite compliance of any type is technically infeasible, the project Site and LID Plan shall at a minimum treat runoff using a structural BMP as well as mitigate the equivalent volume under the Offsite Mitigation Option. The following depicts the design requirements for all other developments.

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Hydromodification

New development and/or redevelopment projects that drain to natural drainage systems in a small part of the Upper Los Angeles River watershed shall control post-development peak storm water runoff discharge rates, velocities, and duration (peak flow control) to mimic pre-development hydrology and to prevent accelerated stream erosion and to protect stream habitat.  These controls should be consistent with the Hydromodification Control Plan developed by the County of Los Angeles, Department of Public Works.

Source Control Measures

Source control measures are low-technology practices designed to prevent pollutants from contacting stormwater runoff or to prevent discharge of contaminated runoff to the storm drainage system.  This section addresses source control measures consisting of specific design features or elements. These control measures have been developed for specific types of sites or activities that have been identified as potential significant sources of pollutants in stormwater. When appropriate, the source control measure requirements discussed in this section shall be incorporated in the design plans in conjunction with any other operational source control measure such as good housekeeping, and employee training to optimize pollution prevention.

Some of the measures presented in this section require connection to the sanitary sewer system.  Connection and discharge to the sanitary sewer system without prior approval or obtaining the required permits is prohibited.  Contact the WPD staff to obtain information regarding obtaining sanitary sewer permits from the appropriate City office.  Discharges of certain types of flows to the sanitary sewer system may be cost prohibitive and may not be allowed.  The designer is urged to contact the appropriate City offices prior to completing site and equipment design of the facility.

Source control measures and associated design features specified for various sites and activities are summarized in the table below and the fact sheets presented in the "Source Control Measures" document for each source control measure.  These sheets include design criteria established by the City to verify effective implementation of the required measures.

BMP Prioritization and Selection

BMP Selection

As the primary goal, BMPs shall be designed to manage and capture stormwater runoff on-site.

The order of priority specified below shall apply to the Priority Development Projects identified in the “Priority Development Projects” section above. Each BMP type shall be implemented to the maximum extent feasible.

1. On-site infiltration, bioretention, evapotranspiration from green roof, and/or rainfall harvest and use.
2. If the above is infeasible, then on-site biofiltration, off-site groundwater replenishment, and/or off-site retrofit; and/or
3. If the above are infeasible, then on-site treatment. A LID mitigation plan shall be prepared to show that on-site stormwater management techniques are properly sized to mitigate the volume of water produced by the SWQDV.

If infiltration, bioretention, green roof, rainfall harvest and use, on-site biofiltration, permeable surface, or a combination of, were technically infeasible to achieve a complete on-site compliance, partial compliance shall be implemented on-site to the Maximum Extent Practicable (MEP) before proposing any off-site measures. Off-site measures may be proposed for the remaining unmitigated stormwater volume. Any remaining runoff that is infeasible to manage on-site and off-site may be mitigated using on-site treatment system.

Infiltration

Infiltration refers to the physical process of percolation, or downward seepage, of water through a soil’s pore space. As water infiltrates, the natural filtration, adsorption, and biological decomposition properties of soils, plant roots, and micro-organisms work to remove pollutants prior to groundwater replenishment. Infiltration BMPs include infiltration basins, infiltration trenches, infiltration galleries, bioretention without an underdrain, dry wells, and permeable pavement. Infiltration can provide multiple benefits, including pollutant removal, peak flow control, groundwater recharge, and flood control. However, conditions that can limit the use of infiltration include soil properties, proximity to building foundations and other infrastructure, geotechnical hazards (e.g., liquefaction, landslides), and potential adverse impacts on groundwater quality (e.g industrial pollutant source areas, contaminated soils, groundwater plumes). To ensure that infiltration would be physically feasible and desirable (i.e., not have adverse impacts), a categorical screening of site feasibility criteria must be completed prior to the use of infiltration BMPs following the guidelines above.

