Modeling Helps With Phased Development of Sewer System

Sept. 1, 2005
The City of San Diego in California is the state’s most southerly urban area, and is experiencing the same inexorable population pressures as most Californian cities.

The City of San Diego in California is the state’s most southerly urban area, and is experiencing the same inexorable population pressures as most Californian cities. One area that remains relatively undeveloped, however, is Otay Mesa in the south of San Diego County - because of a lack of major infrastructure, notably freeways and sewerage, and the capital to put this in place. Construction of sewerage infrastructure to meet current land plans would cost an estimated $120 million - beyond the city’s immediate funding capability.

The solution proposed by consultants PBS&J for its client, the City of San Diego Metropolitan Waste Water Department (MWWD) involved using Wallingford Software’s InfoWorks CS software solution to optimize the development by reducing the initial outlay while making the most of benefits to residents and businesses. PBS&J was brought in for its specific expertise in hydraulic modeling and master planning. Senior Project Manager Daniel Brogadir proposed using a dynamic hydraulic model to develop a phased improvement program that maximized use of the existing facilities and minimized capital layout.

The existing (Phase one) system consisted of the backbone city network, and temporary pump stations and force mains capable of conveying up to 2 mgd of sewage. An interceptor sewer takes flows alongside the ocean to a wastewater treatment plant at Point Loma, discharging treated effluent to the Pacific Ocean.

Portions of the existing Otay Mesa Trunk Sewer run westerly to the edge of the West Mesa area, with a temporary pumping station taking flows north to the adjacent Otay Valley Trunk Sewer. The existing city master plan proposals raised security, environmental and geotechnical concerns, so the second phase of the project moved to considering alternatives.

These included extending the Otay Mesa trunk sewer, transforming the temporary pumping station into a permanent one, taking flows through a linking force main sewer into a 1050mm gravity sewer and connecting into the coastal interceptor sewer. PBS&J was assigned to update the city’s master plan, performing an alignment study of alternatives, and preparing environmental documents for the preferred project alternative.

At the heart of this strategy was the decision to use InfoWorks CS. A dynamic model would provide decision-makers with a more accurate picture of wastewater flow conditions within the existing collection systems.

The project focused on evaluating the available capacity of the existing Otay Valley Trunk Sewer for receiving unplanned flows from the adjacent drainage basin. Short-term conditions were assessed with the aim of maximizing capacity, thus facilitating the planned Otay Mesa expansion. Longer-term flow projects were developed to minimize and defer capital outlays.

InfoWorks CS provided the tools to estimate the quantity of wastewater generated and the profile of the hydraulic stream in the system. The software allowed modelers to systematically calibrate and validate the parameters that control the simulation, including friction loss coefficients and stormwater infiltration rates. Unit generation rates and diurnal patterns for various land uses could be validated. It was also possible to examine temporal variations in flow patterns throughout the week, weekend and holiday periods.

The initial study approach examined the demographic growth projections and identified Phase Two alignment alternatives, which were put through a screening process and a phased implementation plan set out. The growth projections showed that a substantial increase in population and flows was likely for all areas.

Based on results of the coarse screening process, the alignment studies examined in detail four alternatives with a gravity sewer from Otay Mesa Road to San Ysidro Boulevard common to all. Calibrated city-wide InfoWorks models were refined for the study area, and models of each of the four selected alignments were developed for use in final screening of the alternatives and development of the project implementation plan.

‘The whole project became extremely expensive when sized for ultimate buildout,” Brogadir said. “By 2030 the pumping station would need to take 34 mgd, and the force main would have to be very big, so the ultimate cost would be over $100 million. A lot of the projections for the future may not come to fruition - land use may change and development may slow - so we wanted to look at a phased implementation plan.”

The plan chosen involves initially expanding the existing 2 mgd temporary pumping station to take 4 mgd, acquiring land for a new pumping station and its construction in the earliest phase. Flows will continue to be pumped to the Otay Valley Trunk Sewer, but it is acknowledged that there will be a threshold limit.

The InfoWorks CS model is ensuring that the capacity of the Otay Valley trunk sewer is fully used but not exceeded for this interim phase. It is anticipated from initial results that flows can continue to be sent north to Otay Valley for five to 10 years.

“The model is very accurate - we could really look at squeezing capacity from that area,” Brogadir said.

The new gravity portions of the system are being constructed. These trunk sewers will allow construction of the West Mesa portion of the system within the next two years. Later phases will increase capacity and will involve expanding the existing pump station as sewer flows reach the existing 4 mgd threshold capacity. Phase 2D will involve the ultimate connection of the East Mesa system to the new portion of the Phase Two system in the west.

“It is critical to have good hydraulic modeling,” Brogadir said. “We can make sure the line can take the extra capacity, which is important as Otay Valley is also building out, an expansion that the city wants.”

Ultimately, the study enabled San Diego to reduce capital outlay in the first few years to about $20 million, with another $55 million planned in phased construction over 20 years.

“By utilizing the powerful predictive tools available in InfoWorks CS, San Diego was able to arrive at a cost-effective and practical solution to the challenges it faced in developing the Otay Mesa area,” Brogadir said. WW

Flowmeter Receives WEF 2005 Innovative Technology Award

Marsh-McBirney’s Flo-Dar Radar Velocity/Area Flowmeter was selected as the recipient of the Water Environment Federation’s (WEF) 2005 Innovative Technology Award-Instrumentation category.

The WEF Innovative Technology Awards are presented annually to associate members who have introduced new, innovative products or services related to the construction, operation, or maintenance of water pollution control facilities. The products or services must demonstrate an innovative aspect using new ideas, methods, alterations and/or unique changes from existing systems. The Instrumentation category includes scientific instruments used either in the laboratory or for control of the different treatment process.

The Marsh-McBirney Flo-Dar Flowmeter is the world’s first radar velocity/area flowmeter. The meter’s “above-the-flow” sensor monitors open channel flow while eliminating the need for confined space entry and maintenance issues associated with traditional submerged-style sensors.

For additional information, visit the company’s website at

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