U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

Assessment of Landslide Hazards in Kaluanui 

and Maakua Gulches, Oahu, Hawaii, following 

the 9 May 1999 Sacred Falls Landslide

By Randall W. Jibson and Rex L. Baum

Denver Federal Center 

Box 25046, MS 966 

Denver, Colorado 80225

Open-File Report 99-364

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial 

standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for 

descriptive purposes and does not imply endorsement by the U.S. Government.

1999

science for 

uses

a changing world
ASSESSMENT OF LANDSLIDE HAZARDS

IN KALUANUI AND MAAKUA GULCHES, OAHU, HAWAII,

FOLLOWING THE 9 MAY 1999 SACRED FALLS LANDSLIDE

By Randall W. Jibson and Rex L. Baum

U.S. Geological Survey

Denver, Colorado

Background

About 2:30 p.m. on Sunday, 9 May 1999, a landslide killed seven hikers and injured many 

more at Sacred Falls State Park, near Hauula on the north shore of Oahu, Hawaii (Figure 1). One 

of the injured hikers later died of injuries received in the landslide. Governor Ben Cayetano of 

Hawaii ordered that the park be closed due to concern about continuing landslide hazard near the 

falls. Subsequently, Bill Meyer, District Chief for the U.S. Geological Survey (USGS) Water 

Resources Division in Honolulu contacted Tim Johns, Chair of the Board of Land and Natural 

Resources of the Hawaii Department of Land and Natural Resources (DLNR) and offered 

assistance in assessing slope stability in the park. Mr. Johns accepted the offer, and two 

landslide specialists from the USGS Geologic Hazards Team in Golden Colorado were sent to 

the site. On Friday, 14 May 1999, we visited the Sacred Falls landslide site with Glenn Bauer, Ed 

Sakoda, and Gary Moniz of DLNR. The ground investigation involved inspecting the impact 

area, estimating the volume of the deposit, and gathering data to help reconstruct the event. On 

Monday, 17 May 1999, we conducted an aerial reconnaissance of Kaluanui Gulch (Sacred Falls 

State Park) and Maakua Gulch in a commercial helicopter provided by DLNR. We inspected the 

source and path of movement of the Sacred Falls landslide of 9 May and reconnoitered the full 

length of both valleys to get an overview of ongoing landslide hazards there. This report gives 

our observations and conclusions about the Sacred Falls landslide, broadly assesses the ongoing 

hazard in the Kaluanui and Maakua Gulches, and suggests methods for more detailed assessment 

of landslide hazards here and along other trails in state parks on Oahu. Observations and 

conclusions in this report are based on a very brief investigation and thus are preliminary in 

nature.

Sacred Falls Landslide of 9 May 1999

The Sacred Fall landslide is properly classified as a rock fall. The source of the Sacred Falls 

rock fall of 9 May 1999 is at an elevation of about 800 feet above sea level on the southeast 

canyon wall directly above the plunge pool of Sacred Falls (Figure 2). The source is easily 

visible because it is the only place on that part of the slope where there is no vegetation and fresh 

rock is exposed (Figure 3). The source area consists of a scar of freshly exposed rock about 15- 

20 feet wide by 10 feet high on a nearly vertical slope. The thickness of the slab of rock that 

failed appears to have been about 3-6 feet. The source is on a part of the valley wall that forms a 

promontory. Above the scar is a 3-foot overhang of intact rock that was created when the 

landslide material fell from beneath it. Below the scar is a track of uprooted or otherwise 

disturbed vegetation and impact marks left by the falling landslide material.
0 

I

2000 

l

4000 feet

Figure 1. Topographic map showing Kaluanui gulch, Maakua gulch, and Sacred Falls 

(from USGS Hauula 1:24,000 quadrangle, contour interval 40 feet, north up). Area of 

Figure 3 shown.
Figure 2. Topographic map showing approximate 

source area and path of 9 May rock fall (from Hawaii 

Department of Land and Natural Resources, contour 

interval 25 feet). Location shown in Figure 1.

Figure 3. Source area (indicated by arrow) of 

Sacred Falls rock fall.

