HYDRAULIC HYDRODYNAMIC MODELING AS AN EFFECTIVE MANAGEMENT TOOL FOR LARGE COLLECTION SYSTEMS - THE L.A. STORY Fernando Gonzalez, Adel Hagekhalil, Bryan Trussell, City of Los Angeles Bureau of Sanitation Eric Fontenot, Danish Hydraulic Institute, Inc., and Devang Parikh, Map Vision Technologies, Inc. ABSTRACT 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. As a result, City staff involved in planning, operation, and maintenance activities related to the collection system have increased their level of efficiency and effectiveness to design responses in case of severe damage to the collection system, meet the operational needs of the system, and plan the future conveyance needs of the collection system service area. The presentation will describe the model as a tool that effectively integrates planning, routine operation, system optimization, and emergency preparedness as well as its fabrication and calibration process showing results that confirms the validity of using fully hydrodynamic models for the simulation of large urban sanitary sewer collection systems. The presentation will also show the results of successful simulations of the City s large and complex system that includes many pipe interconnections, pumping plants, large catchment basins, special hydraulic structures, and reserve flow conditions. KEYWORDS Collection Systems Planning Tools, Dynamic Hydraulic Modeling, Risk Management, MIKE URBAN modeling INTRODUCTION
The City of Los Angeles has a large and complex wastewater collection system serving close to 4 million people in about 550 square miles of service area. The network consists of roughly 6,500 miles of sewers varying in size from 8 to 150 inches in diameter. Of those sewers, approximately 700 miles are classified as primary sewers (larger than 16 in diameter) which collect flow from smaller upstream secondary sewers. Primary pipes then route flow to approximately 50 miles of larger trunk lines termed outfall sewers by the City. These outfall sewers then discharge flow to one of the City s wastewater treatment plants, Donald C. Tillman (DCT) and Los Angeles Glendale (LAG) Water Reclamation Plants upstream, and then finally the larger Hyperion Treatment Plant. In addition to the City, the collection system and treatment plants serve about 23 contract agencies. With such a large and complex system, hydrodynamic modeling is an essential tool for effectively managing the collections system s operation. Four years ago, as part of an ongoing commitment to protect the public health and environment, the City of Los Angeles together with DHI, Inc. began developing a series of hydrodynamic models of the City s collection system. The first model developed consisted of the outfall sewers for both dry and wet weather conditions. This model is currently being used as a planning tool, capable of determining the demand for future capacity on a reach by reach or pipe by pipe basis. The model also tests the system s response to severe wet weather conditions, identifying any capacity constraints in the system. The model is being used to optimize the operation of the collection system by the rerouting of flows and the storage of peak flows during extreme conditions. Thus, the City s operational procedures have shifted from a reactionary to a preventative approach. Over the past two years, the model development has continued with the addition of the primary collection system to the model. This further increases the City s ability to analyze the collection system by including all of the main sewers in the system. The next step in the model development is to create and calibrate a wet weather model of the primary system; this step is currently in development. There are future plans to eventually extend the model to include the entire collection system, perhaps on a smaller sub-basin scale for the secondary sewers due to computational concerns. DRY WEATHER CALIBRATION The calibration of the dry weather primary model proceeded in two main phases. First, the primary network was imported into the MIKE URBAN geodatabase from the City s asset database. Several procedures were used to locate and correct errors in the network. Flow splits, diversion structures, inverted siphons and pumping stations were added to the model based on as-built drawings. Once the network was imported and corrected, estimates of sanitary sewer flows were added to the system. The sewer flow estimates were then compared to existing gauging data and adjusted to match historically gauged flow in the system. The import and calibration process was broken down into primary planning basins, making for more manageable sections and allowing a consistent evaluation of the calibration s progress. Figure 1 shows the primary collection system and basins in the model. Figure 1 The Primary Collection System and Basins
The network was imported from an existing City GIS shape file which had no interrelation between links (pipes) and nodes (maintenance holes). A customized import bridge was created to import the network information into the MIKE URBAN geodatabase; automatically creating a geometric network and the relationships between links and nodes in the system. Once the initial import was completed, several techniques embedded in MIKE URBAN were used to identify any errors or gaps in the network. The Project Check tool allows for a quick analysis of any errors based on inconsistencies between the network data, including missing data. For example, any manhole invert found to be higher than the associated pipe invert was flagged as an error (See Figure 2a). Longitudinal profiling was also used on a reach by reach basis to identify any significant change in inverts (See Figure 2b). Additional techniques were used to locate errors, such as flow tracing, flow capacity analysis, and network connectivity tools. After identifying and correcting network errors, the model was ready for calibration. Figure 2a Project Check Tool Invert Error Figure 2b Longitudinal Profiling Error
The flows for the hydrodynamic model were obtained from the City s existing Sewer Flow Estimating Model (SFEM). The SFEM calculates sewage flows based on a 1997 Southern California Association of Governments (SCAG) model, existing and future land use, and industrial point source discharges, all of which are embedded into the City s sewer network. Average dry weather flows calculated from the SFEM combined with diurnal patterns obtained from gauging data were then imported into the hydrodynamic model. The simulated flows are then compared to gauge data, and the SFEM estimated flow magnitude is adjusted until a good agreement between the simulated and gauged flows is achieved. The gauge data was collected from 2004 to 2006. The primary model consists of approximately 11,600 links and 11,400 nodes and contains over 600 point source loads, representing the secondary systems tributary to the associated primary link. The model was calibrated to over 500 gauges in the system, or approximately one gauge per every 1.3 miles of sewer pipe. The dry weather flow model was deemed calibrated when the volumetric error was below 10% and coefficient of correlation (R 2 ) was greater than 0.9. A statistical analysis tool in MIKE VIEW, a data and results viewing program, was used to calculate the volumetric error and the coefficient of correlation between the model results and gauged flow. Meeting the pre-defined calibration criteria ensures that the hydrodynamic model is reliable and can be used with confidence as a planning tool. Figure 3 shows an example of a good fit between the gauge data and the modeled data at a particular location. Additional verification of model results with current collection systems gauging will further increase the confidence of the model. Figure 3 Comparison of Gauge Data and Modeled Data
39004014 3900401439004015A (39004014 -> 39004015) 33.95 [cfs] 2.5 2.4 2.3 2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 [] 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 03:00:00 1/1/2005 Model Series Data Series Low er Calc Threshold 06:00:00 09:00:00 12:00:00 15:00:00 18:00:00 21:00:00 R2 = 0.932 The primary sewer network is a complex system consisting of flow splits, pumping plants, special hydraulic structures, and alternate flow conditions. Special consideration of the operational conditions of the collection system during the gauging period is being considered. Due to the network s immense size, the system is in a constant state of flux. Sewer pipes are constantly being constructed and diversion plates are being added or removed. The calibration of the model was performed with the sewer operational conditions representing 2005. Several large sewers were removed during the calibration process and an effort was made to remove any other system changes performed after 2005. After the calibration was completed, the network was reconfigured to match the current settings and was verified by comparing to current gauge data. Additional challenges in the calibration effort included in pipe sediment, poor hydraulics, special diversion structures, and the inclusion of dry weather urban runoff. There are several locations in the collection system where low velocities have led to sediment build up in the sewer pipe. This leads to an error with the calculated flow from the gauging data, since the calculation assumes a clean pipe. In order to address this issue, a trustworthy gauge downstream of the sedimentation location was used to determine the total contributing flow. An estimate of the sediment was then determined by a field crew at every gauged location. The lower sections of the circular pipe were then removed in slices, until the water depth matched the gauged depth and the total flow downstream matched the reliable gauge. This created a panhandle type pipe shape illustrated in Figure 4. Figure 4 Circular pipe with debris and associated specifications
The calibration of the dry weather primary model is only one step in the development of the City s intended modeling capabilities. Currently, additional flow monitors and rain gauges are in place to capture data from future rains for the calibration of the wet weather primary model. This is an ongoing project that, weather permitting, will be completed within the following two years. Long term plans for the modeling development include the addition of secondary sewer pipes to the City s hydrodynamic model. This will allow for a full analysis of the City s hydraulic needs, including future relief efforts and current optimization procedures. THE RISK MANAGEMENT TOOL The City is using the developed hydrodynamic models as tools to analyze the collection system. The existing outfall model has been in use for several years, allowing an analysis of the major flows throughout the system. With the completion of the primary dry weather model, additional detail of the collection system can be analyzed. City planners are developing primary basin master plans with the aid of the model. Planners use the model as a capacity analysis tool and to develop relief alternatives where capacity constraints are identified. The use of the MIKE URBAN hydrodynamic model has greatly aided the City s planning efforts, allowing for future capacity planning and forecasting, system optimization, special operational conditions, and emergency alternative analyses including extreme weather conditions. The existing wet and dry weather outfall model has been a vital tool for the City s collection system. The model has been used to optimize the systems flow settings, analyzing various flow splits and their effect at a system wide scale. Additionally, the system was analyzed under extreme wet weather conditions. System bottlenecks were identified
and flagged as potential future projects. Figure 5 shows depth over diameter of the entire outfall system during one such run. The model calibration was also verified with every model run. As in the field data showed similar results to modeled runs, the confidence of the model s accuracy has continued to grow. Figure 5 Depth over Diameter of the Entire Outfall System Case Studies The outfall model has also been used for several specific projects. An analysis was performed during the pre-design phase of several rehabilitation efforts to determine if the decreased flow capacity would greatly hinder the outfall system as a whole. For example, the modeling results of the Central Outfall Sewer (COS) revealed that the desired circular in pipe alternative would decrease the overall capacity of the collection system and that the existing elliptical shape would need to be maintained for future flows. Another example of the model being used as a planning tool is regarding the system changes at the DCT Water Reclamation Plant. The plant recently upgraded to include a nitrification/denitrification process which resulted in a decrease in the plants overall capacity. The outfall model was run with this decreased plant flow capacity to view the initial impact. It was determined that the system would not be able to handle the decreased flow capacity during wet weather conditions and that some water storage would be needed to attenuate the storm s peak. Further analysis using the outfall model yielded a specific flow and storage volume to prevent any adverse conditions in the
downstream collection system. Figure 6 shows the attenuation of the storm just downstream of the DCT wastewater reclamation plant. In the Figure, the curved lines represent the water level downstream of DCT while the straighter lines represent the volume stored onsite at DCT. Figure 6 Real Time Control (blue) and uncontrolled storage (red) Completion of the primary system dry weather model provides an additional planning tool that allows for a more detailed investigation of the collection system. As a part of the City s preparation for the future, the primary collection system has been divided into 24 primary basins. Capacity analysis reports on each primary basin are being performed. This includes a description of the basin s attributes, the age and condition of the existing pipe, the current and future flow demands, and the need for possible capacity increases and possible corresponding relief alternatives. Prior to the dry weather primary model, all capacity analysis was done using a static flow accumulation model, where peak flows were cumulatively added. Since all peak flows do not occur at the same time, this is a very conservative analysis. The application of the MIKE URBAN model allows for an analysis using a dynamic model. Flows are routed throughout the system and peak flows do not necessarily correspond, allowing for a much more accurate representation of the performance of the collection system. From a risk management perspective, the MIKE URBAN model is far less conservative and allows the City to determine, with a greater accuracy, which relief projects are truly needed. In addition, the dynamic nature of the model allows for an understanding of other flow conditions which would not be identified using a static procedure. Backwater conditions or other hydraulic impedances can now be located and analyzed using the hydrodynamic model. Figure 7 shows a profile of a pipe reach with flow from a typical dry weather flow model run. Figure 7 Profile of a Primary Pipe Reach with Dry Weather Flows
Future Plans The next steps in the development of the City s modeling efforts are already underway. Additional gauges are being installed prior to this year s winter season in order to collect ample data for the calibration of a wet weather primary model. Upon completion of the wet weather model, the City will have yet another tool in the planning process. The wet weather model can be used to test the primary collection system for extreme wet weather conditions, identifying system constraints and possible problem areas. In addition, the development of the wet weather model will help the City to determine the source of the wet weather contribution. The wet weather flow model will be developed using the Rainfall Dependent Inflow and Infiltration model, a continuous hydrologic model that tracks both the direct inflow and infiltration (I/I) components of wet weather flow separately. By simulating the I/I components separately, it can be determined which wet weather flow component dominates in a particular basin, and how to best mitigate those extraneous flows. The understanding of the I/I potential in each basin will further aid the City s understanding and planning process. Future plans for the City s modeling team include possibly combining the entire collection system into one model, perhaps being divided into sub-basins to decrease the intense computational needs. Inclusion of the entire system would allow for a thorough and detailed investigative review of the collection system. CONCLUSION The City of Los Angeles is constantly striving to improve our capability to protect the public health and environment. The organization of a modeling team with a network of tools to analyze the collection system provides a proactive measure towards preventing sanitary sewer overflows and allows for an understanding of the hydraulics of the system. Anticipating future flows and relative relief dates to the highest degree of accuracy possible decreases the City s fiscal expenditures while increasing the relative safety of the public. The calibration of the existing outfall model began the model development process and has proven to be an essential element in planning and designing on a large scale. With the addition of the newly calibrated dry weather primary model, a further step has been taken toward a detailed future planning of the collection system. With these tools, the City has the ability to forecast any potential problems and switches from a reactive damage control methodology to a preventative total preparedness planning. REFERENCES Southern California Association of Governments. Land use and future growth study. 1997 Danish Hydraulic Institute, DHI. MIKE URBAN User Manual. Copenhagen, Denmark