The “Ala Wai Canal Flood Risk Management Project” was launched several years ago by the U.S. Army Corps of Engineers (USACE) to address the increasing risk of high-impact storm and flooding events in the Ala Wai watershed. Flash-flooding conditions can materialize within an hour in the upper portion of the Ala Wai watershed, creating significant damage downstream. USACE estimates a major flood in the watershed could damage 3,000 structures and cost more than $1.14 billion.
A project Feasibility Stage was initially completed in December 2017 when the Chief of Engineers for USACE submitted the Chief’s Report to Congress, but initial plans for an engineering solution were met with criticism from local and state stakeholders.
Oceanit was contracted by the Honolulu City Council, Permitted Interaction Group (PIG), as an environmental consulting interface for the City, the U.S. Army Corps of Engineers, and the local communities to assist with the Ala Wai Flood Mitigation Project.
The objective of the Ala Wai project is to improve safety from a low probability, high impact rain event by mitigating flooding in Waikiki and the Ala Wai Watershed valleys of Makiki, Manoa, and Palolo.
This climate project is of critical importance to protect the State’s primary university campus (UH Manoa), the economic engine that is Waikiki, and the neighborhoods of the watershed from the likely severe damage that a 100-year flood and storm event would cause.
The term “100–year flood” was coined to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given calendar year. Likewise, the term “100–year storm” is used to define a rainfall event that statistically has this same 1-percent chance of occurring. Read more about these terms here.
Stakeholder interaction during the early planning phases of the project was limited and created tension between the parties involved in executing the work, approving the work, and living in areas where work would affect daily life. Utilizing our Design Thinking methodology, Oceanit’s goal was to vastly improve the communication, cooperation, and understanding between all the stakeholders involved in this critical project. By hosting a series of community meetings, both in-person and virtually, we were able to build up conversation among diverse stakeholders, receive and integrate their feedback, and offer updated insights to the community, City, and USACE.
The U.S. Army Corps of Engineers (USACE) shifted from an initial detention model designed to retain water in detention basins in the middle watershed to a conveyance model that put more water in the lower watershed. This created concern within the community, especially from residents that would be affected. To address this concern, Oceanit listened to community input and developed the SWIFT (Subsurface Watershed Inundation Flow Technology) design concept to remove water from the watershed completely and bypass the Ala Wai Canal.
The SWIFT concept balances stakeholder priorities, which include protecting Waikiki at a reasonable cost without sacrificing our communities or ecosystems and bypassing the already delicate Ala Wai Canal.
To learn more, download the Ala Wai community outreach meeting presentation:
Questions from November 17, 2020 Ala Wai Flood Mitigation Project Community Stakeholders Meeting
Question 1: How much confidence is there in the 100-year modeling in forecasting the flood events. The difference between the Oceanit’s model and that of the Corps is what is concerning and the reason for my previous question about the confidence in the modeling. (~26 min.) (Jeff Herzog’s answer ~30:33 min.)
Answer: Oceanit has performed numerous studies on the Ala Wai Watershed and has built a number of models that forecast the flood water surface elevations for various storm events. The results USACE shows in its Hydrologic Engineering Center’s River Analysis System (HEC-RAS) 1D hydrologic and hydraulic models is consistent with the results from Oceanit’s prior 1D models. We have started to review the USACE more comprehensive HEC-RAS 1D/2D models and have not identified any issues that impact our confidence in the modeling.
Question 2: With the SWIFT tunnels, will there be any potential impact to the subsurface natural dykes and groundwater system?
Answer: Brierley Associates consulted its local Brierley geotechnical colleague, Don Painter, who has practiced geotechnical engineering and performed subsurface investigations all around Honolulu for decades on the anticipated subsurface conditions. Don pointed out that literally hundreds of miles of water tunnels were built all over Hawaii, Maui, Oahu and Kauai for Sugar Water including the Pali, Likelike and H3 Tunnels. Don pointed out that most of the old tunnels were “bald-headed” (meaning unlined) in rock because the rock is so stable. Don also built the recent KK Sewer Tunnel on the Windward Side commissioned in~2018 in a complex multiple collapsed caldera volcanic terrain.
Don reported he does not anticipate any impact. During this next phase of work, Brierley will perform an alignment-specific review and summary of available background geology and geotechnical information in order to develop an updated conceptual understanding of subsurface conditions for each of the alignments. Based on the alignment-specific review and summary of available background geology and geotechnical information, Brierley will produce a GIR Memorandum as the geotechnical basis of design and updated understanding of subsurface conditions for development of an updated conceptual designs for each of the two alignments.
Question 3: Will it be “clean”water passing through the SWIFT tunnels going directly into the nearshore marine ecosystem? i.e., any debris or especially sediments that will be carried directly into the marine ecosystem? (~48 min.)
Answer: The stormwater being discharged from the SWIFT tunnels will be significantly cleaner than the stormwater that goes into the Ala Wai Canal and makes its way out through the canal and the harbor into the nearshore marine ecosystem. The water is captured in the upper watershed before it has a chance to pick up significant sediments. The weir used to “capture” the stormwater will help to remove some of the debris.
Question 4: What is the timetable for Oceanit’s activities? Timetable to implement the plan discussed? Schedule to integrate EDR with the SWIFT proposal and dates that will be done?
Answer: We estimate it will take us approximately three months to perform the next steps outlined in the presentation.
Question 5: Can you show the halving of the Ala Wai detention basin. Why reduce the size of that?
Answer: The USACE discovered as part of its effort to update its Hydrologic Engineering Center’s River Analysis System (HEC-RAS) 1D hydrologic and hydraulic modeling to more comprehensive HEC-RAS 1D/2D models to advance design that half of the Ala Wai Golf Course was needed to provide an outlet for overland flows from Kapahulu to minimize localized flooding. As a result, USACE reduced its plans for the Ala Wai Golf Course Detention Basin.
Question 6: Why split the 12-ft diameter tunnel into three 7-ft diameter tunnels at the manifold?
Answer: The 12-ft diameter main tunnels that start in the upper watershed are connected to near-coast outfall manifolds. The ocean outfalls from these manifolds will be constructed by installing multiple, approximately 7-ft diameter, pipes using Microtunnel Boring Machine (MTBM) methods. This is the maximum diameter available for drilling tunnels offshore underwater using unmanned Microtunnel Boring Machines. Three 7-ft diameter pipes are needed to handle the volume of water from the 12-ft diameter main 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 to 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 so does resistance to the flow, which reduces the effectiveness of the tunnel. This is a limitation that was 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 would 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 will the water be 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 will 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 ʻoʻopu (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 will 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 will 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 the 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.
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 that the water that travels through the tunnels and gets discharged into 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 one 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 complete. We may be able to share this data around mid-July.