In the discussion that follows, developed by AECOS Inc. and Oceanit Laboratories Inc. as part of a Total Maximum Daily Load (TMDL) stream assessment study, the 48.4 square-mile basin is divided into seven hydrologic units termed sub-basins. Each sub-basin is, in effect, a small watershed acting as a point source to Waikele Stream, or in some cases, from one reach of the stream to a lower one. The sub-basins are basic units in developing a hydrologic model for the TMDL calculations. Sub-basin boundaries are determined from topography and known constructed drainageways that may alter the normal path of runoff. Sub-basins are chosen, in most cases, as that part of the watershed above an established water quality station. In that way, the quality and quantity of discharge from a particular sub-basin can be attributed to data obtained at the WQ station. It is the purpose of this section of the report to describe the reaches and branches of Waikele Stream as well as watershed that each drains.
Photograph 1.
Wai`anae Mountain lies to the west of the great central plain of O`ahu. The middle part of this plain has been a pineapple growing region for nearly a century. And the lower slopes have supported sugar cane for even longer.
Photograph 2.
Ko`olau Mountain lies to the northeast of the great central plain of O`ahu. Here, looking across Waikele Gulch and up into Waikakalaua Gulch, the central plain appears not to be very flat.
The upper slopes are forested for the most part, and heavily utilized by the U.S. Army for training purposes. The most common trees here are Christmasberry (Schinus terebinthifolius), Formosan koa (Acacia confusa), African tulip (Spathodea campanulata), ironwood (Casuarina equisetifolia), Moluccan albizia (Paraserianthus falcataria), kukui (Aleurites moluccana), silky oak (Grevillea robusta), monkeypod (Samanea saman), and Chinaberry (Melia azedarach). A little lower down, several types of eucalyptus trees are most abundant: swamp mahogany (Eucalyptus robusta), paperbark (Melaleuca sp.), and blue gum (Eucalyptus sp.). Wherever land slope is of sufficient low relief to support agriculture, pineapple (Ananas comosus) dominates the landscape (see Photo 1).
These upper and upper-middle reaches of Waikele Stream comprise Sub-basin 5. Two sampling locations provide water quality data for this sub-basin. Both are below some agricultural and military uses of the land. TMDL Sta. 5b was located downstream of a culvert structure, part of an abandoned road crossing, on Wheeler Army Airfield not far from Kunia Road. Although the stream bed retains pools of water, flow in this part of Waikele Stream is intermittent. TMDL Sta. 5a was also located on Wheeler, downstream of 5b, at a culvert beneath the road to the Base stables. This part of Waikele Stream lies at the bottom of a wide, 40-foot deep gulch, in most places overgrown by Guinea grass (Panicum maximum), with a shrub/tree canopy dominated by Christmasberry and koa-haole (Leucaena leucocephala; see Photo 3).
A short distance further on, the Waikele stream bed meanders within a broadened gulch floor and disappears into a "wetland" overgrown with tall grasses. The stream bed then descends into the deeper gulch of Waikakalaua Stream, which brings perennial flow to the system. Waikele Stream is therefore perennial (although interrupted) downstream of the confluence. The stream occupies a steep-sided gulch incised into the Central O`ahu Plain. The bottom of the gulch is a Military Reservation. The more even ground beyond the margins of the gulch was formerly in agriculture (pineapple and sugar cane, Saccharum officinale), but is now mostly fallow, with Mililani Town developing across the eastern side. This area of existing and former agriculture, sub-urban development, and military reserve land comprises Sub-basin 3. The middle reach of Waikele Stream here was sampled at TMDL Sta. 3, located within the now abandoned Lualualei Naval Ammunition Depot, Waikele Branch upstream of the Kipapa-Waikele confluence (see Photograph 7 below).
|
Photograph 3. Waikele Stream as it passes through Wheeler Army Airfield. The stream bed is a mixture of rocks and silty-sand deposits and was realigned by extensive construction within the gulch during the 1940's. This photograph was taken just upstream of TMDL Sta. 5a. |
Southward from the prominent ridge spur of Pu`u Kanehoa (seen in the near background, center to left side of photo 1) numerous normally dry gulches drain the heights to as far south as Pu`u Kaua. Gulches located yet further south are tributary to Honouliuli Stream and West Loch of Pearl Harbor. These gulches, including Manuwaiahu, Huliwai, and Ekahanui, feed into Poliwai Gulch. These slopes, in the rain shadow of the Ko`olau mountain, receive only about 40 to 60 inches of rainfall a year, most in seasonal storms, so the streams are all intermittent, as is Waikele Stream all of the way to its confluence with Waikakalaua Stream.
