Ala Wai Flood Mitigation Project Status Update

Uncategorized

Oceanit is continuing to work with the community and City Council Permitted Interaction Group (PIG) to improve safety from a low probability, high impact rain event, mitigate the risk of flooding Waikiki and the Ala Wai Watershed, and preserve the shared Federal Funds for the Ala Wai Flood Mitigation Project. COVID-19 has not stopped our progress, but has understandably slowed the response time from many local government entities. The hosting of online community meetings allows us to continue the conversation, receive feedback, and offer updated information to the community and all stakeholders.

The U.S. Army Corps of Engineers (USACE) shifted from a detention model designed to retain water in detention basins to a conveyance model that put more water in the lower watershed. This created a concern. Based on community input and to address this concern, Oceanit developed a SWIFT (Subsurface Watershed Inundation Flow Technology) design concept to remove water from the watershed and bypass the Ala Wai Canal. The SWIFT concept balances stakeholder priorities that include protecting Waikiki at a reasonable cost without sacrificing our communities or ecosystems and bypassing the already delicate Ala Wai canal. Please see the video below for a visual representation and explanation of this technology:

Oceanit presented models and data during the Ala Wai Town Hall Meeting showing the effect of a 100-year storm event with no flood mitigation features implemented and the same event after implementing the SWIFT tunnels. Questions raised during the meeting have been documented below along with responses. If you have any questions related to the Ala Wai Flood Mitigation Project and the SWIFT tunnels, please email us at [email protected] and our team will provide answers on this page.

Questions About the Location of the Tunnels

Question 1: Where will the tunnels be built in the upper watershed and how long are the tunnels? Where are the tunnel openings and service shafts located?   Can we see what the entrances look like?

Answer: This is a conceptual design to describe the proposed tunnels in terms of a set of integrated ideas and concepts about what it should do, how it should behave, and what it should look like to help manage costs, assess risks, and evaluate the potential success of the tunnels. The tunnels are concrete pipes underground. Access wells need to be located at strategic locations on the tunnel trace for construction and maintenance. From a planning standpoint, we found the optimum tunnel lengths and tunnel entrances are as follows:

  • Makiki – 9,100 feet – below the confluence of Laneaole and Kanaha streams near the intersection of Wilder and Makiki streets
  • Manoa– 12,500 feet – downstream of the Woodlawn Bridge close to the boundary of Noelani Elementary School property
  • Palolo– 11,200 feet – near the crossing of Palolo Avenue over Palolo Stream

Further details be determined during the design phase.

Question 2: What about the idea of a tunnel further up by the Manoa Cemetery in the upper watershed? Inlets can be way upstream.

Answer: We investigated moving the tunnel entrance both higher and lower in Manoa Valley to better address the overflow of the stream at Woodlawn. Based on updated HEC-RAS modeling results, we found relocating the tunnel entrance higher in Manoa Valley does not help with this issue. As we move the entrance higher in the valley, the length of the tunnel increases and the resistance to the flow increases reducing the effectiveness of the tunnel. This is a limitation that w as considered in the flow calculations. The optimum location of the tunnel entrance to maximize flow was reached after considering several tunnel entrances in the valley.

Question 3: How about another entrance to the tunnels from the lower areas to help drain McCully & Moilili?

Answer: The tunnels collect stormwater in the upper watershed and use the force of gravity to drive the water flows towards the ocean. Tunnel entrances in the lower watershed will require the use of pumps to push the water into the tunnel and a larger tunnel to carry the extra water being inserted. We are currently investigating the feasibility of this idea.

Question 4: How far offshore does the water get discharged through the tunnels? Where are the tunnel inlets and outlets in the ocean?

Answer: This is a conceptual design to describe the proposed tunnels in terms of a set of integrated ideas and concepts about what it should do, how it should behave, and what it should look like to help manage costs, assess risks, and evaluate the potential success of the tunnels. Further details be determined during the design phase.

 

Questions About Weirs

Question 1: What is a Weir?

Answer: A ‘weir’ is a low dam built across a creek or river to raise the level of water upstream or regulate its flow.

Question 2: How do you design the tunnels so that you don’t deprive the streams of the water they need 99% of the time when there is no 100-year flood?

Answer: We have proposed the use of a weir that runs across the stream to control the flow of water in Manoa Stream. The weir will allow the stream water to flow normally until such time the water level in the stream rises to that of a 20-25 year flood event. At this point, the weir “siphons” off the excess water and sends it to the tunnel entrance.

Question 3: How do the tunnels affect aquatic life, for instance the ‘o’op (Hawaiian freshwater goby) which move up and down the stream.

Answer: We have proposed the use of a weir to control the flow of water in Manoa Stream. The weir will allow the water to flow normally until such time the water level in the stream rises to that of a 20-year flood event when the weir “siphons” off the excess water and sends it to the tunnel entrance. Since no water will flow into the entrance of the tunnel until it reaches the 20-25 year flood level, this should minimize the impact on aquatic life in the stream.

Question 4: If a weir goes across the stream, how high & wide are they?

Answer: The exact dimensions will be determined during the design phase. This is a conceptual design to describe the proposed tunnels in terms of a set of integrated ideas and concepts about what it should do, how it should behave, and what it should look like to help manage costs, assess risks, and evaluate the potential success of the tunnels. Further details be determined during the design phase.

Question 5: About how long is the weir parallel to the stream? What about maintenance? Is the weir cement?

Answer: Further details will be determined during the design phase. This is a conceptual design to describe the proposed tunnels in terms of a set of integrated ideas and concepts about what it should do, how it should behave, and what it should look like to help manage costs, assess risks, and evaluate the potential success of the tunnels. Further details be determined during the design phase. The weir will be made out of concrete. The weir won’t require additional annual maintenance except normal, periodic maintenance to remove the debris that might accumulate over time.

 

Impact of SWIFT Tunnels

Question 1: Will the tunnels prevent the flooding in the lower watershed and the over-topping of the Ala Wai Canal?

Answer: No, the tunnels are designed to convey a portion of the flood waters from the middle watershed to the ocean, bypassing the lower watershed. The tunnel size is restricted by the limitations in micro-tunneling capabilities.   The main goal is to remove enough water from a 100-year flood event to match that of a 20-25-year flood event at the Ala Wai Canal, the current design capacity of the canal. The flooding of the lower watershed by local rainfall is not affected by the tunnels.

The tunnels are not a single standalone solution for the total flooding problems, they are part of a bigger solution. The Corps has a set of solutions separate from the tunnels to protect the watershed that they are still working on.

Question 2: Any idea what the Corps’ proposed solutions to protect the watershed are?

Answer: The Corps is still working on the modifications being documented in their Engineering Documentation Report (EDR). The EDR will include the technical analysis of the modifications being recommended by the Corps, updated economic analysis and cost estimates, and flood inundation animations. We defer to the Corps on this question as we do not have details on the Corps’ proposed solutions.

Question 3: Will the use of the tunnels eliminate the need for flood walls?

Answer: This will depend on what additional features the Corps includes in its Engineering Documentation Report. There will be flood walls required along the mauka side of the canal to protect Iolani School and Ala Wai Elementary. Other walls are under consideration by the Corps to facilitate conveyance of water to detention basins located in the lower watershed.

Question 4: Does the modeling with tunnels eliminate flooding or only reduce the depth of the flooding in the lower watershed around the Ala Wai including Waikiki and mauka of the Ala Wai? If the flooding isn’t eliminated, then the amount of flood water being diverted directly to the ocean is inadequate.

Answer: The tunnels alone do not eliminate the flooding in the lower watershed. When the Corps shifted from a detention model (use of detention basins in the upper watershed) to a conveyance model, it transferred a lot more storm water to the lower watershed. The tunnels help reduce the depth of the stormwater in the lower watershed. The tunnels do not change the rain input below their tunnel intakes. This is a 100-year storm event. All water from rainfall below the tunnel intakes and local rainwater from the lower watershed (Waikiki, Kaimuki, Moiliili, McCully) will still accumulate and be sent to the ocean through the Ala Wai Canal. Inadequate capacity of the canal could cause localized flooding.

Waikiki-There will be significant raining throughout the watershed during a 100-year storm event including Waikiki. The water in Waikiki represents the water that accumulated after the storm drains reached the point where they could no longer dump water into the Ala Wai Canal.

Question 5: What can be done to remove more of the storm water in the lower watershed?

Answer: This will depend on what additional features the Corps includes in its EDR. When the Corps shifted from a detention model (use of detention basins in the upper watershed) to a conveyance model, it transferred a lot more storm water to the lower watershed. We can either widen the Ala Wai Canal to increase the flow area/conveyance capacity of the canal or provide alternative additional paths for flood water conveyance by connecting the Ala Wai Canal to the ocean at strategic points in the canal.

Question 6: Is the Corps going to consider or use the SWIFT tunnels as part of its proposed solution?

Answer: The Corps is currently preparing its Engineering Documentation Report. The features under consideration for this report were locked in by the Corps before the SWIFT tunnel idea was developed. The Corps has indicated it will consider including the SWIFT tunnel as part of an Engineering Value Study to be done after the publishing of its Engineering Documentation Report.

Question 7: Have you considered other alternatives to reduce the flood water in Waikiki?

Answer: There are flow adjustment measures that can be used to reduce the water surface elevations through reductions in the peak volume of water. These include above ground storage areas, porous pavement, residential rain gardens, rain barrels, and bio-swales. Whether these are needed and used depends on the results of the Corps’ Engineering Documentation Report.

Question 8: The video mentioned a reduction capacity equal to the Ala Wai Canal (in terms of olympic swimming pools), that amount of volume is relatively small. A 2000 CFS pump can drop the height of the Ala Wai canal by 4 feet in approximately 70 mins. So why is SWIFT better than a flood gate or flood lock and centrally located pump station with inlets stretching to the critical junctions?

Answer: This assumes a canal dimension of 2 miles by 200 feet and a drawdown of 4 feet. There may be more surface water that might come from the sides, and increased ground water flow into the canal. These estimates need to be analyzed with the potential additional water and other subsurface water table variations that may affect structures in the lower watershed adversely.

Question 9: Is NOAA fine with the tunnels?

Answer: As this is a conceptual design, we have not presented the SWIFT tunnel concept to NOAA yet. We can report the water that travels through the tunnels and gets discharged in the ocean is much cleaner than the water that gets discharged by the Ala Wai Canal.

 

Questions About Tunnel Configuration

Question 1: What is the cost per tunnel?

Answer: As a conceptual design, there are currently lots of unknowns. As a result, the initial cost estimates are very high having included lots of contingency built into the estimates. The estimates will be formalized as the unknows become known. This should result in lower cost estimates. The current estimate for the three 10-foot diameter tunnels is about $710 million. This breaks out as follows:

  • Makiki – $202 million
  • Manoa – $266 million
  • Palolo – $242 million

Question 2: Have you evaluated different tunnel configurations?

Answer: We put different tunnel configurations into the HEC-RAS models to evaluate the effect. We found that by eliminating the Makiki tunnel, increasing the Manoa tunnel to a 12-foot diameter, and leaving the Palolo tunnel at a 10-foot diameter, we can achieve better results than the three tunnels and at a lower cost. The estimated cost of the two-tunnel configuration is about $580 million, or about $130 million less, and breaks out as follows:

  • Manoa – $338 million
  • Palolo – $242 million

We also ran a model using a single 14-foot diameter tunnel in Manoa without the Makiki and Palolo tunnels. The flood mitigation results are comparable to the three-tunnel case and not as good as the two-tunnel configuration. However, we do not have an estimated cost of 14-foot tunnel at this time. We continue to look at different options.

 

Access to Data

Question: Can Oceanit produce graphs and charts that demonstrate the duration of the storm from start to finish (time) vs CFS and total volume at all major confluences and junctions?

Answer: The model runs on the U.S. Army Corps of Engineers (Corps) HEC-RAC software. The base data in the model that Oceanit used in its calculations belongs to the Corps. We have been asked by the Corps to wait to share any hydrographs, curves, confidence levels, etc. until after their current review process for their Engineering Documentation Report is done. We may be able to share this data around mid-July.