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Better Safe than sorry

Better Safe than sorry

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    San Francisco's sewer system master plan is being designed to protect the city's sewer system against the rising tides of the next 30 years. One short-term option is to retrofit outfalls with duckbill valves that protect against backflow. Photo: Tideflex Technologies

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    Carollo Engineers examined local surface water supplies, irrigation demands, and sea-level rise as part of Santa Barbara, Calif.'s water supply planning study. The study looked at precipitation trends to determine whether to modify surface water supply management to accommodate increased variability brought by climate change. At this time, Santa Barbara will continue current operations management while tracking precipitation patterns for future planning. Photo: City of Santa Barbara, Calif.

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    San Francisco flooding on the riseThis chart maps historical monthly mean higher high water (MHHW) levels relative to the San Francisco city datum and estimated 2001 Intergovernmental Panel on Climate Change (IPCC) projections of future high tides. Long before the projections of even higher tides exceed the weir levels, flooding will occur. For a larger version of this graph, visit the “article links” page under “resources” at www.pwmag.com. Source: Carollo Engineers

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: