Enterprise-Wide Sewer Design
The Public Works Department of Anne Arundel County is a true power user of Haestad Methods’ SewerCAD. Their goal? To develop an extended period simulation model for all 38,000 pipes of their wastewater collection system. Along the way they have been maximizing the technology and fully capitalizing on SewerCAD’s extensive functionality. Their sewer system, some of which dates back to the 1920s, consists of approximately 38,000 pipes over an excess of 1,300 miles (2092 km), and serves a major population center between Washington, D.C. and Baltimore, MD, United States.
The diverse terrain of Anne Arundel County has challenged the ingenuity of county engineers George Albright and Laura Layton. Taking full advantage of the data management, system analysis, and design functionality in SewerCAD, they have successfully met these challenges and are well on their way to completing this ambitious project.
Building the Model
As with many modeling projects, Anne Arundel County was faced with building a model from various data sources. The physical attributes for the wastewater collection system are maintained in a GIS which is regularly updated as new sections of the sewer network are created. Using SewerCAD’s Shapefile Connection feature, the engineers can update the model from the shapefiles, ensuring that the model is always based on the latest information.
The main source of sanitary loading data comes from water billing records, which provide flows at metered locations throughout the county. The flows are modified to reflect the return to the sanitary system and are then assigned to the appropriate manholes and tracked in a tax account database (Excel and Access). SewerCAD’s Database Connections feature is then used to connect the external databases to SewerCAD to dynamically update the loading information.
Planning Future Development
Using the Scenario Management feature in SewerCAD, the county engineers set up three scenarios to reflect the sewer network in its various stages of development. The first scenario includes the present day, existing conditions model and is used to calibrate the model and perform everyday operational modeling tasks. The second includes existing flows plus allocated flows and is used to help the engineers track developments in progress and avoid overestimating the network’s capacity to handle the new flows they will generate. The third scenario incorporates existing, allocated, and proposed flows and provides the engineers with a tool to plan for the future.
Simulating Pump Station Performance
The county’s main goal in using SewerCAD is to conduct time-based analysis using SewerCAD’s Extended-Period Simulation (EPS) functionality. An EPS analysis will give the engineers a clear picture of how the system functions over the course of a day, week, or year, and will enable them to accurately analyze pump station cycling and make informed decisions about pump station upgrades.
“Our model runs in steady state right now, but as we continue to develop it, we plan on running it in extended period mode,” said Albright. “We are entering tank levels and other operational data—always planning ahead for EPS analysis.”
Verifying Designs
As part of a new subdivision design submitted to the county, engineers used the automated design capabilities of SewerCAD.
“We looked at the proposed pump station to determine how the additional flows would affect the downstream gravity line. Since there was a diversion in the line, we needed to verify that it could handle the extra flow,” said Albright. “Once the model was set up with SewerCAD’s diversion feature, we ran the simulation and used the color-coding to highlight all the lines over capacity. We then ran SewerCAD in design mode to automatically resize those pipes. This process let us quickly verify the consultant’s work.”
As the collection system gets closer to capacity, the Public Works Department will have to start requiring developers to replace pipes downstream of proposed sites to support the increased loads.
“SewerCAD will give us the tools to support this, as well,” said Layton. “We’ll be able to see what’s over capacity and give design recommendations to the developers. It will be a valuable tool to help solve disputes and provide graphical representations of what’s going on so people can visualize and understand the system.”
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