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Post Clean BMPs

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Part One: Introduction

1.1 Purpose and Applicability

1.2 Organization of Guideline 

 Part Two: Environmental Performance Standards

2.1 Flow Control and Erosion Protection

2.2 Water Quality Protection

2.3 Groundwater Quality Protection

Part Three: Design and BMP Requirement for All Projects

3.1 Project Design and Selection of BMPs Generally

3.2 Performance Requirement for BMPs in Combination

3.3 Preference for Natural BMPs

3.4 Source Control BMPs

3.5 Minimize Impervious Surfaces

3.6 Buffer Zones

3.7 Conserve Natural Areas

3.8 Storm Drain Tiles and Signage

3.9 Additional BMPs Where Necessary

3.10 Infiltration BMPs

3.11 Examples of BMPs

Part Four: Additional Design BMP Requirements for Priority Development Projects

4.1 Priority Development Projects

4.2 Additional BMP Requirements

 Part Five: Other Requirements

5.1 Discharge Prohibitions

5.2 Construction Phase Requirements

5.3 Industrial Facility General Permit Coverage

5.4 BMP Maintenance

 Part Six: Alternatives To On-site BMPs

6.1 Shared Structural Treatment BMPs

6.2 Waivers Where Structural Treatment BMPs are Infeasible

 Part Seven: Reference Materials


1.1 Purpose and Applicability

1.1.1 Purpose
County staff in the Departments of Public Works and Planning and Land Use are required to evaluate and make recommendations to decision makers concerning various public and private land development applications. Staff in these Departments also make decisions on certain land development applications on behalf of their respective Department Directors, where the Director has jurisdiction over an application or where a condition of an approved application requires that the Director’s approval be obtained.

This document provides guidelines for County staff to follow in making the above recommendations or decisions, to ensure that development (including redevelopment) projects (1) include adequate post-construction storm water "best management practices" (BMPs), and (2) do not cause significant adverse impacts on water quality. Staff may also provide this guideline to project designers and applicants.

1.1.2 Development Projects and County Actions Subject to this Guideline
County staff should apply this guideline (1) during the design of covered public land development projects; (2) during CEQA review of covered public and private land development projects before providing any discretionary County approval; and (3) prior to approval of subsequent modifications to a covered public or private land development project.

Covered Public Land Development Projects
This guideline applies to all County land development projects except small projects in rural areas, and to all non-County public land development projects that require a County permit. For public projects that have already been designed and funded and for which environmental reviews have already been completed, and for road construction projects in rural areas (defined below), the principles of this guideline shall be applied to the extent practicable.

Covered Private Land Development Projects
This guideline applies to all private land development projects in a defined "urban" portion of the County, and to private land development projects in rural parts of the County if an on-site storm sewer any part of which is under ground will serve those rural projects.

For purposes of this guideline the "urban" area of the County includes all lands that are west of the designated service boundary for the County Water Authority or that are within the service territory of another public water supply authority, including any future annexations. In general, the following County communities are included within the CWA boundary; Sweetwater, Spring Valley, Valle de Oro, Jamul, Crest/Dehesa, Bostonia, Alpine, Lakeside, Ramona, San Dieguito, North County Metro, Valley Center, Bonsall, Fallbrook, and Rainbow. The present "urban" area of the County also includes some additional developed areas east of the County Water Authority boundary. A map attached as Exhibit "A" to this guideline defines the parts of the unincorporated County that are presently considered to be "urban." All other parts of the unincorporated County are presently considered "rural."

1.1.3 Requirements Implemented by this Guideline
This guideline implements CEQA and applicable County ordinances. CEQA requires the County to impose conditions on development projects to prevent significant adverse impacts on the environment. The County Stormwater Quality Management Ordinance (Stormwater Ordinance) requires that all land development projects in the County include BMPs to reduce pollution in storm water to the "maximum extent practicable" (MEP). (MEP is defined in section 3.1, below.) County ordinances regulating grading and the development approval process specifically require compliance with the Stormwater Ordinance as a condition for various County approvals. The Stormwater Ordinance authorizes the issuance of guidance that can be used to determine whether proposed BMPs are adequate.

This guideline also anticipates more detailed and more stringent regulatory requirements that the County must develop and implement in the near future to satisfy Regional Water Quality Control Board (RWQCB) Order 2001-01. It is County policy to apply these additional requirements throughout the County now, where practicable, to protect water quality.

1.1.4 Anticipated Revisions to this Guideline
This guideline may be revised as the County develops further programs to implement RWQCB Order 2001-01. Revisions may include more detailed guidelines concerning acceptable BMPs for specific types of projects.

1.2 Organization of Guideline

Guidelines and discussion. In the remainder of this document, guidelines are set out in italics, and related discussion is in plain text.

Part Two of this guideline sets out environmental performance standards that covered projects should meet. Project applicants must provide a report to the County explaining how these standards will be met.

 Part Three of this guideline sets out project design and BMP requirements that all covered projects should meet. Sections 3.1 through 3.3 discuss BMP selection generally. Sections 3.4 through 3.8 address specific project elements that require BMPs. Project proponents must propose specific BMPs for their projects to address each of these elements. County staff must then determine whether those BMPs will reduce pollutants in storm water to the maximum extent practicable (MEP). County staff may request information on alternative BMPs from the project applicant to assist in making this MEP determination.

Part Four of this guideline sets out additional "structural treatment" BMP requirements that apply only to the Priority Development Projects identified in section 4.1. The classification rules and BMP standards for Priority Development Projects set out in Part Four are mandated in RWQCB Order 2001-01, at section F.1(a).