The landslide material cascaded down a 70°- 

80° slope and entered a dry waterfall chute that 

points directly at the downstream end of the 

plunge pool at the base of Sacred Falls. The lip 

of this waterfall chute is at an elevation of about 

470 feet; below this point the chute forms a 

slight overhang with its surface being 2°-4° out 

of vertical. Below the lip of the waterfall chute, 

the landslide debris would have lost contact with 

the ground surface and been hi free fall over the 

last part of its descent to the impact area at the 

edge of the pool at the base of Sacred Falls, 

which lies at an elevation of about 320 feet. 

Thus, the landslide material fell a total of about 

480 feet: the first 330 feet it cascaded down a 

precipitously steep waterfall chute, and the last 

150 feet it was airborne and fell straight down to 

the impact zone. The landslide debris thus 

would have been traveling at a very high speed 

when it hit the impact area, probably 100-150 

feet/second (about 70-100 miles/hour).

Accurately delineating the area of the 

deposit is somewhat difficult because of the 

large number of pre-existing boulders in the 

impact zone. The deposit from the 9 May rock 

fall filled a shallow channel that formerly 

drained the plunge pool and appears to extend 

30-40 feet downstream from the northeast edge 

of the pool (Figure 4). The deposit has a 

maximum width of about 20 feet and averages 

2-3 feet hi thickness. Scattered freshly broken 

fragments of rock lay on the ground 

surrounding the deposit. The total volume of 

the 9 May deposit appears to have been less 

than 50 cubic yards. The deposit consisted 

mainly of rounded, weathered boulders of basalt 

(lava rock) and a few freshly broken boulder 

fragments (Figure 5). Soil partially covered 

some of the deposit and occupied space between 

the boulders. Pieces of shrubs and small trees 

that were growing hi the source area and along 

the path of the rock fall made up the remainder 

of the deposit. The boulders had apparently 

weathered hi place to a rounded shape high on
In-place boulder 

broken by impact 

of tailing rock

Impact mark on 

canyon wall. 20-25 

ft above canyon 

floor

Impact mark on 

canyon wall. 15ft 

above canyon floor May 9 lock-tall deposit

^-Dry chute (waterfall 

channel)

Edge of plunge pool 

beneath overhanging 

rock

the valley wall prior to falling to the valley 

floor. Except for the size of the boulders and 

the covering of loose soil and brush on the 

deposit, the deposit of the 9 May rock fall 

looked remarkably similar to the older deposit 

of boulders at the downstream edge of the 

plunge pool. Most boulders in the 9 May 

deposit were smaller than 2 feet across. Many 

of the boulders in the older deposit were much 

larger, up to 3-5 feet across.

Figure 4. Sketch map (not to scale) showing location 

of 9 May rock-fall deposit in relation to Sacred Falls, 

plunge pool, and other features.

Several observations on the ground confirm 

that the landslide material was in free fall 

during the final part of its descent and that 

many rocks bounced on impact. We saw many 

impact marks and freshly broken rocks on the

surface of the older boulder deposit and on the opposite (northwest) valley wall. We saw two 

impact marks on the northwest valley wall, which consisted of rounded, shallow depressions 

several inches in diameter exposing fresh rock. One of the marks was about 15 feet above the 

valley floor and the other was 20-25 feet above the valley floor. It is unlikely that any of the 

rocks bounced high enough to make these marks, rather they resulted from impact by airborne 

falling rocks. Numerous impact marks were on the valley floor; most consisted of small areas of 

crushed rock and small depressions on the faces of basalt boulders. The farthest impact point of 

a rock from 9 May is about 120 feet downstream from the impact zone (Figure 4).

Causes of the Sacred Falls Landslide

The landslide occurred without triggering from any discernable external conditions. Things 

that commonly trigger landslide movement include intense or prolonged rainfall, earthquake 

shaking, recent steepening of a slope, or applying a load to the top of a slope. None of these 

things occurred prior to the 9 May landslide. The week prior to the slide is reported to have been 

abnormally dry, and no changes in slope geometry or load have occurred. The smallest 

earthquake required to trigger this type of slope failure is in the magnitude-4 range (Keefer, 

1984), which would have been recorded by local seismic networks and felt strongly throughout

the surrounding area, neither of which

occurred.

