Usgs geologic Investigations Series i-2761, Oahu

S

outh of Kahuku, along the windward coast to Hauula, the narrow coastal plain is

marked by deeply-carved bays separated by prominent rocky points that reach seaward.

The embayments of Malaekahana and Laie Bay consist of relatively pristine sandy beaches,

while those farther south at Laie and Hauula have been severely altered by development and

suffer from chronic erosion. This is a strong trade wind environment, and eolian features,

both modern and relict, are pervasive throughout the region. Small emerged carbonate islets

formed of lithified sandstone (eolianite) lie offshore of Kalanai and Laie Points. The shoreline

is characterized by an extensive fringing reef complex associated with a broad, shallow, and

generally smooth reef flat. Near Hauula the beach is highly developed with seawalls and revet-

ments, many of which are aged and appear to be failing

in their attempt to protect seaside properties. Vertical

relief along this coastline is relatively low, but does

increase slightly towards the south and near the rocky

outcrops of Makahoa and Laie Points. Several streams

that empty the Koolau Range transport upland sedi-

ments to the coastal zone during intense rainfall events.

This is reflected in the beach sediment composition.

Chronic erosion and beach loss south of Laie Beach

Park is responsible for the difference in the Overall

Hazard Assessment (OHA) of moderate (4) for the

northern half of this region and moderate to high (5) for

the southern portion. Tsunami flood heights of 27 and 23

ft  associated with the 1946 and 1957 tsunamis were

recorded just south of Kahuku. Flash floods and over-

flowing streams in this region are common. For example,

in 1929, 11 in of rain falling in 11 hr brought floods to the

lowlands, and on March 31, 1965, 4.5 in of rain was

recorded in 1.5 hr at Punaluu! For these reasons, a high

ranking is given to both the tsunami and stream flooding

hazards. High waves become a significantly lesser threat

south of Kahuku, as the coast becomes increasingly shel-

tered from winter swell. As a result, it is ranked moder-

ately low.

The hazard due to storms, however, is ranked moderately high south of Kalanai Point and

moderately low to the north where the coast is more sheltered because storms usually

approach from the east-southeast and have brought high winds and waves to this coast in the

past. Erosion is significantly greater in the south near Hauula and Punaluu where shore-hard-

ening structures, installed to stop coastal erosion, have exacerbated beach loss. Reflected

waves off seawalls, revetments, and groins along Hauula and Punaluu often splash up onto the

seaside properties and the coastal road at high tide, removing what little sand remains.

Erosion of the embayed beaches in the north is ranked moderately low to low because it is

partly mitigated by long rocky points that provide protection from extreme wave energy. Sea-

level rise and volcanic/seismic threats are ranked moderately low along the entire region, as

they are in most portions of northern Oahu where they are relatively low compared to other

areas in Hawaii.

63

Laie

Laie

Narrow beaches are being rapid-

ly lost to sediment difficiencies,

sea-level rise, and waves and cur-

rents along the low coastal plain

between Hauula and Laie.

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Kahana

D/B

fr

S

D/B

frw

D/B

fr

B

e

w

/Sew

Rfr

D/B

fr

S

D

/B

frw

Rfr

Hazard 

Ty

pe:

G          CS                    

T     SF   W     S     E     SL    V/S                 OHA

Island of Oahu

Coastal Hazard Intensity

21°34'N

Hazard Intensity:

EXPLANATION

[For explanation of hazard types, see

 “Notes on Specific Hazards” in the Introduction]

Overall Hazard Assessment  (OHA)

G - Geology:

fr-fringing reef;  br-barrier reef;  e-embayed coast;  w-wetland

B-Beach;  S-Stream;  R-Rocky;  H-Headland;  D-Developed

Low - 1       2          3        4 - High

<20% -1      2       3 - >45%

W - High Waves

SF - Stream Flooding

S - Storms

E - Erosion

SL - Sea Level

V/S - Volcanic/Seismic

- No Data

T - Tsunami

CS - Coastal Slope

1         2          3         4          5         6          7

Low

High

Base Credit:  USGS 1:50,000 Laie, Hawaii 5421  III  W733  Edition 1-DMA

Prepared in cooperation with Hawaii Coastal Zone Management Program,

Hawaii Office of Planning and the National Oceanic & Atmospheric Administration

157°53'W

SCALE:  1: 50,000

Contour Interval = 40 meters

Cartography by Manoa Mapworks, Inc.

0

0

0.5

1 mile

0.5

1

1.5 km

K

ahana

1

2 2 2 2 4 1 2

5

64

Kahana

T

he Kahana coastline is dominated by low-lying, narrow beaches with broad shallow

fringing reefs extending offshore. This region is extensively developed and its narrow

beaches suffer from chronic erosion and are nearly devoid of sand. Seawalls, revetments, and

groins have been constructed along Punaluu and Kaaawa to protect coastal property at the

expense of the sandy beach that provides a natural buffer for active shorelines. A broad reef

flat extends seaward to a well developed fringing reef complex. Streams transport surface

runoff and upland sediments to the coastal areas at Punaluu and Kalae Oio Beach Parks.

Steeper rocky ramparts and basalt boulder piles surround Kahana Bay. The beach at the

mouth of Kahana Stream is the widest in the region and consists of very fine mixed volcanic

and carbonate sand that grades into carbonate-dominated sand offshore. A wetland exists at

the stream mouth and fish ponds are still visible along the southern flanks of the bay. After

heavy rainfall, Kahana Stream transports enough upland sediment to create a large plume of

suspended silt that extends into the center of the bay.

The Overall Hazard Assessment (OHA) is moderate to high (5) north and south of Kahana

Bay, where the lower coastal slopes are more susceptible to tsunami inundation. Inside

Kahana Bay, the OHA is moderate (4) while on the north side it is low (3) and along the south-

ern side out to Mahie Point it is very low (2), due to the reduced threat of waves and flooding

along the steeper rocky cliffs that flank the bay sides. The tsunami and stream flood hazards

are high along the Kaaawa coastal zone and at the mouth of the Kahana Stream, where the

coastal slopes are low. On both

sides of the bay, these threats

are reduced by the presence of

steep rocky headlands. The

stream flooding hazard is high

due to stream floods and flash

floods that frequently inundate

the coastal plain. For example,

in 1963, 19 in of rain in a 24 hr

period brought torrential

floods to the lowlands. The

high wave threat is ranked

moderately low throughout the

region because it is relatively

sheltered from winter swell.

High winds and waves associ-

ated with hurricanes and trop-

ical storms as well as intensive

trade wind storms passing

through the area, however, have caused considerable damage along this coast. In fact, the high-

est winds on Oahu during Hurricane Iwa (1982), which passed to the west of the island, were

recorded along this portion of coast and are thought to have been accentuated as they

descended over the steep windward cliffs of the Koolau Range.As a result, the storm hazard is

moderately high, except at the steep headlands surrounding Kahana Bay, where it is moder-

ately low. Erosion is ranked high along the entire coast except at the beach inside Kahana Bay,

where it is downgraded to low. Sea-level rise and volcanic/seismic hazards are moderately low

throughout the region as they are in most of the northern half of Oahu.

Very little sand remains along 

the narrow beaches of Kaaawa.

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