6 detention basins in Makiki, Manoa, & Palolo streams
3 multi-purpose detention basins
1 in-stream debris catchment structure
4 ft. high concrete wall around the Ala Wai Canal
Flood warning system
The United States Army Corps of Engineers (USACE) plans to build these structures to protect us from the 100-year storm. This plan is estimated to cost $345 million. Design plans and details are in Final EIS. You can also find this information on the USACE's website.
What is the Ala Wai Flood Project?
What is the 100-year storm?
A 100-year storm has a 1% chance of hitting every year. This means we should see a storm of this magnitude approximately every 100 years. This goes the same for a "5-year storm" or a "20-year storm" etc.
A 5-year storm has a 20% chance of hitting every year. We should experience a 5-year storm approximately once every 5 years.
This model is the USACE's 100-year storm based off their data. You can find this in the Final EIS. Many areas within the Ala Wai Watershed are completely flooded and underwater. YES, this is a frightening image. However, let's look at the other storm renderings from the EIS to see how the USACE's data adds up.
5-Year Storm vs. 500-Year Storm in the Draft EIS: Appendix B
1. There is not a large difference between the area flooded in the 5-year and 500-year storm. This should raise a few questions because the USACE claims the flood in pink has a 20% chance of hitting every year.
2. The 5-year storm shows 75% of Waikiki underwater. According to this model, we should have seen flooding of this magnitude once every 5 years.
3. How many times have you seen Waikiki flood to the amount in pink? The answer is never. The Ala Wai Canal is almost 100 years old and we have never seen a storm close to this model.
4. We aren't saying a storm like this cannot happen. Anything can happen. However, can we trust the USACE's data to accurately depict the damage of a 100-year storm if their modeling is exaggerated?
Isn't storm protection a good Thing?
Yes! We are not against flood mitigation or protecting Waikiki. However, the USACE's current plan is deeply flawed.
Many areas such as upper Palolo Valley show a total of $7,000 property damage. That amount does not justify the large detention basins in the Pukele and Waiomao areas. We could mitigate flooding using less expensive, nonstructural measures;
Large upstream detention basins will have a footprint of 300 feet long and will require the excavation of up to 1000 feet of natural stream per detention basin.
The upstream detention basins are proven to become silt pits with problematic maintenance issues. This affects water quality and downstream flows. The basins can easily be breached on very small storms by becoming plugged from debris from fallen trees coming from miles upstream;
The USACE's basic means for flood protection is by detention (holding water) which is suitable for the mainland with watersheds that are thousands of miles long and no place for the water to go;
The Ala Wai Watershed is about 5 miles long from ridgeline to ocean which is a short distance. The watershed is also relatively small. The Ala Wai Canal is a little over a quarter of a mile from the ocean. It makes no sense to be building up concrete walls when the simple objective is to get the water to the ocean;
Storm protection is great. However, if it is not done correctly, we will end up paying more for structures that are UNWANTED and INEFFICIENT in the purpose it was meant to serve.