Usgs geologic Investigations Series i-2761, Oahu

Makua

58

Makua

T

he Makua coast from Makaha Beach Park to the remote and rugged Kaena Point, is

mostly rocky and narrow. Emerged Waimanalo-age carbonate reefrock borders the

coastline near Makaha while, beneath the steep headlands of Kaena, basalt boulders dominate

the coast. Features that appear to be alluvial fans or rock slides along the Kaena coast reveal

that these cliffs are actively eroding. A few isolated pocket beaches exist between these rocky

outcrops. At Makua, a platform of subtidal beachrock supports a perched sandy beach. The

low-lying coastal road, which was overwashed by storm waves from Hurricane Iniki, gives way

to a foot trail along the steep headland northwest of Kaena Point State Park. Scattered fring-

ing reefs surround the rocky points and small sandy embayments. Large sand fields offshore

of Makaha Beach Park merge with smaller elongated sand channels to the west. This region is

dominated by high wave energy from North Pacific swell, especially in the winter and spring,

however it is largely sheltered from active trade winds. The Makua coast is dominated by a

fairly arid climate and stream flooding, while not a perennial threat, does occur from time to

time.

The Overall Hazard Assessment (OHA) is moderate to high (5) along Keana Point State

Park beach , Makua beach, and south of Ohiki-Lolo where the threat of tsunami is high and

stream flooding is moder-

ately  high, except along

Kaena Point State Park

where it is moderately low

because of a lack of coastal

streams. The OHA is mod-

erate (4) along the steeper

headlands between Kaena

Point State Park and

Makua and near Ohiki-

Lolo where the tsunami

hazard is moderately high

and stream flooding is low.

The threat of high waves,

up to 20 and 25 ft on the

outer reefs, comes annually

during the winter and

spring, and as a result, is

ranked high. The Makua

coast is vulnerable to both Kona storms and hurricanes tracking to the west of the Hawaiian

Islands, and in the past has experienced considerable flooding, overwash, and wind damage

from these events.As a result, a high hazard ranking has been given to storms along this coast.

Erosion is ranked moderately low along the rocky cliffs in the western portion of Kaena State

Park and south of the park to Ohiki-Lolo. Erosion is ranked moderately high for the lower-

lying regions and beaches just inside Kaena Point State Park, and the entire shoreline south of

Ohiki-Lolo, except for the steep-sided and rocky Keaau Beach Park, where it is ranked low.

Sea-level rise is also a low to moderately low threat due to the rocky coastline. The

volcanic/seismic hazard is ranked moderately low here as it is for the whole northern half of

Oahu, from Makaha to Kaneohe Bay, which is removed from the Molokai Fracture Zone and

other known regions of seismicity.

Sandy beachs are separated by rocky headlands along the

Makua Coast.

---

59

Kaena Point

A

long the north coast, from Kaena Point to Mokuleia, broad inter-

tidal and subtidal wave-abrasion platforms are carved into

Waimanalo-age limestone, reflecting the long history of great wave activi-

ty along this shoreline.A low-lying platform of fossil reefrock is elevated 3-

6 ft above mean sea level and extends from Mokuleia to within 0.5 mi of

Kaena Point. Isolated sandy beaches are found at breaks in the rocky bench

and widen toward Mokuleia where they connect with small offshore sand

fields. Modern intertidal erosion cuts into the elevated limestone testifying

that wave energy and bioerosion are high at the shoreline. Offshore lies an

extensive fringing reef complex. At Kaena Point, a partially vegetated

Holocene dune field lies near sea level and is active in the trade winds and

southerlies. Efforts to reduce erosion along the Mokuleia shoreline by

implementing seawalls have essentially failed and instead have led to sub-

stantial beach loss.

The Overall Hazard Assessment (OHA) along the Kaena Point coast

increases from moderate (4) at the point to high (6) along the low-lying

sandy beaches of Camp Erdman and Mokuleia Beach, where the coastal

slope is lowest and chronic erosion is diminishing Mokuleia’s sandy beach.

Between Kaena Point and Camp Erdman, where stream flooding is great-

est, and along the hard limestone shoreline west of Mokuleia Beach, the

OHA is moderate to high (5). Tsunami and stream flooding are ranked

high along the lower slopes between Camp Erdman and Mokuleia, while

towards Kaena Point they are only moderately high because of the higher

coastal slope. Tsunami flood heights of 36, 30, and 22 ft were observed at

Kaena Point in 1946, 1952, and 1957, respectively. Stream flooding at

Kaena Point is reduced further to moderately low, as this region is far

removed from the drainage of the Kuaokala uplands. The hazard due to

high waves is high throughout this region on the North Shore, where win-

ter swells commonly reach heights between 20 and 25 ft. Kaena Point, at

the northwestern tip of Oahu, is vulnerable to Kona storms and high trade

winds, as well as hurricanes. As a result the storm hazard is ranked high

along the western half of the region, where hurricane force winds from

hurricanes passing along the west shore of Oahu may be encountered. It is

ranked moderately high toward the east, where the coast becomes shel-

tered from hurricane and Kona storm energy. Erosion is high along the

isolated sandy beaches of Camp Erdman and Mokuleia, but moderately

low along the hard limestone shoreline and rocky Kaena Point. The sea-

level rise and volcanic/seismic hazards are ranked moderately low.

Kaena Point

The rugged, rocky coast between Kaena Point and Mokuleia experiences

trade winds nearly 70% of the year.

---