Executive Summary
At 8:00 p.m. on December 31, 1987, when the National Weather Service (NWS) issued a flood warning for the eastern part of Oahu, Hawaii, few New Year's Eve revelers in the area imagined that nearly $34 million in flood damages would occur before torrential rains subsided the next morning. Although no lives were lost and the amount of damages might not be considered severe by some standards, the flooding proved significant because it occurred without warning and affected densely populated urban watershed areas.
The severe weather that caused the News Year's Eve flooding culminated an unusually wet December that had already seen more than five times the average rainfall expected for the month. Minor damage and disruption to telephone and power services had already occurred as a result of the earlier rains.
The drenching rains responsible for the New Year's Eve flood commenced about 3:00 p.m. on Thursday, New Year's Eve, but rain had fallen throughout the day. The forecast called for continued thundershower activity, with heavy downpours expected; however, the torrential rainfall and resulting floods were not anticipated even as late as 4:40 p.m., when NWS forecasters told officials of the Oahu Civil Defense Agency that there were no data to indicate an imminent threat of flooding.
The flood rains were produced by a cold front that had weakened into a shear line, a significant cloud and rain producer that acted as a center of strong low-level convergence between weak east-southeasterly winds to the south of the flood zone and fresh north or northeast winds to the north. When lifted along the southern rampart of the Koolau Mountains, this shear line produced steady rains of 2 to 4 inches per hour over the already saturated watersheds of southeastern Oahu.
Forecasting of the torrential rains was made difficult by the unavailability of adequate weather radar in the region. In addition, high clouds masked the actual rain clouds, limiting the effectiveness of satellite imagery in depicting local weather. For this reason, a flood warning was not issued until flooding had already commenced in some regions.
Rainfall totals were impressive. In the region immediately windward of the Koolau Mountains, the precipitation was in excess of that expected for a 100-year storm (a storm of an intensity expected to recur only once every 100 years) and was probably as much as would occur in a 200-year storm. Rainfall measured more than 20 inches in many mountain locations over a 24-hour period. In many cases, accurate totals were not available, since some raingauges malfunctioned or their capacity was overwhelmed.
Two types of flooding resulted from these rains. Flash flooding occurred in the Hawaii Kai area and in Waimanalo, a relatively low-lying region. Farther north in the Kailua region, overtopping of a flood control levee produced comparatively slower but more pervasive flooding.
Reports of property damage, household evacuations, and transportation disruptions were already being fielded by police in the Hawaii Kai and Waimanalo areas by 8:00 p.m., when the NWS issued its first flash flood warning. Because of the holiday, locating emergency response personnel was difficult, but by 9:00 p.m. authorities of the Oahu Civil Defense Agency had activated their emergency operations center, had begun to respond to distress calls, and had authorized the opening of the first emergency shelter in Hawaii Kai.
Major flooding and accompanying debris flows in Hawaii Kai commenced by 9:00 p.m. Blockage of drainage systems by rocks and debris caused unanticipated diversions of floodwaters, resulting in extensive damage to many upland neighborhoods not accustomed to flooding. Meanwhile, in Waimanalo, a low-lying region, floodwaters inundated homes with up to 5 feet of swirling water at the peak of the runoff.
Flooding in Kailua began around midnight, as the levee protecting the region from the Kawainui Marsh was overtopped and canals draining the area were overwhelmed. Residents had no warning that flooding was imminent, since flash flood warnings extended only to Waimanalo, not farther up the coast, and media attention and emergency response efforts up to that time were focused on the Hawaii Kai area.
As the evening proceeded and the flooding began to displace some residents and cut transportation routes, preventing others from returning home after the evening's festivities, the Red Cross opened several shelters, eventually serving almost 1,100 people on New Year's Eve and New Year's Day.
In all, more than 1,250 homes sustained some form of damage, with over 300 homes receiving major structural damage. Unfortunately, only about a tenth of these residences were covered by flood insurance through the National Flood Insurance Program. Only those homeowners whose mortgages are secured through a federal loan program (e.g., the Federal Housing Administration) or through a federally insured bank or savings and loan are required to purchase such insurance in specified flood zones; most other residents of Oahu chose not to buy flood insurance.
