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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
4—
Hydrology and Hydraulics
STREAM RUNOFF DATA AND RECURRENCE INTERVALS
Instantaneous peak discharges for Niu, Hahaione, Inoaole, Kuliouou, Makawao, and Maunawili streams were calculated by the U.S. Geological Survey and are summarized in Table 6 (R. Nakahara, U.S. Geological Survey, Honolulu, personal communication, 1988). Only the Makawao Stream has a continuous recording streamgauge (no. 2540). The Inoaole, Kuliouou, and Maunawili streams have a crest-measuring stake from which instantaneous peaks are calculated using a slope-area method (R. Nakahara, personal communication, 1988).
The historical records of the Inoaole, Kuliouou, Makawao, and the Maunawili streams allow for the calculation of a recurrence interval (flood frequency) for the floods that occurred on these streams. However, the Niu and Hahaione streams have no streamflow gauges or crest gauges and therefore have no historical record from which to determine a recurrence interval. Therefore, the only record available for determining a recurrence interval for the New Year's Eve floods in the southeast Oahu area (east Honolulu) is the Kuliouou Stream, for which there are only 17 years of streamflow data. Using the U.S. Water Resources Council's (1977) procedures, peak runoff value is determined to have had a recurrence interval in excess of 500 years. However, it should be noted that records of less than 20 years' duration are not reliable in the determination of recurrence intervals (Linsley et al., 1982).
The data from the Makawao stream, the only continuous recording streamgauge and the only hydrograph resulting from this storm, are presented in Table 7 and plotted in Figure 11. Note that the instantaneous peak discharge of 3,100 cfs occurred at 10:45 p.m. A comparison of the December 31, 1987 streamflows with historical events for the Makawao stream is presented in Table 8.
Some streamgauge stations have questionable rating curves due to streambed variability during storms and backwater effects from the ocean. Such is the case for
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
TABLE 6 Estimated Instantaneous Peak Discharges for Niu, Hahaione, Inoaole, Kuliouou, Makawao, and Maunawili Streams
Stream
Station No.
Drainage Area (square miles)
Years of record
Instantaneous Peak Discharge (cfs)
Recurrence Interval (years)
Measurement Method Used
Measurement Site
Niu (Kupaua Valley)
__
0.97
__
6,420
__
Slope-area
As indicated in Figure 1
Hahaione
__
0.91
__
3,450
__
Slope-area
As indicated in Figure 1
Inoaole
2,488
1.21
29
1,400
11.1
Slope-area
Station no. 2488
Kuliouou
2,479
1.18
17
4,700
>>500
Slope-area
Station no. 2479
Makawao
2,540
2.04
29
3,100
10.8
Recorded
Station no. 2540
Maunawili
2,605
5.34
29
5,710
14.3
Slope-area
Station no. 2605
Source: Data obtained from USGS Honolulu district office on February 29, 1988. Recurrence intervals were calculated using the USGS annual peak flow frequency analysis, which followed U.S. Water Resources Council Guidelines Bulletin 17-B.
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
TABLE 7 New Year's Eve Flood Hydrograph at Makawao Stream (Streamgauge No. 2540)
Date
Time
Discharge (cfs)
December 31, 1987
6:00 p.m.
418
6:30
640
7:00
1,670
7:30
1,220
8:00
1,170
8:30
1,960
9:00
2,250
9:30
2,040
10:00
2,350
10:30
2,630
10:45
3,100
11:00
2,690
11:30
2,590
12:00 p.m.
1,380
January 1, 1988
12:30 a.m.
1,740
1:00
1,810
1:30
1,820
2:00
274
2:30
67
3:00
20
3:30
52
the records obtained at the Waimanalo streamgauge (station no. 2490). Using the Gumbel Type I distribution, the annual maximum instantaneous streamflows for Kuliouou (station no. 2479), Inoaole (station no. 2488), Waimanalo (station no. 2490), Makawao (station no. 2540), and Maunawili (station no. 2605) streams were determined and are plotted in Figures 12 to 16.
