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Wet Weather Hydrodynamic Modelling as a Planning Tool for the City of Los Angeles

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Abstract:

As part of its ongoing commitment to protect the public health and environment, the City of Los Angeles' Bureau of Sanitation has taken proactive measures to minimize and mitigate spills from the wastewater collection system network. MIKE URBAN was used to develop a hydrodynamic wet weather model that the City has started to use as a tool to help quantify inflow and infiltration to assess the sanitary sewer systems capacity. This paper presents a summary of the steps performed and the situations encountered during the model development and calibration process. In addition, this paper showcases the City's application of the wet weather model to help manage its large wastewater collection system. The model includes over 700 miles of the City's Primary Sewer and Outfall systems; consisting of over 11,600 pipe network links, over 11,400 nodes, over 950 dry weather point source loads, and over 900 wet weather catchments.

The wet weather model is a representation of the hydrologic loading of inflow and infiltration during wet weather events, the sanitary dry weather flow loadings, and the hydrodynamic routing of flows through the sewer system. The model was calibrated to gauged flows, depths, and velocities at 93 flow monitoring locations throughout the system. The calibration process employed rainfall measurements using the Los Angeles County ALERT rain gauge system, with the addition of a radar component to adjust gauge data to temporally and spatially distribute the rain over the region, thereby compensating for gaps created by rain gauge distribution. After calibration, the model was run under various design storm loading conditions to assess the sewer systems capacity and identify system bottlenecks and flow constraints.

The calibrated wet weather MIKE URBAN model is used to review and validate the City's Trigger Flow Levels, validate existing analyses in Primary Sewer Basin Master Plans, prioritize the City's Wastewater Capital Improvement Program, and evaluate sewer capacity availability requests to assess the impacts of proposed developments.

The modelling results have enabled the wastewater planning team to prioritize Wastewater Capital Improvement Program projects. Large sewer relief projects have been displaced along the improvement schedule based on the modelled anticipated relief year. As a result, the revised project sequence has improved cash flow, making funds available to address other wastewater treatment plant or collection system projects. This is especially critical given the current economic climate.

Modelling has been used to identify system responses to challenging flow conditions in cases of severe damage, especially when emergency conditions arise as a result of excessive wet weather flow, wastewater treatment plant shutdown, and/or pipe structural failure. Modelling results have led to the construction of wet weather attenuation tanks at the Donald C. Tillman Wastewater Reclamation Plant (DCTWRP). These modelling results enabled the plant to develop and implement a wet weather operation plan to effectively utilize the storage for wet weather peak flow attenuation, using real time controls to appropriately manage the flow in-and-out of the storage. The operation plan minimizes the probability of wastewater overflows due to a severe rain storm event.

Hydrodynamic Modelling has become an essential tool to effectively manage the system's operation. Bureau staff involved in planning, operation, and maintenance activities related to the collection system have exponentially increased their level of efficiency and effectiveness. Hydrodynamic modelling has made it possible to design, operate, and maintain the wastewater collection system to reduce sanitary sewer overflows by the maximum extent feasible. All Bureau operation procedures are now oriented towards forecasting, prevention, and total preparedness rather than purely mitigation measures and damage control.

Keywords: Collection System planning Tools; GIS Applications; Hydraulic Modelling; Infiltration and Inflow Modelling; Risk Management; Wastewater; Wet Weather Modelling

Document Type: Research Article

DOI: https://doi.org/10.2175/193864711802766308

Publication date: 2011-01-01

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