There has been much speculation about the 

cause of the Sacred Falls landslide. It has been 

suggested, for example, that the landslide may 

have been triggered by sonic booms, lowflying helicopters, or excessive drying of the 

slope material. There is no evidence that any 

of these things caused the landslide of 9 May, 

and it is highly unlikely that they could have 

had any significant effect on the stability of the Figure 5. Sacred Falls rock-fall deposit.
slope. Although sonic booms and low-flying aircraft sound loud and powerful, the vibrations 

they cause in slopes are so minute as to be undetectable. The vibration caused by Sacred Falls is 

many times greater than those that would be caused by sonic booms and aircraft. The idea that 

drying of slope materkls contributed to failure is likewise unfounded: there are no precedents for 

landslides occurring because they were too dry. Rather, water is the nemesis of landslides; all 

other things being equal, the wetter a slope is, the less stable it becomes.

Many landslides throughout the world occur without a discernable trigger. Slope materials 

gradually weaken over time under the influence of long-term weathering, infiltration of water, 

root wedging, and other physical processes. Eventually, if the strength falls below what is 

necessary to maintain the stability of the slope, a landslide will occur. The timing of such 

landslides is somewhat random, at least from a human point of view, which indicates that 

accurate prediction of when such events will occur is impossible.

Current Hazards at Sacred Falls Landslide Site

We saw no indication of additional potential landslide material near the source of the Sacred 

Falls landslide that appeared particularly likely to fall in the immediate future. The scar is 

relatively clean and free from loose debris that could come down in the aftermath of the main 

landslide. There may be additional rock fragments trapped in the vegetation along the landslide 

track that could come down during rainstorms, but such fragments, if they exist, are probably 

small and few in number. The overhang created by the landslide is certainly less stable than 

before the landslide, but it appears to be firmly anchored to the slope, and we saw no fractures or 

other indications of impending failure. However, this newly created overhang should be 

periodically inspected for fresh cracks or other indications of potential instability.

Reconnaissance of Kaluanui Gulch

The walls of Kaluanui Gulch show abundant evidence of having produced rock falls over a 

long period of time. Stable canyon walls become covered with vegetation ranging from a thin 

layer of fungus or lichen to thick shrubbery; areas that fail and form landslides remove the

vegetation and expose fresh rock. We saw a 

few fresh scars that appeared to be very 

recent (within a few weeks) at various 

locations, principally on the southeast wall. 

Many dozens of scars appeared to be 

moderately recent (within a few months or 

years), and evidence of even older scars was 

also visible (Figure 6). Clearly, rock falls 

from both walls of the canyon is a common 

occurrence in both the recent and long-term 

history of the canyon. The sizes, shapes, and 

locations of the scars suggests that the Sacred 

Figure 6. Fairly recent rock-fall scar on the wall of Falls landslide of 9 May was of typical size

Kaluanui gulch. and form. Some of the scars formed by
material falling and leaving overhanging slabs, such as occurred on 9 May, and this creates an 

area of decreased stability that will eventually fail again.

Deposits in the valley bottom also indicate that rock falls and other landslides are common at 

and downstream from Sacred Falls and that these processes have been active for a long time. 

Most of the trail up the canyon to Sacred Falls was built on rock-fall and other landslide deposits. 

These deposits form steep wedges that join the near-vertical canyon walls to the stream channel 

in the floor of the valley. The deposits consist of soil and rock fragments that range in size from 

pebbles to large boulders. Many of the large boulders deposited by previous rock falls lie on the 

ground surface in the midst of a forest of mature trees, which indicates that rock falls have been 

occurring in this valley for some time.

Reconnaissance of Maakua Gulch

Maakua Gulch has walls that appear steeper and higher than those of Kaluanui Gulch. We 

saw several landslide scars on the canyon walls, but there appeared to be significantly less scars 

than in Kaluanui Gulch. The vegetation on the walls of Maakua Gulch appeared to be thicker 

and more well-developed than in Kaluanui Canyon. Several parts of the gulch do have arers that 

have been subject to recent landslides, but other reaches appear to have been more statte for 

some time.

Assessment of Landslide Hazards in Kaluanui and Maakua Gulches

Both Kaluanui and Maakua Gulches are areas of very high hazard from rock falls. The 

canyon walls are hundreds of feet high and are extremely steep (70°-90°), and the valley bottoms 

are only 50-100 feet wide. The slope material is volcanic rock that weathers deeply and is 

fractured, which makes it very susceptible to failure. Rock falls can occur either as a remit of 

being triggered by conditions such as intense or prolonged rainfall or earthquake shaking, or in 

long-term conditions of gradual degradation of slope strength. The narrowness of the valley 

floors tightly constrains the area of potential impacts and thus increases the likelihood of the rock 

falls causing damage in the valleys.

