News & Updates

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Nuuanu Dams

The Board of Water Supply (BWS) is making available information on the Nuuanu Reservoir No. 1 Dam and the Nuuanu Reservoir No. 4 Dam as a community service. Please be sure to check out the General FAQ for commonly asked questions. Information is current as of the time it was posted online and is subject to change and update as new information becomes available.

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Nuuanu Reservoir No. 1 Dam (Fact Sheet)

Nuuanu Reservoir No. 1 Dam is located on the Ewa-side of Pali Highway above Pulelehua St. by Oahu Country Club.

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BWS operation of Nuuanu Reservoir No. 1 Dam:

Because Nuuanu Reservoir No. 1 has no outlet works other than its emergency spillway, BWS regularly siphons water from the reservoir as part of its normal operations. This provides capacity in the reservoir for run-off. BWS continues to monitor and drain water from the reservoir.

What level does the water need to reach to require mandatory evacuation?

When the water level reaches a foot below the top of the dam, a mandatory evacuation notice will be issued. (The City Department of Emergency Management (DEM) and the State Department of Land and Natural Resources (DLNR) are currently reviewing evacuation triggers and will update their plans as necessary.) BWS is working with DEM to coordinate the evacuation plan, which includes public evacuation notification and designation of evacuation centers, only if needed.

If I need to be evacuated, how will BWS notify me?

BWS will work with the City Department of Emergency Management (DEM) and other city agencies to provide notice to the affected residents. Should evacuation become mandatory, personnel from the Honolulu Police Department, Honolulu Fire Department, and DEM will notify the community.

In the event of a mandatory evacuation, where can I find an evacuation center?

Locations of evacuation centers would be provided to community leaders and the media to share with the public. Stay tuned to local news media and social media channels for up-to-the-minute information. Residents would need to bring everything they might need for at least 24 hours – food, water, bedding, toiletries, etc. The evacuation shelters would only provide a place to stay.

Do you have a list of streets in the evacuation zone so I can see if I live in that area?

BWS worked with DEM to determine the extent of the evacuation zone. See list of streets currently identified as part of the Nuuanu Reservoir No. 1 dam evacuation zone, and reference the current evacuation zone map and description below. Subject to change.

NUUANU RESERVOIR NO. 1 DAM EMERGENCY EVACUATION PLAN

• If evacuation should become necessary, the City Department of Emergency Management (DEM) would notify residents in the flood zone of the potential need to evacuate their properties due to possible flooding from Nuuanu Reservoir No. 1.

• BWS constantly monitors and siphons excess water from the reservoir to maintain storm flow capacity. Occasionally, due to severe weather events, rainfall rates could exceed the standard siphoning capacity. In that case, additional pumps can be started to reduce the water level.

• In the event that water levels reach 1 ft. below the top of the dam, a mandatory evacuation notice will be issued to downstream areas. This evacuation could affect as many as 10,000 residents. (Please see the red outlined area on the map.) Potential shelter locations would be identified.

Disclaimer: Information shown on the map is approximate and should be used as a guideline for emergency response. While the best available data has been utilized as inputs into a dam failure computer model, the final modeled product results should be interpreted as “best available estimates” of the evacuation areas. The computer model assumes that the dam fails from a defect in the embankment structure, while full at maximum capacity, under dry (no rain) conditions and no discharge through the spillway. Dam breaches during flooding or other hazard events could differ or be larger than the area identified. Should evacuations be initiated, listen to instructions from your local emergency management/civil defense agency, and directives from the police and fire departments. Based on the anticipated dam hazard or multiple hazards facing the community, this evacuation area may be altered by the local emergency management agency. The dam evacuation area is different from FEMA flood zone maps or the Tsunami Evacuation Maps (those can be viewed at: gis.hawaiinfip.org). Although the modeled dam failure area utilized to create the evacuation zone maps were conducted by PDC for the State DLNR, the displayed evacuation map is the product and property of the local County Emergency Management or Civil Defense Agency. Any usage or alteration of this map should be cleared with that respective county agency.

For More Evacuation Zone Information:
Contact the local Emergency Management/Civil Defense Agency or visit http://dlnreng.hawaii.gov/dam.

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Who should I contact for more information?
Call the BWS Communications Office at 748-5041 or email contactus@hbws.org.

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Nuuanu Reservoir No. 4 Dam (Fact Sheet)

Nuuanu Reservoir No. 4 Dam is located on the Diamond Head-side of Pali Highway above Nuuanu Pali Dr.

