The TMDL Sub-Basins

To better understand the Kapa‘a Stream watershedA, the entire 1.1 square-mile (711 acre; 288 ha) basin is divided into hydrologic units termed sub-basins. Each sub-basin is, in effect, a small watershed acting as a point source to Kapa‘a Stream, or in some cases, from one reach of the stream to a lower one. The sub-basin boundaries are determined from topography and known constructed drainageways that may alter the normal path of runoff. In the Kapa‘a watershed, man-made drainages have altered this natural flow to such an extent that the original stream bed no longer exists in most places. A sub-basin map displays the boundaries of the 14 sub-basins. The following table gives some basic statistics for each sub-basin and the text below summarizes conditions and land uses in each.

Sub-basin  Area  Land Use (%)
1 %100000
2 %100000
3 %00
4 %100000
55.822.67.8 %600400
31.612.84.4 %100000
124.950.617.6 %69400
8 %810019
9 %000100
10 %001000
40.916.65.8 %100000
147.559.720.7 %780220
71.128.810.0 %100000

Sub-basin 1: Headwaters
This steeply sloped, 45-acre parcel is completely covered with a mesic scrub forest consisting primarily of octopus tree (Schefflera actinophylla). Slopes are generally steep, averaging 11 percent (11 feet vertical in 100 feet horizontal) and terminate along the H-3 Freeway. USGS topographic maps show the origin of Kapa‘a Stream arising in a steep gulch on the northwest side of the slope of Ulamawao, but no evidence of a stream bed can be found along this forested ravine. The bottom of this ravine and slopes close to the freeway support a variety of trees, primarily java plum (Syzygium cumini), Christmasberry (Schinus terebinthefolius), fiddlewood (Citharexylum caudatum), monkeypod (Samanea saman), and, in the lowest areas, hau (Hibiscus tiliaceus). Although more typical of the nearby Kane`ohe watershed slopes, some albizia (Paraserianthes falcataria) and African tulip (Spathodea campanulata) trees are present, as well as scattered ironwood (Casuarina equisetifolia). There exist only minimal bare soil slopes since cuts created when the freeway was constructed have long since overgrown with trees and grasses. As-built drawings for the H-3 freeway indicate a culvert draining the ravine into the stream channel adjacent to Sub-basin 6. However, sediment deposits since the construction of the freeway have apparently buried this conduit as no trace could be found on the lower or outlet side of the roadway fill.

Sub-basin 2: Upper Freeway
This sub-basin occupies only 35 acres on the very steep side-slope (slope averages 25%) of Ulumawao and is covered primarily by non-native mesic scrub (mostly octopus trees). The H-3 Freeway is the only developed portion of this sub-basin. A single __ inch drainage culvert under the freeway connects this sub-basin to Kapa‘a stream near the top of Sub-basin 7 where the stream disappears into a hau thicket. Small portions of this sub-basin may drain directly to the freeway right-of way and enter the stream through freeway drains at several locations between water quality Station 6 and the H-3 undercrossing drain at Station 5 [?].

Sub-basin 3: Ameron Quarry, Phase II
Ameron International quarry operations occupy both sides of upper Kapa‘a Valley. The Phase II quarry on the south side of the freeway comprises sub-basin 3 and covers about 35 acres. Runoff from surrounding up-gradient lands and within the active quarry area of sub-basin 3 is controlled on site through a series of drainage swales, holding basins, sumps, and pumps. This system is designed to capture the runoff from a 10-inch 24-hour storm completely and either retain on-site or pump to a pond at higher elevation in Sub-basin 7. This pond stores water for later use in dust control, irrigation, and as process water for plant operations. If runoff exceeds the volume of the retention pond, excess is drained into the Phase I quarry pit. Keeping all runoff internal to the quarry operations is a continual maintenance challenge, but a condition of Ameron's NPDES permit. For the purpose of this TMDL it is assumed that all drainage from quarry operations is kept internal to the quarry.

