Chapter 16.104
ESTABLISHMENT OF STORMWATER BMPS

Sections:

16.104.010    Progression of stormwater BMPs.

16.104.020    Requirements to implement stormwater BMPs.

16.104.030    Selection of structural stormwater BMPs.

16.104.040    Structural stormwater BMPs for projects not generating primary pollutants.

16.104.050    Structural stormwater BMPs for projects generating primary and secondary pollutants.

16.104.060    Efficiency standards for structural stormwater BMPs.

16.104.070    Alternative stormwater BMPs.

16.104.080    Description of treatment control BMP categories.

16.104.090    Description of pollutant of concern categories.

16.104.010 Progression of stormwater BMPs.

Site design BMPs reduce the need for source and/or treatment control BMPs, and source control BMPs may reduce the amount of treatment control BMPs needed. Throughout all the following sections, all priority projects shall consider and incorporate and implement where expressly required by this division, and if not so required where determined applicable and feasible by the Director of Development Services, stormwater BMPs into the project design, in the following progression:

A. Site design BMPs.

B. Source control BMPs.

C. Treatment control BMPs. (Ord. 569 § 2, 2002)

16.104.020 Requirements to implement stormwater BMPs.

At a minimum, priority projects must implement LID site design BMPs and source control BMPs, and must implement treatment control BMPs unless a waiver is granted based on the infeasibility of all treatment control BMPs. BMPs must achieve certain performance standards set out in the Municipal Permit Section D.1.(d)(4) through (6). Selection of BMPs from the menus included in this division, using the rules set out in this division, must fulfill these requirements. (Ord. 671 § 46, 2008; Ord. 569 § 2, 2002)

16.104.030 Selection of structural stormwater BMPs.

Using the treatment control BMP selection matrix, each priority project shall select a structural treatment BMP by comparing the list of pollutants for which the downstream receiving waters are impaired (if any), with the pollutants anticipated to be generated by the project (as identified in Table 1). Any pollutants identified by Table 1 which are also causing a Clean Water Act Section 303(d) impairment of the receiving waters of the project shall be considered primary pollutants of concern. Priority projects that are anticipated to generate a primary pollutant of concern shall meet all applicable requirements of establishing stormwater BMPs, and shall select a single or combination of stormwater BMPs from Table 3 which maximizes pollutant removal for the particular primary pollutant(s) of concern.

Alternatively, a project proponent may elect to implement a combination of LID BMPs that either disperse and infiltrate, or direct to bioretention facilities, the flows from all impervious areas on site. These BMPs are presumed to provide maximum extent practicable treatment for all pollutants of concern; therefore, no further documentation of the treatment BMP selection process is required. (Ord. 671 § 47, 2008; Ord. 569 § 2, 2002)

16.104.040 Structural stormwater BMPs for projects not generating primary pollutants.

Priority projects that are not anticipated to generate a pollutant for which the receiving water is Clean Water Act Section 303(d) impaired shall meet applicable standard requirements of establishing stormwater BMPs, and shall select a single or combination of stormwater BMPs from Table 3 which are effective for pollutant removal of the identified secondary pollutants of concern, consistent with the maximum extent practicable standard defined in Attachment C of the Municipal Permit. (Ord. 671 § 48, 2008; Ord. 569 § 2, 2002)

16.104.050 Structural stormwater BMPs for projects generating primary and secondary pollutants.

A. Where a site generates both primary and secondary pollutants of concern, primary pollutants of concern receive priority for BMP selection. For such sites, selected BMPs must only maximize pollutant removal for the primary pollutants of concern.

B. Where a site generates only secondary pollutants of concern, selected BMPs shall target the secondary pollutant of concern determined to be most significant for the project.

C. Selected BMPs must be effective for the widest range of pollutants of concern anticipated to be generated by a priority project (as identified in Table 1), consistent with the maximum extent practicable standard defined in Attachment C of the Municipal Permit. (Ord. 671 § 49, 2008; Ord. 569 § 2, 2002)

16.104.060 Efficiency standards for structural stormwater BMPs.

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 Director of Development Services when a feasibility analysis has been conducted which exhibits that implementations of treatment control BMPs with a high or medium removal efficiency ranking are infeasible. (Ord. 671 § 50, 2008)

16.104.070 Alternative stormwater BMPs.

Alternative stormwater BMPs not identified in Table 3 may be approved at the discretion of the Director of Development Services, provided the alternative BMP is as effective in removal of pollutants of concern as other feasible BMPs listed in Table 3.