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BWS operation of Nuuanu Reservoir No. 4 Dam:

BWS keeps the reservoir at least half full, or at a water depth of approximately 30 feet to maintain adequate flood control storage capacity, by keeping the middle gate of the intake tower open. Water from the reservoir drains via a 24-inch diameter pipeline through the dam and into Lulumahu Stream and ultimately Nuuanu Stream.

What level does the water need to reach to require mandatory evacuation?

When the water level reaches a foot below the top of the dam, a mandatory evacuation notice will be issued. (The City Department of Emergency Management (DEM) and the State Department of Land and Natural Resources (DLNR) are currently reviewing evacuation triggers and will update their plans as necessary.) BWS is working with the DEM to coordinate the evacuation plan, which includes public evacuation notification and designation of evacuation centers, only if needed.

If I need to be evacuated, how will BWS notify me?

BWS will work with the City Department of Emergency Management (DEM) and other city agencies to provide notice to the affected residents. Should evacuation become mandatory, personnel from the Honolulu Police Department, Honolulu Fire Department, and DEM will notify the community.

In the event of a mandatory evacuation, where can I find an evacuation center?

Locations of evacuation centers would be provided to community leaders and the media to share with the public. Stay tuned to local news media and social media channels for up-to-the-minute information. Residents would need to bring everything they might need for at least 24 hours – food, water, bedding, toiletries, etc. The evacuation shelters would only provide a place to stay.

Do you have a list of streets in the evacuation zone so I can see if I live in that area?

BWS worked with DEM to determine the extent of the evacuation zone. See list of streets currently identified in the 96813 zip code and currently identified in the 96817 zip code for the Nuuanu Reservoir No. 4 dam evacuation zone, and reference the current evacuation zone map and description below. Subject to change.

NUUANU DAM NO. 4 DAM EMERGENCY EVACUATION PLAN

• If evacuation should become necessary, the City Department of Emergency Management (DEM) would notify residents in the flood zone of the potential need to evacuate their properties due to possible flooding from Nuuanu Reservoir No. 4.

• BWS keeps the water level in the reservoir constant by letting water out via a 24-inch diameter pipeline through the dam. Occasionally, due to severe weather events, rainfall and inflow rates could exceed the discharge capacity of the 24-inch pipeline. In that case, additional pumping units can be brought in to help reduce the water level in the reservoir.

• In the event that water levels reach 1 ft. below the top of the dam, a mandatory evacuation notice will be issued to downstream areas. This evacuation could affect as many as 12,000 residents. (Please see the yellow-outlined area on the map.) Potential shelter locations would be identified.

Disclaimer: Information shown on the map is approximate and should be used as a guideline for emergency response. While the best available data has been utilized as inputs into a dam failure computer model, the final modeled product results should be interpreted as “best available estimates” of the evacuation areas. The computer models assume that the dam fails from a defect in the embankment structure, while full at maximum capacity, under dry (no rain) conditions and no discharge through the spillway. Dam breaches during flooding or other hazard events could differ or be larger than the area identified. Should evacuations be initiated, listen to instructions from your local emergency management/civil defense agency, and directives from the police and fire departments. Based on the anticipated dam hazard or multiple hazards facing the community, this evacuation area may be altered by the local emergency management agency. The dam evacuation area is different from FEMA flood zone maps or the Tsunami Evacuation Maps (those can be viewed at: gis.hawaiinfip.org). Although the modeled dam failure area utilized to create the evacuation zone maps were conducted by PDC for the State DLNR, the displayed evacuation map is the product and property of the local County Emergency Management or Civil Defense Agency. Any usage or alteration of this map should be cleared with that respective county agency.

For More Evacuation Zone Information:
Contact the local Emergency Management/Civil Defense Agency or visit http://dlnreng.hawaii.gov/dam.

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Who should I contact for more information?
Call the BWS Communications Office at 748-5041 or email contactus@hbws.org.

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Frequently Asked Questions

1. What forms of communication will the BWS use to inform residents about an evacuation?

If we determine there may be an elevated level of risk (prior to the need for evacuation) the City and BWS will disseminate information through television, radio, HNL.info, and our official social media accounts to explain the situation and what actions MAY be required in the future. If evacuations are necessary, in addition to the above, the City would sound the outdoor warning sirens and send emergency responders to the affected area to alert residents using their PA systems and vehicles sirens in the affected areas. Wireless Emergency Alerts (WEA) will be sent to mobile phones in the affected area.