The implementation of infiltration BMPs may be deemed feasible or infeasible at a project site due to existing site conditions. To assist in the determination of compliance feasibility, a categorical screening of specific site information shall be carried out to assess site conditions. Categorical screenings must be verified by a site-specific geotechnical investigation report and/or hydrologic analysis conducted and certified by a State of California registered professional geotechnical engineer or geologist and approved by LADBS. Refer to the County of Los Angeles Department of Public Works Geotechnical and Materials Engineering Division’s Guidelines for Geotechnical Investigation and Reporting Low Impact Development Stormwater Infiltration document for testing methods that can be used to determine the insitu infiltration rates.

Assessing Site Infiltration Feasibility

Assessing a site’s potential for implementation of Low Impact Development Best Management Practices (LID BMPs) and infiltration BMPs requires both the review of existing information and the collection of site-specific measurements. Available information regarding site layout and slope, soil type, geotechnical conditions, and local groundwater conditions should be reviewed as discussed below. In addition, soil and infiltration testing is required to be conducted to determine if stormwater infiltration is feasible and to determine the appropriate design parameters for the infiltration BMP.

Geotechnical Considerations and Report Requirements:

When the on-site infiltration system is technically feasible, a Stormwater Infiltration Study, prepared by a professional Geotechnical or Civil Engineer licensed in the State of California and/or a Certified Engineering Geologist, shall be included with the LID report. The study shall include the field percolation test data, the design infiltration rate, the historical and explored groundwater depths, and the infiltration system location and setback recommendations. Refer to the current Department of Building Information Bulletin, “Guidelines for Stormwater Infiltration”. 

For projects where infiltration is infeasible, the Stormwater Infiltration Study shall specify the reasons for infeasibility, and include supporting field data and documents. The technical infeasibility may result from conditions that were listed in the "Consideration of Redevelopment Projects" section. Infiltration is also deemed infeasible for projects located on a hillside grading area, or in a liquefaction zone, and will not require a Stormwater Infiltration Study, unless an infiltration system is proposed.

Infiltration projects located in the Upper Los Angeles River Area (ULARA) must comply with the ULARA Watermaster Requirements, including boundaries, requirements, and approvals.

Design Criteria and Requirements

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Unless specifically stated, the following criteria and requirements listed below are required for the implementation of all infiltration

 BMPs. Provisions not met must be approved by the City of Los Angeles.

• Infiltration BMPs have been designed and constructed to promote uniform ponding and infiltration.
  Where necessary, a sediment forebay or separate pretreatment unit (e.g. vegetated swale, filter strip, hydrodynamic device, etc.) is located between the inlet and infiltration BMP. The sediment forebay has a volume greater than or equal to 25% of the total design volume.
• Sediment forebay has a minimum length to width ratio of 2:1 and is designed to conduct flow to the infiltration BMP.
• Any embankment slopes (interior and exterior) are not steeper than 3:1 (H:V) unless approved by the City of Los Angeles.
• The bottom of the infiltration bed is native soil and has been over-excavated to at least one foot in depth.  It is recommended that the excavated soil be amended with 2 – 4 inches of coarse sand before being replaced uniformly without compaction.
• The hydraulic conductivity (K_sat) of the subsurface layers is sufficient to ensure the maximum drawdown time of 48 hours.
• Provided overflow safely conveys flows to the downstream stormwater conveyance system, an additional BMP, or an alternatively acceptable discharge point.
• Where the infiltration system is placed underground, an observation well is provided for inspection/maintenance purposes.
• Porous pavement facilities consist of various layers of material. The top layer consists of either asphalt or concrete with a percentage of voids of at least 15%. This layer is followed by a washed stone reservoir layer or a thick layer of washed aggregate with 25-35% voids. Two transition layers are also present. The depth of each layer and the specific materials used  shall be determined by a licensed civil engineer.
• Dry wells shall be filled with 3/4 – 1 inch washed crushed rock, recycled concrete aggregate, or open-graded gravel (i.e. gravel with a small percentage of small particles). If a perforated pipe has been installed in the well, perforations are 3/8” and are smaller than the fill gravel. A woven geotextile shall be placed over the top of the drywell to prevent sediment clogging.

