The Otay Mesa Trunk Sewer: helping development in southern San Diego County


Courtesy of Innovyze

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The City of San Diego in California is the state's most southerly conurbation, sited just a few miles from the Mexican city of Tijuana – so close that a trolley bus runs every ten minutes down to the busy border at San Ysidro. A sprawling, pleasant mix of ultra-modern medium-rise blocks, wide palm-lined boulevards and Spanish-style dwellings, San Diego 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 – not because of a lack of desire to build, but because of a lack of major infrastructure, notably freeways and sewerage, and the capital to put this in place. Construction of the required sewerage infrastructure to facilitate development of the area to the level needed 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 forcemains 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, and then discharges treated effluent to the Pacific Ocean.

Portions of the existing Otay Mesa Trunk Sewer – constructed in Phase One of the project - 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 for the region proposed eliminating the pumping station and extending the Otay Mesa Trunk Sewer as a gravity sewer, initially 24' in diameter but getting larger as it approached the coastal interceptor sewer, following the line of a canyon. There were security, environmental and geotechnical concerns, however, as the final length of sewer would have been 78' in diameter, running close to the Mexican border in landslide-prone materials.

The second phase of the project therefore moved to considering alternatives – namely, extending the Otay Mesa trunk sewer, transforming the temporary pumping station into a permanent one and taking flows through a linking force main sewer into a 1050mm gravity sewer and connecting into the coastal interceptor sewer. PBS&J was assigned the task of updating 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 revised 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 goal was to develop a rational, defendable platform from which to make policy decisions in the light of quantifiable risks.

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.

Since the model simulated the response of the collection system over time, subject to the fundamental governing equations of flow, considerable refinement of the model parameters was possible.

The initial study approach examined the demographic growth projections and identified Phase Two alignment alternatives. Alternatives were put through a screening process and a phased implementation plan set out. The future growth projections looked at the sub-basins of Otay Mesa – East Mesa and West Mesa - and also the combined basins, from a start point in 2000 through to 2030 and ultimate buildout of the basin. From this it was clear that a substantial increase in population and flows was likely for all areas.

Future flow projections used a number of scenarios to make estimates. These were: projected flow with 9% annual growth; projected flow based on 75 GPD per residential capita and 1500GPD per acre for light industrial use; projected flow at ultimate development buildout based on 5000 GPD for industrial uses in 2045 or beyond; and projected flow based on the SANDAG (San Diego Association of Governments, the regional planning agency) projected growth rate.

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 for development of the project implementation plan. Continuous model updates were based on rezoning and development schedules.

Model loading involved setting separate sub-catchments for residential and non-residential (employment) populations, with calibrated generation rates of 75GPD per capita for residential and 25GPD per capita for non-residential inhabitants. Separate standard diurnal patterns were established for each group.

Mr Brogadir comments: ‘The whole project became extremely expensive when sized for ultimate buildout. By 2030 the pumping station would need to take 34MGD, and the force main would have to be very big, so the ultimate cost would be over $100million. 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 2MGD temporary pumping station to take 4MGD, acquiring land for a new pumping station and construction of the proposed station in the earliest phase. Flows will continue to be pumped north to the Otay Valley Trunk Sewer, but it is acknowledged that there is a threshold where this will not be able to take more flows.

The InfoWorks CS model is being used for the delicate task of ensuring that the capacity of the Otay Valley trunk sewer is fully utilized 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 ten years. ‘The model is very accurate – we could really look at squeezing capacity from that area,' Mr Brogadir adds.

The sewer faces a constraint at the I805 interstate crossing, which was flagged as the first portion of the Otay Valley Trunk Sewer that would go over capacity, and model results and metering figures had to be used to determine the available capacity in the critical reach. The hydraulics of the critical reach were modeled along with potential hydraulic conditions. ‘This stretch was very interesting hydraulically,' says Mr Brogadir. ‘There were backwater flow effects and a lot of transitions.'

The new gravity portions of the system are currently being constructed as Phase 2A and 2B. These trunk sewers will allow construction of the West Mesa portion of the system within the next two years. Later phases will increase the capacity and will involve expansion of the existing pump station as sewer flows reach the threshold capacity of the existing 4MGD capacity station. The phased increases in capacity will involve replacement of the existing pumping station with a new 8MGD capacity station, which will then be expanded in phases to 12MGD and, if development projections are realized, eventually up to 34MGD.

Phase 2D will involve the ultimate connection of the East Mesa system to the new portion of the Phase Two system in the west, a stage that has proved problematic and critical to the whole project. The timing depends on the construction of a new freeway, SR (State Route) 905, which is vital for the area's development. This portion of the sewer cannot be constructed until the freeway is complete, but it is currently delayed, which makes the ability of InfoWorks CS to finely judge the capacity of the existing system even more vital.

‘It is critical to have good hydraulic modeling,' says Mr Brogadir. ‘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 city to reduce its capital outlay in the first few years to about $20 million, with another $55 million planned in phased construction over the next 20 years, by mapping the deferred implementation of the scheme. It has meant that development in the Otay Mesa area has been able to continue by maximizing use of the existing infrastructure and has enabled tools to be developed that can easily analyze changing development conditions. In all, it has proved a win-win situation for the city, residents, businesses and developers.

Mr Brogadir said: ‘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.'

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