Sub-basin 4: Kapaa Landfill, Phase-III
Sub-basin 4 is a small (24 acre), steeply sloped (18 percent) basin draining directly to the H-3 Freeway. Foliage is largely introduced koa-haole (Leucaena leucocephala) and Guinea grass (Panicum maximum). Near the summit of Ulumawao there are patches of native forest consisting of a scrub form of `ohi`a lehua (Metrosideros polymorpha) and an abundance of `akia (Wikstroemia cf. oahuensis). Unfortunately much of this area is utilized by off-road vehicles causing severe erosion in some places. Runoff ends up in Kapa‘a Stream through a culvert near the Ameron entrance gate (WQ Station 4). A photo shows clearly the difference between runoff from the red-dirt eroding areas off H-3 and that emanating from the upper quarry road.

Photo by Eric Guinther (June 5, 2002)

Photo 1
Dr. S. Montgomery examines `ohi`a blossoms for native insects near the peak of Ulumawao

Sub-basin 5: Kapaa Landfill, Phase-II
This 56 acre parcel encompasses both the upper part of the City & County of Honolulu, Kapaa Landfill (Phase II) and relatively undisturbed slopes up to the ridgeline of Ulumawao at around 600 ft. elevation. Slopes average about 12 percent, but are highly variable due both to the natural terrain at the summit and man-made fill and terraces over the landfill. Drainage from this sub-basin flows to a circumferential drainage channel constructed around Kapaa Landfill Phase-III (Photo 2). Runoff in the drainage ditch is transported downslope to a drainage pipe, passes under Kapaa Quarry Access Road via a 48-inch culvert (outlet is WQ Station 3), and discharges into Sub-basin 8 over a concrete energy dissipation structure (see Photo 6, below).

Landfill swale (photo by Bob Bourke) gutter

Photo 2
Runoff from Kapaa Landfill, Phase II is captured by rock-lined swales and transported to a buried conduit that empties into Sub-basin 8 .

Vegetative cover over this sub-basin consists primarily of Guinea grass and other grasses planted on the landfill cap. Less disturbed slopes are dominated by Java plum and monkeypod trees. At the ridgeline there are patches of native plants such as `ulei (Osteomeles anthyllidifolia), `ilima (Sida fallax), and `akia. Slopes are high at landfill steps and in the upper reaches near the summit. Historical references describe a spring in this sub-basin, but no surface expression of this spring can now be found. Primary sources of pollution in runoff from this sub-basin would be sediment from the natural and landfill surfaces. Natural slopes are impacted by off-road vehicles, resulting in soil erosion.

Sub-basin 6: Upper Valley North
Sub-basin 6 consists of approximately __ acres along the top of the watershed on the north side of H-3 Freeway, adjacent to the active Ameron quarry. A graveled road (Photo 3) provides access from the quarry operations area to the hills and cliff face overlooking the quarry pit. This road is graded and maintained to capture all runoff from above, and is therefore functionally part of Sub-basin 7.

Looking towards Ulumawao from the saddle at the upper end of Kapa‘a Valley (photo by E. Guinther) gutter

Photo 3
This road, here close to the Kane`ohe Watershed slope near the head of Kapa‘a Valley, intercepts runoff from above and directs it into Ameron operations (Sub-basin 7). This redirection is necessary because overburden from quarry operations is stored further upslope.

Sub-basin 6 is only the narrow remaining part of the slope below the access road and including the "saddle" at the upper end of Kapa‘a Valley. Views from the watershed boundary are: to the south into the HPU campus (Kane`ohe watershed); and to the west into the National Veterans Cemetery (Kawa watershed). The gravel road is graded to intercept runoff from quarry operations (in this area, mostly overburden storage) during heavy rains, so all land above the road drains back into the quarry operations area along road surface and drainage ditches. During the early part of the TMDL study, the gravel road had become deeply eroded and storm water flows had broken through a berm separating it from adjacent Kapa‘a Stream. This was possibly the source of much of the gravel found in the stream bed during the first several storm events. With subsequent repair, the road now channels all runoff into Ameron's runoff control system. The downslope edge of the gravel road is planted with trees and irrigated to promote their growth. From this elevation down to the stream bed bordering the freeway the foliage consists of an overstory of Java plum and octopus trees above miscellaneous grasses and shrubs. No significant sources of erosion or pollutants are readily apparent from this sub-basin. A stream bed is, at best, weakly defined at the bottom of the ravine, and may not be the original bed of Kapa‘a Stream.

