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LONG RANGE SEWER REHABILITATION PLAN
City & County of Honolulu
(page 2 of 3)
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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.
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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.
Design flows were then generated for the entire
collection system. The flows were used to determine system adequacy,
and subsequently to develop alternatives for system improvements. The
analyses involved extensive hydraulic modeling work to identify
existing and future flow and capacity-related deficiencies. Non-problem
sewers and facilities were identified and eliminated from further
detailed analyses. Once the problems were assessed, alternative
solutions were developed and evaluated. Selected alternatives were
recommended for inclusion into the Long Range Sewer Rehabilitation
Capital Improvements Program (CIP).
Structural Condition Assessment
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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 |
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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.
Results from all of the three condition assessment
efforts were compiled and used to develop the City’s Long Range Sewer
Rehabilitation Plan.
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