Mitigation of Rock-Fall Hazards

Rock-fall hazards can be mitigated in three broad categories: prevention, retention, and set 

back. Prevention involves selective removal of potential landslide material (scaling) or 

physically strengthening the slope by using rock bolts, screening, retaining structures, or 

grouting (cement). This does not appear to be a viable option in this case because of aesthetic 

concerns (marring the canyon walls) and the practicality of actually getting the necessary 

equipment to the unstable slopes. Retention involves building retaining structures at the bases of 

slopes to catch material that falls. Such structures in this case would have to be very strong to 

catch rocks falling at high speeds from near-vertical, very high slopes. Again, aesthetics and the 

narrowness of the valley seem to rule out this option: there is simply not enough space at the
base of the slopes to build such structures. Set-back involves limiting access to areas near the 

bases of slopes that are within range of falling and bouncing rocks. This clearly is not possible 

because the narrowness of the valleys is such that anywhere on the valley floor is within range of 

falling rocks from either valley wall.

Because none of the possible mitigation measures appears viable, the situation becomes 

relatively straightforward: the only way to totally eliminate the possibility of additional 

landslide-related deaths or injuries is to close public access to the canyons. If access is allowed, 

then there is a continuing probability (not yet quantified) of eventual injuries or fatalities. The 

fact that very few fatalities have occurred over the past few decades suggests tha* this 

probability, on a per-visit basis, is low, but it is, nonetheless, significant.

Proposed Methods of Detailed Rock-Fall Hazard Assessment

The Sacred Falls rock fall has raised concern about the safety of hiking trails in state parks or 

other state-managed lands on Oahu. Consequently we were asked to suggest methods of rockfall hazard assessment for these areas. Field conditions in the steep canyons of Oahu severely 

limit application of field-based methods, such as the one developed by Harp and Noble (1993), 

which involves measuring the characteristics of fractures in rock outcrops. Outcrops in Hawaii 

are commonly steep and inaccessible to a geologist working on the ground, and mary are 

covered by vegetation. Consequently we propose a method that relies mainly on acrid and 

photographic studies of the areas. This approach is similar to the rock-fall hazard assessment 

conducted in Yosemite National Park following a fatal rock fall there (Wieczorek and ethers, 

1992, 1998).

A three-phase study could identify hazardous areas along each trail and rank the relative 

hazards: Phase 1, reconnaissance using aerial photographs; Phase 2, field checking using 

helicopter- and ground-based observations; and Phase 3, assignment of hazard rankings and 

identification of areas where rock-fall abatement is feasible. An optional Phase 4 would use 

records or estimates of trail usage and previous incidents to estimate the probability of hikers 

being injured or killed by a rock fall or other type of landslide.

Phase 1. Reconnaissance for evidence of active or recent rock fall could be conducted from 

a study of aerial photographs. Such a study would require vertical and oblique color photographs 

of the trails and surrounding areas. The photographs should have enough overlap to allow 

stereoscopic viewing and oblique photographs would be needed for both walls of each canyon. 

Vertical photographs would be used mainly for viewing canyon bottoms and as an aid in 

transferring data between the oblique photographs and maps. The oblique photographs would 

provide a more detailed view of the steep canyon walls than is possible using vertical 

photographs alone. Scale of the photographs should be large enough to clearly show rock fall 

scars (source areas and paths) as small as 2-3 feet across (scale of about 1:10,000 or greater). 

Rock fall areas could be plotted on topographic maps of the area to show their locations relative 

to the trails. Such photographs would create a record that could be consulted in the future and 

would significantly reduce the amount of helicopter time needed to map rock-fall evidence along 

and above the trails. The reconnaissance phase would result in a map showing suspected rockfall source areas and recent rock-fall paths or tracks.
Phase 2. Field checking from helicopter flights over the trail areas and ground observations 

along the trails would be used to confirm and correct observations made from the aerial 

photographs. These observations would be similar but much more detailed to those we 

performed in connection with the Sacred Falls rock fall. Ground observations would focus on 

presence and approximate age of landslide deposits in the valley bottoms and along the trails, 

and availability of buffer space from possible rock-fall impact areas. Where visibility allowed, 

the valley walls could be checked for presence or absence of rock-fall scars or potential source 

areas. Aerial observations would focus on identifying active or recent scars and potential rockfall source areas as well as recent rock-fall tracks. These ground and aerial field observations 

would be compared directly to the preliminary map of rock-fall areas from Phase 1 and would 

result in a revised map of rock-fall source areas and recent scars and tracks. This phase would 

also result in qualitative estimates of the frequency or relative ages of rock falls in the area.

