Usgs geologic Investigations Series i-2761, Oahu
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- A bird’s-eye view of Mokapu Peninsula, home to the Kaneohe Bay Marine Air Corps Station.
- Waimanalo T
- Fine, white sands border the nearly perfect cresent of Waimanalo Bay.
- The Koko Head coast is largely rocky, except for the central portion sur- rounding Sandy Beach.
- Very narrow beaches are slowly being lost to chronic erosion and shoreline hard- ening along Maunalua Bay
S outh of Kualoa Point, the coast is deeply embayed and bordered by isolated beaches of gravely mud, fish ponds, small mangrove forests, and shallow deltas of upland sediment deposited by the many rivers emptying the Koolau Range. This low-lying coastal plain is especially prone to, and has experienced, considerable damage from flash floods generated by intense runoff from the very steep slopes of the Koolau Range. In November of 1970, 11.5 in of rainfall were recorded in 4 hr in Kahaluu. Near Kahaluu, where Kaneohe Bay reaches its greatest depths, the fringing reef lies far offshore and is, in form and function, Hawaii’s only barrier reef. A very shallow and broad subtidal shelf of fossil reef rock extends nearly a mile offshore, just seaward of the barrier reef.A prominent sand bar,Ahu o Laka Island, in the cen- tral portion of the coast parallels the shore and becomes exposed at low tides. Mokolii (Chinaman’s Hat) Island, an erosional remnant of Koolau basalt, lies offshore near the sea- ward edge of the barrier reef. Wave energy at the shoreline is generally very low, but along the outer reef it is higher and dominated by trade wind swell, and under moderate storm condi- tions such as during the distant passage of Hurricane Fernanda in 1993, waves overwash the coastal road along the Kualoa Point. The Overall Hazard Assessment (OHA) for Kaneohe is moderate to high (5) except for the short stretch of coastline sheltered behind Kualoa Point that is ranked moderate (4). Tsunami activity in Kaneohe has historically been very low. It is ranked moderately high in the north- ern portion where Kaoio and Kualoa points are exposed to swell. South of Kualoa Point the tsunami threat is ranked moderately low where the coast is more sheltered. The low-lying Kaneohe Coast has had 20 major stream floods since 1936. As a result, the stream flooding hazard is ranked high throughout the area. The hazard due to high waves is ranked moder- ately low throughout the area, because it is largely reduced by the barrier reef complex and broad reef flat bordering the shoreline. The threat of storms along this low-lying, east-facing coastline is ranked high, because it faces tropical storms and trade wind storms that approach from the east. Erosion threatens the sparse sandy beaches that remain north of Kualoa Point and hence has been ranked high there. Little data exists on erosion rates within Kaneohe Bay, where development has changed the shoreline, and so a ranking has not been given. Hazards associated with sea-level rise are relatively low along this entire coastal segment. The volcanic/seismic hazard is moderately high in the south, due to its proximity to the Molokai Seismic Zone, and moder- ately low in the north. 65 Kaneohe Kaneohe
A sand spit extending west from Kualoa Point functions like a barrier island to the Molii Fish Ponds. 66 Mokapu T he Mokapu Peninsula, site of the Kaneohe Bay Marine Corps Air Station, separates Kaneohe Bay from Kailua Bay. It was formed by a group of secondary volcanic eruptions that produced Ulupau Crater, Puu Hawaiiloa, and Pyramid Rock sometime since the middle Pleistocene. The shoreline is steepest at the rocky headland where cliffs seaward of Pyramid Rock and Ulupau Crater are pounded by high waves. A long beach lies at the north end of the peninsula, and some active dunes exist above isolat- ed basalt boulder outcrops. Several fish ponds occupy the low-lying isth- mus of the peninsula, which may have been submerged by a higher rela- tive sea level during the late Holocene, isolating the headland from the rest of Oahu making Mokapu an island. The shoreline is low and flat along the Kaneohe side and heavily developed with seawalls, fish ponds, seaplane ramps, and dredged channels. A basaltic outcrop, Moku o Loe (Coconut Island) and an emerged limestone platform, Kekepa Island, lie within Kaneohe bay along the northwest side of Mokapu Peninsula. The Kailua side of the peninsula consists primarily of a small rocky limestone escarp- ment between Kapoho and Kii Points with some beachrock and perched beaches. The north end of Kailua Beach begins at the mouth of the Kawainui Canal, which drains the Kawainui Marsh. An extensive fring- ing reef borders the entire coastline. The Overall Hazard Assessment (OHA) north of Pukaulua Point on Mokapu Peninsula is moderate to high (5) reflecting the low coastal slope and proximity to the drainages of the