The Flooding in Mapunapuna?

by Panos Prevedouros PhD, FixOahu, September 1, 2017

Quoted in Alexander Zane's story on KHON about the repeated flooding in Mapunapuna:

Several years ago, the city installed a duckbill drainage system to get storm water out and keep ocean water from coming in. But according to Panos Prevedouros, chair of the UH Manoa Dept. Of Civil and Environmental Engineering, anything more would come with a hefty price. “Basically you’d need to have a storage system and a pump system to pump the storm water out of it. Essentially close the connection. You have to cut the cord with the ocean so the ocean never comes in,” Prevedouros explains. As for the option of installing a pump system to get water out of the area, Panos Prevedouros says it probably won’t happen. “I’m pretty sure that the area does not generate significant county taxes to justify a huge investment. That’s probably what the problem has been all this time. If the area was upgraded with more expensive real estate so that the county can collect more taxes than a more sophisticated solution could be put in place, so it’s a trade-off kind of thing,” Prevedouros said.

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A couple more things to add here:

Duckbills can get clogged by debris and then they remain open allowing ocean water to intrude. In all likelihood they do not work as City claims because the area flooded during king tides in July this year.

Given the relatively low property value of this area, it makes sense to provide incentives for small businesses to relocate and return this large parcel to nature. That is, treat this area as a small estuary: The Mapunapuna pond.

In addition, Charles Hunt saw this story and contacted me with the following important perspective and information:

An added perspective that occurs to me is that such systems will become increasingly necessary, what with progressive sea-level rise, so the City will eventually have to start installing some. Mapunapuna could be an early prototype for the City to gain experience with system designs, learning which systems and consultants have “proven out” in other locales, which systems offer best cost/reliability characteristics, etc. Or – if tax-base is an overriding consideration – perhaps another locale like Waikiki or Downtown will become the first prototype.

Here’s a link to the gateway page for my report, there’s a full PDF of the report available at the page. We installed some instrumentation to record water levels and the pump on-off duty cycle and were able to tell a few stories about drainage from the watershed and how the pump-out system operated during a couple of rain storms over the year of study:

Hunt, Charles D., Jr.; De Carlo, Eric H., Water-Resources Investigations Report 99-4171 : Hydrology and Water and Sediment Quality at James Campbell National Wildlife Refuge near Kahuku, Island of Oahu, Hawaii, 2000.

Hunt also suggested that a camera system can be used to monitor the operation of the duckbills.

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Totally Unrelated:

2016: UH Seeks Believers to Photograph King Tides for Sea Level Rise Propaganda