20.68.040 Specific requirements.
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. Street and Road Projects. Street and road construction projects with construction costs greater than $500,000 and add at least 10,000 square feet of impervious surface shall follow the city of Cudahy’s green streets policy manual (available from the city clerk).
B. Planning Priority Projects. The remainder of planning priority projects shall prepare a LID plan to comply with the following:
1. Retain stormwater runoff on site for the stormwater quality design volume (SWQDV) defined as the runoff from:
a. The eighty-fifth percentile 24-hour runoff event as determined from the Los Angeles County eighty-fifth percentile precipitation isohyetal map; or
b. The volume of runoff produced from a three-quarter-inch, 24-hour rain event, whichever is greater.
2. Minimize hydromodification impacts to natural drainage systems as defined in the municipal NPDES permit.
3. To demonstrate technical infeasibility, the project applicant must demonstrate that the project cannot reliably retain 100 percent of the SWQDV on site, even with the maximum application of green roofs and rainwater harvest and use, and that compliance with the applicable post-construction requirements would be technically infeasible by submitting a site-specific hydrologic and/or design analysis conducted and endorsed by a registered professional engineer, geologist, architect, and/or landscape architect. Technical infeasibility may result from conditions including the following:
a. The infiltration rate of saturated in situ soils is less than three-tenths 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 best management practices (BMPs) in retaining the SWQDV on site.
b. Locations where seasonal high groundwater is within five to 10 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 on-site volume retention requirement.
4. If partial or complete on-site retention is technically infeasible, the project site may biofiltrate one and one-half times the portion of the remaining SWQDV that is not reliably retained on site. Biofiltration BMPs must adhere to the design specifications provided in the municipal NPDES permit.
a. Additional alternative compliance options such as off-site infiltration and groundwater replenishment projects may be available to the project site. The project site should contact the approving agency to determine eligibility.
5. The remaining SWQDV that cannot be retained or biofiltered on site must be treated on site 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. One-fifth inch 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. (Ord. 690 § 4 (Exh. A), 2018).