Soil and Vegetation Requirements

Soil and vegetation to be incorporated in infiltration facilities shall be selected by a licensed landscape architect.  In general, drought and flood resistant plant species native to California should be selected when possible. Soil media should be selected to not restrict performance requirements. Selected soils shall therefore have a higher hydraulic conductivity than the underlying soil, shall be able to support the selected plant palette, and shall be graded to provide adequate filtration as to not clog underlying soils.

Construction Requirements

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To preserve and avoid the loss of infiltration capacity, the following construction guidelines shall be adhered to:

• The entire area draining to the infiltration facility is stabilized before construction of the infiltration facility begins, or a diversion berm is placed around the perimeter of the infiltration site to prevent sediment entrance during construction.
• Infiltration BMPs shall not be used as sediment control facilities during construction.
• Compaction of the subgrade with vehicles and/or equipment is minimized. If the use of heavy equipment on the base of the facility cannot be avoided, the infiltrative capacity shall be restored by tilling or aerating prior to placing the infiltrative bed.
• Where pervious pavement is to be installed, installation of the pavement shall be scheduled as the last installation at a development site. Vehicular traffic is prohibited for at least two days following installation. Site materials shall not be stored on pervious pavement.

Operations and Maintenance

• Frequent inspections of the infiltration facilities shall occur to ensure that surface ponding infiltrates into the subsurface completely within the design drawdown time following storms. If vector breeding is taking place at a site as a result of contained stormwater or inadequately maintained BMPs, the Greater Los Angeles County Vector Control District has the ability to fine site owners for violating the California Health and Safety Code (Section 2060 – 2067).
• Regular inspections shall take place to ensure that the pretreatment sediment removal BMP/forebay is working efficiently. Sediment buildup exceeding 50% of the forebay sediment storage capacity shall be removed.
• The infiltration facility shall be maintained to prevent clogging. Maintenance activities include checking for debris/sediment accumulation and removal of such debris.
• Facility soil (if applicable) shall be maintained. Flow entrances, ponding areas, and surface overflow areas will be inspected for erosion periodically. Soil and/or mulch will be replaced as necessary to maintain the long-term design infiltration rate for the life of the project.
• Site vegetation shall be maintained as frequently as necessary to maintain the aesthetic appearance of the site as well as the filtration capabilities (where applicable). This includes the removal of fallen, dead, and/or invasive plants, watering as necessary, and the replanting and/or reseeding of vegetation for reestablishment as necessary.
• Pervious pavement areas that are damaged or clogged shall be replaced/repaired per manufacturer’s recommendation as needed.
• Follow all proprietary operation and maintenance requirements

Calculating Size Requirements for Infiltration BMPs 

The main challenge associated with infiltration BMPs is preventing system clogging and subsequent infiltration inhibition. In addition, infiltration BMPs must be designed to drain in a reasonable period of time so that storage capacity is available for subsequent storms to prevent standing water that results in vector risks or plant mortality. Infiltration BMPs should be designed according to the requirements listed in the “LID Sample Design Calculations'' section of this manual.

Infiltration BMP Types 

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• Surface Infiltration BMPs: These BMPs rely on infiltration in a predominantly vertical (downward) direction and depend primarily on soil characteristics in the upper soil layers.  These infiltration BMPs include:
  • Infiltration Basins:  An infiltration basin consists of an earthen basin constructed in naturally pervious soils with a flat bottom typically vegetated with dry-land grasses or irrigated turf grass.  An infiltration basin functions by retaining the design runoff volume in the basin and allowing the retained runoff to percolate into the underlying native soils over a specified period of time.
    
  • Infiltration Trenches:  Infiltration trenches, which are similar to basins, are long, narrow, gravel-filled trenches, often vegetated, that infiltrate stormwater runoff from small drainage areas. Infiltration trenches may include a shallow depression at the surface, but the majority of runoff is stored in the void space within the gravel and infiltrates through the sides and bottom of the trench.

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  • Infiltration Galleries: Infiltration galleries are open-bottom, subsurface vaults that store and infiltrate stormwater. A number of vendors offer prefabricated, modular infiltration galleries that provide subsurface storage and allow for infiltration. Infiltration galleries come in a variety of material types, shapes and sizes.
  • Bioretention: Bioretention stormwater treatment facilities are landscaped shallow depressions that capture and filter stormwater runoff. These facilities function as a soil and plant-based filtration device that remove pollutants through a variety of physical, biological, and chemical treatment processes. The facilities normally consist of a ponding area, mulch layer, planting soils, plantings, and, optionally, a subsurface gravel reservoir layer.