KAWAINUI MARSH FLOODING
To understand the cause of flooding in the Kailua area, it is important to understand the dynamic processes of the events that took place during the storm in the Kawainui Marsh area. Adjacent to the Coconut Grove area in Kailua, the Kawainui Marsh drains 11.2 square miles on the windward side of the Koolau Mountains (U.S. Army Corps of Engineers, 1956). Outflow from the marsh discharges into the man-made Oneawa Canal (also known as Kawainui Canal) and then into the northern end of Kailua Bay. On New Year's Eve water overflowed the Kawainui marsh levee and flooded more
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 11 Hydrograph of the New Year's flood recorded at Makawao Strewn (station no. 2540).
TABLE 8 Summary of Significant Historical Discharges at Makawao Stream (Streamgauge No. 2540) for the Period 1958 to 1987.
Date
24-hr Discharge (cfs)
Ranking
6-hr Discharge (cfs)
Instantaneous Discharge Ranking
Peak (cfs)
Ranking
February 4–5, 1965
256
3
887
3
6,000
1
December 17–18, 1967
442
2
1,480
2
2,490
5
November 26, 1970
148
5
480
5
3,000
4
February 14–15, 1985
240
4
747
4
3,940
2
December 31, 1987
677
1
2,010
1
3,100
3
Source: Data compiled by Iwao Matsuoka, Hydrologist, U.S. Geological Survey, Honolulu district office.
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 12 Gumbel Type I frequency distribution of an annual maximum instantaneous streamflow for Kuliouou Stream (station no. 2479). Period of record is 1970 through 1986.
Figure 13 Gumbel Type I frequency distribution of an annual maximum instantaneous streamflow for Inoaole Stream (station no. 2488) for period of record 1958 through 1986.
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 14 Gumbel Type I frequency distribution of an annual maximum instantaneous streamflow for Waimanalo Stream (station no. 2490) for period of record 1963 through 1986.
than 300 homes in the immediate downstream Coconut Grove area, which is located between the marsh and the ocean. It is believed that sometime between 11 p.m. and midnight floodwaters began overflowing the compacted earth-fill levee that protects the Coconut Grove residential area. Three-fourths of the 6,850 foot-long levee was overtopped during the night (U.S. Army Corps of Engineers, 1988e). However, residents confirmed that the Oneawa Canal did not overflow and that no signs of blockage occurred at the bridges along the drainage canal.
The flooding of the Kawainui Marsh and the Coconut Grove area to the east of the Kawainui Marsh on New Year's Eve resulted from a complicated overland flow event in the upstream watershed, the hydraulic routing process of the 740-acre marsh, and an overtopping of the levee. A detailed engineering analysis of the dynamic processes of the marsh under flooding conditions is required to determine the modifications needed to prevent a recurrence of the floods. Such an analysis is beyond the scope of this report.
According to a U.S. Army Corps of Engineers (1956) design memorandum, the
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 15 Gumbel Type I frequency distribution of an annual maximum instantaneous streamflow for Makawao Stream (station no. 2540) for period of record 1958 through 1986.
maximum flood stage in the Kawainui Marsh should be 7.35 foot mean sea level (msl), which is 2.15 feet below the original top of the levee (elevation 9.50 feet msl), built more than 20 years ago. Although the elevation of the top of the levee at the time of the New Year's flood is unknown, a U.S. Army Corps of Engineers survey conducted in March 1988 indicated that the top of the levee was 8.5 feet msl (U.S. Army Corps of Engineers, 1988f). Therefore, the levee had apparently settled approximately 1 foot, and water began to overflow the levee near the Oneawa Canal and near the southeastern portion of the levee at an elevation of 8.5 feet msl.
Research and detailed engineering analysis are not the primary objective of this postdisaster study team's mission. Therefore, certain important parameters—such as the time history of sedimentation and debris flow deposition in the Kawainui Marsh since its original construction in 1966, inflow retardation due to floating vegetal mat in the marsh, and backwater and tidal effects on the Oneawa drainage channel—were not investigated. Any of these factors could have contributed to the flooding in the Coconut Grove area in Kailua.
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 16 Gumbel Type I frequency distribution of an annual maximum instantaneous streamflow for Maunawili Stream (station no. 2065) for period of record 1958 through 1986.
