Climate change is not only expected to cause more hot weather days in Los Angeles; scientists forecast there will be more wet ones as well.
A rise in extreme wet-weather events could push the capacity of the storm drain and wastewater collection systems to their limits. But up until recently, the city’s civil and environmental engineers had not factored in the effects of climate change into their computer-assisted models.
“While climate change data exists, the integration between climatologists and planners is just starting to develop at the city level,” said Bryan Trussell, a civil engineer in the City of Los Angeles Department of Public Works, Bureau of Sanitation, Wastewater Engineering Services Division.
“This is one of the first projects to incorporate its impacts into advanced planning efforts for a major utility,” he said.
The result is a first-time study for Los Angeles, which did not focus on its hydraulic systems as a whole, but narrowed its scope to two study areas. Engineers selected 1) the San Pedro storm drain system and 2) the wastewater collection system leading to the Venice Pumping Plant. They are located along the bay and the coastline, respectively, and have been vulnerable to spills or flooding in the past during heavy storms.
The hydrological models Trussell works with are already capable of pinpointing weak sections during heavy storms where the system is over capacity and, as a result, spills or floods in the streets. The models use historical local rain gauge data to forecast storm characteristics, particularly the 24-hour period of a 10-year storm, a heavy storm that might occur once in a decade.
Consultants at CH2M Hill who have worked with cities such as Alexandra, Va., and Louisville, Ky., on similar studies were asked to calculate new rainfall intensity duration frequency (IDF) curves for the Los Angeles basin with the global climate change data factored in. CH2M Hill worked with CLIMsystems, developers of the computer program SimCLIM, to combine IPCC climate models with local rainfall data to come up with the IDF curves, said Kathy Freas, global water resources director for CH2M Hill.
The results were analyzed for target years 2025, 2050 and 2090, based on the current state of the city’s hydraulic infrastructure.
Trussell said the engineers focused on the worst case greenhouse gas emissions scenario for the purposes of the study. Known in the scientific community as A1FI, this scenario is one of six developed by the Intergovernmental Panel on Climate Change (IPCC). The outcome is based, on among other factors, an estimated 4-degrees Celsius (3.6-degrees Fahrenheit) rise in the global temperature.
San Pedro Storm Drain System
The San Pedro storm drain system is just one component of Los Angeles’ entire storm drain network, which includes 1,500 miles of underground pipes, 100 miles of open channels and 60 outfalls, or pipes, that discharge into the ocean, bays or river flood control channel.
Climate change is projected to cause more extreme wet weather events, increasing over the decades. During previous storms, this portion of the San Pedro system had been subject to spills or flooding. The model, without climate change data, already showed weak points during severe storms. Analyzing the data, engineers compared the two results and found that the spilling or flooding in certain places did worsen according to 2090 projections, but the difference was not major. In addition, the San Pedro system in its current state is expected to be able to accommodate the increase in peak flow. In 2090, at the outfall, the flow is projected to increase nearly 12 percent, from 600 cubic feet per second to 670.
But the biggest difference came with the projected rise in sea level. At current sea levels, the 60-inch diameter outfall, or pipe, that discharges the rainwater from the San Pedro system into the harbor is already partially submerged. As the water rises, it makes it more difficult for the rainwater to flow down the diagonally slanted pipe into the ocean due to pressure from the back flow. The models project the pipe has the potential to be completely submerged underwater.
“The differences in the sea level rise are not that much for the LA basin in 2050,” said Rajat Chakraborti, CH2M Hill project engineer. The rise is greater, he said, from 2050 to 2100.
After 2050, the climate change models have a lot of uncertainty in the results, he said. A difference between a 15-inch sea level rise or a 55-inch rise is projected for the year 2100. The first calculation is based on IPCC’s Assessment Report 4. The second is based on the California Interim Guidance (2011), which substitutes for Assessment Report 5, until it is released. In 2013, the SimCLIM model is expected to be updated using Assessment Report 5 guidance.
Coastal Interceptor Sewer and Venice Pumping Plant
The city’s wastewater collection system ranks among the largest in the nation, accounting for four million residents, in addition to another 600,000 in neighboring cities which Los Angeles services.
The Coastal Interceptor Sewer system is a network of public sewers along the coast that carry wastewater, urban runoff and rainwater into the Venice Pumping Plant. This network was chosen by the engineers for the study because of the criticality of the plant, since all flow from the plant must be pumped.
Trussell said the study did not factor a sea level rise component into its analysis of the Coastal Interceptor Sewer and Venice Pumping Plant because the model ends before it reaches the Hyperion treatment plant, the last stop before the outfall discharges the treated water into the ocean. And the plant is situated 20 feet above sea level.
While rainwater is generally not treated before being discharged into the ocean and bay, urban runoff is treated at the Hyperion plant. Trussell noteed the wastewater collection system is not designed to collect rainwater, but its flows double in a 10-year storm. The rainwater enters the public sewer system through manhole covers, pipe seals and joints.
The current system is expected to accommodate the increase in peak flow in 2090 projected at an additional three cubic feet per second, a difference of less than two percent.
While there are slight increases in spills and floods projected in 2090, Trussell said one current upgrade will improve the system’s capacity to handle increased loads. The Venice Pumping Plant will receive a second force main to accommodate peak flows when carrying water to the ocean outfall.
“When we’re fixing something now, we’re accounting for the future,” said Trussell.
Utilizing the Data
The pipes used for wastewater collection have an estimated 80-year lifespan, but half in the city have already reached 50.
Trussell said the information from the models can help engineers make informed decisions for the future. But at the same time, there are more urgent issues with maintenance and operations of the existing wastewater collection and storm drain systems. The question is whether to further invest in infrastructure based on the theoretical results, he said.
With the initial study demonstrating that the city engineers can factor in climate change data in their hydrological modeling, the next step is to examine the system on a larger scale. How will the rise in sea level affect other areas? What about the flooding impact in other parts of the city? How much of an increased flow will the outfalls need to accommodate?
Once these questions are answered, the city of Los Angeles will be more prepared to handle climate change impacts by making adjustments to their wastewater collection and storm drain systems, ensuring the aging infrastructure can continue to play its important role for generations to come.