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  • Permeable Pavements: Permeable (or pervious) pavements contain small voids that allow water to pass through to a stone base. They come in a variety of forms; they may be a modular paving system (concrete pavers, modular grass or gravel grids) or poured-in-place pavement (porous concrete, permeable asphalt). All permeable pavements with a stone reservoir base treat stormwater and remove sediments and metals to some degree by allowing stormwater to percolate through the pavement and enter the soil below.
• Multi-Directional Infiltration BMPs: These BMPs take advantage of the hydraulic conductivities (Ksat) of multiple soil strata and infiltration in multiple directions.  They may be especially useful at locations where low Ksat values are present near the surface and soils with higher permeabilities exist beneath. A Multi-Directional Infiltration BMP may be implemented to infiltrate water at these lower soil layers, thus allowing infiltration to occur at sites that otherwise would be infeasible.  These infiltration BMPs typically have smaller footprints and include, but are not limited to:

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  • Dry Wells: A dry well is defined as an excavated, bored, drilled, or driven shaft or hole whose depth is greater than its width. Drywells are similar to infiltration trenches in their design and function, as they are designed to temporarily store and infiltrate runoff, primarily from rooftops or other impervious areas with low pollutant loading. A dry well may be either a drilled borehole filled with aggregate or a prefabricated storage chamber or pipe segment.
  • Hybrid Bioretention/Dry Wells: A bioretention facility with dry wells is useful in areas with low surface-level hydraulic conductivities that would normally deem a bioretention BMP infeasible but have higher levels of
    permeability in deeper strata. By incorporating drywells underneath the bioretention facility, water is able to be infiltrated at deeper soil layers that are suitable for infiltration, if present. This hybrid BMP combines the aesthetic and filtration qualities of a bioretention facility with the enhanced infiltration capabilities of a dry well.

Pretreatment Requirements

Drainage areas resulting in high sediment loading rates to the infiltration facility shall require pretreatment to reduce sediment loads and avoid system clogging. Examples of appropriate pretreatments may include: 

• sedimentation/settling basins
• baffle boxes
• hydrodynamic separators
• media filters
• vegetated swales
• filter strips

Capture and Use 

Capture and use, commonly referred to as rainwater harvesting, collects and stores stormwater for later use, thereby offsetting potable water demand and reducing pollutant loading to the storm drain system.  Partial capture and use can also be achieved as part of a treatment train by directing the overflow to a bioretention system to provide additional volume reduction and water quality treatment in instances where the quantity of runoff from a storm event exceeds the volume of the collection tank.

In the City of Los Angeles, the use of collected stormwater will primarily be limited to irrigation of landscaped surfaces. However, as new guidelines and guidance become available the potential for other uses of collected stormwater will be considered. Capture and use BMPs that are designed with the intent to use captured stormwater for indoor or consumptive purposes will be reviewed on a case-by-case basis to ensure that all treatment, plumbing, and Building and Safety codes are met.

Assessing Site Capture and Use Feasibility

As with infiltration BMPs, assessing a site’s potential for implementation of capture and use BMPs requires both the review of existing information and the collection of site-specific measurements. Available information regarding the site’s landscaped area should be reviewed as discussed below. In addition, human health concerns should be prioritized, particularly with regards to vector control issues arising from the addition of standing water on site. 

Landscaped Area Assessment

The capture and use BMPs shall be sized for the Estimated Total Water Use  (ETWU) from October 1 - May 31st for irrigating the proposed project landscape area. If the ETWU is greater than or equal to the stormwater volume produced by the stormwater quality storm event (i.e ETWU 8 months > V_M) no further mitigation is required. If only partial capture & use is available, other volume reduction measures shall be combined to achieve full compliance.

Los Angeles County Department of Public Health Requirements

Projects implementing rainfall or urban runoff capture and distribution systems must obtain approval from the County of Los Angeles, Department of Public Health Guidelines for Alternate Water Sources - Indoor & Outdoor Non-Potable Uses and Approval and Use of Cisterns for Rainfall / Run-Off Capture and Distribution.