2. Suggestion from a resident: Please develop a website where the public can receive real-time updates that depict flood levels and relevant information.

Some real-time reservoir water levels are available online and links are posted on the individual reservoir pages in the DLNR dam inventory system (dams.hawaii.gov). Currently, there are real-time reservoir water level gauges at Nuuanu Reservoir No. 1 and Nuuanu Reservoir No. 4. DLNR is working on a site to display the water levels for these reservoirs and other dams throughout the state and will advise the public when this feature is available on their website, floods.hawaii.gov.

3. How much notice will residents have to evacuate?

If there is an elevated level of risk, BWS and the City will communicate with residents as much as possible prior to the need for evacuation. Residents should be prepared to leave immediately upon receiving notification to evacuate.

4. How much time would one have to evacuate if the dam breaks?

BWS and the City would alert and warn residents before an actual breach occurs. Residents should plan on evacuating prior to the breach. The amount of time of the flow from the breach will depend on your location. Residents should plan their evacuations based on instructions from emergency officials rather than the occurrence of a breach event.

Maximum water depths from a dam break under 3 scenarios were estimated as follows: Kimo Street bridge in 8-11 minutes, the Puiwa Road bridge in 14-15 minutes, the Pali Highway bridge in 23-24 minutes, and the Beretania Street bridge in 33-56 minutes. Maximum water depths were estimated to occur at Country Club Road in 4 minutes, Wyllie Street in 19 minutes, North Kuakini Street in 27 minutes and H-1 in 29 minutes.

5. If you live in a high-rise, what floor or height level are you considered safe from a dam failure?

While there is no official guidance on vertical evacuation, in other communities, moving up to higher floors during a flood is generally the guidance given.

6. In case of an evacuation, does the BWS have an "action plan" to deal with the traffic congestion that will occur?

The City plans to send emergency responders to establish traffic control points to assist with safe and expeditious traffic flow. Furthermore, the evacuation zones span large stretches of land from the mountain towards the ocean but are relatively narrow going east to west, enabling a fast evacuation from the hazardous areas.

7. What type of "action plan" does the BWS have to help evacuate residents that are disabled?

Emergency responders will assist as they are able to but depending on circumstances may not be available to immediately assist all residents. We urge everyone make plans ahead of any emergency events to take care of themselves, their families, friends, and neighbors. Make a plan, build an emergency supply kit, and stay informed.

8. What is the expected depth of flooding?

Residents should be aware as to whether they are in a dam evacuation zone. Dam break computer simulations were conservatively analyzed (with reservoirs at full water levels), and used to develop evacuation maps that identify boundaries from which properties outside of the evacuation zone are anticipated to have minimal impact from a dam failure.

Should a dam failure occur, the resulting depth of flooding and potential flood damage will depend on multiple factors; such as but not limited to how much water is in the reservoir at the time of failure, height of the dam embankment, how much flooding from other sources are occurring at the time of the peak flood, topography at the location relative to the dam, distance from the dam, distance from the downstream floodway channel, capacity of the downstream channel, potential for blockages along the downstream channel, and other factors.

For the Nuuanu area, a failure at either Nuuanu Reservoir No. 1 or 4 would result in a flood with significant flood depths positioned along the drainage ways (streams) in excess of 7 feet, based on the reservoirs being full at the time of failure and assuming no blockages occur downstream. Flood waters at this depth will do significant damage to anything in its way. However, flood depths will vary from these maximum depths throughout the inundation area down to several inches, dependent on the previously listed factors. Flood water of just 6”-12” can move vehicles.

In a Dam Break Analysis for Nuuanu Reservoir No. 4 prepared by Oceanit in 2008 for the US Army Corps of Engineers and DLNR, maximum water depths from a dam breach were modeled for three scenarios: 500-year storm, 100-year storm and “sunny day” event. Based on the computer models, maximum water depths were estimated to be 24-28 feet at the Kimo Street bridge; 32-43 feet at the Puiwa Road bridge, 34- 42 feet at the Pali Highway bridge and 21-29 feet at the Beretania Street bridge. The higher depths are for a dam break during a 500-year storm. In an Individual Assessment Report for Nuuanu Reservoir No. 1 prepared by the Pacific Disaster Center (PDC) in 2014 for DLNR, computer models estimated that maximum water depths from a “sunny day” dam break would be approximately 5 feet at Country Club Road, 8 feet at Wyllie Street, 6 feet at North Kuakini Street and 13 feet at the H-1.