Part Five of this guideline summarizes other stormwater requirements, not related to post-construction BMPs that are applicable to development projects. These requirements are contained in the County Stormwater Ordinance or are mandated by RWQCB Order 2001-01, and are included here for reference. Many of these underlying requirements (e.g., prohibitions on non-storm water discharges) are not limited to new development and redevelopment projects.

Part Six of this guideline discusses off-site alternatives to on-site BMPs. County staff should consider whether to require the use of off-site BMPs when on-site BMPs are infeasible or what off-site BMPs would be environmentally preferred.



All projects subject to this guideline should meet the performance standards set out in this Part and the more specific technical guidelines set out in Part Three (and if applicable, Part Four) of this document. Evaluations of environmental impacts (e.g., from increased flows) should consider both the direct impacts of single project, and the cumulative impacts or other anticipated projects in the same area. 

2.1 Flow Control and Erosion Prevention
2.1.1: Post-construction peak runoff flow rates and velocities from the project site shall be maintained at levels that will not cause a significant increase in downstream erosion.

2.1.2: Priority Development Projects (see Part Four below) must control the post-development peak storm water runoff discharge rates and velocities to maintain or reduce pre-development downstream erosion, and to protect stream habitat.

2.1.3: Measures to comply with 2.1.1 and.1.2 shall not disrupt flows and flow patterns that are necessary to support downstream wetlands or riparian habitats. Diversion of runoff to regional facilities shall not be allowed to deprive immediate downstream habitats of the minimum flows and /or over-bank flow events they need.

Project proponents shall provide satisfactory evidence that discharge volume and velocity meet these criteria. . Staff may determine that a drainage study, analysis of downstream flows, a soils report (erosion and runoff), and proposed mitigation measures are necessary.

Minimizing the amount of new impervious surface created, retaining or constructing vegetated swales and buffers, and the use of velocity reducers and energy dissipation can help to achieve these standards. Where these measures are not sufficient to achieve these standards, runoff must be captured and released in a more controlled manner. "Hardening" natural downstream areas to prevent erosion is not an acceptable technique for meeting these performance standards.

2.2 Water Quality Protection

Pollutants in non-storm water and storm water discharged from the project (or discharged to waters of the state within the project area) shall:

2.2.1 Not cause or contribute to an exceedance of receiving water quality objectives;

Whether a project meets this standard will depend on the waters affected by the project, on the water quality objectives established for those waters at the time the project is proposed, and on the amount and type of pollutants discharged by the project. The question is whether increased pollution from the project (together with pollution from other sources) would be likely to result in water quality violations that would not otherwise occur.

2.2.2 Not significantly degrade receiving water quality;

Projects that cause some degradation of water quality after the use of appropriate BMPs are not necessarily in violation of this performance standard—staff must determine whether those anticipated releases would cause "significant" degradation. 2.2.3 Be reduced to the MEP; and

Whether this standard is met is both a technical and an economic determination. If the project fulfills the requirements in Part Three (and if applicable Part Four) of this guideline, it should be deemed to have fulfilled requirement 2.2.3.

2.2.4 Not cause or contribute to a condition of "pollution", "contamination" or "nuisance."

Whether a project causes or contributes to a condition of "pollution, contamination, or nuisance" depends on the special meanings those terms have in the state Water Code, section 13050 subsections (k), (l) and (n). "Pollution" is an unreasonable interference with a beneficial use assigned to a specific water body in the RWQCB Basin Plan. "Contamination" involves a threat to public health. A "nuisance" is a condition that affects a considerable number of persons, and "is injurious to health, or is indecent or offensive to the senses, or an obstruction to the free use of property, so as to interfere with the comfortable enjoyment of life or property."

To support County determinations and findings related to these standards, staff should require applicants to submit a report that identifies affected receiving waters, identifies applicable water quality objectives and pollutants of concern, and estimates post-construction discharges rates (with all BMPs in place), and explains why those projected pollutant loads would not cause a violation of these standards.

2.3 Groundwater Quality Protection

2.3.1: Structural treatment BMPs intended primarily to infiltrate run-off into ground water shall not cause or contribute to an exceedance of applicable groundwater quality objectives as set out in the RWQCB "Basin Plan" for the San Diego area..

2.3.2: The project shall not significantly degrade ground water quality.

Structural treatment BMPs intended primarily to infiltrate run-off into ground water include infiltration trenches and infiltration basins. Guideline 2.3.1 does not apply to BMPs such as grassy swales, detention basins, vegetated buffer strips and constructed wetlands which allow incidental infiltration, but which are not designed primarily to function as infiltration devices.



Sections 3.1 and 3.2 below discuss BMP selection generally. Sections 3.3 through 3.8 set out specific projects elements each of which must be addressed through BMPs. Staff must then determine whether those BMPs will reduce pollutants in storm water to the MEP. Staff may request information on alternative BMPs from the project applicant to assist in making this determination.

Section 3.9 addresses design adjustments if the BMPs initially proposed by an applicant are found to be inadequate. Section 3.10 sets out limitations on the use of infiltration BMPs. Finally, Section 3.11 provides examples of BMPs used in combination to address several land development situations.

The following references provide additional information on BMP designs: California Storm Water Best Management Practices Handbooks; Caltrans Storm Water Quality Handbook: Planning and Design Staff Guide; Manual for Storm Water Management in Washington State; The Maryland Storm water Design Manual; Florida Development Manual: A Guide to Sound Land and Water Management; Denver Urban Storm Drainage Criteria Manual, Volume 3 – Best Management Practices and Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters

3.1 Project Design and Selection of BMPs Generally

3.1.1 Choosing Effective BMPs
BMPs must be effective. The following criteria shall be used to determine if a BMP is effective:

o BMPs must prevent or reduce the discharge of the pollutants identified at a site for reduction;

o BMPs must comply with other regulations as well as Stormwater regulations.

