Minimize Flood Damage

While some sort of seasonal flood related damage occurs nearly every year, the flooding and associated landslide events of the 1998 El Nino represent the most recent significant flooding within San Mateo County. San Mateo County has helpful information and standards that will help prevent and mitigate against flood damage.

Causes of Flooding in San Mateo County

Flooding occurs when climate (or weather patterns), geology and hydrology combine to create conditions where river and stream waters flow outside of their usual course and “overspill” beyond their banks.  In San Mateo County, the combination of these factors, augmented by ongoing development, create chronic seasonal flooding conditions.  San Mateo County spans a wide range of climatic and geologic regions that result in considerable variations in precipitation due the effect of living on a Peninsula.  This geographical area can produce several different weather patterns in any given day.  San Mateo County receives approximately 22 inches of rain on average each year.  The US average is 37 inches.  Flooding is most common from October through April when storms from the Pacific Ocean bring intense rainfall to the area.  Larger floods result from heavy rains that continue over the course of several days, augmented when the soil is near saturation from previous rains.

Riverine flooding and urban flooding are the two types of flooding that primarily affect San Mateo County.  Riverine flooding is the overbank flooding of rivers and streams, the natural processes of which add sediment and nutrients to fertile floodplain areas.  Urban flooding results from the conversion of land from fields or woodlands to parking lots and roads, through which the land loses its ability to absorb rainfall.

Factors that Can Lead to Flooding

Maintaining the flow capacity in streams that cross County properties requires cooperation and assistance to prevent flooding and bank erosion.  Following are some suggestions and information for understanding the ways that floodplains function and how the County regulates the floodplain in order to protect property and lives, while affording County citizens the ability to obtain floodplain insurance.

Do not dump or throw anything into ditches or streams. A plugged channel cannot carry water, and when it rains, the excess water must go somewhere.  Trash and vegetation dumped into a stream degrades water quality of both the stream itself and its receiving waters, and every piece of trash contributes to flooding.  All surface water management agencies that serve the urban areas of the County have adopted and enforce regulations that prohibit the dumping of material into any natural or manmade component of the drainage system.  Additionally, the County as a whole has adopted and enforces regulations that prohibit the illegal dumping of material, including material dumped into ditches, streams or other drainageways.  Please report any observations of the dumping of debris or other objects into streams, drainageways, or rivers to the San Mateo County Code Enforcement Section at 650/363-4825, Department of Public Works 650/363-4100, and Environmental Health Services Division 650/372-6200.

Remove debris, trash, loose branches and vegetation.  Keep banks clear of brush and debris to help maintain an unobstructed flow of water in stream channels.  Do not, however, remove vegetation that is actively growing on a stream bank.  Streamside vegetation is tightly regulated by local, state and federal regulations.  Before undertaking any removal of streamside vegetation, contact the San Mateo County Planning and Building Department at 650/599-7310 or San Mateo County Department of Public Works 650/363-4100.  Please report any observations of the clearing of vegetation or trees on stream banks to the San Mateo County Code Enforcement Section at 650/363-4825.

Obtain a floodplain development permit and/or building permit, if required.  To minimize damage to structures during flood events, the County requires all new construction in the floodplain to be anchored against movement by floodwaters, resistant to flood forces, constructed with flood-resistant materials and flood-proofed or elevated so that the first floor of living space, as well as all mechanical and services, is at least 1-foot above the elevation of the 100-year flood.  These standards apply to new structures and to substantial improvements of existing structures.  The County defines a Substantial Improvement as any reconstruction, rehabilitation, or addition to an existing structure, the cost of which exceeds 50% of the structures value using $300.00 per square foot as replacement cost.  See Section 6820, San Mateo County Zoning Regulations:  Flood Hazard Areas Regulations, and San Mateo County Building Regulations Ordinance, Division VII, Section 9022.3, and using the average replacement cost vs. the cost of the new construction will determine if the projects trips the 50% Rule and falls into the category of a substantial improvement project.  Additionally, most other types of development within the floodplain also require a floodplain development permit, such as grading, cut and fill, installation of riprap and other bank stabilization techniques.  County staff is available to undertake site visits (there may be a fee imposed), if requested, to review flood, drainage, sewer or retrofitting issues.  Contact the San Mateo County Planning and Building Department at 650/599-7310 or 650/599-7311 for further information and prior to undertaking any activity within the floodplain or if you see non-permitted building or filling in the floodplain.

Recognize the natural and beneficial functions of floodplains to help reduce flooding.  Floodplains are a natural component of the San Mateo County environment.  Understanding and protecting the natural functions of floodplains help reduce flood damage and protect resources.  When flooding spreads out across the floodplain, its energy is dissipated, which result in lower flood flows downstream, reduced erosion of the streambank and channel, deposition of sediments higher in the watershed and improved groundwater recharge.  Floodplains are scenic, valued wildlife habitat, and suitable for farming.  Poorly planned development in floodplains can lead to streambank erosion, loss of valuable property, increased risk of flooding to downstream properties and degradation of water quality.

Reduce risk of damage to homes.  Practical and cost-effective methods for reducing or eliminating the risk of flooding are available to property owners whose homes have experienced damage from flooding in the past, or may experience damage in the future.  Such techniques include elevation of the home, relocating the home to higher ground, constructing floodwalls or berms, flood-proofing and protecting utilities.  For further information, contact the San Mateo County Planning and Building Department at 650/599‑7310 or 650/599-7311.

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