SEDIMENT, DEBRIS, AND LANDSLIDES FROM THE STORM
Aerial photographs taken during postdisaster reconnaissance indicate that the upper watersheds of the three valleys (Niu, Kuliouou, and Hahaione) on southeastern Oahu provided an excellent environment for the growth of trees and vegetation (G. Kekuna, Oahu Civil Defense Agency, personal communication, 1988). Floodways in the upper valleys had an accumulation of dead trees or debris from prior floods, as seen in Figures 17a–e. The intermittent streamflows of Niu, Kuliouou, and Hahaione streams, coupled with local rainfall, support a significant density of trees and vegetation. Small valleys such as these can be a source of debris that clogs downstream channels, with disastrous results during large floods.
During the torrential rains of New Year's Eve 1987, numerous debris flows occurred. Some of them developed when trees and vegetation were eroded and transported downstream. As floodwaters scoured and widened the natural channels, the eroded materials were transformed into debris flows. Other flows originated as
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 17a
Figure 17b
Figure 17a–e Floodways in upper Niu Valley.
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 17c
Figure 17d
Figure 17e
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
slope failures but were transformed immediately into debris flows by the torrential rains, transporting sediment and rock fragments large distances downstream. Several debris flows in the Hahaione Valley were of this type.
In Niu Valley the debris plugged the underpass of a bridge on Halemaumau Street and flooded lower Niu Valley (Figures 18a and b). In Kuliouou Valley the debris completely sealed off the debris basin (Figures 19a and b) and forced the water to change its course. In Hahaione Valley the culvert immediately downstream from the debris basin clogged and overflowed. The floodwaters then changed their course and flowed down through Kahena Street. The turbulence of the flood gouged a channel 10 to 20 feet deep along Kahena Street and carried asphalt and cars to the bottom of the street (Figures 20a–c).
It is estimated that 10 to 15 landslides occurred during the New Year's Eve storm event in each of these three valleys (J. Costa, U.S. Geological Survey, Vancouver, personal communication, 1988). Most of the landslides appear to have originated on steep straight-sided hillslopes (Figures 21a–c). U.S. Geological Survey geologists (J. Costa, personal communication, 1988; S. Ellen, R. Iverson, T. Pierson, U.S. Geological Survey, Menlo Park/Vancouver, personal communication, 1988) observed that none of these landslides reached a channel in the valley floor except one in the upper Niu Valley that directly contributed to the debris flow (Figures 22a–c).
It is apparent that the debris flows generated by the torrential New Year's Eve rainfall were the major cause of damage in the three valleys. Therefore, a method of determining the quantities of debris flow that might be generated by severe storms should be an essential factor in the development of a hazard-mitigation program. This would greatly improve the planning and design criteria for debris basins and other flood containment structures.
STORM DRAINAGE STANDARDS
The design procedures that have evolved for the storm drainage facilities in the city and county of Honolulu are presented in a sequence of three reports entitled Storm Drainage Standards published in 1957, 1979, and 1986 (City and County of Honolulu, Department of Public Works, 1957, 1969, 1986). The storm drainage facilities for southeastern Oahu were designed using the criteria presented in these reports.
In its introduction the 1957 report cites "the need and demand for adequate drainage facilities to protect property against storm waters." The report also states that the "city has a responsibility of protecting properties against flooding conditions" and calls for the adoption of a "Master Plan for Drainage'' to be prepared by the city's planning commission. The recommended design criteria in the 1957 Storm Drainage Standards report call for a recurrence interval (frequency or return period) varying from 10 to 50 years, as shown in Table 9.
The recommended design criteria in the March 1969 Storm Drainage Standards report are shown in Table 10. Comparison of Tables 9 and 10 indicates that any area of 100 acres or less that contributes to a highway culvert or bridge is required to be
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 18a Bridge over east fork of Niu stream at Halemaumau Street.
Figure 18b View of east fork of Niu stream above Halemaumau street.
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 19a
Figure 19b
Figures 19a and b This debris basin on Kuliouou Stream (photo taken 1/8/88) was completely sealed off by storm debris.
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 20a
Figure 20b
Figures 20c
Figures 20a–c Kahena Street, Hahaione Valley on January 2, 1988. Flood waters gouged a 10–20 ft channel in the roadway. (Photo courtesy of T. Giambelluca).
