§ 9-606

This Chapter specifies requirements and standards for storm drainage in development projects. The following general requirements apply to all development, unless otherwise specified in this Title:

(a)

Drainage Facilities. All development shall provide drainage facilities within and downstream from the project site to carry storm water runoff both tributary to and originating within the development site. The tributary area upstream from the development shall be considered as being developed in accordance with the General Plan or applicable Specific Plan, even if undeveloped. Storm water runoff shall be conveyed into a terminal drain or may be retained in a retention basin on-site as provided by this Chapter.

(b)

Approval of Drainage Facilities by Public Works. The design of drainage facilities required by this Chapter shall be approved by the Director of Public Works prior to issuance of a building permit or approval of the Final Map or Parcel Map, whichever occurs first. Submission of the drainage plans for approval shall be the

responsibility of the owner or developer of the site on which the development is proposed. All plans and drainage reports shall be prepared and signed by a California-registered civil engineer.

(c)

Drainage Facilities for Increased Runoff. Where a development will cause an increase in the storm water runoff, the developer shall provide drainage facilities to attenuate the flow rate and concentration of storm water discharged onto other properties to the pre-project condition. The design may consist of off-site drainage facilities to convey runoff to terminal drainage, or, if provided for in this Chapter, retention of runoff in ponds within the development site, or the use of detention basins to control the flowrate and areal concentration. The allowable discharge may be increased by the Director of Public Works when the developer can demonstrate in a drainage report that:

(1)

The increased discharge will not exceed the capacity of the downstream drainage facilities (all undeveloped land in the tributary watershed must be considered as being developed when determining the capacity of downstream drainage facilities); and

(2)

The proposed discharge will not increase the water surface elevation in upstream properties.

(d)

Design. Storm Drainage Facilities shall be designed in conformance with any Master Drainage Plan, Specific Plan, Special Purpose Plan, Master Plan, or area drainage plan adopted by the Board of Supervisors or approved by the Director of Public Works.

(e)

Discharge to Sanitary Sewer Prohibited. Disposal of surface and storm waters into sanitary sewer lines, leach lines, or seepage pits of individual sanitary sewage disposal systems is prohibited.

(f)

Watercourses and Flood Control Channels. If a development is affected by watercourses, channels, streams, or creeks for which the 100-year flood plain has not been established, the developer shall determine the 100-year flood plain limits and shall dedicate drainage easements conforming substantially with the flood plain limits plus such additional rights-of-way as shall be required by the Director of Public Works for access, structures, or channel changes.

(g)

Detention Basins and Retention Basins. If orderly and reasonable development of an area consistent with the General Plan or applicable Specific Plan requires detention of storm water, the developer shall provide facilities for detaining runoff in accordance with the County's Improvement Standards. Retention ponds not maintained by a public agency may only be used where allowed by this Chapter and shall be designed in accordance with the County's Improvement Standards.

(1)

Private retention storm drainage facilities shall be inspected on a complaint basis only to ensure that the drainage basin is being maintained as designed and approved. The property owner shall reimburse the County for all time, services, and materials needed to abate the violation, if it is determined from the inspection that the retention basin is out of compliance.

(2)

Private detention or retention basins that are found to be out of compliance shall be brought into conformance within 90 days. Failure to bring the on-site drainage improvements into compliance may result in revocation of any underlying use permit(s).

(h)

Drainage Report. Developers shall submit a drainage report for all development projects. This report must be submitted at the same time as the grading/drainage plan or subdivision improvements plans are submitted for plan check, whichever occurs first. Drainage reports shall be prepared by a Californiaregistered civil engineer in accordance with the County's Improvement Standards.

(i)

Development Projects Using Existing Buildings. Development projects using existing buildings and not increasing runoff may be served by the existing drainage system except where modifications are needed to comply with the National Pollutant Discharge Elimination System requirements.

(j)

Capacity of Existing Drainage System. If an existing drainage system is required to serve a development project but does not have capacity to serve the development project, the developer shall provide for additional capacity either by constructing off-site drainage facilities or contributing funds to the serving agency or district for drainage system expansion. The serving agency or district shall determine which option is appropriate.

(k)

Drainage Flood Control Improvements. Where a development project or subdivision is subject to a flood hazard, the developer or subdivider shall provide such flood control works, drainage facilities, or other improvements sufficient to provide all structures or building sites, both existing and proposed within the subdivision, with 100-year flood protection.

(l)

Easements for Drainage and Flood Control. The developer or subdivider shall provide easements for all flood control and drainage facilities sufficient to encompass such facilities and to provide for their maintenance, operation, and improvement.

Within an urban community, development projects shall be served by an existing public agency or district providing drainage services as follows:

(a)

Subdivisions. Subdivision drainage facilities shall discharge into a terminal drain, except that an agency retention basin may be an acceptable alternative where permitted by a Master Plan, a Special Purpose Plan, or Specific Plan, or a private on-site retention basin may be allowed in Rural Residential zones if parcels are two acres or greater and:

(1)

There are no terminal drainage facilities on-site or within 200 feet; and

(2)

The site conditions of soil permeability and groundwater level allow a retention basin design to meet the County's Improvement Standards.