Koolau Range. The OHA is reduced to moderate (4) for the short stretch near Pukaulua Point where the coastal slope steepens, and is further reduced to moderate to low (3) at the rocky Mokapu Point where the coast is steepest and farthest removed from potential stream flooding originating in the Koolau range. A 21 ft runup, recorded at Mokapu Point, was generated by the tsunami of 1946. However, because the barrier reef complex and broad reef flat of Kaneohe Bay help to dissipate high wave energy, the tsunami hazard is ranked moderately low north of Pyramid Rock. At the low-lying beach on the north edge of the peninsula, tsunami flooding is ranked high, and at the rocky headland at Mokapu Point where the slope is steeper, it is ranked moderately high. Stream flooding is ranked high along the low coastal plain of Kaneohe, but is reduced to the south, where the Mokapu headland becomes removed from the drainage of the Koolau Range, and along the steep slopes at Mokapu Point. The wave hazard is ranked moderately low along the entire portion of Mokapu because it is relatively sheltered from winter swell. Combined with high winds, the storm hazard is ranked high throughout the region except at the steep, rocky Mokapu Point, where it is moderately high. Erosion data is unavailable for the Kaneohe coast. However, along Mokapu Point, erosion is ranked moderately low because of the hard rocky headland. The threat of sea-level rise is moderately low along the Kaneohe coast and low at the steep rocky point. The volcanic/seismic hazard here resembles that of southern Oahu, which is ranked moderately high in accordance with its proximity to the Molokai Seismic Zone. Mokapu
A bird’s-eye view of Mokapu Peninsula, home to the Kaneohe Bay Marine Air Corps Station. S outh of Mokapu Point to Kapoho Point, the rocky volcanic headland gives way to emerged reefrock and eolianite, and then to the long, sandy beaches of Kailua, Lanikai, and Waimanalo. Canals that drain the interior valley, separate these beaches from isolated outcrops of basalt and emerged limestone. The densely developed coastal plains of Kailua, Lanikai, and Waimanalo are gently sloping and believed to have been submerged by a higher relative sea level between 1,500 and 4,000 years ago. Relict and modern dune ridges and fos- sil beaches compose most of the coastal plain at Kailua and Waimanalo. Popoia Island, an emerged limestone islet, lies along an extensive fringing reef complex offshore. The wave energy here is moderate.A substantial proliferation of seawalls and revetments south of Alala Point has accelerated beach loss in Lanikai, no doubt by impounding sand. Revetments, emplaced in front of the airfield at Waimanalo, also appear to have exacerbated beach loss on the north side of Waimanalo Beach and possibly led to impacts in south Lanikai. The Overall Hazard Assessment (OHA) along the Kailua coast reflects the variable nature of alternating steep, rocky outcrops, stream mouths, and low-lying sandy beaches. The OHA increases from moderate to low (3) along Mokapu Point to moderate (4) between Kii Point and Fort Hase Cove where stream flooding is moderately low. Along the low-lying sandy beaches of Kailua and Lanikai, where stream flooding and erosion are ranked high in places, the OHA is moderate to high (5), except at Alala and Wailea Points where it is moderate to low (3) due to the lower tsunami, stream flooding, storm and erosion hazards at those steep, rocky headlands. South of Wailea Point, the OHA is ranked high (6) due to the high tsunami hazard along Waimanalo. Tsunami inundation along the Kailua waterfront historically has not been as high as in Waimanalo, and as a result, the tsunami hazard is ranked moderately high along Kailua and Lanikai exept at the Alala and Wailea Points and high in Waimanalo. Stream flood- ing is ranked low at the north end of Mokapu Point, moderately low near the south side of Mokapu Point, and high along the low-lying developed coastline of Kailua, Lanikai, and Waimanalo, where prolonged and flash flooding are frequent, year-round occurrences. During the winter of 1987, for example, a slow-moving storm brought about 2–5 ft of flood- ing to Kawainui Marsh (just west of the town of Kailua) and surrounding area, while in 1970, over 11 in of rain fell on Waimanalo in only 4 hr. The Kailua coast, facing east and protected by the extensive fringing/barrier reef offshore, is relatively sheltered from winter north swell, so the high wave hazard is ranked moderately low. Erosion is low and moderately low along the rocky outcrops. It is also moderately low in the central portion of Lanikai, where the beach is presently accreting at the expense of beach loss along its margins. At stream mouths and especially along the northern and southern ends of Lanikai, where there is chronic beach loss, the erosion hazard is ranked high. Sea-level rise is ranked moder- ately low here as it is along most of Oahu’s shores, where rates of rise are moderate. The volcanic/seismic threat is ranked moderately high in Kailua, due to its proximity to the Molokai Seismic Zone. 67 Kailua Kailua
