Tuesday, May 18, 2010

Manoa Flooding: Lessons Not Learned

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.

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.