350-Year Storm Led to $800K Sewage Spill Fine, City Says

December 19-22, 2010, 7.85-inches of rain fell in the City of La Mesa over a 72-hour period. 

This amount of rain over this time period is characterized by the National Oceanic and Atmospheric Administration (NOAA) as a 350-year storm event. 

Significantly more rain fell in La Mesa from this storm than in most other areas of San Diego County. The City’s sanitary sewer collection system was not designed to handle the high volume of rain water inflow and infiltration that entered it and capacity was exceeded in several locations resulting in spills. As a result, the Regional Water Quality Control Board issued an Administrative Civil Liability complaint against the City for $948,816 for alleged violations of Statewide General Waste Discharge Requirements.

Nearly 80% of the spill came from one location which the City’s 2008 sewer flow model did not previously identify as having any capacity issues. Flow modeling is used to determine what sewer pipes need to be replaced and then programmed in a capital improvement replacement plan. 

Following the spill, City staff conducted an investigation and discovered that a bypass pipe installed in 1992 was siphoning flows from a 21-inch pipe to a smaller 15-inch pipe and likely was a major contributing factor to the spill. Bypasses are often installed and necessary to redirect flows in advance of future projects. 

Typically once the future improvements are constructed the bypass is left in place. There was no previous evidence of a any problems within these pipes. The bypass pipe was subsequently abandoned in 2012.

The City of La Mesa has been working systematically to reduce sewer inflow and infiltration since 2001. Some inflow and infiltration is normal, expected and taken into account when sewer models are created and pipes are designed, but not to the volume generated by a 350 year storm. 

In 2001, the City submitted an application to the State Water Resources control board for a $14.5M State Revolving Fund loan to replace 56,000 linear feet of the City’s oldest sewer pipes in an effort to reduce sewer inflow and infiltration. The loan was approved in 2006 and the work was completed in four phases in 2009.

Since 2002, over 86,000 linear feet of sewer pipe has been rehabbed or replaced at a cost of $20,842,000 to improve the reliability of the sewer system, reduce inflow and infiltration and prevent spills. This amounts to approximately 11% of the City’s total wastewater collection system. 

The City of La Mesa took all reasonable and prudent steps in the design maintenance and operation of its sewer collection system. The City of La Mesa’s sewer collection system is designed and built in accordance with industry standards and practices to accommodate typical and reasonably expected sewer flows. 

The rain event in December 2010 was anything but typical. When modeling and designing a sewer collection system an engineer must make some assumptions in an effort to come up with a reasonable design expected to protect the environment and be practical to build. A sewer system could be designed to withstand a 350 year storm event, but it would be far from practical to build.

On December 11, 2013 the San Diego Regional Water Quality Control Board approved a penalty of $801,462 against the City of La Mesa for the December 2010 sewer spill. The City was successful in negotiating down the original penalty almost $150,000 and in getting almost half of the remaining penalty ($387,606) suspended in return for completing a wetland restoration project within the Alvarado Channel.

That portion of the Channel is currently overgrown with invasive vegetation, subject to transients, graffiti and illegal dumping. When completed, the channel project will be a benefit to the community.

City Manager Dave Witt said "While the city is not pleased to be in this position, it is acknowledged that a significant sewage spill did occur in La Mesa in December 2010 as a result of a major rain storm. The settlement agreement was negotiated with the regional board staff in accordance with established standards and is consistent with similar actions in other communities. The settlement provides an opportunity to meet a large portion of the penalty with a project which will provide a direct benefit to our community.”