Chronic beach loss along north Lanikai since the 1980’s has left the shoreline lined with seawalls. In the distance, a wide beach at the Kailua Beach Park is a favorite island playground. 68 Waimanalo T he long Waimanalo Beach, backed by vegetated dune ridges, lies on a developed coastal plain that narrows south toward Kalona Beach Park.There it abruptly changes to a rocky limestone coast with isolated basalt boulder deposits and small pocket beaches south to Kaupo Beach Park. A large basalt flow extends towards Kaohikaipu Island (Rabbit Island), just north of the last sandy beach found on the windward coast at Makapuu Beach Park. The relatively low topography at Waimanalo abruptly steepens at Makapuu where the southern end of the Koolau Range abuts the shore and culminates at Makapuu Head. The wide fringing reef of Waimanalo works to effectively dissipate trade wind wave energy. The fringing reef is absent at Makapuu Beach enabling larger waves to reach the shore there. Stream flooding poses one of the greatest threats along the Waimanalo coastline, where intense rainfall over the steep Koolau Range may generate high rates of surface runoff that collects on the low coastal plain. In November of 1970, for example, 11.5 in of rain fell in only 4 hr, and in March of 1958 13.8 in of rainfall were recorded in 24 hr, flooding Waimanalo with 3 ft of standing water. To the north, beach loss occurs at the 800 ft-long Bellows revetment, and along the southern shore where coastal properties are protected by seawalls, beaches suffer sand deficien- cy and chronic erosion. The Overall Hazard Assessment (OHA) varies between high (6) along Waimanalo Beach, where erosion and stream flooding are highest, to mod- erate to high (5) south of Kalona Beach Park, where erosion and stream flooding are moderately low. The OHA is further reduced to moderate to low (3) south of Makapuu Point, where few streams exist and runoff is low. The tsunami hazard for the Waimanalo coast is high along the low-lying embayment south to the headland at Makapuu Point. The storm hazard is also ranked high along this portion of the coast, where hurricanes, such as Hurricane Kate in 1976, have brought waves as high as 15 ft to these shores. Winds up to 82 mph, associated with Hurricane Iwa in 1982, were also recorded in the Waimanalo area. Trade wind gales are also common, though not annual events. Stream flood hazard is high north of Kaupo Beach Park, where flash flooding such as in March of 1958, can inundate the town of Waimanalo with several feet of water. It is moderately low between Kaupo Beach Park and Makapuu Point and low south of the point. The threat of high waves is moderately low in the northern portion, which is partially sheltered from winter swell and protected by the exten- sive fringing reef offshore. To the south of Kaupo Beach it is ranked high, due to its greater exposure to trade wind swell and both winter and south- ern wave energy that refracts towards Makapuu Beach. Erosion is high to moderately high throughout the beach system of Waimanalo, where retreat of the coast is intense, but moderately low south of Kaupo Beach Park where it is mitigated by the rocky basalt coastline. Sea-level rise, while ranked only a moderately low hazard for Waimanalo and Makapuu Beaches, is ranked even lower for the rocky, resistant headlands at Kaupo Beach Park and Makapuu Point. The volcanic/seismic hazard is ranked moderately high being so close to the Molokai Sesimic Zone. Waimanalo
69 Koko Head T he steep and rocky headland of Makapuu Point (see Waimanalo map) flattens out westward to a coastal terrace but remains rocky and partly covered by perched beaches and dune fields before reaching Sandy Beach Park. West of Sandy Beach to Hawaii Kai, the coastline is steep and rocky, with cliffs and small embayments. Sandy Beach is the most prominent beach system in this region and maintains a small sand field offshore. Sandy Beach is famous for its intense shorebreak. The shore- line northeast of Sandy Beach consists primarily of basaltic boulders and volcanic outcrops but to the west is bordered by a low-lying bench that contours the coast through Hanauma Bay. Above lie steep cliffs of Pleistocene volcanic tuff deposits. Several blow holes near Holona Point provide exciting views of water plumes that are sent billowing high into the air as waves force water through naturally formed cracks in the basalt and tuff. A broad reef flat inside Hanauma Bay offers divers access to the most beautiful and perhaps most sensitive wildlife spectacle on Oahu. Around Koko Head, the Portlock coast is heavily developed along steep cliffs and is devoid of sandy beaches below. Seawalls