3.1.2 Using BMPs to the "Maximum Extent Practicable" (MEP)

The BMPs selected and installed pursuant to this guideline must reduce pollution in runoff to the MEP. Effective, technically feasible BMPs should not be omitted from a project proposal unless other equally effective BMPs are used instead to serve the same purpose, or where the rejected BMP would be so costly in the context of the project as to be impracticable. The following criteria shall be used to determine whether a BMP is practicable:

o BMPs must be technically feasible (considering area soil, geography, water resources, and other resources available).

o BMPs must be compatible with the area’s land use, character, facilities, and activities;

o BMP implementation costs should not exceed benefits obtained from pollution reduction;

3.1.2 Other BMP Selection Criteria

The criteria that follow should be used in assessing the appropriateness of BMPs for a particular project. Selection of BMPs for a project is a function of assessing project type, size, post-construction activities and other factors. Project proponents shall identify all impacts relating to pollutants of concern and , and provide satisfactory evidence using these criteria that the specific BMPs proposed will mitigate such impacts to the maximum extent practicable. Site Factors

BMPs can be an addition or detraction from any project, depending upon the creativity of the project designer. Wet ponds and wetlands are frequently used to create a waterfront effect in residential developments, and dry extended detention areas can serve as attractive parks or sports fields available for public use most of the year. The following site factors must be used in selecting BMPs.

a) Steep slopes restrict the use of some BMPs. Porous pavement must be situated in sites with slopes of 5 % or less. Swales should only be used if their slope is less than 5 percent; and because of slope stability concerns, infiltration trenches and filter strips are not practical when slopes exceed 20%.

b) A high water table acts as an effective barrier to infiltration and can sharply reduce the ability of an infiltration BMP to drain properly. If the height of the seasonally high water table extends to within 4 feet of the bottom of an infiltration BMP, the site is not considered suitable (even with runoff filtration or pretreatment).

c) Soil type and permeability limit use of infiltration BMPs. These BMPs shall not be used when infiltration rates are less than 0.27 inch per hour, as defined by the least permeable layer in the shallow soil profile. This excludes most "C" and "D" soils (_______________ Soil Classification System), which cannot exfiltrate enough runoff through the subsoil. Extremely permeable sandy soils may not maintain adequate water levels in wet ponds.

d) Infiltration BMPs divert runoff back into the soil and may cause local seepage or contamination. These BMPs shall not be located within 100 feet of a building foundation or a drinking water well.

e) BMPs must be designed to address rainfall volume and intensity during both wet weather and dry weather seasons. Wet ponds require some continuous flow (dry weather water source) to keep them from stagnating or developing odor and vector problems.

f) Some sites are too intensively developed or limited in area to allow for some BMPs, such as pond BMPs and porous pavement, which require a large surface area and buffer area.

g) Infiltration BMPs must be designed to completely drain within 2 to 3 days after a storm. If the infiltration rates of the underlying soils are slow, depth and footprint of the infiltration BMP must be adjusted to achieve this standard.

h) Land uses may dictate that only certain BMPs can be applied, and most BMPs are not broadly applicable for all development sites. Porous pavement can only be used for sites with parking lots not expected to receive heavy car or truck traffic, or much sediment.

i) Infiltration BMPs may be clogged by large loads of sediment generated during construction, and shall not be installed until all of the land to be disturbed by construction is effectively and permanently stabilized. To prevent clogging after construction, a pre-treatment device must be used to filter sediment and other coarse particles before they reach the infiltration BMP.

j) Landscaping can be an effective BMP option provided continued maintenance and protection are assured. Pollutants of Concern

Storm water runoff from a site has the potential to contribute pollutants associated with the proposed future use of the site such as silt, oil and grease, suspended solids, metals, gasoline, pesticides, and pathogens. Any pollutant that may be released from a site in storm water or non-stormwater is a pollutant of concern. However, project proponents must identify and project reviewers should focus special attention on the control of potential pollutants that exhibit one or more of the following characteristics:

a) Current loadings or historic deposits of the pollutant are impacting the beneficial uses of a receiving water

b) Elevated levels of the pollutant are found in sediments of a receiving water and/or have the potential to bioaccumulate in organisms therein

c) Detectable inputs of the pollutant are at concentrations or loads considered potentially toxic to humans and/or flora and fauna. Effective Removal of Pollutants of Concern

BMPs must effectively address the "pollutants of concern" at the site. The nature and concentration of the pollutant being removed, removal mechanisms, and the amount of runoff being treated all affect the potential removal rate that can be achieved with a given BMP.

a) Pollutants such as sediment and lead can be removed effectively by common BMP removal mechanisms, including settling and filtering.

b) Soluble pollutants such as nitrate, phosphate, and some trace metals are more difficult to remove and require biological and/or chemical mechanisms, such as uptake by bacteria, algae, rooted aquatic plants, organic material, terrestrial vegetation, or soils. Costs

The appropriateness of a BMP for a particular site can be affected by economic feasibility considerations, which encompass short- and long-term cost factors. Short-term costs include installation costs for both materials and labor. Long-term costs include maintenance and replacement. To sustain proper function, some BMPs require low-level maintenance on a regular and frequent basis, whereas other BMPs require infrequent maintenance of a more extensive nature. Maintenance costs include the proper disposal of accumulated material. In selecting a control method, all cost elements—construction, installation, and maintenance—associated with the BMP should be considered. Watershed area

The feasibility of a particular BMP depends on the contributing watershed area and the reasonably expected flow at the portion of the project being protected.

a) Wet pond BMPs generally requires a significant contributing watershed area of greater than 10 acres, and in locales such as Southern California, a dry weather source of water.

b) Infiltration and vegetative BMPs are generally applicable for catchments less than 10 acres, due to space, economic, or flow volume constraints.

c) Contributing watershed area is not generally limited to the development project site. By using local topography and hydraulic design, the contributing watershed area may be increased or decreased to better accommodate a particular BMP.

d) Additional runoff generated away from the development project may be routed to a BMP, thereby increasing total catchment area and making pond options more practicable.

e) Portions of the total runoff from a development project site may be diverted to smaller, individual BMPs, thereby decreasing the contributing watershed area and making infiltration and vegetative BMPs more practical. Offsite diversion is prohibited.

f) Where appropriate, post-construction BMPs (including but not limited to regional facilities) should be considered with project requirements to preserve or enhance open space. Project proponents may wish to put their BMPs in open space. This shall only be allowed when the County can determine that such usage will be an enhancement to the open space and will be consistent with the reasons for which the open space is being preserved. Environmental Impacts

a) Impacts to downstream aquatic life must be avoided by maintaining the natural low flow levels experienced during the dry weather season. Infiltration BMPs can contribute significantly to groundwater recharge and may be able to help the watershed better mimic its past hydrologic behavior. Vegetative BMPs such as swales and filter strips can also help.

b) Stream bank erosion must be controlled to avoid large sediment loads to receiving waters and impacts on the habitat downstream. Extended detention ponds and full exfiltration BMPs can reduce erosive storm flows enough to keep downstream channels and banks relatively stable.

Some BMPs can create wetland or open water areas that will be utilized by waterfowl, marsh birds, and other wildlife. Shallow marshes and wet ponds are particularly well suited for this role, if relatively small investments are made in landscaping design and plant selection. These applications must consider a dry weather source of water, unless a seasonally wet area is desired. Terrestrial wildlife habitat may be created through the incorporation of BMPs such as wet ponds, extended detention ponds, infiltration basins, and filter strips, and judicious planting of trees, shrubs, and grasses which provide food and cover for the target wildlife.

3.2 Performance Requirement for BMPs in Combination

3.2.1: The project proponent must propose, and after any adjustments required by the County must implement and maintain, BMPs in each of the areas set out in sections 3.3 through 3.8 below. The cumulative effects of the measures implemented must be (1) to limit increases in post-development runoff from the site to the maximum extent practicable; and (2) to reduce post-construction discharges of pollutants from the project to the maximum extent practicable.

The BMP design for a project must address all potential sources of pollution. In addition, BMPs must address specific requirements in the RWQCB Order as set out below. Variation among projects prevents specification of a set of "minimum" BMPs.

3.3 Preference for Natural BMPs

3.3.1: BMPs which incorporate natural systems or approaches shall be considered and shall be utilized whenever practical.

The Stormwater Ordinance establishes a mandatory preference for the use of natural BMPs such as constructed or natural wetlands, grassed swales, biofilters, wet ponds, and vegetated filter strips. The use of natural BMPs can conserve natural areas, can remove pollutants from impervious areas before they reach water, and can maximize infiltration, provide retention, and reduce runoff velocities.

Hardened alternative BMPs can be used instead if natural BMPs are not practical, but most projects in the County should be able, with appropriate design and perhaps some project downsizing, to make good use of natural BMPs. Staff should require project applicants to explain why any significant opportunities to use natural BMPs have not been exploited.

Natural BMPs are "structural" BMPs. They require provisions for maintenance as set out in section 5.4 below.

3.4. Source Control BMPs

3.4.1: All sources of potentially significant post-construction pollutant loading shall be identified and the introduction of pollutants from these sources into storm water and non-storm water discharges shall be prevented or reduced to the maximum extent practicable.

3.4.2 The project must be designed and built to protect slopes and channels from eroding.

3.4.3: Outdoor materials storage and trash storage areas must be properly designed. Detached single-family residences are exempt from this requirement.

To ensure compliance with guideline 3.4.2, project proponents should be required to provide satisfactory evidence that each of the following measures has been provided in the proposed project design:

a) Convey runoff safely from the tops of slopes and discharge in a manner so as not to cause downstream erosion.

b) Stabilize all disturbed slopes

c) Utilize natural drainage systems to the maximum extent practicable

d) Control or reduce flow to natural drainage systems to the maximum extent practicable

e) Stabilize permanent channel crossings.

f) Vegetate slopes with native or drought tolerant vegetation.

g) Install energy dissipaters, such as riprap, at the outlets of new storm drains, culverts, conduits, or channels that enter unlined channels to minimize erosion, with the approval of all agencies with jurisdiction, e.g., the U.S. Army Corps of Engineers, RWQCB, and the California Department of Fish and Game

Improper storage of materials outdoors may provide an opportunity for toxic compounds, oil and grease, heavy metals, nutrients, suspended solids, and other pollutants to enter the storm water conveyance system. Where proposed project plans include outdoor areas intended for storage of materials that may contribute pollutants to the storm water conveyance system, a proper design of such areas should include the following BMPs:

a) Materials with the potential to contaminate storm water must be: (1) placed in an enclosure such as, but not limited to, a cabinet, shed, or similar structure that prevents contact with runoff or spillage into the storm water conveyance system; or (2) protected by secondary containment structures such as berms, dikes, or curbs.

b) The storage area must be paved and sufficiently impervious to contain leaks and spills.

c) The storage area must have a roof or awning to minimize collection of storm water within the secondary containment area.

d) Personal storage areas for detached single-family residences are exempt from this requirement.