Photograph 4.
Scene typical of the central O`ahu Plain: pineapple planted on the low-sloping ground with a tree-filled gulch dissecting the adjacent landscape. Where properly graded, there exists little opportunity for run-off directly into the gulch from the fields. Increasingly, fields are being converted to roads and houses.
These normally dry gulches that feed into Poliwai Gulch (including Ekahanui) constitute Sub-basin 2. This sub-basin includes steep slopes of the Wai`anae and extensive areas of pineapple cultivation (see Photograph 4) along Kunia Road. Ekahanui Gulch drains through the middle of the Hawaii Country Club golf course. Sampling (TMDL Sta. 2) for water quality was established where Poliwai Gulch joins Ekahanui Gulch, just above the opening into Waikele Gulch. However, water flow was not observed here on any visits.
The upper reaches of Waikakalaua Stream drain Sub-basin 8, having an area of __ ac and extending from the Ko`olau crest to TMDL Sta. 8 at an elevation of about 720 ft (220 m). This sampling site was located within a new construction area (minimal grading completed) for "Launani Valley" above the end of Wikao Street. It is noteworthy that this sub-basin is very long and narrow, representing one of numerous gulches that drain these slopes, separated in many instances by equally narrow ridges (interfluves). Waikakalaua is squeezed between drainages for Kaukonahua Stream and tributaries to Kipapa Stream (see below). Kaukonahua drains into Lake Wilson (Wahiawa Reservoir) and eventually to the north shore of O`ahu.
Sub-basin 8 is essentially the heavily forested, moderately to deeply dissected slopes of the leeward face of the Ko`olau. Below this sub-basin is Sub-basin 6, comprising the lower, remainder of the Waikakalaua drainage to it's confluence with Waikele Stream at an elevation of about 570 ft (175 m). Sampling for this sub-basin was accomplished at TMDL Sta. 6, located within Wheeler Army Base a short distance downstream of Kamehameha Highway bridge over this stream.
Photograph 5.
Waikakalaua Stream in the vicinity of TMDL Sta. 6. The stream is rapidly eroding its banks in this area, perhaps aided by the small side-channel shown on the right which drains Mililani Town in the vicinity of Kamehameha Highway and Waimaku Drive. Note that the tree growing at the confluence has it's roots exposed to a depth of over 4 feet.
Sub-basin 6 ( _ ac) comprises low-sloping ground of the Central O`ahu Plain cut by the steep-sided and narrow gulch (800-1000 ft wide) of Waikakalaua Stream. Housing developments occupy much of the floor of this part of the gulch, presumably because the surrounding lands above the gulch were, until recently, highly valued for agriculture. In more recent years, sub-urban Mililani Town has spread northwards towards the gulch.
The upper reaches of Kipapa Stream, with numerous tributaries in narrow, steep-sided gulches draining forested lands, comprises Sub-basin 7. The following description by Ron England (ETI, 1993) evokes a useful image of the uppermost reaches of these central Ko`olau streams:
|
The stream[s] begin as a series of steep hanging valleys that coalesce at approximately 518 m [1700 ft] elevation into sinuate, rock and cobble channel. The channel follows an extremely convoluted course through a catchment composed of heavily forested razorback ridges, until emerging into the benchlands of central Oahu...
... In its far upper reaches the Kipapa Stream consists of scattered pools connected by subsurface flow that emerges occasionally in areas of bedrock exposure. Beginning at approximately 381 m [1250 ft] elevation permanent flow appears, consisting of shallow riffle/runs connecting long, deep pools. |
The riparian vegetation along the upper reach of Kipapa is decribed (ETI, 1993) as dominated by guava (Psidium cattleianum), `ohi`a (Metrosideros polymorpha), and koa (Acacia koa) with a thick understory of ferns. At somewhat lower elevation (244 m or 800 ft), `ohi`a, guava, ginger (Hedychium coronarium), ti (Cordyline fruticosa), kukui, and Christmasberry predominate. Mass wasting (hillslope landslides) was evident on the steep slopes, potentially contributing to substantial inputs of soil and rock during storms.
