LONG RANGE SEWER REHABILITATION PLAN
City & County of Honolulu
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TECHNICAL APPROACH

FAINC embarked on the most comprehensive sewer system assessment program ever attempted in the State of Hawaii to systematically evaluate the hydraulic, structural and operational condition of the system.

Hydraulic Condition Assessment
For the hydraulic condition assessment, the sewer system inventory was updated to provide the necessary information required to identify the major elements of the collection system and allow the formation of the critical sewer network. The system was subdivided into numerous collection areas, or basins, that would serve as the analytical units for a comprehensive sewer flow monitoring program. The flow monitoring program provided the basic data required to understand the hydraulic operation of the collection system. The flow and rainfall data were analyzed, identifying problem and non-problem areas, and prioritizing the needs for additional detailed assessments.

The City’s flow monitoring network included 65 permanent monitors, 50 temporary monitors and 25 recording rain gages. Over 6 years of continuous sewer flow and rainfall data were collected and used for the analyses. Characteristics of normal dry weather flows, as well as wet weather flows for each collection basin, were derived from analyses of the flow monitoring data. The analyses determined key factors that characterized the "leakiness" of each basin. These factors indicated the amount of I/I attributed to a given basin, how quickly the flows entered the system, and the duration of wet weather impacts.

The extensive monitoring network included flow monitors and recording rain gages

A major objective of this effort was the determination of reasonable design flows that the collection system should be able to convey. The most significant factor affecting the system’s hydraulic adequacy was the allowance for wet weather flows. It was evident that the City’s existing Sewer Design Standards were inadequate in terms of providing sufficient peak wet weather flow capacity. The approach used for establishing design wet weather flows involved a cost performance analysis. Optimal wet weather design levels were determined by evaluating the reduction in wet weather SSOs in comparison with the cost of the required system improvements. Based on this analysis, system capacities capable of handling the 2-year design storm provided the optimal containment level, since higher magnitude events resulted in only a marginal improvement in SSO containment.

Structural Condition Assessment

The second component of the assessment effort addressed the structural condition of the sewer system. Evaluation of the sewer system inventory enabled the identification of the structurally critical sewers. The critical sewers were assessed through manhole entry and closed circuit TV (CCTV) inspections. This included approximately 3000 manhole entry inspections and 130,000 feet of CCTV inspection, covering about 15 percent of the collection system. Recommended actions on the various defects were prioritized, addressing the most severe problem first, and programming other problems based on defect severity. The most severe defects requiring immediate action were identified for emergency repair or inclusion on current CIP lists. Sewers with moderate defects were included in the Long Range Sewer Rehabilitation CIP. Minor defects and non-problem lines were identified for continued monitoring to track their condition over time. Non-critical sewers are to be addressed by the City’s on-going preventive maintenance program.

Severely corroded pipe

Severely corroded pipe viewed through CCTV inspection

Operational Condition Assessment
The third component of the system assessment was the operational condition assessment. This program focused on high-priority operational problem areas, addressing known locations of repeated SSOs and areas requiring frequent maintenance that tie-up the City’s maintenance staff and equipment. Alternative solution options include physical repairs of the system when appropriate, more frequent maintenance if physical fixes were not practical, or source control if the problem could be traced to third party activities.