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Not everybody is sold on the concept of global warming and climate change. Yet scientific evidence is mounting. Carbon dioxide levels in the atmosphere are at their highest in 800,000 years, and the average global surface air temperature has risen at a rate of 0.23°F each decade over the last 50 years. By 2100, it is anticipated to be between 3° to 7° F warmer, according to projections by the United Nations-endorsed Intergovernmental Panel on Climate Change (IPCC).

Future climate change could overwhelm current U.S. water, stormwater, and wastewater infrastructure, due to such factors as more frequent and intense storm events, rising mountainous snowlines, earlier spring runoff from snowmelt, increased melting of land ice (specifically in Greenland and Antarctica), and thermal expansion of the marine mixed layer in oceans.

Given the long lead times required to plan for and build water infrastructure, it is critical that water utilities include climate-change mitigation and adaptation strategies into their planning scenarios now.


The IPCC publishes assessment reports that help evaluate potential impacts on climate change. These reports consider various scenarios as a basis for estimating the rate of change in atmospheric concentrations of greenhouse gases (GHGs). Some show that by reducing GHG emissions, we can slow or prevent future climate change.

A mitigative planning strategy incorporates GHG reduction measures to prevent future climate change in long-term planning and designs for infrastructure and water management. Such strategies include, but are not limited to:

Increasing energy efficiency of facility pumps and treatment processes

  • Reducing truck transport of materials, e.g., chemicals and solids
  • Implementing renewable energy use (onsite production or purchasing).
  • While GHG legislation has been developing abroad over the last couple of decades, California adopted the United States' first GHG regulation in September 2006. The state's Global Warming Solutions Act regulates GHG emissions from both public and private agencies to reach 1990 levels by 2020 and 80% below 1990 levels by 2050 (see January 2009 issue, “Emission remission,” page 37).

    The regulation, however, does not mandate publicly owned water and wastewater treatment works unless they (1) generate more than 1 MW of power and emit more than 2,500 metric tons of carbon dioxide equivalent (CO2e) emissions or (2) have stationary combustion sources that emit more than 25,000 metric tons of CO2e combined.

    The U.S. EPA is following this lead and in March 2009 proposed a mandatory GHG reporting rule using the same emission threshold for stationary combustion sources. Approximately 13,000 entities nationwide would be affected by this rule, comprising 85% to 90% of annual GHG emissions.

    It is likely that, in the near future, wastewater treatment plants will be mandated to report and reduce emissions.


    In some regions, adaptation may be more urgent than mitigation. Adaptation includes incorporating current and predicted regional and local climate change impacts into planning for water systems. Some climate changes have already happened:

    Annual precipitation. A recent study led by the Climate Research Division of Environment Canada looked at two data sets of global rainfall from 1925 through 1999, which showed increased average annual precipitation in temperate regions of the U.S. Northern Hemisphere. Since about 1970 through 2000, the average tended to remain above the 20th-century mean and about 5% more than the previous 70 years. In the Western mountains, approximately 74% of weather stations showed an increase in annual precipitation falling as rain rather than snow from 1949 through 2004.

    Projected changes in total annual precipitation have varied widely across models and emissions scenarios. Therefore, long-term planning should be based on current precipitation trends analyzed on a monthly basis. Monthly trending is important for evaluating water supply in terms of precipitation falling as rain versus snow, as well as observing the timing of precipitation events throughout the year.

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.


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.

  • High tides will get higher. San Francisco's combined sewer system includes weirs along the city's perimeter for managing overflows. Some of these weirs are situated along the city's bayside and are 1.8 to 4 feet below the city datum (a point of reference that is 8.07 feet above the mean tidal level).

    The San Francisco Bay experiences two tidal cycles each day, resulting in a higher high tide and a lower low tide. The higher high tide is of concern due to potential backflow into the sewer system through the bayside weirs. The existing weirs experience backflow during extreme storm events, and rising sea levels will exacerbate this. The above chart shows historical monthly mean higher-high water (MHHW) levels of high tides, as well as estimated levels using 2001 IPCC sea level rise projections for 2050 and 2100. The levels at which a portion of the city's combined sewer overflow weirs exist are overlaid to show how the MHHW is approaching them over time.