A "trash storage area" refers to an area where a trash receptacle or receptacles are located for use as a repository for solid wastes. Loose trash and debris can be easily transported by the forces of water or wind into nearby storm drain inlets, channels, and/or creeks. Staff should required that all trash container areas meet the following BMP requirements.

a) Trash container areas must be designed to either prevent contact with runoff or spillage to the storm water conveyance system; or to include protection by secondary containment structures such as berms, dikes, or curbs.

b) The trash storage area must be paved and sufficiently impervious to contain leaks and spills, with drainage from adjoining roofs and pavement diverted around the area(s).

c) Trash storage areas must be screened or walled to prevent off-site transport of trash.

d) Residential trash storage areas for detached single-family residences are exempt from this requirement.

3.5 Minimize Impervious Surfaces

3.5.1: The project must include site design and landscape characteristics that maximize infiltration, provide stormwater retention, slow runoff, and minimize impervious land coverage, to the MEP.

Reducing impervious surfaces is an effective and preferred means to prevent increased runoff and increased pollution from development, and is a cornerstone of the County’s programs to reduce water pollution from new development. Therefore, proposed projects should be closely scrutinized to ensure that measures such as the following have been included to reduce impervious surfaces to the MEP. Design and landscape characteristics that can be used to meet this guideline include the following. Note however that all General Plan, Specific Plan, zoning, subdivision design, traffic safety and other development regulations must be complied with:

a) Direct rooftop runoff to pervious areas such as yards, open channels, or vegetated areas, and avoid routing rooftop runoff to the roadway or the storm water conveyance system.

b) Use permeable materials for private sidewalks, driveways, parking lots, golf cart paths, trails, or interior roadway surfaces.

c) Reduce overall lot imperviousness by promoting alternative driveway surfaces and shared driveways that connect two or more homes together.

d) Reduce the overall imperviousness associated with parking lots by providing compact car spaces, minimizing stall dimensions, incorporating efficient parking lanes, and using pervious materials in spillover parking areas.

e) Provide reduced width sidewalks and incorporate landscaped buffer areas between sidewalks and streets. These must comply with regulations for the Americans with Disabilities Act and other life safety requirements and will require provision for maintenance as set out is section 5.4 below.

f) Design residential streets for the minimum required pavement widths. The radius of cul-de-sacs should be the minimum required to accommodate emergency and maintenance vehicles.

g) Minimize the number of residential streets and cul-de-sacs and incorporate landscaped areas to reduce their impervious cover.

h) Use smaller lot sizes in return for the creation of larger open space areas.

i) Reduce building density.

3.6 Buffer Zones

3.6.1: Implement buffer zones for natural water bodies where feasible.

3.6.2: Where buffer zone implementation is infeasible, implement other buffers such as trees, lighting restrictions, access restrictions, etc.

This guideline is mandated by RWQCB Order 2001-01, and is also intended to be consistent with current County practices implementing CEQA; state and federal species, habitat and wetlands protection programs; and the County Resource Protection Ordinance. The ability of buffer zones to help to protect water quality should be considered when determining whether a project meets the requirements of this guideline.

3.7 Conserve Natural Areas

3.7.1: Conserve natural areas where practicable, to protect water quality.

RWQCB Order 2001-01 requires the conservation of natural area "where feasible" for priority development projects, also referred to as "SUSMP" projects. (See Part Four, below.) That Order does not establish the conservation of natural areas as a specific requirement for all development projects. However, the County has long pursued development policies intended to conserve natural areas. therefore, staff shall apply this guideline to all development projects where economically practicable. Other environmental considerations, code requirements, or County programs, such as species and habitat protection requirements, may result in more stringent requirements to preserve natural areas.

The following design and development characteristics must be considered under this guideline:

a) Concentrate or cluster development on portions of a site while leaving the remaining land in a natural undisturbed condition.

b) Limit clearing and grading of native vegetation at a site to the minimum amount needed to build lots, allow access, and provide fire protection.

c) Maximize trees and other vegetation at each site by planting additional vegetation, clustering tree areas, and promoting the use of native and/or drought tolerant plants.

d) Promote natural vegetation by using parking lot islands and other landscaped areas.

e) Preserve riparian areas and wetlands.

Reasons for not pursuing a project alternative that would better conserve natural areas must be clearly identified and explained as part of the preliminary approval process.

3.8 Storm drain tiles and signage

3.8.1: Storm drain inlets and access points to creeks and channel must be marked to discourage illegal dumping, with markings indicating the receiving water by name.


a) All storm drain inlets and catch basins within the project area shall have a tile placed with prohibitive language (such as: "NO DUMPING – DRAINS TO ________") and/or graphical icons to discourage illegal dumping.

b) Signs and prohibitive language and/or graphical icons, which prohibit illegal dumping, must be posted at public access points along channels and creeks within the project area.

c) Legibility of tiles and signs must be maintained.