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 21a Hahaione Valley
Figure 21b Kuliouou Valley
Figure 21c Kuliouou Valley
Figures 21a–c Landslides in Hahaione Valley and the Kuliouou Valley.
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
Figure 22a
Figure 22b
Figure 22c
Figures 22a–c Landslides in the Niu Valley.
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
TABLE 9 Recommended Recurrence Interval, 1957
Drainage Area (acres)
Less Than
Greater Than
Recurrence Interval
100
—
10
300
100
20
640
300
30
—
640
50
Source: City and County of Honolulu (1957).
TABLE 10 Recommended Recurrence Interval, 1969
Drainage Area (acres)
Recurrence Interval (years)
100 or less
10
100 or less with sump, tailwater effect, and for the design of roadway culverts and bridges
50
100 or greater and all streams
Based on maximum recorded flood peaks
Source: City and County of Honolulu (1969).
TABLE 11 Recommended Recurrence Interval, 1986
Drainage Area (acres)
Recurrence Interval (years)
100 or less
10
100 or less with sump, tailwater effect, and for the design of roadway culverts and bridges
50
100 or greater and all streams
100
Source: City and County of Honolulu (1986).
designed using a 50-year recurrence interval. Furthermore, all other areas greater than 100 acres and all streams are required to use a recurrence interval based on maximum recorded flood peaks. When compared with the 1957 report, these changes are significant.
The recommended design criteria in the March 1986 Storm Drainage Standards report are shown in Table 11. The major difference from previous years is the
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The New Year's Eve Flood on Oahu, Hawaii December 31, 1987–January 1, 1988
requirement of using a recurrence interval of 100 years for drainage areas greater than 100 acres.
The most striking aspect of these reports is the lack of any discussion of sediment and debris, which can quickly clog and render inoperative the most carefully designed storm drainage system. Only in the March 1986 report is there the brief statement that ''debris barriers should be provided upstream of the intake to prevent clogging. Where required, boulder basins shall be provided upstream of the debris barrier" (p. 12). However, other than referring to a 1949 U.S. Soil Conservation Service report and to a U.S. Bureau of Reclamation engineering monograph (no date given), no design criteria are given. Therefore, all of the designs by the city and county of Honolulu's Division of Engineering are based only on clear water flows (A. Thiede, Department of Public Works, Honolulu, personal communication, 1988). That is, no consideration is given to the possibility that the flows in the storm drainage channels may carry sediment and debris that add bulk to the flow, increase its volume, and can potentially block artificial and natural stream channels.
Calculation of the impact of sediment and debris on Oahu's streams requires time-series data from a long history of storms. The volumes of sediment and debris produced by each storm event as it occurs should be measured and recorded. Unfortunately, these data have not been and are not being collected.
POSTDISASTER STUDIES
Preliminary geophysical and engineering investigations of the New Year's Eve flood began during the early recovery period. In late January and early February 1988, the U.S. Army Corps of Engineers compiled rehabilitation letter reports for Kuliouou Stream (U.S. Army Corps of Engineers, 1988a), Hahaione Stream (U.S. Army Corps of Engineers, 1988b), Omao and Maunawili streams (U.S. Army Corps of Engineers, 1988c), and Niu Stream (U.S. Army Corps of Engineers, 1988d). Concurrently, the U.S. Soil Conservation Service also implemented rehabilitation programs for Waimanalo (J. Lum, U.S. Department of Agriculture, Soil Conservation Service, personal communication, 1988). Preliminary peak discharges that occurred during the New Year's Eve flood from Niu, Kuliouou, Hahaione, and Maunawili streams were estimated by the U.S. Geological Survey (J. Nakahara, personal communication, 1988). These values are tabulated in Table 6. In early February 1988 the USGS investigated the landslides that occurred in the Niu, Kuliouou, and Hahaione valleys. A month later a team of two geologists and one hydrologist (S. Ellen, R. Iverson, and T. Pierson, personal communication, 1988) from the USGS conducted more extensive fieldwork on the landslide sites.
Representative terms from entire chapter:
storm drainage
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