Vector Control Considerations

A vector is any insect, arthropod, rodent, or other animal that is capable of harboring or transmitting a causative agent of human disease. In the City of Los Angeles, the most significant vector population related to stormwater is mosquitoes.

Vector sources occur where conditions provide habitat suitable for breeding, particularly any source of standing water. This means that stormwater BMPs, especially those of the capture and use type, can be breeding grounds for mosquitoes and other vectors resulting in adverse public health effects related to vectors and disease transmission. Because of this, efforts shall be made to design, capture, and use BMPs that do not facilitate the breeding of vectors.  Vectors should be considered during the preparation of stormwater management and maintenance plans and during preconstruction planning to avoid creating possible public health hazards.
Oversized capture and use BMPs designed to hold captured stormwater for longer than 72-hour periods will require additional treatment such as filtration or disinfection to protect the collection tanks from fouling, to prevent the breeding of vectors, and/or to improve the quality of water for reuse applications.  These BMPs must have appropriate vector control measures incorporated into the design of the system to exclude vector access and breeding (i.e., observation access for vector inspection and treatment). They should be approved by the County of Los Angeles Department of Public Health. These scenarios will be reviewed on a case-by-case basis.

If vector breeding is taking place at a site as a result of contained stormwater or inadequately maintained BMPs, the Greater Los Angeles County Vector Control District has the authority to fine site owners for violating California Health and Safety Code (Section 2060 – 2067).

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Capture and Use of BMPs

Capture and Use refers to a specific type of BMP that operates by capturing stormwater runoff and holding it for efficient use at a later time. On a commercial or industrial scale, capture and use BMPs are typically synonymous with cisterns, which can be implemented both above and below ground. Cisterns are sized to store a specified volume of water with no surface discharge until this volume is exceeded. The primary use of captured runoff is for subsurface drip irrigation purposes. The temporary storage of roof runoff reduces the runoff volume from a property and may reduce the peak runoff velocity for small, frequently occurring storms. In addition, by reducing the amount of stormwater runoff that flows overland into a stormwater conveyance system, less pollutants are transported through the conveyance system into local streams and the ocean. The onsite use of the harvested rainwater for non-potable purposes conserves City-supplied potable water and can recharge local groundwater aquifers when directed to unpaved surfaces.

Siting Requirements and Opportunity Criteria

Drainage areas implementing capture and use BMPs must pass the feasibility guidelines in accordance with the siting requirements set forth in the "Capture and Use Feasibility" section of this page. This screening process must be approved by a site-specific geotechnical investigation report and/or hydrologic analysis conducted and certified by a State of California registered professional civil engineer, geotechnical engineer, geologist, or other qualified professional.

Capture and use BMPs designed for these extended holding times will require additional treatment such as filtration or disinfection to protect the collection tanks from fouling, to prevent the breeding of vectors, and/or to improve the quality of water for reuse applications. These scenarios will be reviewed on a case-by-case basis. 

Irrigation / Dispersal of Captured Stormwater

A developer is required to hold harvested stormwater for the purpose of irrigation during dry periods.  Calculations in line with the California Department of Water Resources Model Water Efficient Landscape Ordinance AB 1881 (also refer to City of Los Angeles Irrigation Guidelines) shall be provided. Captured stormwater should be used to offset the potable irrigation demand that would occur during the rain season (Oct 1 – May 30, 8 months) yielding to an evapotranspiration value of 25.41-inches (Reference Evapotranspiration Zones); see also LID Sample Design calculations for examples. If the volume of captured stormwater exceeds the Estimated Total Water Use for the rain season (ETWU8), excess stormwater shall, at a minimum, establish a schedule to release captured stormwater over vegetated landscape area.

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Design Criteria and Requirements

Unless specifically stated, the following criteria and requirements listed below are required for the implementation of all capture and use BMPs. Provisions not met must be approved by the City of Los Angeles.

