Page 2 of 5
Geology and Hydrology
|
...basin form... The Kawai Nui Basin slopes 1 to 3 degrees seaward (Allen-Wheeler, 1981), ranging in elevation from 6 meters (20 feet) above to just below present sea level (Dames and Moore, 1961). The surface of the marsh lies at an average elevation of about 1.5 meters (5 feet) above sea level (Athens et al., 1992; Wilson Okamoto & Assoc., 1994). This sureface consists of a mat of vegetation and peat that ranges from 30 cm to over a meter (1 to 3 feet) in thickness, and is floating on less than 1 m to over 5 m (3 to 15 feet) of water and water-sediment slurry. This vegetation mat has a profound impact on basin hydrology and water quality.1 The marsh lies near the center of the ancient Ko‘olau caldera. Stearns and Vaksvik (1935) described throat brecceas out-cropping between Kane`ohe and the marsh and concluded that these were the site of the main vent of the Ko‘olau volcano. Adams and Furumoto (1965) showed with seismic profiles that the volcanic plug lies at three to four kilometers depth in this area.2 Changes in sea level have alternatively left the area submerged and emergent. During an emergent period, stream erosion destroyed the caldera and carved the fluted valleys (Stearns, 1966) now evident in the Nu‘uanu Pali. A higher stand of sea level resulted in the deposition of both consolidated and unconsolidated marine sediments. Re-emergence of the area to near its present level allowed for the formation of the barrier beach dunes (Stearns, 1935) that separated the marsh from Kailua Bay. The Coconut Grove area of Kailua now occupies a portion of this barrier.2 A summary of the geology of the entire area is found in Takasaki et al. (1969). Much of the history of the gradual transformation of Kawai Nui from a marine embayment (resembling present-day Kāne‘ohe Bay) to a vegetation-choked wetland is revealed in sediment cores. [MORE: CLICK HERE] ...The flow of water through the marsh... The largest natural water source into Kawai Nui Marsh is Maunawili Stream. A smaller drainage, Kahana‘iki Stream, feeds the marsh less than 1/6 of the water that enters via Maunawili Stream. Another small intermittent stream (Kapa‘a Stream) enters the marsh near the present location of the rock quarry. The original drainage into Kawai Nui Stream then to Kaelepulu Stream was blocked by construction of a long dike along the northeast edge of the marsh in the 1950s, but the canal to Kawai Nui Stream still remains on the makai side of the dike, serving as a drainage for the Coconut Grove area of Kailua. A wide channelized drainage ("Kawainui Canal" or Oneawa Channel) was constructed in the north corner of the marsh. The upper streams and remnant ponds in the marsh are fresh water, while the salinity of water within Oneawa Channel is affected by tidal influence. Patterns of water flow and circulation within the marsh are poorly understood. The amount of open water left in the marsh varies considerably with patterns of rainfall runoff. A large central pond, ranging in depth from three feet to more than ten feet in places, has remained partly open and free of floating vegetation. Heavy rainfall in early May, 1977 produced extensive flooding in the upper marsh, resulting in a substantial (more than 100%) increase in the amount of open water by clearing
|
vegetation. However, this and other small ponds in the marsh are covered to some degree by a mat of water hyacinth that opens or moves during periods of heavy rainfall or high winds.3
Kawai Nui Marsh receives drainage from an area of about 25 km2 within the Kailua watershed (Wilson Okamoto & Assoc., 1994). Most of the input is from the sub-basins of Maunawili Stream (14.5 km2), Kahana‘iki Stream (4.9 km2), and Kapa‘a Stream (3.1 km2). The total freshwater input to Kawai Nui Marsh is estimated at 6.8 mgd (25,700 m3/day) (Drigot et al., 1982) to 9.5 million gallons per day (35,900 m3/day) (Wilson Okamoto & Assoc., 1994), with 23,800 m3/day (6.3 mgd) discharged to Kailua Bay through the Oneawa Channel and 12,100 m3/day (3.2 mgd) lost to evapotranspiration.1
The basin acts as a flood control reservoir, ponding runoff from major rainfall events that is gradually discharged to Kailua Bay. The basin is also a sediment trap and sink for nutrients and pollutants, reducing negative impacts associated with runoff on Kailua Bay. Rapid urbanization of the Kailua watershed after 1966 increased soil erosion and produced higher rates of basin sedimentation, steadily decreasing the volume of the basin and water storage capacity. Wilson Okamoto & Assoc. (1994) calculated an average annual sediment yield from the Kailua/Maunawili watershed of 1049 short tons/mi
Once increased urbanization and higher population exceeded waste water infrastructure capacity in the 1960's, partly treated effluent was discharged directly into Kawai Nui Marsh. Nutrient loading stimulated rapid growth of exotic vegetation, negatively impacting water quality and expanding the floating vegetation mat through nutrient loading. Expansion of the vegetation mat resulted in the closing of open water channels and increased sediment trapping, further decreasing basin storage capacity. In addition, the vegetation mat retards flow through friction and damming, ponding water in the basin and retarding drainage. Reduced storage capacity and ponding may have contributed to flood waters topping the levee and flooding homes in the Coconut Grove area on January 1, 1988.1
Historically, a portion of Kawai Nui Marsh was a 450-acre fishpond kept clear of encroaching vegetation by the communal efforts of the ahupua`a residents. The marsh drained through wetlands along Kawai Nui Stream. Much of the the drainage from Maunawili and Kahanaiki was diverted into taro lo`i. Milkfish, mullet, aholehole and o‘opu were taken from the Kawainui pond and drainage. When the repetitive clearing of vegetation ceased, the natural process of ecological succession continued uncontrolled. Since that time, the original pond has shrunk to a fraction of its former size through sedimentation and encroachment of vegetation.3
A staff gauge set up in 2002 in an open pond at Nā Pōhaku o Hauwahine provided a record of water level in the marsh for a year. U.S. Geological Survey measurements of marsh and waterway water levels are also discussed there. Most pertinent for Kawai Nui was USGS 16264600 ("Kawainui Marsh nr levee sta 15+00"), apparently no longer maintained by USGS. |
|
 Where dat? |
|||
| Move to: [KO‘OLAU NET] | [AHUPUA‘A] | ||||
WebmasterThis webpage edited and reposted Mar. 30, 2011. Copyrights of cited text retained by original authors.