    As a result of this high-tide analysis, the master plan recommends that approximately 30 duckbill valves be placed along the bayside of the city to prevent backflow of seawater into the combined sewer system during times of high tides.

    Long-term solutions for potential sea level rise include installing additional pumping. The city is also initiating a Low Impact Development program that over time may help to reduce combined sewer overflows or discharges. At some point in the future, when the tides become higher than the overflows on a daily basis in winter, the combined sewer overflows or discharges will need to be pumped out during the storm and coincident high tide. One solution is to modify the conveyance of wet weather flows by directing it to the three existing treatment plants/wet weather facilities. High-rate clarification will be required at the plants to treat the higher flow rates.

    The master plan also recommends that sea level and precipitation patterns be investigated for potential impacts every five years. These findings should be incorporated in the planning and design of new infrastructure.

    San Francisco's combined sewer system has served the city for more than 100 years. The last comprehensive master plan, adopted in the 1970s, addressed ways to upgrade the system to meet stringent regulatory requirements. This new master plan, which should be approved this fall, will update the aging infrastructure so it can better sustain climate changes.

    — Deslauriers is a project engineer, Lechowicz is an environmental analyst, McDonald is a project manager, and Holmes and Clinton are assistant project managers with Carollo Engineers in Walnut Creek, Calif. Loiacono is a project manager with the San Francisco Public Utilities Commission.

    From mitigation to adaptation

    Congress considers funding for water projects anticipating climate change.

    The Obama Administration has primarily supported mitigation efforts that reduce greenhouse gas emissions, with the American Recovery and Reinvestment Act of 2009 providing:

    $8.6 billion for energy efficiency and renewable energy projects
  • $5 billion for weatherization assistance
  • $2.4 billion for energy research and development
  • $810 million for transportation efficiency and alternative fuels
  • $1.4 billion that's expected to be available for industrial carbon sequestration projects
  • Extended or increased tax credits for energy efficiency, renewable energy, fuel refueling systems to homeowners and businesses. Advanced energy tax credits are also offered to carbon sequestration facilities.
  • One potential funding source has been proposed specifically for climate change adaptation projects in the Water System Adaptation Partnerships Act (H.R. 2969) presented to the U.S. House of Representatives June 19. If signed into law, this bill will provide 50% matching grants to water and wastewater projects promoting climate change adaptation, including:

    Water use efficiency

  • Water quality improvements
  • Source water protection
  • Green infrastructure
  • Energy efficiency and renewable energy
  • Advanced treatment technologies
  • Climate change studies or assessments.
  • Funding will be prioritized based on which projects address the most immediate risk of climate change-related impacts to water quality. At press time, the last major Congressional action was to refer the bill to the Committee on Transportation and Infrastructure and the Committee on Energy and Commerce.

    Pick a strategy

    Water managers should seek integrated, adaptive approaches to address potential climate change.

    The projected impacts of climate change on water resources management, coastline water, and wastewater infrastructure dictate that water and wastewater agencies should adopt an adaptive strategy for addressing climate change. Two approaches may be taken:

    A “reactive” approach refers to short-term solutions. For example, retrofitting coastal outfalls to prevent backflow due to rising sea levels (see picture at immediate right) or speeding up long-term plans as necessary.

  • A “proactive” approach refers to preparing preventive, adaptive, long-term planning solutions. For example, re-examining the intensity-duration-frequency curves of design storms used to model and design collection/conveyance systems and re-examining water storage management decisions. Both public works management and elected officials should be involved in these approaches. Managers should prioritize their capital improvement plan, keeping in mind that their facilities and infrastructure will be pushed to capacity as weather conditions change. Elected officials need to stay informed on climate change issues, and inform the public of the changes underway and those projected, to provide better support for their community's evolving needs.