• Fertilizers, pesticides, or herbicides on landscaped areas shall be minimized.
• Above-ground cisterns are not allowed.
• To minimize clogging to the irrigation system, a filtration system shall be incorporated prior to the tank connection. Examples of appropriate filtration may include: sedimentation/settling basins; baffle boxes; hydrodynamic separators; media filters; vegetated swales; filter strips.
• Overflow outlet is provided upstream of the tank inlet and is designed to disperse overflow onsite. Dispersal and overflow must be through an approved landscape area where erosion or suspension of sediment is minimized, or through a high flow biotreatment BMP. Overflow from the tank into the storm drain system is not allowed.
• For landscape applications, a subsurface drip irrigation system, a pop up, or other approved irrigation system, has been approved and installed to adequately discharge the captured water.
  If a pumping system is used, a reliable pump capable of delivering 100% of the design capacity is provided. Pump must be accessible for maintenance and is operable within 20% of its best operating efficiency. A high/low-pressure pump shut-off system is installed in the pump discharge piping in case of line clogging or breaking.
• If an automated harvesting control system is used, it is complete with a rainfall or soil moisture sensor. The automated system must be programmed to not allow for continuous application on any area for more than 2-hours.
• Dispersion is directed so as not to knowingly cause geotechnical hazards related to slope stability or triggering expansive (clayey) soil movement.
• Cisterns do not allow UV light penetration to prevent algae growth.
• Cistern placement allows easy access for regular maintenance. If cistern is underground, manhole shall be accessible, operational, and secure.
• Refer to County of Los Angeles, Department of Public Health Guidelines for Alternate Water Sources - Indoor & Outdoor Non-Potable Uses and Approval and Use of Cisterns for Rainfall / Run-Off Capture and Distribution for additional guidelines and requirements.
• Provide observation access for vector inspection and treatment.

Operations and Maintenance

• Cistern components, including spigots, downspouts, and inlets will be inspected 4 times annually to verify proper functionality. Parts will be repaired or replaced as needed.
• Cisterns and their components will be cleaned as necessary to prevent algae growth and the breeding of vectors.
• Dispersion areas will be maintained to remove trash and debris, loose vegetation, and rehabilitate any areas of bare soil.
• Effective energy dissipation and uniform flow spreading methods will be employed to prevent erosion and facilitate dispersion.
• Cisterns will be emptied as necessary to prevent vector breeding, unless exclusion devices are implemented to prevent vector access. If vector breeding is taking place at a site as a result of contained stormwater or inadequately maintained BMPs, the Greater Los Angeles County Vector Control District has the ability to fine site owners for violating the California Health and Safety Code (Section 2060 – 2067).

Biofiltration 

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Projects that have demonstrated they cannot manage 100% of the water quality design volume onsite through infiltration, bioretention,  and/or capture and use BMPs may manage the remaining volume through the use of  biofiltration/biotreatment BMP. A  biofiltration/biotreatment BMP shall be sized to adequately capture 1.5 times the volume not managed through infiltration and/or capture and use.

Biofiltration BMPs are landscaped facilities that capture and treat stormwater runoff through a variety of physical and biological treatment processes. Facilities normally consist of a ponding area, mulch layer, planting soils, plants, and in some cases, an underdrain. Runoff that passes through a biofiltration system is treated by the natural adsorption and filtration characteristics of the plants, soils, and microbes with which the water contacts. Biofiltration BMPs include vegetated swales, and planter boxes. Biofiltration can provide multiple benefits, including pollutant removal, peak flow control, and low amounts of volume reduction through infiltration and evapotranspiration.

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Biofiltration Types

Biofiltration BMPs rely on various hydraulic residence times and flow-through rates for effective treatment. As a result, a variety of BMPs are available.

Bioretention with Underdrain: Bioretention facilities are landscaped shallow depressions that capture and filter stormwater runoff. As stormwater passes down through the planting soil, pollutants are filtered, adsorbed, and biodegraded by the soil and plants. Because they are not contained within an impermeable structure, they may allow for infiltration. For sites not passing the infiltration feasibility for reasons other than low infiltration rates (such as soil contamination, expansive soils, etc.), an impermeable liner may be needed to prevent incidental infiltration.