Storm drain tiles and signs are highly visible source controls that are typically placed directly adjacent to storm drain inlets. The tile contains a brief statement that prohibits the dumping of improper materials into the storm water conveyance system. Graphical icons, either illustrating anti-dumping symbols or images of receiving water fauna, are effective supplements to the anti-dumping message. Naming the receiving water also has proven to increase the effectiveness of this signage by making it more personal (i.e. "flows to San Dieguito River Park")

3.9 Additional BMPs Where Necessary

3.9.1: If the measures proposed for implementation under guidelines 3.2 through 3.8 above are not adequate to ensure compliance with the performance standards in Part Two above, additional BMPs sufficient to meet those performance standards must be included in the project.

Two approaches can be taken to meet this guideline: proposed BMPs can be intensified, or additional treatment BMPs can be added. Additional treatment BMPs can be engineered structures, or enhanced or constructed "natural" features. Potentially useful treatment BMPs include the following:

a) Utilize "natural" structural treatment BMPs such as vegetated swales, vegetated buffer strips, wet ponds, bioretention facility, constructed wetlands, and foundation planting

b) Utilize structural infiltration BMPs such as infiltration basin, infiltration trenches, dry-wells, and cisterns, so long as ground water is not jeopardized.

c) Utilize structural filtering BMPs such as oil/water separators, catch basin inserts, storm drain inserts, media filtration, continuous flow deflection/ separation systems, catch basin screens, clarifiers, desilting basins, and filtration systems

d) Utilize structural flow BMPs such as extended/dry detention basins and normal flow storage/ separation systems.

3.10 Infiltration BMPs

3.10.1: Infiltration BMPs shall not adversely affect designated beneficial uses for ground water.

Structural treatment BMPs intended primarily to infiltrate ground water include infiltration trenches and infiltration basins. This term does not include BMPs such as grassy swales, detention basins, vegetated buffer strips and constructed wetlands that are not designed primarily to function as infiltration devices, but which allow incidental infiltration.

Infiltration BMPs shall not be used for areas of industrial or light industrial activity, areas subject to high vehicular traffic (25,000 average daily traffic (ADT) on main roadway or 15,000 or more ADT on any intersecting roadway), automotive repair shops, car washes, fleet storage areas (bus, truck, etc.), or nurseries.

County staff shall require all projects using structural treatment infiltration BMPs to implement one of the following protective measures:

Option 1: Runoff shall undergo pretreatment such as sedimentation or filtration before infiltration or satisfactory evidence must be provided that project source control measures will adequately clean runoff for infiltration.

Option 2: The vertical distance from the base of any infiltration structural treatment BMP to the seasonal high groundwater mark shall be at least 10 feet. Where groundwater basins do not support beneficial uses, this distance requirement may be reduced.

Option 3: Infiltration BMPs shall not be located within 100 feet horizontally of any water supply wells.

Combination of these measures, or additional measures may be required by the County on a case-by-case basis to protect ground water quality, based on CEQA review.

3.11 Examples of BMPs

Application of these guidelines could, for example, lead to design requirements and permit conditions such as the following:

i. A commercial development might have loading/unloading dock areas where material spills could be quickly transported to the storm water conveyance system. The project should be required to cover loading dock areas or design drainage to minimize run-on and runoff of storm water. Direct connections to storm drains from depressed loading docks (truck wells) should be prohibited.

ii. A vehicle repair facility could handle oil and grease, solvents, car battery acid, coolant and gasoline from the repair/maintenance bays that can negatively impact storm water. Design plans should show repair bays indoors or designed to prohibit storm water contact; the drainage system should be designed to capture all wash water, leaks and spills with a sump for collection and disposal; and direct connection to the storm drain system should be prohibited.

iii. A project that includes vehicle/equipment washing/steam cleaning has the potential to contribute metals, oil and grease, solvents, phosphates, and suspended solids to the storm water conveyance system. The project plans should include an area for washing/steam cleaning of vehicles and equipment. The area should be self-contained and/ or covered, equipped with a clarifier, or other pretreatment facility, and properly connected to a sanitary sewer (with the required permit).

iv. A restaurant project could include various outdoor activities, equipment, accessory washing/steam cleaning, which have the potential to contribute metals, oil and grease, solvents, phosphates, and suspended solids to storm water. This type of project should include an area for the washing/steam cleaning of equipment and accessories that are self-contained, equipped with a grease trap, and properly connected to a sanitary sewer. If outdoors, this areas should also be covered, paved, and have secondary containment.

v. A gas station or auto repair shop project would have fueling areas with the potential to contribute oil and grease, solvents, car battery acid, coolant and gasoline to storm water. The project should include overhanging roof canopy for the fuel dispensing area, with the canopy and any downspouts designed to prevent drainage across the fueling area. The fuel dispensing area must be paved with Portland cement concrete (asphalt concrete is prohibited) with a 2% to 4% slope to prevent ponding, separated from the rest of the site by a grade break that prevents run-on of storm water. This protected area should extend the length at which any proposed hose and nozzle assembly may be operated plus 1 foot.

vi. A parking lot project (or portion of a project) would typically be a source of pollutants such as heavy metals, oil and grease, and polycyclic aromatic hydrocarbons that are deposited on parking lot surfaces by motor-vehicles, and that can be transported to storm water. Parking lots should be required to reduce impervious land coverage of parking areas, and to infiltrate clean runoff before it reaches storm drain system. (Parking lots subject to Part Four, below, should also be required to treat runoff before it reaches storm drain system to remove oil and petroleum hydrocarbons, and to ensure adequate operation and maintenance of these treatment systems to prevent system fouling and plugging.)