Sub-basin 7: Ameron Quarry, Phase I
This 125 acre sub-basin, along with Sub-basin 3, are hydrologically removed from the rest of the watershed by man-made drainageways. Sub-basin 7 includes all of the active (Ameron Phase I) quarry site at this location since about 1972 (the C & C landfill occupies the earlier quarry). The quarry pit represents the source of an estimated ___ tons of rock removed from during the past 30 years and provided substance to a majority of all concrete structures on the Island of O`ahu.

The bed of Kapa‘a stream lies adjacent to this sub-basin (along the boundaries with Sub-basins 2, 3, and 4). However, little or no runoff enters the stream from either Sub-basin 3 or 7. Runoff from these sub-basins is controlled completely by Ameron through a series of swales, ponds, and pumps designed to contain a 10-inch, 24-hour rainfall. Water received in excess of the containment-pond capacity is pumped into the deep main quarry pit. Consequently, the stream contains water flow only during relatively heavy rains. The sources of run-off to this reach are primarily valley slopes above (Sub-basins 1 and 6) and directed runoff from H-3 Freeway (in Sub-basins 1 and 2). The stream bed adjacent to Sub-basin 7 is deeply incised between the quarry road and the freeway, with bank plantings of Erythrina trees maintained by Ameron as a screen along the freeway.

photograph by Eric Guinther photograph by Bob Bourke

Photo 4
These settling ponds at Ameron Kapaa Quarry catch all runoff from roadways, product storage areas, and rock crushing plant operations upslope. From here, water is pumped upslope to a deep storage pond (turquoise green pond seen in aerial photo of quarry) where it is stored for use on site.

Photo 5
Any runoff captured along the entrance road to the quarry is directed to a concrete sump basin located just inside the Ameron main gate. Water is then pumped to the settling pond above the quarry. In the event of pump failure, this sump is emptied by a pumper truck as shown above.

At the point where Kapa‘a Stream flows through a culvert under Ameron's entrance road (WQ Station 6) the stream is joined by culverts from H-3 Freeway drainage. From this point to the bottom of Sub-basin 7 at Ameron's main gate, the stream bed and much of the banks have been stabilized over the years with concrete pours. At the Ameron Gate (WQ Station 4) the stream is the discharge point for another 24-inch culvert draining the nearby H-3. This freeway drainage (from Sub-basin 4) is often red tinged by sediment eroding from the slopes above the freeway where off-road vehicles have created trails all over the hillside.

Sub-basin 8: Spring
This 21-acre sub-basin comprises the valley floor between lateral drainages that are mostly intercepted on the west by Ameron (Sub-basin 7) and on the east by Kapaa Quarry Access Road (Sub-basin 14). Included is Kapa‘a Stream from the vicinity of the Ameron main gate (WQ Station 4). At the lower end of a 10-foot culvert beneath the road to the Kalaheo green-waste facility, a summer trickle flow occurs and Kapa‘a Stream becomes, by definition, perennial. Elevation at this small spring is approximately 115 feet, slightly below the 120-foot water table measured at the bottom of the quarry pit (Nance, 2002). This trickle, estimated at about one liter per minute (0.03 cfm) in late summer, feeds into a plunge pool (see Photo elsewhere) approximately 30 feet downstream. This small pool is home to prawns (Macrobrachium lar) and an unidentified poecilid fish, indicating permanence. Thick vegetation marks the stream channel further downstream, where eventually Kapa‘a Stream crosses beneath the H-3 Freeway.

The bottom of the culvert under H-3 freeway and much of the area just downstream consists of coarse sand and gravel through which minor stream flows would easily infiltrate. An aerial photo indicates that much of this area was covered with sediment to form a delta (source?). This photo was taken not long after the 1989 New Year's Eve (18-inch) rainfall and could represent results from that flood.

Photo 6 (by Bob Bourke) gutter

Photo 6
Energy dissipation structure in Sub-basin 8 is located below a culvert outlet draining Sub-basin 5 (the Kapaa Landfill, Phase II).

