Chapter 16.107
TREATMENT CONTROL BMPS

Sections:

16.107.010    Treatment control BMPs – Objectives.

16.107.020    Design to treatment control BMPs standards.

16.107.030    Structural treatment control volume-based BMPs.

16.107.040    Structural treatment control flow-based BMPs.

16.107.050    Limited exclusions.

16.107.060    Locating BMPs near pollutant sources.

16.107.070    Restrictions on use of infiltration BMPs.

16.107.010 Treatment control BMPs – Objectives.

A. Minimizing a development’s detrimental effects on water quality can be most effectively achieved through the use of a combination of site design, source and treatment control stormwater BMPs. Where projects have been designed to minimize, to the maximum extent practicable, the introduction of anticipated pollutants of concern that may result in significant impacts to the receiving waters through the implementation of site design and source control stormwater BMPs, the development would still have the potential for pollutants of concern to enter the stormwater conveyance system. Therefore, priority projects shall be designed to remove pollutants of concern from the stormwater conveyance system to the maximum extent practicable through the incorporation and implementation of treatment control BMPs.

B. In meeting the requirements in this section, priority projects shall implement a single BMP or combination of stormwater BMPs that will remove anticipated pollutants of concern, as identified by the procedure of this division, in site runoff to the maximum extent practicable. Treatment control BMPs with a high or medium pollutant removal efficiency for the project’s most significant pollutant of concern shall be selected. Treatment control BMPs with a low removal efficiency ranking shall only be approved by the City when a feasibility analysis has been conducted that exhibits that implementation of treatment control BMPs with a high or medium removal efficiency ranking is infeasible. Treatment control BMPs must be implemented unless a waiver is granted to the project by the Director of Development Services based on the infeasibility of any treatment control BMP. (Ord. 671 § 65, 2008; Ord. 569 § 2, 2002)

16.107.020 Design to treatment control BMPs standards.

All priority projects shall design, construct and implement structural treatment control BMPs that meet the design standards of this section, unless specifically exempted by the limited exclusions. Structural treatment control BMPs required by this section shall be operational prior to the use of any dependent development, and shall be located and designed in accordance with the requirements of this division. (Ord. 569 § 2, 2002)

16.107.030 Structural treatment control volume-based BMPs.

Volume-based BMPs shall be designed to mitigate (infiltrate, filter, or treat) either:

A. Volume of runoff produced from a 24-hour 85th-percentile storm event, as determined from the local historical rainfall record (0.6 inch approximate average for the San Diego County area). This volume is not a single volume to be applied to all of San Diego County as the size of the 85th percentile is different for various parts of the County. Project proponents may calculate the 85th percentile using local rain data. In addition, isopluvial maps contained in the County of San Diego Hydrology Manual may be used to extrapolate rainfall data to areas where insufficient data exists; or

B. Volume of runoff produced by the 85th-percentile 24-hour runoff event, determined as the maximized capture urban runoff volume for the area, from the formula recommended in Urban Runoff Quality Management, WEF Manual of Practice No. 23/ASCE Manual of Practice No. 87 (1998); or

C. The volume of annual runoff based on unit basin storage volume, to achieve 90 percent or more volume treatment by the method recommended in California Stormwater Best Management Practices Handbook – Industrial/Commercial (1993); or

D. The volume of runoff, as determined from the local historical rainfall record, that achieves approximately the same reduction in pollutant loads and flows as achieved by mitigation of the 85th-percentile 24-hour runoff event. Under this volume criterion, hourly rainfall data may be used to calculate the 85th-percentile storm event, where each storm event is identified by its separation from other storm events by at least six hours of no rain. (Ord. 569 § 2, 2002)

16.107.040 Structural treatment control flow-based BMPs.

As an alternative to volume-based BMPs, structural treatment controls may be designed as flow-based BMPs. Flow-based BMPs shall be designed to mitigate (infiltrate, filter, or treat) either:

A. The maximum flow rate of runoff produced from a rainfall intensity of 0.2 inch of rainfall per hour for each hour of a storm event; or

B. The maximum flow rate of runoff produced by the 85th-percentile hourly rainfall intensity, as determined from the local historical rainfall record, multiplied by a factor of two, for each hour of a storm event; or

C. The maximum flow rate of runoff, as determined from the local historical rainfall record, that achieves approximately the same reduction in pollutant loads and flows as achieved by mitigation of the 85th-percentile hourly rainfall intensity multiplied by a factor of two, for each hour of a storm event. (Ord. 569 § 2, 2002)

16.107.050 Limited exclusions.

A. Proposed restaurants, where the land area for development or redevelopment is less than 5,000 square feet, are excluded from the numerical sizing criteria requirements listed for structural treatment control volume-based BMPs.

B. Where significant redevelopment results in an increase of less than 50 percent of the impervious surfaces of a previously existing development, and the existing development was not subject to SUSMP requirements, the numeric sizing criteria discussed for structural treatment control volume-based BMPs apply only to the addition, and not to the entire development. (Ord. 569 § 2, 2002)

16.107.060 Locating BMPs near pollutant sources.

Structural treatment control stormwater BMPs should be implemented close to pollutant sources to minimize costs and maximize pollutant removal prior to runoff entering receiving waters. Such BMPs may be located on- or off-site, used singly or in combination, or shared by multiple new developments, pursuant to the following requirements:

A. All structural treatment control BMPs shall be located so as to infiltrate, filter, and/or treat the required runoff volume or flow prior to its discharge to any receiving water body supporting beneficial uses;

B. Multiple postconstruction structural treatment control BMPs for a single priority development project shall collectively be designed to comply with the design standards for structural treatment control volume-based BMPs;

C. Shared stormwater BMPs shall be operational prior to the use of any dependent development or phase of development. The shared BMPs shall only be required to treat the dependent developments or phases of development that are in use;

D. Interim stormwater BMPs that provide equivalent or greater treatment than is required by structural treatment control volume-based BMPs may be implemented by a dependent development until each shared BMP is operational. If interim BMPs are selected, the BMPs shall remain in use until permanent BMPs are operational. (Ord. 569 § 2, 2002)

16.107.070 Restrictions on use of infiltration BMPs.

At a minimum, use of structural treatment BMPs that are designed to primarily function as infiltration devices shall meet the following conditions. These conditions do not apply to structural treatment BMPs which allow incidental infiltration and are not designed to primarily function as infiltration devices (such as grassy swales, detention basins, vegetated buffer strips, constructed wetlands, etc.).

A. Urban runoff from commercial developments shall undergo pretreatment to remove both physical and chemical contaminants, such as sedimentation or filtration, prior to infiltration.

B. All dry weather flows shall be diverted from infiltration devices except for those nonstormwater discharges authorized pursuant to 40 CFR 122.26(d)(2)(iv)(B)(1): diverted stream flows, rising ground waters, uncontaminated ground water infiltration (as defined at 40 CFR 35.2005(20)) to stormwater conveyance systems, uncontaminated pumped ground water, foundation drains, springs, water from crawl space pumps, footing drains, air conditioning condensation, flow from riparian habitats and wetlands, water line flushing, landscape irrigation, discharges from potable water sources other than water main breaks, irrigation water, individual residential car washing, and dechlorinated swimming pool discharges.

C. 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.

D. The vertical distance from the base of any infiltration structural treatment BMP to the seasonal high groundwater mark shall be at least 10 feet or as determined on an individual, site-specific basis by the Director of Development Services. Where groundwater does not support beneficial uses, this vertical distance criterion may be reduced, provided groundwater quality is maintained.

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

F. 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.); plant nurseries; and other high threat to water quality land uses and activities as designated by the Director of Development.

The horizontal distance between the base of any infiltration structural BMP and any water supply wells shall be 100 feet or as determined on an individual, site-specific basis by the Director of Development Services. (Ord. 569 § 2, 2002)