The site for every planning priority project shall be designed to control pollutants, pollutant loads, and runoff volume to the maximum extent feasible by minimizing impervious surface area and controlling runoff from impervious surfaces through infiltration, evapotranspiration, bioretention and/or rainfall harvest and use.
(A) A new single-family hillside home development shall include mitigation measures to:
(1) Conserve natural areas;
(2) Protect slopes and channels;
(3) Provide storm drain system stenciling and signage;
(4) Divert roof runoff to vegetated areas before discharge unless the diversion would result in slope instability; and
(5) Direct surface flow to vegetated areas before discharge, unless the diversion would result in slope instability.
(B) Street and road construction of 10,000 square feet or more of impervious surface shall follow USEPA guidance regarding Managing Wet Weather with Green Infrastructure: Green Streets (December 2008 EPA-833-F-08-009) to the maximum extent practicable.
(C) The remainder of planning priority projects shall prepare a LID Plan to comply with the following:
(1) Retain stormwater runoff onsite for the Stormwater Quality Design Volume (SWQDv) defined as the runoff from:
(a) The 85th percentile 24-hour runoff event as determined from the Los Angeles County 85th percentile precipitation isohyetal map; or
(b) The volume of runoff produced from a 0.75 inch, 24-hour rain event, whichever is greater.
(2) Minimized hydromodification impacts to natural drainage systems as defined in the Municipal NPDES permit.
(3) When, as determined by the City, 100 percent onsite retention of the SWQDv is technically infeasible, partially or fully, the infeasibility shall be demonstrated in the submitted LID Plan. The technical infeasibility may result from conditions that may include, but are not limited to:
(a) The infiltration rate of saturated in-situ soils is less than 0.3 inch per hour and it is not technically feasible to amend the in-situ soils to attain an infiltration rate necessary to achieve reliable performance of infiltration or bioretention BMPs in retaining the SWQDv onsite.
(b) Locations where seasonal high groundwater is within five to ten feet of surface grade;
(c) Locations within 100 feet of a groundwater well used for drinking water;
(d) Brownfield development sites or other locations where pollutant mobilization is a documented concern;
(e) Locations with potential geotechnical hazards;
(f) Smart growth and infill or redevelopment locations where the density and/or nature of the project would create significant difficulty for compliance with the onsite volume retention requirement.
(4) If partial or complete onsite retention is technically infeasible, the project Site may biofiltrate 1.5 times the portion of the remaining SWQDv that is not reliably retained onsite. Biofiltration BMPs must adhere to the design specifications provided in the Municipal NPDES Permit.
(a) Additional alternative compliance options such as offsite infiltration may be available to the project site. The project site should contact the City to determine eligibility. Alternative compliance options are further specified in County of Los Angeles LID Standards Manual 2009 or as may later be amended.
(5) The remaining SWQDv that cannot be retained or biofiltered onsite must be treated onsite to reduce pollutant loading. BMPs must be selected and designed to meet pollutant-specific benchmarks as required per the Municipal NPDES Permit. Flow-through BMPs may be used to treat the remaining SWQDv and must be sized based on a rainfall intensity of:
(a) 0.2 inches per hour, or
(b) The one year, one-hour rainfall intensity as determined from the most recent Los Angeles County isohyetal map, whichever is greater.
(6) A multi-phased project may comply with the standards and requirements of this section for all of its phases by: (a) designing a system acceptable to the City to satisfy these standards and requirements for the entire site during the first phase, and (b) implementing these standards and requirements for each phase of development or redevelopment of the Site during the first phase or prior to commencement of construction of a later phase, to the extent necessary to treat the stormwater from such later phase. For purposes of this section, MULTI-PHASED PROJECT shall mean any planning priority project implemented over more than one phase and the site of a multi-phased project shall include any land and water area designed and used to store, treat or manage stormwater runoff in connection with the development or redevelopment, including any tracts, lots, or parcels of real property, whether developed or not, associated with, functionally connected to, or under
common ownership or control with such development or redevelopment.
(Ord. 4654, passed 3-10-14)