A desilting basin was constructed at the base of the energy dissipation device (Photo 6) where outflow from Sub-basin 5 empties into Sub-basin 8. According to C & C maintenance workers, this area is graded annually to remove excess vegetation and prevent blockage of storm runoff. However, this basin was almost filled with sediment in 1995 (pers. obs, Bourke) and is now (2002) completely filled. Some of the material filling the basin is not from runoff but appears to have been spread in from the direction of the adjacent asphalt recycling operation. Aerial photos suggest that the footprint of the asphalt recycling operation in Sub-basin 9 has expanded in recent years to cover more of this area. Below the delta the stream flows down into Sub-basin 11 and looses elevation rapidly.

Sub-basin 9: Light Industrial Park
Sub-basin 9 is mostly a plateau constructed between 19__ and 19__ from tailings and overburden of the original Kapa‘a quarry operation nearby and possibly covering residential refuse. In 19__ the plateau totaled about __ acres, but by 19__ this had been extended by unknown sources to almost 45 acres [?]. It is evident that the steep side slopes of the fill continue to receive minor additions of materials from a variety of sources. Although there is little vegetation on the surface of the plateau, runoff tends to be minimal due to the absence of slope across much of the surface. Percolation into the fill appears to be rapid except where surfaces have been paved or compacted. Around these paved surfaces, particularly in the light industrial area, there are no drainage systems and runoff appears to flow off the edge at multiple locations.

This entire area is leased from Kaneohe Ranch by Ameron and sub-leased to various tenants. Approximately __ acres of land encompasses a small business complex consisting of quonset huts, two large warehouses, and miscellaneous out-buildings.

Sub-basin 10: New Green-Waste Site
This "site" consists of about 21 acres of low sloping ground representing an old landfill covering residential waste dumped here over a wetland between 19__ and 19__. The land surface is covered with Guinea grass and koa-haole shrub and the remnants of previous construction storage and solid refuse. The stream-side boundary of Sub-basin 10 with Sub-basin 13 is demarcated by larger trees growing along the sides of the old land-fill. Presently some of the site is being developed as a green-waste recycling facility. Site Drainage is not evident due to the level topography and the fact that most rainfall probably percolates into the fill. A canal separating this parcel from Kapaa Quarry road (upper end is WQ Station 2) is usually stagnant, receiving little surface water flow during rains. However, this canal probably does receive ground water flow from upland areas through Sub-basin 10. A sample of groundwater weeping from the side of the Sub-basin 10 landfill above the canal was collected for analyses.

Sub-basin 11:
Sub-basin 11 consists of __ acres mostly above the H-3 Freeway draining through several 24 to 36" inch culverts below the Kalaheo Landfill access road and H-3 Freeway into an additional __ acres along Kapa‘a Stream below Sub-basin 8. Above the freeway, slopes are relatively steep (24 percent) and about a third of the land is developed into roads, stock-piles, and catchment ponds as part of the Ameron quarry operation. Some of the drainage from the Ameron controlled property is directed into a catchment pond on site, but a portion of the drainage proceeds down the access road and under the freeway through drainage culverts. The major portion of this sub-basin above the freeway consists of dryland scrub, dominated by koa-haole on the slopes of Mahinui ridge.

As drainage flow passes off and under the freeway right-of way it joins Kapa‘a Stream flow from Sub-basin 8. The stream bed here is narrowed between the footing of the freeway and fill land supporting a light industrial area (that is, Sub-basin 9). Foliage is extremely thick in this part of the stream and the area was not accessed on foot. The stream drops through dense vegetation including monotypic stands of giant elephant grass for over about a quarter mile. Near the end of the basin the stream intermittently receives input from Kaleheo Landfill run-off (Sub-basin 12) through an 8-foot culvert beneath the H-3 Freeway. Approximately 500 feet downstream of this confluence in a dense stand of elephant grass, Kapa‘a Stream reaches a permanent pond at a gravel berm marking the junction between this sub-basin and Sub-basin 13 (see Photos elsewhere). At this point the stream bed elevation is about 24 feet.