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Planter Boxes: Planter boxes are biodetention treatment control measures that are completely contained within an impermeable structure with a perforated underdrain (they do not infiltrate). They are similar to bioretention facilities with underdrains except they are situated at or above ground and are bound by impermeable walls. Planter boxes may be placed adjacent to or near buildings, other structures, or sidewalks.

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Bioinfiltration: Bioinfiltration facilities are designed for partial infiltration of runoff and partial biotreatment. These facilities are similar to bioretention devices with underdrains but they include a raised underdrain above a gravel sump designed to facilitate infiltration and nitrification/denitrification. These facilities can be used in areas where there are little to no hazards associated with infiltration, but infiltration screening does not allow for infiltration BMPs due to low infiltration rates or high depths of fill.

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Vegetated Swales: Vegetated swales are open, shallow channels with dense, low-lying vegetation covering the side slopes and bottom that collect and slowly convey runoff to downstream discharge points. An effective vegetated swale achieves uniform sheet flow through the densely vegetated area for a period of several minutes. The vegetation in the swale can vary depending on its location and is the choice of the designer. Most swales are grass-lined.

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Filter Strips (to be used as part of a treatment train): Filter strips are vegetated areas designed to treat sheet flow runoff from adjacent impervious surfaces such as parking lots and roadways, or intensive landscaped areas such as golf courses. While some assimilation of dissolved constituents may occur, filter strips are generally more effective in trapping sediment and particulate-bound metals, nutrients, and pesticides. Filter strips are more effective when the runoff passes through the vegetation and thatch layer in the form of shallow, uniform flow. Filter strips are primarily used to pretreat runoff before it flows to an infiltration BMP or another biofiltration BMP.

Site Requirements and Opportunity Criteria

Sites with plans to implement biofiltration/biotreatment systems for the management of stormwater must first be screened for infiltration and capture and use BMP feasibility. Biofiltration should be implemented to treat all runoff onsite to the maximum extent feasible at sites incapable of implementing infiltration and/or capture and use BMPs as a result of the feasibility process set forth in this document.

Sites implementing biofiltration BMPs must have sufficient area available to ensure that BMPs produce adequate contact time for filtration to occur. For biofiltration BMPs with underdrains, sufficient vertical relief must exist to permit vertical percolation through the soil media to the underdrain below. For biofiltration BMPs with incidental infiltration, it must be demonstrated that there are no hazards associated with infiltration (i.e. infiltration screening does not allow for infiltration BMPs due to low infiltration rates or high depths of fill).

Biofiltration systems may integrate storage modules below the filtration soil media in-lieu of the conventional gravel media with the perforated drainage pipe. This would detain measurable stormwater volume that will be used for irrigation by capillary rise/ passive irrigation within an approved growing media profile. If the entire stormwater quality volume is retained in the storage module, and meets the irrigation demand for the landscape area above, then the system is deemed a full capture and use system, and the planting media depth may be shallower than the biofiltration media. If the irrigation demand is not met, capture and use, and biofiltration combination, or a hybrid system may be designed. A smaller surface area than the conventional biofiltration system is anticipated because the available storage volume will be subtracted from the stormwater quality volume, and the biofiltration system above will be sized for 1.5 times the remaining portion only.

Biofiltration Sizing

Except for swales, biofiltration facilities can be sized using the simple sizing method. The design capture volume must be completely infiltrated within the drawdown time shown in the LID Sample Design Calculations.  

Design Criteria and Requirements 

Unless specifically stated, bioretention facilities can be sized using the simple sizing method. The design capture volume must be completely infiltrated within the drawdown time shown in the LID Sample Design Calculations. 

Operations and Maintenance

Biofiltration areas require annual plant, soil, and mulch layer maintenance to ensure optimum infiltration, storage, and pollutant removal capabilities. In general, biofiltration maintenance requirements are typical landscape care procedures. The following operations and maintenance practices will be adhered to:

• Facility soil will be maintained. Flow entrances, ponding areas, and surface overflow areas will be inspected for erosion periodically. Soil and/or mulch will be replaced as necessary to maintain an infiltration rate at or near the initial K_sat,designvalue for the duration of the project.
• Site vegetation will be maintained as frequently as necessary to maintain fire protection, public safety, and the aesthetic appearance of the site as well as the filtration capabilities. This includes the removal of fallen, dead, and/or invasive plants, watering as necessary, and the replanting and/or reseeding of vegetation for reestablishment as necessary. Swales and filters will be mowed as necessary.
• BMP inlets will be inspected and maintained to ensure even flow enters the facility. Sediment collecting at the inlet will be removed as necessary.
• Proprietary devices will be inspected and maintained in accordance with the requirements of the manufacturer.  