9. Would electrical be affected?

Electricity may be affected if the flooding impacts the infrastructure. It’s important to note that most of the flow will go into the stream as the stream bed is deeper than the areas on its banks. Flooding will occur when water cannot be contained in the stream channel. So, it’s possible that electrical poles and wires may not be impacted if they are not in areas prone to flooding from the stream.

10. Would the rushing water be strong enough to cause power polls to fall?

It is possible that with the flow of water and debris, utility poles or other structures will be affected.

11. Will the BWS provide any financial assistance to residents whose homes are damaged by a possible dam break or stream overflow?

All claims made against the Board of Water Supply (BWS), including claims relating to water damage from dam events, are evaluated at the time presented. While anyone may file a claim, the submission of a claim form does not guarantee future financial compensation, as each claim is reviewed and adjudicated based on its individual merits.

12. What is the purpose of the BWS operating the reservoir dams in Nuuanu valley? What would be the cons of closing it?

When the dams in Nuuanu were transferred to BWS by Presidential Proclamation and Hawaii Legislative resolution and Governor’s Executive Order in the early 1920’s, the reservoirs provided potable water to downtown Honolulu. With the discovery of more reliable groundwater sources, however, the reservoirs were no longer needed for water service and became largely storm and debris detention basins for Nuuanu Valley. Unfortunately, the dams were never transferred from BWS to a more appropriate government agency. Closing any of the four reservoirs could result in more flood events for Nuuanu Valley because the 4 reservoirs retain flood flows that would otherwise flow through Nuuanu and other area streams and lets them out slowly so as to not overwhelm the stream channels and overflow onto the stream banks and adjacent properties.

13. What type of assessments were made on "reservoir four" and explain the outcome?

In addition to the 2008 Oceanit Dam Break Analysis, a Phase I Inspection Report was prepared by Gannett Fleming (GF) in August 2008, a Phase II Dam Safety Inspection Report was prepared by Ernest Hirata & Associations (EHA) in May 1999, a Phase I Dam Safety Inspection Report was prepared by EHA in November 1993 and a Phase I Inspection Report was prepared by the US Army Corps of Engineers (USCOE) in April 1978.

The purpose of the GF Ph. I Inspection Report was to conduct an assessment of the general physical condition of Nuuanu 4 dam and its operations. The GF report recommended several primary and secondary priority actions be taken:  investigate reported seepage from the dam; remove trees and overgrown vegetation from the dam, spillway and outlet discharge channel; rehabilitate the intake tower sluice gates and trash racks; prepare an operations and maintenance (O&M) manual for the dam; perform a stability analysis of the dam; provide better access to the toe of the dam and update the emergency action plan (EAP) for the dam.

The purpose of the 1999 EHA Ph. II Inspection Report was to evaluate the safety of the dam; identify potential public health, safety and welfare areas, assess the dam property visually, structurally, hydraulically and hydrologically; and identify areas needing immediate remedial action and relevant costs. EHA found the Nuuanu 4 dam did not appear to be an immediate hazard to the public health, safety and welfare, but recommended continuing efforts to regularly clear & maintain vegetation from the dam, spillway and outlet channel; install and maintain weirs and piezomieter to measure and monitor seepage from the dam; and maintaining the water level at just above half full.

The primary purpose for the 1993 EHA Ph. I Dam Safety Inspection Report was to evaluate the integrity of the dam and appurtenant structures (spillway, intake tower and access bridge, etc.). In their report, EHA recommended clearing and regularly maintaining vegetation on and around the dam, spillway and outlet discharge channel; improving and maintaining the access bridge to the intake tower; investigating the condition of the lowest sluice gate; preparing formal O&M procedures and an EAP for the facility; implementing a comprehensive monitoring program; conducting a comprehensive topographic survey to locate areas of seepage, monitoring instruments (piezometers, monitoring wells, weirs, etc.) and their elevation, and to generate cross sections of the dam for interpretation of monitored data; and installing toe drains to help monitor and measure seepage through the dam.

We do not have a copy of the 1978 USCOE Ph. I Inspection Report, so do not know what it’s purpose was or what came out of the report.

14. Why doesn't the BWS maintain our valley streams?

According to Hawaii Revised Statute 46-11.5, Maintenance of drainageways is the responsibility of the property owner and enforcement powers are provided to the Counties.

