Manoa Flood: Lessons Not Learned

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A major flood occurred on October 30, 2004 that caused several million dollars worth of damage to UH-Manoa (including the loss of some precious library resources), serious damage to some homes and vehicles, minor damage to Noelani Elementary School, and extensive stream and neighborhood cleanup efforts.

In 2006 the Army Corps of Engineers finished an analysis and issued a report. Although debris was an issue causing additional clogging, the Corps investigation reveals a significant lack in stream capacity for a 10-year event. Yet nothing has happened to address the situation and debris has been accumulating.

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Here is their main conclusion: “Results indicate that Manoa Stream, between Kahaloa Drive and Woodlawn Drive has insufficient capacity to contain the flood waters caused by the October 30, 2004 storm. Flood damage was increased by debris blockage of the East Manoa Road and Woodlawn Drive bridges. Frequency analysis determined the October 30, 2004 flood event to be about a 4% chance or 25-year flood event.

“An analysis of bridge and channel capacities determined that Manoa Stream can safely carry about 4,500 to 5,000 cfs [cubic feet per second] within the banks, but the East Manoa Road and Woodlawn Drive Bridges can only safely pass flows in the range of 2,800 to 3,900 cfs without bridge blockages. [In other words, the bridges severely limit the capacity of the stream.] Flows in this range have a magnitude less than the 10% chance or 10-year flood.”

Conclusion: Unless one of the alternatives proposed is constructed, a repeat flooding of lower Manoa and UH is quite likely.

Panos Prevedouros PhD is a professor of traffic engineering at the University of Hawaii – Manoa and a candidate for Honolulu

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  1. The report says:

    Alternatives analyzed included levees or floodwalls along selected portions of the
    stream channel between Manoa District Park and Woodlawn Drive and creating an
    artificial channel between East Manoa Road and Woodlawn Drive. Of these
    alternatives, the channel drop structure at Woodlawn Drive Bridge has the best potential
    for increasing the capacity here in the short-term with the least amount of maintenance,
    aesthetic, bridge structure, and drainage problems to mitigate.

    The parts subject to mitigation appear to be under City control but parts of the stream’s length are under State control.

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