RWQCB Order 2001-01 also mandates that, no later than August, 2002, the County must enact regulations, which will require the use of appropriately sized "Structural Treatment BMPs" ("STBMPs", as defined in paragraph 4.1 below) at all "Priority Development Projects" ("PDPs", as defined in paragraph 4.2 below). (The County may determine to enact such regulations earlier than that date.) While applications related to PDPs which are approved prior to the enactment of such new regulations are not required to use STBMPs, they are encouraged to do so.

The purpose of this Part Four is to provide guidance to applicants and to staff reviewing applications for PDPs, which either (1) elect to utilize STBMPs, or (2) may be presented to decision makers after regulations requiring STBMPs have been enacted and therefore must comply with such regulations. The guidance provided in this Part Four is in addition to the requirements set out elsewhere in this Guideline.

4.1 Priority Development Projects

Pursuant to RWQCB Order 2001-01, section F.1.b.(2)(a), the development (including redevelopment) project categories subject to this Part are as follows:

i. Residential development of 100 or more housing units,

ii. Residential development of 10 to 99 housing units,

iii. Commercial development where the land area for development is greater than 100,000 square feet,

iv. Automotive repair shops,

v. Restaurants where the land area for development is greater than 5,000 square feet.

vi. Hillside development, on a slope of 25% or greater, where there are known erosive soil conditions, if more than 5,000 square feet of impervious surface will be created,

vii. Projects within or directly adjacent to Environmentally Sensitive Areas or discharging directly to such areas, if more than 2,500 square feet of impervious surface is created or the imperviousness of the site is natural condition is increased by more than 10%,

viii. Parking Lots >5,000 square feet or >15 parking spaces,

ix. Streets, roads, highways, and freeways if more than 5,000 square feet of impervious surface is created by the project, and

x. Retail gasoline outlets.

4.2 Additional Requirements

A "structural treatment BMP" is a runoff interception feature added to a project either (1) to capture stormwater for treatment, infiltration, or evaporation; or (2) to treat stormwater as it flows through the BMP. Structural treatment BMPs can use natural processes for treatment or infiltration.

4.2.1: Structural treatment BMPs must be implemented to remove pollutants of concern from storm water and non-storm water.

4.2.2: Required structural treatment BMPs shall be implemented prior to discharging urban runoff into receiving waters supporting beneficial uses.

4.2.3: Structural treatment BMPs shall be sized, designed, and located so as to infiltrate, filter, or treat the 24 hour volume indicated on the 85th percentile isopluvial map prepared by the County for use with this guideline, prior to its discharge to any receiving water body supporting beneficial uses.

4.2.4. If structural treatment BMPs intended primarily to infiltrate ground water are used in the project, those BMPs must meet the eight specific requirements set out in RWQCB Order 2001-01 section F.1.b.(2).(i).

These four additional requirements are in addition to the basic source control BMP, impervious surface, buffer area and natural area conservation requirements of Part Three.

RWQCB Order 2001-01 sets out specific requirements that the County must impose on structural infiltration BMPs at priority development projects. Those requirements are:

Urban runoff shall undergo pretreatment such as sedimentation or filtration prior to infiltration.

All dry weather flows shall be diverted from infiltration devices.

Pollution prevention and source control BMPs shall be implemented at a level appropriate to protect groundwater quality at sites where infiltration structural treatment BMPs are to be used.

Infiltration structural treatment BMPs shall be adequately maintained so that they remove pollutants to the maximum extent practicable.

The vertical distance from the base of any infiltration structural treatment BMP to the seasonal high groundwater mark shall be at least 10 feet. Where groundwater basins do not support beneficial uses, this vertical distance criteria may be reduced, provided groundwater quality is maintained.

The soil through which infiltration is to occur shall have physical and chemical characteristics (such as appropriate exchange capacity, organic content, clay content, and infiltration rate), which are adequate for proper infiltration durations and treatment of urban runoff for the protection of groundwater beneficial uses.

Infiltration structural treatment BMPs shall not be used for areas of industrial or light industrial activity; areas subject to high vehicular traffic (25,000 or greater average daily traffic on main roadway or 15,000 or more average daily traffic on any intersecting roadway); automotive repair shops; car washes; fleet storage areas (bus, truck, etc.); nurseries; and other high threat to water quality land uses and activities as designated by each Copermittee.

Infiltration structural BMPs shall not be located within 100 feet horizontally of any water supply wells.

A discussion of limitations and guidance for infiltration practices is contained in, Potential Groundwater Contamination from Intentional and Non-Intentional Storm water Infiltration, Report No. EPA/600/R-94/051, USEPA (1994).



This Part summarizes other stormwater requirements, not related to post-construction BMPs that are applicable to development projects. These requirements are contained in the County Stormwater Ordinance or are mandated by RWQCB Order 2001-01, and are included here for completeness. Many of these requirements (e.g., prohibitions on non-storm water discharges) are not limited to new development projects.

5.1 Discharge Prohibitions

All public and private development projects (and all other stormwater dischargers in the County) are subject to the discharge prohibitions set out below. Additional discharge prohibitions can be imposed in land development-related permits when necessary to protect water quality.

Sections 67.805 & 67.806 of the County’s Stormwater Ordinance prohibit all non-stormwater discharges that are not specifically authorized. These sections authorize discharges pursuant to an RWQCB permit; discharges that are specifically authorized by the County as necessary to protect public health and safety; and discharges from: water line flushing or other potable water sources, landscape irrigation or lawn watering; stream flows, rising ground water, ground water infiltration to storm drains, uncontaminated pumped ground water, foundation or footing drains (not including active groundwater dewatering systems), crawl space pumps, air conditioning condensation, springs, non-commercial washing of vehicles, natural riparian habitat or wet-land flows, swimming pools (if dechlorinated), fire fighting activities, and any other water source not containing Pollutants.