OFFSITE MITIGATION MEASURES, AND ON-SITE TREATMENT

Offsite Mitigation Measures

Managing Offsite Drainage:  Locations and sources of offsite run-on to the site must be identified early in the design process. Offsite drainage must be considered when determining appropriate BMPs for the site so that the drainage can be managed. By identifying the locations and sources of offsite drainage, the volume of water running onto the site may be estimated and factored into the siting and sizing of onsite BMPs. Vegetated swales or storm drains may be used to intercept, divert, and convey offsite drainage through or around a site to prevent flooding or erosion that might otherwise occur.

The option for offsite mitigation shall be exercised after the following conditions have been met:

1. All allowed stormwater management techniques below have been exhausted and therefore deemed technically infeasible.
   1. infiltration
   2. capture and use
   3. treated through a biofiltration system

Offsite project BMPs such as off-site groundwater replenishment, and/ or off-site retrofit should be located as close as possible to the project site, on private and/or public land, and should address a mix of land uses like those included in the proposed project. The offsite project shall not be located within Waters of the U.S. and it shall be demonstrated that equivalent pollutant removal is accomplished prior to discharge to Waters of the U.S.

Remaining runoff that can’t feasibly be managed onsite, the project shall implement offsite mitigation in either:

1. The public right of way immediately adjacent to the subject development and/or;
2. Within the same sub-watershed as the proposed project (as defined as draining to the same HUC-12 hydrologic area as defined  by the MS4 Permit)

Construction of an offsite mitigation project(s) shall achieve at least the same level of water quality protection as if all runoff was retained onsite and also be sized to mitigate the volume from the onsite and the tributary area from the adjacent street (from the crown of the street to the curb face for the entire length of the development site). All City Departments will assist the developer, when and where feasible, permitting and implementation of LID BMP projects within the public right of way.

Construction work in the public right-of-way will be the responsibility of the developer and may require both a Revocable Permit and an A or B Permit for construction from the Bureau of Engineering (BOE). The developer will also be required to record a Covenant and Agreement with the Los Angeles County Recorder’s office to confirm the owner assumes full responsibility for perpetual maintenance of the onsite and offsite BMP(s) executed by the Covenant and Agreement.  The type of BOE permit required depends on the scope of the project. Please refer to BOE’s Development Services Permit Manual for additional information.

On-Site Treatment Measures

Any remaining runoff that is infeasible to manage on-site and/ or off-site, using the measures described in the “Infiltration Feasibility” section above, may be mitigated on-site using flow-through modular treatment system including sand filters, or other proprietary BMP treatment systems that are certified for “Basic Treatment” under the Washington State Department of Ecology’s TAPE Program; or an approved future BMP certification developed by the State of California. Flow-through treatment device shall be sized and designed to filter or treat either:

1. The maximum flow rate of runoff produced from a rainfall intensity of 0.2 inch of rainfall per hour. For each hour of storm event; or
2. The maximum flow rate of runoff produced by the 85th percentile hourly rainfall intensity (for each hour of a storm event), determined from the Los Angeles County 85th percentile hourly rain event, multiplied by a factor of two.

Green Infrastructure Projects

In an effort to assist developers the City has recently approved and adopted a series of Green Street Standard Plans. These plans provide a series of standards that developers can implement utilizing the public right of way immediately adjacent to the development. These standard plans provide general requirements for Green Streets, parkway swales in major/secondary highways, parkway swales in local/collector streets, parkway swales with no street parking, vegetated stormwater curb extensions, and interlocking pavers for vehicular and pedestrian alleys.  

Additional information on the City’s Green Street Standard Plans can be found at the Bureau of Engineering’s Standard Plans site referencing Green Street Standard Plans.