Phase 3. Assignment of hazard rankings would be based on factors observed in the previous 

phases. These include relative abundance of rock-fall scars and potential source areas and the 

relative age or frequency of rock-fall activity. Hazard increases with opportunity for rock fall to 

occur. Thus, abundant fresh and recent rock-fall scars characterize high hazard zones, absence 

of fresh or recent scars and other potential rock-fall source areas characterize low hazard zones. 

Intermediate zones can be defined as needed to subdivide the range of observed rook fall 

abundance and frequency between the high and low hazard zones.

Once rock-fall hazard near the trails has been classified, segments of trails near high-hazard 

zones can be considered for various types rock-fall mitigation. As discussed above in the section 

on mitigation of rock-fell hazards, several methods are available, but only a few may be feasible 

along segments of most trails. Selective removal of loose or overhanging rock may be feasible 

in a few areas, but in general would be very difficult in steep, high-walled canyons. Where space 

allows, trails may be relocated to move them away from high hazard zones. Buffer zones can be 

defined by mapping the extent of rock-fall deposits downslope from active rock-fall areas, and 

trails may be rerouted beyond the edge of the deposits, provided the new route is acceptable on 

other grounds.

Phase 4. Estimates of probability of fatalities or injury could be used to aid in decisions 

about closing a trail or keeping it open to hikers. The first step in this phase would be to decide 

upon an acceptable level of risk (on a per-visit basis, how many rock-fall related injuries or 

deaths is DLNR and the community willing to accept). The next step is to estimate the 

likelihood of an injury or fatality based on existing conditions and the historic number of 

incidents. For example, to estimate the rate of fatalities due to rock fall at Sacred Falls, divide 

the total number of fatalities on record by the total number of visitors since records on such 

incidents have been kept (not just since the first rock-fall incident). Per-visit rates of fatalities 

and injuries can be compared to other statistics as an aid in determining how risky hiking a given 

canyon or trail is compared with other activities. It also provides a means of comparing the 

relative risk of different trails.
Summary and Conclusions

The Sacred Falls landslide of 9 May 1999 involved a very small volume (-50 cubic yards) of 

weathered rock, soil, and vegetation that plunged nearly 500 feet onto a narrowly constricted 

canyon floor that was crowded with visitors. One lesson to be learned from this event is that 

very small landslides can cause catastrophic results if they occur in times and places where many 

people are present. The following conclusions can be made in light of our observations and 

expertise:

 The Sacred Falls landslide occurred as a result of long-term, gradual degradation of the slope 

rather than by being triggered by external factors.

 The ongoing level of landslide hazard does not appear to have been significantly increased by 

the Sacred Falls landslide.

 The continuing (long-term) level of landslide hazard in the Kaluanui and Maakua Gulches is 

very high because of the steep, high canyon walls; narrow valley floors; and ongoing slope 

weathering and rock fall.

 Traditional methods to mitigate rock-fall hazards are not viable in these steep, narrow canyon 

environments.

 Detailed assessment of rock-fall hazards is possible using a combination of ground, aerial, 

and aerial-photographic investigations.

References

Harp, E.L., and Noble, M.A., 1993, An engineering rock classification to evaluate seismic rockfall susceptibility and its application to the Wasatch Front: Bulletin of the Association of 

Engineering Geologists, v. 30, p. 293-319.

Keefer, D.K., 1984, Landslides caused by earthquakes: Geological Society of America Bulletin, 

v. 95, p. 406-421.

Wieczorek, G.F., Morrissey, M.M., lovine, G., and Godt, J., 1998, Rock-fall hazards in the 

Yosemite Valley: U.S. Geological Survey Open-file Report 98-467, 1:12,000.

Wieczorek, G.F., Snyder, J.B., Alger, C.S., and Isaacson, K.A., 1992, Rock falls in Yosemite 

Valley, California: U.S. Geological Survey Open-File Report 92-387, 38 p., 2 app., A pi, 1 

disk.