Rainfall intensity/distribution. The frequency of extreme 24-hour precipitation (storm) events in the continental United States has, on average, increased 22% to 26% since 1948, according to a 2007 study by the Environment California Research and Policy Center. The largest increases occurred across New England, New York, the Great Lakes area, and the upper Midwest, in addition to Louisiana, New Mexico, Northern Washington, and Southern California. Both general circulation and regional climate models project the intensity of precipitation is likely to increase around the world, with the most significant increases occurring in the middle to high latitudes.
Global simulations show the percentage increase in extreme precipitation is greater than the percentage increase in mean rainfall. Vietcheslav Kharin and Francis Zwiers, research scientists with the Canadian Centre for Climate Modelling and Analysis in Victoria, British Columbia, predict that extreme, 24-hour storm events will increase by a factor of two or more by the end of the 21st century. This means that water agencies should anticipate that 24-hour events with return periods of 10, 20, 50, and 100 years will occur at least twice as often by the year 2100.
Sea level. Melting land ice and thermal expansion are contributing to sea level rise, which is likely to accelerate in the future. According to the IPCC's 2007 Fourth Assessment Report, the mean sea level could rise 7 to 23 inches by 2100 (relative to 1990 levels). However, these projections are based on physical models that don't reproduce the current rate at which polar ice caps are melting.
During the last two years there have been major advances in the science of sea level rise. CALFED Independent Science Board studies estimate a mean sea level rise of 20 to 55 inches (midrange 28 to 39 inches) by 2100 and recommend that 55 inches be used for long-term climate change adaptation planning. The IPCC's more conservative estimates should be used for short- to mid-term planning purposes.THE SAN FRANCISCO APPROACH
Because coastal wastewater facilities and sewer infrastructure are especially vulnerable to rising sea levels and extreme storm events, the San Francisco Public Utilities Commission (SFPUC) is preparing for the worst. It is one of the first agencies to initiate a 30-year sewer system master plan that includes readiness for — and prevention of — flooding and backflow caused by the potential effects of climate change.
The commission is working with Carollo Engineers Inc., Brown and Caldwell, and Metcalf and Eddy to:
- Develop a long-term strategy for the management of the city's wastewater and stormwater
- Address specific challenges facing the system such as aging infrastructure, flood-prone neighborhoods, odor control, and biosolids disposal options
- Maximize system reliability and flexibility.
Carollo Engineers reviewed scientific, peer-reviewed papers published in the last 15 years to project changes in Northern California precipitation patterns and sea levels. It was determined that:Extreme storm events will increase.
The city's frequency of extreme rainfall events (and for periods of up to 24 hours) is expected to increase in the long term, but the total average annual precipitation will either remain the same, or will not increase significantly.
Project engineers analyzed hourly rainfall recorded at the National Weather Station rain gage, located in the city, and developed rainfall statistics for each storm event from 1907 through 2004, including total storm depth, peak intensity, and duration. For the design storms — storms whose intensity, duration, and frequency (IDF) do not exceed the design load for a storm drainage system or flood protection project — they analyzed the past 30 years of rainfall to consider factors such as climate change and El Niños.
The project engineers developed and compared IDF curves based on information provided by the State Climatologist's Office. They determined that the current IDF curve is conservative enough to be used in the master plan as the basis for sewer system design.
Sea levels are already rising. The average rate of mean sea level rise has an estimated range between 0.07 to 0.11 inches/year. Yet San Francisco's rate of sea level rise for the past decade has been 0.13 inches/year, exceeding these estimates.