O&M of Private Stormwater Measures

A stormwater treatment measure must be regularly maintained to ensure proper function. With proper and regular care, stormwater treatment measures can last for decades and continually provide water quality and flow reduction benefits. If not well maintained, a clogged stormwater measure can result in flooding of your property, standing water and mosquitoes, and other malfunctions of the measure, which may be expensive to fix. For instructions on how to properly care for your stormwater treatment measure, please refer to your Property Operations and Maintenance Agreement as well as the SMCWPPP Green Infrastructure Operations and Maintenance Guidebook.

A stormwater treatment measure is designed to capture sediment and pollution. Over time, these pollutants can build up in different areas of the treatment measure system. Regular inspection and maintenance activities will improve the measure’s ability to treat stormwater and provide benefits to the watershed.

Common problems include:

  • Sediment Buildup – Runoff from roads or other impervious surfaces can carry sediment into your stormwater treatment measure. Sediment can build up over time and prevent adequate flow or reduce storage capacity.
  • Erosion – As water moves through the stormwater treatment measure, scouring and channelization can reroute the flow of water and cause unnecessary ponding or circumvention of drainage features.
  • Trash Accumulation – Runoff during storm events can carry trash into your stormwater treatment measure which can clog the feature and prevent the feature from functioning properly.
  • Plant Loss – Impacts on plant life can occur from lack of water or too much water, over pruning, or other issues. Plant health is an important feature in stormwater treatment systems that require biotreatment or use evapotranspiration to assist with removal of water.
  • Weed Growth – Weeds growing at inlets or outlets can impede water flows and capture sediment. Weeds can also crowd out other species that are more conducive to stormwater treatment.
  • Soil Compaction – Walking in or through stormwater features can contribute to soil compaction which inhibits water infiltration and plant growth while increasing erosion.
  • Mulch loss – During significant storm events that exceed treatment capacity, mulch can be washed away. Improper mulch cover can contribute to increased erosion and weed growth while reducing the treatment area’s ability to retain water.
  • Standing Water – Standing water can result from poor drainage or poorly maintained irrigation systems. Excess water can contribute to plant death and result in the breeding of mosquitos or other vectors.

Any of these problems can lead to less effective treatment of stormwater runoff but regular inspection and maintenance can easily prevent these problems and their associated effects on stormwater quality. Tips on how to maintain Stormwater Treatment Measures can be found in the SMCWPPP Green Infrastructure Operations and Maintenance Guidebook.

Key Dates/Timelines

Monthly during the Wet Season (October to April of every year) – Recommended visual inspections and maintenance as needed.

October  – Courtesy notice and reminder regarding Annual Reporting and Inspection (if required) will be sent out by the County Planning & Building Department

By December 31st each year – Complete and submit Annual Report Forms to San Mateo County with pictures (as needed) of stormwater treatment measures.

At least once every 5 years – Full inspection of stormwater treatment measures completed by County staff.