 

Table 2. Site Design and Source Control Stormwater BMP Selection Matrix

Priority Project Category

LID and Site Design BMPs(1)

Source Control BMPs(2)

Requirements Applicable to Individual Priority Project Categories(3)

a. Private Roads

b. Residential Driveways and Guest Parking

c. Dock Areas

d. Maintenance Bays

e. Vehicle Wash Areas

f. Outdoor Processing Areas

g. Equipment Wash Areas

h. Parking Areas

i. Roadways

j. Fueling Areas

k. Hillside Landscaping

Detached Residential Development

R

R

R

R

R

Attached Residential Development

R

R

R

Commercial Development > One Acre

R

R

R

R

R

R

Industrial Development > One Acre

R

R

R

R

R

R

R

Automotive Repair Shop

R

R

R

R

R

R

R

Restaurants

R

R

R

R

Hillside Development >5,000 ft.2

R

R

R

R

Parking Lots

R

R

R(4)

Retail Gasoline Outlets

R

R

R

R

Streets, Highways and Freeways

R

R

R

R = Required; select BMPs as required from the applicable steps in Chapter 16.105 or 16.106 PMC. (1) Refer to Chapter 16.105 PMC. (2) Refer to Chapter 16.106 PMC. (3) Priority project categories must apply specific stormwater BMP requirements, where applicable. Projects are subject to the requirements of all priority project categories that apply. (4) Applies if the paved area totals >5,000 square feet or with >15 parking spaces and is potentially exposed to urban runoff.

Table 3. Treatment Control BMP Selection Matrix
Pollutant of Concern	Bioretention Facilities (LID)	Settling Basins (Dry Ponds)	Wet Ponds and Wetlands	Infiltration Facilities or Practices (LID)	Media Filters	High-Rate Biofilters	High-Rate Media Filters	Trash Racks and Hydrodynamic Devices
Coarse sediment and trash	High	High	High	High	High	High	High	High
Pollutants that tend to associate with fine particles during treatment	High	High	High	High	High	Medium	Medium	Low
Pollutants that tend to be dissolved following treatment	Medium	Low	Medium	High	Low	Low	Low	Low

(Ord. 671 §§ 51, 52, 53, 2008; Ord. 569 § 2, 2002. Formerly 16.104.060)

16.104.080 Description of treatment control BMP categories.

All rankings of treatment control BMP pollutant removal efficiency in Table 3 are relative. Ranking of all facilities assumes proper sizing, design, and periodic maintenance. Following are general descriptions of each treatment control BMP category included in Table 3:

A. Bioretention Facilities (Infiltration Planters, Flow-Through Planters, Bioretention Areas, and Bioretention Swales). Facilities are designed to capture runoff and infiltrate slowly through soil media, which also supports vegetation. Bioretention facilities, except for flow-through planters, effectively promote infiltration into native soils. In clay soils, facilities may capture excess treated runoff in an underdrain piped to the municipal storm drain system. Typical criteria: an infiltration surface area at least four percent of tributary impervious area, six-inch average depth of top reservoir, 18-inch soil layer, 12-inch to 18-inch gravel subsurface storage layer.

B. Settling Basins and Wetlands (Extended Detention Basins, “Wet” Basins, Decorative or Recreational Lakes or Water Features Also Used for Stormwater Treatment, Constructed Wetlands). Facilities are designed to capture a minimum water quality volume of 80 percent of total runoff and detain for a minimum of 48 hours. Some wetland designs have proven effective in removing nutrients, but performance varies.

C. Infiltration Facilities or Practices (Infiltration Basins, Infiltration Trenches, Dry Wells, Dispersal of Runoff to Landscape, Pervious Pavements). These facilities and landscape designs capture, retain and infiltrate a minimum of 80 percent of runoff into the ground. Infiltration facilities are generally only feasible in permeable (Hydrologic Soil Group A or B) soils. Volume and area of infiltration facilities depends on soil permeability and safety factor used. Typical criteria: Infiltration facilities should have pretreatment to remove silt to prolong life of the facility. A 10-foot vertical separation from average seasonal groundwater depth is required. Dispersal to landscape may be accomplished in any soil type and generally requires a maximum 2:1 ratio impervious:pervious and concave topography to ensure the first one inch of rainfall is retained.

D. Media Filters (Sand Filters). Filters designed to treat runoff produced by a rainfall of 0.2 inches per hour (or two times 85th percentile hourly rainfall intensity) by slow infiltration through sand or other media. Typical criteria: surface loading rate not to exceed five inches per hour. Entire surface of the sand must be accessible for maintenance.

E. High Rate Biofilters (Tree Wells, Typically Proprietary). Biofilters with specially designed media to rapidly filter runoff while removing some pollutants. Filterra® (proprietary version) recommends surface loading rates of up to 100 inches/hour.

F. High Rate Media Filters (Typically Proprietary). Vaults with replaceable cartridge filters filled with inorganic media.

G. Drainage inserts have low effectiveness in removing pollutants that tend to associate with fine particles and have medium effectiveness in removing coarse sediment and trash. They are sometimes used to augment more effective treatment facilities and are sometimes used alone when more effective facilities have been deemed infeasible. (Ord. 671 § 54, 2008)

16.104.090 Description of pollutant of concern categories.

In Table 3, pollutants of concern are grouped as gross pollutants, pollutants that tend to associate with fine particles, and pollutants that remain dissolved. The different pollutant categories that fall under these groups are presented in Table 4.

Table 4. Pollutants and Associated Particle Sizes
Pollutant	Coarse Sediment and Trash	Pollutants that tend to associate with fine particles during treatment	Pollutants that tend to be dissolved following treatment
Sediment	X	X
Nutrients		X	X
Heavy Metals		X
Organic Compounds		X
Trash and Debris	X
Oxygen-Demanding		X
Bacteria		X
Oil and Grease		X
Pesticides		X

(Ord. 671 § 55, 2008)