(3)

Subdividers shall provide a deposit to finance their proportionate share of the estimated cost of the storm drainage system.

(b)

Development Projects on Existing Lots. Development projects on existing lots shall discharge into a public terminal drain if one is within 200 feet of the nearest property line. A private on-site retention basin may be an acceptable alternative for non-residential parcels or residential parcels that are two acres or greater, in size where:

(1)

There are no terminal drainage facilities within 200 feet,

(2)

The site conditions of soil permeability and ground water level allow a retention basin to meet the County's Improvement Standards, and

(3)

The tributary watershed area to the retention basin is less than 20 acres.

(Ord. No. 4683, § 27, 12-9-2025)

Subdivisions and development projects on existing vacant parcels in rural communities shall be served by a public drainage system or a private on-site retention basin as follows:

(a)

Subdivisions with Parcels Less than Two Acres. For subdivisions with any lots less than two acres in size, the subdivision shall be served by public drainage facilities that discharge into a terminal drain, except that a public agency retention basin may be allowed where there are no terminal drainage facilities within 1,000 feet.

(b)

Subdivisions with Parcels Two Acres and Larger. For subdivisions with all lots two acres or larger in size, the subdivision shall be served by an agency retention basin. Private on-site retention basins may be allowed where there are no terminal drainage facilities within 200 feet, and the site conditions of soil permeability and groundwater level allow a private retention basin design to meet the County's Improvement Standards.

(c)

Development Projects on Existing Vacant Lots. For development projects on existing vacant lots, the development project shall be served by a public drainage system that discharges into a public drainage system, except that a private on-site retention basin may be allowed for non-residential parcels or residential parcels that are two acres or greater in size where:

(1)

There are no terminal drainage facilities within 200 feet, and

(2)

The site conditions of soil permeability and groundwater level allow a retention basin design to meet the County's Improvement Standards.

(Ord. No. 4683, § 28, 12-9-2025)

(a)

I-L, I-P, and I-G Industrial Zones Development within the Limited Industrial (I-L), Industrial Park (I-P), and General Industrial (I-G) zones outside of urban communities shall be served by a public drainage system planned for the entire industrial area which discharges into a terminal drain. An alternative drainage system may be considered in conjunction with a Master Drainage/Special Purpose Plan that demonstrates that the drainage facilities:

(1)

Can manage multiple large storms,

(2)

Cause no drainage impact to neighboring properties,

(3)

Meet stormwater quality standards,

(4)

Include aesthetic features,

(5)

Manage vector control, and

(6)

Are designed to meet County Improvement Standards.

(b)

Warehouse and Truck Terminal Zones. Development within Warehouse (I-W) and Truck Terminal (I-T) zones may be served by a private on-site retention basin if the following exist:

(1)

There are no terminal drainage facilities within 1,000 feet; and

(2)

The site conditions of soil permeability and groundwater level allow a retention basin design to meet the County's Improvement Standards. Otherwise, projects must meet the requirements of Section 9- 606.040(a).

(c)

Freeway Service Commercial Zone. Development within Freeway Service Commercial (C-FS) zones outside of an urban community shall be served by a public drainage system planned for at least each side of the freeway that discharges into a terminal drain.

(d)

Commercial Recreation Zone. Development within a Commercial Recreation (C-R) Zone that are outside of urban communities shall be served by a public drainage system planned to serve the entire commercial recreation area.

(Ord. No. 4683, § 29, 12-9-2025)

Development projects within the General Agriculture (AG), Limited Agriculture (AL) and Agriculture-Urban Reserve (AU) zones may be served by private on-site retention basins.

(a)

Existing Buildings. A subdivision of lots with existing buildings on each proposed new parcel shall not be required to meet the drainage facility requirements specified in this Chapter.

(b)

Remainder Parcels. Undeveloped, designated remainder parcels created under the provisions of this Section shall meet the requirements of this Chapter prior to any grant of approval for a development project or issuance of a building permit on the remainder parcel.

For existing lots in antiquated subdivisions, drainage facilities may consist of a private on-site retention basin if there are no public drainage facilities within 200 feet of the lot and the site conditions of soil permeability and groundwater level allow a retention basin to meet the County's Improvement Standards.

9-606.080 - DEFERRED STORM DRAINAGE FACILITY AGREEMENT FOR FUTURE STORM DRAINAGE SYSTEM IMPROVEMENTS

Where a subdivision or development is required to discharge into a terminal drain and no terminal drainage facilities are available, the Director of Public Works may approve Deferred Storm Drainage Facility Agreements to ensure that subdividers and developers finance their proportionate share of the cost of the future storm system improvements.

Chapter 9-607 - Encroachments into County Rights-of-Way