In anticipation of the more stringent requirements of RWQCB Order 2001-01, this guideline imposes the following additional discharge prohibitions: discharges from water main breaks; non-commercial washing of vehicles other than individual residential car washing; and contaminated ground water infiltration, are all prohibited from discharging to MS4s.

5.2 Construction-Phase Requirements

5.2.1: During construction, all development projects must comply with the state General Stormwater permit for construction activities, if applicable; with the conditions imposed in permits required for construction; and with County ordinances and guidelines for construction activities.

Processing of a project shall include a plan review to determine that appropriate construction storm water measures have been included. For additional information, see separate County guidance on construction activities and State Water Quality Control Board (SWRCB) Order 99-08 on the SWRCB web site (<>).

5.3 Industrial Facility General Permit Coverage

5.3.1: Prior to commencing industrial operations, any new industrial facility subject to California’s statewide General Permit for Storm Water Associated with Industrial Activities must provide evidence to the County that the Notice of Intent required to filed under that general permit has been filed.

Processing of a project shall include the submission and review information concerning any proposed industrial uses for the project (per the Standard Industrial Codes) to determine if coverage under a state industrial permit is required. For additional information see the industrial section of the SWRCB web site (<>). The County has not yet issued guidance for stormwater compliance at industrial facilities; state guidance applies.

5.4 BMP Maintenance

5.4.1: BMPs shall not be considered "effective," and therefore should not be accepted as meeting the MEP standard, unless the proponent provides proof to the County that a mechanism is in place to ensure ongoing long-term maintenance of all structural post-construction BMPs.

This requirement is mandated by the RWQCB. Moreover, experience shows that improper maintenance is a common reason for BMPs to not function as designed, or for water quality protection systems to fail entirely.

At this time, project proponents may meet this requirement in one of three ways. Each approach is subject to conditions.

Public entity maintenance

A public or acceptable quasi-public entity (e.g., the County Flood Control District, a state or federal resource agency, or a conservation conservancy) can agree to assume responsibility for maintenance, repair and replacement of the BMP. The Departments will not recommend that the County agree to accept any maintenance obligations itself unless estimated costs are front-funded or reliably guaranteed, e.g., through a trust fund, bond, letter of credit or similar means. In addition, the County must be protected from liability by appropriate releases and indemnities.

The developer must provide any public entity accepting maintenance obligations sufficient ownership or easement interests to allow maintenance, repair and replacement of BMPs. If structural BMPs are located within a public area proposed for transfer, they will be the responsibility of the developer until the County or other public entity accepts them. Structural BMPs proposed for transfer to any other public entity must be approved by the County prior to installation. The County shall be involved in the negotiation of maintenance requirements with any other public entities accepting maintenance responsibilities. The County must be identified as a third party beneficiary empowered to enforce any such maintenance agreement.

Developer agreement to maintain BMPs

The developer seeking County approval may enter into a contract with the County obliging that developer to maintain, repair and replace the BMP as necessary in perpetuity. Security may be required. The developer may also make separate arrangements for this work to be done by subsequent owners or third parties, but the developer must ultimately be accountable to the County if these third parties fail to perform as the developer sought to arrange.

"County Service Area" with back-up maintenance agreement

The developer can seek to create a County Service Area (CSA) or other funding mechanism to provide funds for BMP maintenance, repair and replacement on an ongoing basis. However, the developer must remain ultimately accountable to the County to pay all costs for BMP maintenance, repair or replacement if funding and maintenance by a CSA are inadequate for any reason.

Unacceptable options

Approaches that attempt to create an enforceable right in the County against persons or entities other than the developer are not, by themselves, acceptable to fulfill this requirement. A back-up maintenance agreement with the developer is also required. 



6.1 Shared Structural Treatment BMPs

All structural treatment BMPs are to be located to infiltrate, filter, or treat the required runoff volume or flow prior to its discharge to any receiving water body supporting beneficial uses. In most cases, on-site BMPs will be the best alternative for protecting all downstream water bodies. However, in some situations, better results may be achieved when structural treatment BMPs are shared by multiple new development projects. The County may elect to allow the use of a shared structural treatment BMP provided construction of the shared structural treatment BMPs is completed (or an equivalent temporary alternative is put in place) prior to the post-construction use of any new development project from which the structural treatment BMP will receive runoff.

6.2 Waivers Where On-site Structural Treatment BMPs are Infeasible

The County is authorized to develop a waiver provision as part of the development of a SUSMP. If developed and implemented, such a provision might allow structural treatment BMP requirements or sizing requirements that are infeasible for a project to be waived with potential cost savings being paid and used for improvements within the same watershed. The County could also develop an impact fee whereby all high priority development projects pay an urban runoff impact fee to the County, sufficient to offset the residual effects of pollution and flow added to runoff as a result of the project.

At the time of this guidance document there is no waiver or impact fee in-place or proposed.



1) Grading Ordinance - Sections 87.101 through 87.717 of San Diego County Code of Regulatory Ordinances On the Web at

2) Storm water Ordinance - Sections 67.801 through 67.811 of San Diego County Code of Regulatory Ordinances On the Web at

3) DLI-LD-H, Procedures for Storm water Regulations Compliance





The Maps referenced in this guideline are available for online viewing at:

"Flood Control"

Hard copy maps have been attached to those guidelines mailed out, but may be difficult to read. Those wishing to fully analyze the attachments are encouraged to utilize the online maps and to utilize their browser’s zoom function to test the maps.

Land Development | Private Development