and minor groins exist along a sand spit at the mouth of Kuapa Pond and the Hawaii Kai coastline. The patchy fringing reef that exists along the Sandy Beach coast, fades away seaward of Hanauma Bay and Koko Head, but is well-estab- lished in Maunalua Bay. The Overall Hazard Assessment (OHA) for the majority of this coast- line is ranked moderate to low (3), due to the steep and resistant nature of the cliffs and headlands that flank the south shore. However, at the low- lying area of Sandy Beach the OHA is elevated to moderate to high (5), where the tsunami, stream flooding, and storm hazards are higher. Between Kaloko and Puu o Kipahula, and just west of Kawaihoa Point, the OHA is moderate (4). The tsunami and stream flooding hazards are high and moderately high, respectively, for the short stretch of low-lying coast between Kaloko and Halona Points. Elsewhere, the coastal slope is suffi- ciently high that the tsunami hazard is moderately low and the stream flooding hazard is low. This coastline borders the 35 mi wide Molokai Channel that is known for its rough sea conditions throughout the year. Winds and waves traveling through the channel are often intensified as they accelerate between Oahu and Molokai. While wave heights do not often surpass 10 to 12 ft, they can build very rapidly as they make landfall from the very deep water of the channel. This makes for intense wave ener- gy as waves stack up at the shore and collide with the steep Sandy Beach and rocky cliffs to the west. The threat from waves is high throughout this coast as it receives substantial swell in winter and summer. The storm haz- ard is greatest for the low coastal areas near Sandy Beach and west of Koko Head, where it is ranked high. For the rocky cliff coastline between, it is moderately high. Erosion is moderately low throughout the entire region, as the rocky shoreline mitigates this threat. The sea-level rise hazard is moderately low throughout the region, except at the steeper headlands at Koko Head where it is reduced to low. The volcanic/seismic hazard here is ranked moderately high in accordance with its proximity to the Molokai Seismic Zone. Koko Head The Koko Head coast is largely rocky, except for the central portion sur- rounding Sandy Beach. 70 Diamond Head T he Diamond Head coastal zone stretches from Waikiki to Hawaii Kai, in the eastern portion of Maunalua Bay. One of Hawaii’s shallowest and widest reef flats exists along Maunalua Bay. Residential and commer- cial development is widespread along the low-sloping terrace comprised of emerged fossil coral reef. Diamond Head Crater and Kupikipikio Point (Black Point), formed by recent volcanic eruptions, are steep headlands that separate the beaches of Waikiki from the narrow sand and gravel beaches of Kahala. Intensive development and construction of seawalls, revetments, and groins along the Kahala and Niu coasts have been accom- panied by chronic beach loss. In many locations, the waterline at low tide coincides with the base of a seawall. Streams and drainage canals carry surface runoff and upland sediment from the Koolau Range and urban- ized valleys to the sea, in some places creating deltas of silt and volcanic sand. The Overall Hazard Assessment (OHA) for the Diamond Head coast- line is moderate to high (5) except along the low sloping coastal areas of Niu Valley and between Wailupe and Kahala which are most susceptible to flooding and wave damage, and are ranked high (6).At the steep headland of Diamond Head Crater, the OHA is moderate to low (4). The tsunami and storm hazards along the Diamond Head coast reflect the influence of topography. They are both ranked high except at Diamond Head, where they are reduced to moderately high. The stream flooding hazard is ranked high east of Kahala, where stream discharge in Niu Valley and Aina Haina have reached rates of 3600 ft 3 /sec. The saddle-like topography west of Waialae Beach Park, directs runoff either toward Honolulu through the Ala Wai canal or east toward the canal at Waialae Beach Park. This area is also far removed from the Koolau Range and the coastal slope is great, so stream flooding is ranked moderately low. The threat of high waves that annually reach this coast in spring and summer is ranked moderately high. Erosion is high along the entire coast except at the steep Wailupe and Kupikipikio Point headlands. The hazard of sea-level rise is moderately low here where it is experiencing low rates of rise compared to other areas in Hawaii. The volcanic/seismic hazard is ranked moderately high as it is along all of Oahu’s southern shores, due to their proximity to the Molokai Seismic Zone. Diamond Head Very narrow beaches are slowly being lost to chronic erosion and shoreline hard- ening along Maunalua Bay between Diamond Head and Hawaii Kai to the southeast. Document Outline
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