HRS [§46-11.5] Maintenance of channels, streambeds, streambanks, and drainageways. Notwithstanding any law to the contrary, each county shall provide for the maintenance of channels, streambeds, streambanks, and drainageways, whether natural or artificial, including their exits to the ocean, in suitable condition to carry off storm waters; and for the removal from the channels, streambeds, streambanks, and drainageways and from the shores and beaches any debris which is likely to create an unsanitary condition or otherwise become a public nuisance; provided that to the extent any of the foregoing work is a private responsibility the responsibility may be enforced by the county in lieu of the work being done at county expense, and any private entity or person refusing to comply with any final order issued by the county shall be in violation of this chapter and be liable for a civil penalty not to exceed $500 for each day the violation continues; provided further that it shall be the responsibility of the county to maintain all channels, streambeds, streambanks, and drainageways unless such channels, streambeds, streambanks, and drainageways are privately owned or owned by the State, in which event such channels, streambeds, streambanks, and drainageways shall be maintained by their respective owners. [L 1986, c 121, §2]

Posted: 03/10/2020

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Hawaii Dam and Reservoir Safety Program

The Hawaii Dam and Reservoir Safety Program administered by the Hawaii Department of Land and Natural Resources (DLNR) Engineering Division exists to protect the health, safety, and welfare of the citizens of the State by reducing the risk of failure of the dams or reservoirs.

What are dams?

A barrier constructed to hold back water resulting in a pool that can be used for a variety of purposes; agriculture, water supply, electricity generation, flood or sediment mitigation.

History of Dams in Hawaii

Since the late 1800’s and early 1900’s many dams were built to support agriculture across the state.

Dam Safety Regulatory Program

• 1980 - US Army Corps of Engineers conducts Phase I inspection of High Hazard dams and develops the Hawaii regulated dam inventory
• 1987 - Dam Safety Act (HRS 179D)
• 1989 - Hawaii Administrative Rules (HAR) 13-190
• 2006 - Kaloko Dam failure
• 2007 - Update “Hawaii Dam and Reservoir Safety Act of 2007” (HRS 179D)
• 2012 - Update “Dams and Reservoirs” (HAR 13-190.1)

Regulatory Size

Greater than 25‐ft high and 16.3 million gallons in volume, but nothing less than 6‐ft tall, or 4.9 million gallons in volume

October 2018 Statistics

Regulates 131 dams and reservoirs statewide: Hawaii (10), Maui (56), Oahu (13), and Kauai (52).

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Question & Answer

1. Why should I care about dams?

Dams provide a great asset to the community by providing stored water for irrigation, drinking, energy generation and flood and sediment control. However, they come with risks if there were to be a dam failure. Not only the flooding of the community, but also the loss of the asset and its benefits.

2. What are risks associates with dams?

Dam failures can be structural, mechanical or hydraulic in nature.

• Structural – foundation defects causes the failure of the embankment
• Mechanical – malfunctioning gates or valves can cause a dam failure
• Hydraulic – overtopping due to inadequate spillway design or as a result of other causes

3. Am I or could I be affected by a dam and what is the dam failure flood inundation area?

• Yes, potentially, if you live downstream of a dam. Flood inundation areas and water flow down natural tributaries can be modeled. There is a potential for stream banks and drainage ways to be overtopped during a dam break due to an increase in flow.
• You can find out if you live in a Dam Evacuation area by visiting http://gis.hawaiinfip.org/fhat/ or http://dlnreng.hawaii.gov/dam/ for more information about the program.
• The evacuation area is a much larger area that may be affected by flooding. The evacuation area is designed by emergency managers to control the movement of people in and out of the potential hazard zone, and determined by the roadway network of a particular neighborhood.

4. Once I determine that my property is in a dam break inundation area, what’s next?

• Know where the dam is located, compared to your location.
• Find out the downstream channel and tributary stream or drainageway downstream of the dam.
• Become familiar with the streets and major roadways that would be closed in the event of a potential dam break.
• Become aware of how to move away from the potential flood wave. Depending on the downstream tributary, which direction to move, including vertical evacuation.

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These photos show the siphon system which helps to maintain the water level in the Nuuanu 1 reservoir.

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State of Hawaii Dam Resources:
- DLNR Inventory Website of Dams in Hawaii
- DLNR Online Dam Evacuation Area Tool (Flood Hazard Assessment Tool)
- DLNR Dam Safety Website

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NOTE:
Information is current as of the time it was posted online and is subject to change and update as new information becomes available.
 

Updated: 10/04/2023 10:05