The 4 million residents of the City of Los Angeles are served by one of the world’s largest and most extensive wastewater collection and treatment systems. This system conveys wastewater generated within the City, and from 29 surrounding municipalities and sewer districts served by the City, to three wastewater treatment/reclamation facilities. The system is owned by the City of Los Angeles and the overall responsibility for operating and maintaining this complex infrastructure lies with the City’s Department of Public Works – and more specifically – the Bureau of Sanitation (BOS). It is the goal of the City to operate this enormous system within both governmental regulatory requirements and established internal goals.
In order to meet these goals the City of Los Angeles BOS developed a wet weather flow model for the primary collection system using DHI’s MIKE URBAN collection system modeling software. During the model build and calibration process, DHI provided engineering support and training to the City.
The City’s primary collection system model includes 18 pumping stations and approximately 700 miles of sanitary sewer comprising pipes with diameters of 15 inches and greater. The wet weather flow (WWF) model consisted of a rainfall dependent inflow and infiltration hydrologic model coupled to the existing dry weather flow network hydraulics model developed using the CS Pipeflow module of MIKE URBAN.
The WWF collection system model was calibrated to 116 flow measuring gauges using measurements of rainfall and wastewater flows in the primary wastewater collection system recorded between 2007 and 2009. The calibrated wet weather flow model is capable of predicting the inflow and infiltration response within the primary collection system from both historical and design precipitation events.
The calibrated WWF model is currently being employed by BOS’s Wastewater Engineering Services Division’s (WESD) planning and modeling staff as a decision support tool in the execution of basin analysis and planning. The model has become an efficient tool for the determination of sewer capacity availability requests (SCARs) to assess the impact of proposed developments. The ability to identify I/I components and route the flow through the system allows the WESD planners to better plan, prepare, and operate the collection system to meet the demands of increased flows.
Modeling scenarios were created to simulate future conveyance needs of the system and to anticipate future wastewater conveyance deficiencies. The model allows the planners to determine the return interval at which capacity issues arise at individual locations. By determining both the location of the capacity limitations and the return interval at which the capacity issues arise, the City can prioritize and test system operating strategies and system improvements to ensure the desired level of service in the sewer system is met in the most cost effective way.
Finally, the modeling results have enabled the wastewater planning team to efficiently re-arrange the project implementation sequencing of the wastewater capital improvement program. Large sewer relief projects have been displaced along the improvement schedule according to the relief year predicted by the model, and as a result, the new project programming has produced an improved cash flow. In total, over $100 million has been made available to other wastewater treatment plant or collection system projects as a result of the revised wastewater capital improvement program.
For more information on the City of Los Angeles Wet Weather Flow model, please contact Fernando Gonzalez, City of Los Angeles, Bureau of Sanitation (fgonzale@san.lacity.org) or Eric Fontenot, Senior Project Manager, DHI Water & Environment (ejf@dhigroup.com). For more information on MIKE URBAN and how it can be used to help meet your collection system modeling needs please visit www.dhigroup.com or send an email to dhi-us@dhigroup.com.