Ultimately, the City aims to develop a 30-year plan for its combined sewer and wastewater treatment system, a major cornerstone of which is the creation and application of a detailed hydraulic model of the sewer system. This will evaluate the effect of various alternatives on sewer system flows, as well as CSO activation frequencies and volumes.
To undertake this work, a set of design storms was needed to properly evaluate system performance. As well as traditional single-event design storms, a continuous rainfall period is required to evaluate CSO operation over several years.
This undertaking was a major challenge, because San Francisco’s climate is heavily influenced by its geography. It is sited on a peninsula that extends into the Pacific Ocean, which keeps temperatures in the city within a range of between 50º and 70ºF (10ºC and 21ºC) in summer and 45º to 60ºF (7ºC and 15.5ºC) in winter. The city’s varied topography also creates distinct ‘microclimates’, so weather can vary considerably between locations.
San Francisco receives around 21.6 inches (549mm) of rain per year, with 95% falling between October and April, and 80% concentrated between November and March. Snow is rare.
Rain gage data
The City has historical rain data from three primary sources: National Weather Service rain gages; San Francisco Hydraulic-Hydrologic Data Acquisition rain gages; and the San Francisco Public Utilities Commission’s rain gage network.
There are two historic NWS gages in San Francisco, one at the Oceanside wastewater treatment plant and the other in Duboce Park. This latter gage, known as the FOB (Federal Office Building, after one of its locations) gage, has a reliable recording period dating back to 1907, and consequently data from it was used to select the typical period.
Citywide rain gage data is also available from the two other sources. Since rainfall can vary substantially from area to area, the typical period was chosen so that it fell within one of two periods for which there is citywide gage data.
Defining storm events
The minimum inter-event time defines the number of dry hours that have to occur between storms before the rainfall can be considered to be a new storm event. This is often calculated statistically by choosing a value that gives a coefficient of variation of 1 for the inter-event times, but this is not as good a fit on the west coast, where the resulting inter-event times are frequently 300 hours in duration.
To avoid having such a large inter-event time, a value of 24 hours was used to define storm events, based on the length of time needed to empty the CSO storage boxes within the city as simulated in the calibrated InfoWorks CS model.
Analyses were undertaken using the hourly rainfall data from the FOB gage. Rainfall statistics were collated for each storm event during the available record period, from 1907 to 2005, including total storm depth, peak intensity and duration.
Annual rainfall statistics, including total annual rainfall depth and number of storms, were also calculated for 1907 to 2004 (the complete 2005 and 2006 rainfall data were not available at the time). Storm depth is important for CSO control measures that involve storage, while peak intensity controls the occurrence of individual CSO activations.
To develop a typical period, a five-year span that closely resembled the long-term average in terms of number and distribution of storms was chosen. Individual storms were then added and removed so that the resultant five-year period matched average trends. The first step in this process was defining the long-term statistics that the typical period should match.
Long-term rainfall trends
A number of factors affect long-term rainfall averages in San Francisco, and it was important to take these into account. Rainfall can vary spatially, so it was important to take account of the fact that one key rain gage had moved twice (though analysis showed that the changes were not significant.)
Global climate phenomena such as El Niño also significantly affect San Francisco, as this produces a marked and rapid warming of the Tropical Pacific Ocean that can significantly increase the amount of rainfall that the City experiences. Spikes in annual rainfall at the NWS gage (1973, 1983 and 1997) have been proved to coincide with particularly strong El Niño years.
The Pacific Decadal Oscillation can also lead to changes in climate patterns. This longer-term phenomenon has been indicated by research that shows temperatures in the North Pacific follow a pattern of warming and cooling taking place over a number of decades. When the PDO Index (a number calculated based on the spatial average of temperatures in the Pacific Ocean north of 20º) is warm, or positive, the southern half of the US receives higher-than-average rainfall. The last change was in 1997, when the PDO shift changed back to warm.
Global climate change also appears to be increasing rainfall in San Francisco, though it is not clear if this trend will continue. Based on this analysis of rainfall factors, the City determined that the typical period should not match overall historical averages but should instead match the averaged characteristics of the past 30 years of rainfall.
This approach will create a more conservative typical period that takes account of recent increases in the overall rainfall and the warmer PDO period. Because climate change models vary so widely in their predictions, it was decided not to hypothesize any increase in rainfall from this source.
Because of the variability in the amount of rainfall experienced during individual storm events, it was important to account for spatial variation when choosing the typical period. To illustrate the potential range of rainfall, factors were created for each rain gage in the City based on the 50 largest storm events in terms of depth, and for a similar number by peak intensity as recorded by the NWS rain gage.
Storm depths and peak one-hour intensities were calculated for these storms for each City rain gage. Depth and intensity factors were calculated by dividing the storm value recorded at a given rain gage by the value at the NWS gage. It was decided that when spatial variation was taken into account a longer simulation period might be better, to account for regional variations – while a year may be considered typical at one gage, it may not be at other points in the system. Selecting a longer period – in this case, five years - minimizes the effects of year-to-year variation.
1977 to 1981 were chosen as the basis of the five-year typical period. These five years fell within the first of two periods containing widespread rain gage data from across the city and closely resembles the long-term period of record.
Although other periods with available data were closer to the 30-year average in terms of overall depth, this period was chosen because it most closely matched the 30-year average in terms of the distribution of storms - in particular intensity and depth ranges. The period was then modified by adding or removing storms to make it more closely match the typical 30-year average.
The calibrated InfoWorks CS model of the San Francisco collection system was used to simulate the five-year typical period. The results of this simulation were evaluated to determine the frequency of CSO activation. Activations occurring within six hours of another overflow are considered the same discharge event, and any separated by more than six hours were considered to be separate events.
The results of the five-year typical period simulation were compared to historical records for the past 20 years and the results were found to agree very closely, indicating that the typical period is suitable for use in predicting average overflows.