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Modeling Bypass Alternatives in Sacramento, California

The City of Sacramento Department of Utilities has been systematically evaluating dozens of sewer basins through master planning efforts, starting with top priority basins. Boyle Engineering Corporation provided master planning, hydraulic modeling, cost estimation, and report preparation services on one such basin project. PIZER, Inc. provided technical assistance to Boyle Engineering by developing and calibrating a HYDRA® model for the project.

Background: When the conveyance system was constructed in this residential area, sewer collector pipes were laid in rear-yard easements. In the intervening years, trees and shrubs have invaded, causing extensive infiltration. Also, the sewer mains were too small, causing severe surcharging.

Project Scope: The primary objective was to recommend projects to improve operations and minimize current maintenance needs in the basin. This included evaluation of existing conditions of the collection system and pump stations, identification of physical problems and hydraulic deficiencies with the facilities, evaluation of alternative solutions for problems, and recommending improvement projects.

Results: Hydraulic analyses of the existing collection system indicated that peak flows could not be conveyed without surcharging throughout the majority of the basin. Recommended improvements, including rehabilitation of portions of the collection system and construction of two alternative relief pipelines was modeled using HYDRA.

Existing System Bypass Scenario A Bypass Scenario B
The existing system is surcharged upstream from the pump station. Below is a hydrograph showing the worst-case scenario upstream from the station: Peak flow 7.56 cfs One alternative included diversion of the north east quarter of the system directly to the pump station, decreasing flow in existing pipelines upstream of the pump station: Peak flow 4.72 cfs Another alternative included diversion of the east half of the system directly to the pump station, routing even more flow through the bypass: Peak flow 2.03 cfs

Boyle Engineering determined the reduction in infiltration and inflow attributable to these improvements, and compared the costs and benefits of each of these improvements.

The City plans to implement the recommendations, including the 24-inch bypass pipeline of Scenario B over the next 3 years as part of their Capitol Improvement Program.

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