Evaluating Alternative Alignments and Maximizing Existing Sewer Capacities Results in Cost Savings for Sewer Improvements at the Design Level

In May 2007, the City of Anaheim (City) updated their Combined Central Anaheim Area Master Plan of Sanitary Sewers (CCAAMPSS) originally completed in September 2006. This 2007 update (Addendum) evaluated alternative pipeline alignments for the proposed build-out condition and included new connections into the regional agency's (Orange County Sanitation District) trunk sewer. This resulted in a significant reduction in build-out flows in downstream sewer mains. In June 2007, the engineering design team (City and Psomas) proceeded with design of the Citywide Sanitary Sewer Improvement Program/Project (CSSIPP) Group 3 in the area downstream of the Addendum improvements. During the preliminary design phase all potential diversions and alternative alignments were explored with the intent to maximize the capacity of the existing system and reduce the needed improvements. Additional preliminary design services included supplementary hydraulic modeling to evaluate the reduced flow rates, verify the various flow diversion strategies, and pipeline alternative alignments. A primary strategy of this additional analysis was to optimize the use of available capacity in the City's existing Interstate-5 Freeway (I-5) 24-inch [61 centimeters (cm)] and 33-inch (84 cm) sewer crossings. With projected build-out flows, the existing 24-inch (61 cm) sewer crossing has a depth over diameter (d/D) ratio of 83%, which exceeds the City's maximum criteria of 75% (equating to a capacity deficiency of 0.93 cubic feet per second (cfs) [0.026 cubic meters per second (m³/s)]; while the larger, 33-inch (84 cm) crossing to the north has a d/D ratio of only 65%, providing over 2.3 cfs (0.065 m³/s) of available capacity. Based upon these results, model flows were diverted from the 24-inch (61 cm) crossing to the 33-inch (84 cm) crossing approximately one mile (1,600 meters) upstream. Selecting this location for the diversion proved to be significant, since it not only balanced the projected flows in the two freeway crossings closer to their design capacities, but also relieved the capacity constraints in other downstream pipelines. The modeling indicated the proposed flow diversions nearly eliminated all remaining local deficiencies noted in the Addendum. This extended evaluation of the City's 2007 Addendum to their 2006 CCAAMPSS at the project design level generated an innovative solution providing a significant cost savings to the City. This is afforded by the recommendation to constructing diversion structures rerouting flow to the existing 33-inch (84 cm) I-5 Freeway sewer crossing. The financial impact of this recommendation to the City should result in savings of approximately $6.9 million (August 2006 dollars) in construction and engineering costs, as well as avoiding impacts from major construction within these heavily traveled roadways. In summary, the analysis demonstrated a strong partnership between the City and Consulting Engineer resulted in an innovative and cost-effective sewer improvement program.