Hydraulic hydrodynamic modeling as an effective management tool for large collection systems - the LA story

There are a many difficult challenges when it comes to managing large and complex wastewater collection systems such as the one the City of Los Angeles owns and operates. The City’s sewer system serves close to 4 million people in about 550 square miles of service area. The network consists of 6,500 miles of sewers varying in size from 8 inches to 150 inches in diameter. Of those sewers approximately 700 miles make the Primary Sewer System that collect from smaller sewers and discharge to 50 miles of large capacity trunk lines called the Outfall Sewer System.  Outfall sewers convey wastewater to two upstream treatment plants and subsequently to the larger Hyperion Wastewater Treatment Plant. Hydrodynamic Modeling is an essential tool to effectively manage the system’s operation, especially when emergency conditions arise as a result of excessive wet weather flow, wastewater treatment plant shutdown, and/or pipe structural failure.

As part of the ongoing commitment to protect the public health and environment, the City of Los Angeles has taken proactive measures to minimize and mitigate spills from the wastewater collection system network. During the past four years, the City’s modeling team, made of City staff working together with DHI, Inc., developed a series of dynamic hydraulic models of the City’s sewer Outfall System. The Outfall Model, run under various conditions, allowed City staff to better plan, prepare, and operate the collection system to meet the demands of increased flow due to growth, special operational conditions, or excessive rainfall. Modeling scenarios were created to simulate flow conditions to design system responses in case of severe damage, to meet the operational needs of the system, and to plan the future conveyance needs of the system.

Thus, all City operation procedures are oriented towards forecasting, prevention, and total preparedness rather than purely mitigation measures and damage control.  Recently, the City’s modeling team has extended the existing Outfall Model of the City’s sewer to include the Primary Sewer lines. The City’s modeling team is responsible for developing and calibrating the City’s primary sanitary sewer collection system to both dry and wet weather flow. The model can be broken down into 24 primary sewer collection system basin master planning models. These 24 models can be aggregated into one master model of the entire 700 miles of the City’s Primary Sewer and Outfall systems, consisting of over 11,600 links, over 11,400 nodes, and over 600 point source loads.

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