Stormwater Program
Stormwater is a valued natural resource, the importance of which is being increasingly recognized through evolving national and state-level permits and greater public awareness. The United States Environmental Protection Agency (U.S. EPA) and California’s State Water Resources Control Board (SWRCB) have tasked local government agencies, such as the County of Humboldt, with administering stormwater management programs to reduce discharge of pollutants to storm sewer systems and associated receiving waters within the County’s municipal separate storm sewer system (MS4) jurisdiction. The SWRCB’s Municipal Stormwater Program webpage notes the “U.S. EPA defines an MS4 as a conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains) owned or operated by a State.”
However, stormwater management extends beyond the water quality flowing from our rooftops, driveways, parking lots and streets into storm sewer systems, managing stormwater quality discharging from construction sites and commercial and industrial facilities is mandated as well. California’s stormwater permitting program regulates discharges from select industrial facilities (the Industrial General Permit [IGP]), from construction-related activities (the Construction General Permit [CGP]), and areas administered by local government (the small MS4 Permit).
The SWRCB adopted the small MS4 Permit, also known as the Phase II Permit, in 2013. The small MS4 Permit has been amended by the SWRCB several times since its original adoption and is currently under review and is being amended, with reissuance of an updated small MS4 Permit anticipated by 2025.
The small MS4 Permit applies to stormwater discharges from MS4s, where designation as a “small” MS4 is related to a region’s population. The MS4 Permit intent is to provide regulatory consistency across the state to reduce discharge of pollutants to storm sewer systems and the environment. For Humboldt County, the SWRCB has stipulated the small MS4 Permit applies to McKinleyville, the unincorporated Eureka area (Myrtletown, Cutten, Ridgewood Heights, Pine Hill, and Humboldt Hill), and Shelter Cove (County of Humboldt Phase II MS4 Boundary Maps). Humboldt County’s MS4 jurisdiction is anticipated to be expanded to several southern Humboldt County communities with permit reissuance.
The small MS4 Permit includes development of programs which invite public participation, encourage public education, promote public involvement and outreach, detect and eliminate illicit discharges (such as improperly connected septic treatment systems, car wash water, and clean ups of spills and illegal dumping), technical review of property development designs, inspections of facilities, and enforcement of ordinances.
Post-Construction Stormwater Management
Low Impact Development
The small MS4 Permit mandates certain development projects comply with post-construction stormwater requirements based on LID standards. LID standards are intended to accommodate a site’s pre-development precipitation runoff by incorporating design techniques which capture, treat, and infiltrate stormwater onsite. These standards came into effect July 1, 2015.
The small MS4 Permit specifies two development project size classes mandating post-construction water management requirements. Projects creating and/or replacing 2,500 to 5,000 square feet of impervious surface (“small projects”) are required to implement at least one site design measure to reduce project site runoff, as described in the LID Stormwater Manual for Humboldt County. An impervious surface is a barrier (such as pavement) which prevents the soil’s natural ability to absorb and infiltrate stormwater. Impervious surfaces include, but are not limited to: roof tops, walkways, patios, driveways, parking lots, storage areas, impervious concrete and asphalt, and any other continuous watertight pavement or covering. Examples of site design measures include disconnection of rooftop drainage from impervious areas, tree planting, rain barrels, vegetated swales, and porous pavement.
Projects creating and/or replacing 5,000 square feet or more of impervious surface (“regulated projects”) are required to implement site design measures based on more detailed procedures and are required to demonstrate compliance with runoff reduction thresholds. Some projects may be required to construct bioretention facilities to promote infiltration of stormwater, while other projects in the larger size category will need to employ source control measures to minimize the contact between pollutants and stormwater runoff.
Procedures, standards, and specifications for implementing small MS4 Permit post-construction requirements are contained in the Humboldt Low Impact Development Stormwater Manual V3.0. This manual is also used by the Cities of Eureka, Arcata, Fortuna, and Trinidad for compliance with the small MS4 Permit. Humboldt LID Stormwater Manual V3.0 and associated documents are available on the North Coast Stormwater Coalition w.
Low Impact Development for Existing Sites
LID measures can, and are encouraged to be, incorporated into existing sites to enhance stormwater retention and infiltration onsite, with LID measures often being considered not only beneficial to water quality, but also attractive assets to a site. Site design measures include tree planting, rain capture and storage for later use, directing rooftop drainage systems to discharge to vegetated areas, and incorporation of permeable driving surfaces, such as driveways constructed using pavers which allow for infiltration of stormwater.