CONCLUSIONS AND RECOMMENDATIONS
In general, the performance of weather forecasters was hampered by the level of technology available, resulting in a lack of adequate predisaster warnings. The unavailability of adequate radar information, the mediocre performance of the raingauge network, and the limits of satellite imagery conspired to leave forecasters without sufficient data to anticipate the flood threat.
RECOMMENDATION: The raingauge network on Oahu needs immediate improvement, including increased raingauge capacity to preclude overflows, increased raingauge density, and perhaps higher-frequency monitoring of existing telemetered raingauges.
RECOMMENDATION: Although installation of the National Weather Service's Next Generation Radar (NEXRAD) in the mid-1990s will address many of the current radar deficiencies, a near-term fix should be considered as well, such as providing the NWS with direct access to radar imagery from Hickam Air Force Base or from the University of Hawaii. Acquisition of a series of inexpensive radars such as that at the University of Hawaii for deployment throughout the island should also be considered.
The New Year's Eve flood resulted from a combination of four factors. First, heavy rains earlier in the month meant the soil was already saturated with moisture. Second, the New Year's Eve storm was an extreme weather event that resulted in 24-hour rainfall totals expected only once every 100 to 200 years. These two factors combined to generate the third factor, the real culprit in the New Year's Eve flood: copious sediment and debris that filled debris basins, blocked drainage channels, and diverted streams from their natural or man-made channels.
A fourth factor was the failure of existing flood control facilities and structures. The Kawainui Marsh was designed as a flood control reservoir, but sedimentation and a lack of systematic dredging reduced the reservoir's capacity, and the levee surrounding the marsh had settled, losing about 1 foot in height. Furthermore, design of the Oneawa Canal, which drains the reservoir, had ignored the backwater effects of ocean tidal action, and design of the debris basins, concrete channels, and roadway crossings associated with the reservoir had ignored possible debris and sediment flows.
RECOMMENDATION: Studies of the volume of debris produced from storms in the urbanized watersheds of Oahu should be conducted, and design criteria for adequately controlling debris loads should be developed for incorporation into the planning and design of flood control works.
The inability of the NWS to issue a flood watch announcement prior to the onset of flooding resulted in a lack of predisaster mobilization by emergency response
agencies, which delayed their ability to respond quickly to flood problems. In addition, the holiday created logistical problems in mobilizing and coordinating emergency personnel. Despite these difficulties, early relief and recovery efforts were generally successful due to adequate predisaster planning.
RECOMMENDATION: Emergency response exercises modeled after the Oahu holiday disaster should be conducted to identify strategies to mobilize resources as quickly as possible.
Although there were two island-wide warning systems available (an air-raid warning system and a tsunami warning system), neither was used in this instance to alert residents of a possible flood threat even though unusual circumstances may have warranted such a use.
RECOMMENDATION: Consideration should be given to enhancing the ability of the present warning systems to alert residents to monitor their radios or televisions for emergency broadcasts. In addition, attention should be given to the content of weather advisory messages and to the most effective way to transmit them to the general public.
In 1970 Honolulu began to map flood-prone areas of Oahu, and in 1980 an official flood map (a Flood Insurance Rate Map) delineating areas of flood risk was adopted as part of the National Flood Insurance Program. This map was revised in September 1987, only 3 months before the New Year's Eve event.
Based on the hazard analysis underlying this map, only about a third of the damage that actually occurred took place in areas where flooding was expected, given the characteristics of the storm. In contrast, over half of the damage that took place occurred in areas where damage was not expected in such a storm. In addition, 15 percent of the damage took place in areas that had not yet been evaluated for their flood potential, even though the map had recently been updated.
RECOMMENDATION: A reassessment of some of the flood zone designations should be conducted, especially in the Kailua area, to determine whether the current hazard designations are appropriate. In addition, flood hazard mapping efforts should be extended to those areas where flood risks are currently unevaluated.
RECOMMENDATION: Although a number of loan and grant programs were made available after the flood, an evaluation should be undertaken to determine what monetary resources homeowners and renters used to repair damage and replace possessions, especially those who were not covered by the National Flood Insurance Program.
