Flood Protection
How Flooding Happens
Flood Preparedness

Understanding Flooding

Every drop of rain follows a different path downhill. Where it goes depends on how much rain falls, what people have done to modify its route through the watershed, and many other factors. If we understand the causes of flooding, we can begin to address its impacts.

Rainfall from peak to valley

The summit of Mount Tamalpais receives the most rainfall. The watershed is drier at lower elevations near the bay.

What happens when it floods?

Flood types

tidal inundation

Tidal Inundation
Rising tides fill creeks and flood lowlands

Extreme high tide events exacerbate creek or stormwater overflow during storms. High tides can back up stormwater pipes and prevent non-tidal areas from draining. Sea level rise will increase the extent and frequency of tidal inundation.

storm overflow

Stormwater Overflow
Stormdrains back up

Stormwater drainage systems quickly convey rainwater through underground pipes to creeks and the Bay. When the stormdrains are obstructed or broken or when the creeks that they lead to are already full, water backs up onto the streets. 


Isolated Ponding
Pools form on the ground

Isolated ponding can occur in any area that doesn’t drain effectively – for example, in a natural depression in the landscape.

creek overflow

Creek Overflow
Creeks spill over their banks

illustration of flood types

Naturally, waterways regularly overflowed onto an adjacent flood plain. Buildings are now often located on these flood plains. The size and slope of a channel, blockages, proximity to the bay, and constrictions obstructing flow such as bridges, utility pipes, or adjacent buildings all influence the frequency and volume of creek overflow.

How have land use changes impacted flooding?

The Southern Marin Watershed alone is home to 32,000 people, over 200 miles of roads and highways, 150 miles of sanitary sewers/waterlines, seven sewage treatment districts, one sewage treatment plant, eight pump stations, and 40 miles of stormwater pipes. It is also home to our dwellings, the commercial areas where we gather, the schools we attend, and the creeks, hills, and wildlife we enjoy. 

Elevation under natural conditions

Elevation under urban conditions

How does flooding impact water quality?

  • Water quality can be impaired by excessive nutrients, pesticides, elevated bacterial levels (such as fecal coliform), sediment from erosion, and sewage spills.
  • Moving water out to Richardson Bay as quickly as possible can have negative impacts on water quality. Waters that move slowly and have an opportunity to infiltrate can be cleansed by physical and biological processes that naturally occur underground.
  • Floods can also overwhelm sewage treatment facilities and wash untreated sewage into creeks, stormdrains, and Richardson Bay. 
  • Stormdrains take untreated runoff from the exteriors of our homes and businesses directly to creeks and Bay.

How do climate change and sea level rise affect flooding?

Dealing with flooding from rain and upstream runoff is already complicated. Sea level rise will make it even more complicated by increasing the frequency and duration of flooding.

When water temperature increases, water expands and takes up more space than cold water. As the planet warms, the water in the  ocean warms, expands, and elevates sea levels. The changing climate has also melted parts of the ice caps at the North and South Poles. As this ice melts and flows into the ocean, it increases the amount of water in the ocean and raises sea levels even more. Sea levels in San Francisco Bay have risen seven inches over the past century. 

Predictions of future sea level rise vary from 12 inches by 2030 to 60 inches by 2100. The Bay Conservation and Development Commission (BCDC) recommends using 36 inches of sea level rise for planning purposes.

Rising sea levels increase the upstream extent of tidal flooding, worsen creek overflow due to backwater effects of elevated high tides, and create larger, stronger waves which erode the shoreline and destroy sensitive marshes. Coastal flooding will have a large impact on cities and habitat.

High tide vs. low tide illustration