The ETI report suggests Kipapa Stream is interrupted (flowing only part of the time) from about the 700 ft (210 m) elevation to and beyond the confluence with Waikele Stream owing to agricultural and domestic withdrawals. This would seem to be more a phenomenon resulting from drought conditions, perhaps influenced by groundwater withdrawals, as there are no stream diversions in upper Kipapa.
The area of Sub-basin 7 is _ ac. The sub-basin delineation ends at TMDL Sta. 7, along Kipapa Stream at the 450 ft (137 m) elevation, just upstream of the H-2 viaduct crossing. Some crop-farming occurs along the floor of the gulch above the TMDL Station (mostly papaya and banana). Former agricultural lands now developed for housing as Mililani Mauka may contribute some drainage to the gulch in Sub-basin 7, but the development drains to Kipapa mostly downstream of the TMDL Station (that is, into Sub-basin 4).
Photograph 6.
Kipapa Stream in the deep gulch beneath the H-2 viaduct. Papaya and bananas dominate field crops in the gulch. The open canopy allows large grasses (here Guinea grass and elephant grass) to grow along the banks of the stream. Moluccan albizia (trees in background with white trunks) are especially abundant in this part of O`ahu. View looking upstream from TMDL Sta. 7.
Downstream from the H-2 viaduct flow may be interrupted or occurring beneath the bed material during the dry season (ETI, 1993). A large chicken farm is located in the gulch upstream from Kamehameha Highway. The riparian vegetation in this area is dominated by grasses, with some hau (Hibiscus tiliaceus, mango (Mangifera indica), and kukui trees present.
Sub-basin 4 (_ ac) encompasses much of Mililani Town, a suburban housing development located on former agriculture lands between Waikele and Kipapa gulches. Next to Sub-basin 1, this sub-basin is the most urbanized of all the Waikele watershed sub-basins. Much of the gulch itself is reserved as military lands, which may have last seen considerable activity and development in the 1940's. Lands east of the gulch are abandoned sugar cane fields, as are those to the south of Mililani Town (west side of Kipapa and east side of lower Waikele Gulch). Diversified agricultural cropping is slowly appearing on these former cane lands. The lower-middle reach of Kipapa Stream was sampled at TMDL Sta. 4, located within the now abandoned Lualualei Naval Ammunition Depot, Waikele Branch upstream of the confluence with Waikele Stream.
Photograph 7.
Road ford with culvert across lower Waikele Stream upstream of the confluence with Kipapa Stream. TMDL Sta. 3 was located on the upstream side of this structure.
Sub-basin 1 comprises the drainages to Waikele Stream below the confluence of Waikele and Kipapa streams. Despite the significant size of the eroded canyon, Waikele Stream itself is in places no more than a few yards (meters) across. Of the two streams at the confluence, Waikele stream appears smaller and of less permanent flow than Kipapa. However, the stream bed below the confluence enlarges downslope from the combined flows. There is an obvious disparity between base flow conditions and infrequent flood flow conditions that must discharge substantial amounts of water. Immediately below the Naval Reservation an area belonging to Oahu Sugar Company where there are pump stations and several large springs. The ETI (1993) report noted that these springs contributed most of the base flow to lower Waikele Stream.
Sub-basin 1 was sampled at TMDL Sta. 1 located immediately upstream of the estuary at the USGS gaging station at Farrington Highway (Photograph 9). Although Waikele Stream passes through Waipahu, relatively little of this urban center drains to the stream. Most of Waipahu drains towards other channels that flow into Pearl Harbor.
Photograph 8.
Waikele Stream above the USGS gaging station weir is a large shallow pool. The banks of the channel are steep and eroding through soft material, but are lined with trees and elephant grass (Pennisetum purpureum). This view is looking upstream from TMDL Sta. 1.
Photograph 9.
Upper end of the estuary of Waikele Stream beneath the Farrington Highway bridge. The channel is concrete-lined for nearly 900 ft, then lined with red mangrove (Rhizophora mangle), extending well out into the West Loch of Pearl Harbor on an extensive stream mouth deposit. This view is looking downstream from TMDL Sta. 1.