Sub-basin 12: Kalaheo Landfill
The Kalaheo landfill is an unlined landfill that received municipal waste from 19__ to 19__, and is presently the site of a green-waste recycling facility. The actual landfill only encompasses about __ acres, but the surrounding lands up to the ridge summit (an additional __ acres) all drain into the facility's runoff catchment system. The average slope of this watershed is steep (24 percent) but the designed drainage system appears to minimize runoff from the landfill. This system consists of two main conduits, one on either side of the landfill, with cross branches at each landfill terrace. The main branches meet below the bottom of the landfill in a large (100' x 200'), five-foot deep, desilting basin. Overflow from this basin is directed under H-3 Freeway via an 8-foot culvert to intersect with Sub-basin 11 about 500 feet upslope of the gravel berm. During the course of the investigation, this desilting basin was observed to overflow on only one occasion. City & County maintenance crews clear out the basin on a biannual schedule.

Sub-basin 13: Kapa‘a Stream Mouth
Sub-basin 13, encompassing about 71 acres, is the lowest portion of the watershed and contains the mouth of Kapa‘a Stream where it flows into Kawai Nui Marsh (WQ Station 1) at an elevation of about 5 feet. Three culverts (42", 42" and 30") connect the stream to __ acres of mostly disturbed uplands on the north side of the H-3 Freeway. The headwalls of these culverts are hidden amongst dense hau along the base of the freeway fill. The slopes above H-3 are part of the ridge known as Mahinui and are covered by a scrub growth of koa-haole, but aggresive fiddlewood (Citharexylum caudatum) is gradually covering significant areas of the hillside.

Near the top of the watershed, at the boundary with Sub-basin 11, a gravel berm crosses the valley bottom. This berm once served as a road bed across the wetland, perhaps providing access to H-3 during construction. Concrete components for the H-3 viaducts in Haiku Valley were manufactured at Kapaa Industrial Park. The berm was intact until 1995 when a channel was opened to allow free flow of water. The opening of the channel through the gravel berm resulted in the creation of a small pond on Kapa‘a Stream. The elevation of this pond is approximately 24 feet. The old road bed forces stream flow to the freeway side of the valley where the water meanders in channels partly within a dense growth of hau (Hibiscus tiliaceus), and through adjacent fields of elephant grass (Pennisetum purpureum). From this point to Kapaa Quarry Road, the lower reach of Kapa‘a Stream flows through a much disturbed wetland marked by pockets of umbrella sedge.

Sub-basin 14: Kapaa Landfill, Phases I & III
This __ acre sub-basin consists of the lower parts of the Kapa‘a Landfill (Phase I; operational from 1964 until about 1972). This section of the landfill occupies the site of the first Ameron quarry operated between 1949 and 1964. This area also once included an ancient adze quarry. The Hawaiian civilization depended upon the stone adze as an all-purpose tool (made in various sizes) to produce religious images, build homes and houses of worship, produce agricultural tools, manufacture weapons of war, and carve and shape the all-important fishing canoes.1

Located on a slope surrounded by the landfill is a heiau situated just above the site of an ancient adze quarry (now buried). Called Pahukini ("many drums"), it is a large, walled structure measuring approximately 120 feet by 180 feet with an adjoining structure on the north side of 32 feet by 38 feet. Clearing and cleaning of the site was overseen by Ameron in 1988 and the site remains under the respectful care-taking of the company. This structure is said to have been built by the legendary chief Olopana, and was known to some informants as Mo`okini ("many lineages") and also Makini ("many deaths as a contraction of make kini). These names suggest that this heiau was of the class designation of po`okanaka (meaning human head or skull) and functioned as a luakini or large heiau where ruling chiefs prayed and human sacrifices were offered.

Closure of the Phase I landfill included construction of a surface water drainage channel that directs flow from the east and north sides of the sub-basin into Kawai Nui Marsh through a desilting basin located alongside Kapaa Quarry Road. Other drainages, including that coming down Kapaa Quarry Access Road from as far up as the H-3 viaduct, flows to and across Kapaa Quarry road and into Kawai Nui Marsh. These two areas are lumped together as Sub-basin 14. Because run-off is diverted away from Kapa‘a Stream, this Sub-basin is technically no longer part of Kapa‘a watershed and is discussed here for historical purposes only.

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1 Modified by Eric Guinther (AECOS Inc.) from text provided by Robert Bourke (Oceanit Laboratories).
A It is recognized that Kapa‘a Valley is a sub-basin of the much larger Kawai Nui / Maunawili watershed. For purposes of this TMDL, however, Kapa‘a is treated as an independent watershed flowing into Kawai Nui Marsh.