FREQUENTLY ASKED QUESTIONS

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 Alert, 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.

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.hawaiʻi.gov). Currently, there are real-time reservoir water level gauges at Nuʻuanu Reservoir No. 1 and Nuʻuanu 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.

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.

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 Nuʻuanu area, a failure at either Nuʻuanu 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 Nuʻuanu 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 Nuʻuanu 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.

When the dams in Nuʻuanu were transferred to BWS by Presidential Proclamation and Hawaiʻi 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 Nuʻuanu 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 Nuʻuanu Valley because the 4 reservoirs retain flood flows that would otherwise flow through Nuʻuanu 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.

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.

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.

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.

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.

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.

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.

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

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 Nuʻuanu 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 Nuʻuanu 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.

According to Hawaiʻi 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]