Natural History, Hydrology and Water Quality of Enchanted Lake – Kaelepulu Pond
The Kaelepulu watershed once incorporated all of the present day Kawainui (7,175 acres) and Kaelepulu ( 3,450 acres) watersheds. With only one natural outlet, large storm events would commonly cause flooding across the low elevation sand dune separating the waterbodies from Kailua Bay. In response to the growth of Kailua Town across the sand dune, the USACE constructed the Oneawa Canal (1952) to drain the marsh to the west end of Kailua Bay, and in 1966 completed building the Kawainui Levee. The levee protected Kailua Town from flooding but also separated Kaelepulu Stream from its primary water source of 10 to 15 cubic feet per second. Also in the late 1960s, the 190-acre Kaelepulu Pond, surrounded by an additional 90 acres of marsh, was dredged and filled around its perimeter to create the urban community of Enchanted Lake. This resulted in the 100-acre pond that we see now. The Enchanted Lake Residents Association (ELRA) purchased 89 acres of the pond from Bishop Estate and has managed it since 1989. The City and County of Honolulu (City) owns all of the storm drains leading into the lake and the main channels of the Kaelepulu Stream, and Kawainui Stream. These water bodies receive storm drain flow from most of Kailua and channel this flow to the Kaelepulu Stream mouth at the east end of Kailua Bay.
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Kaelepulu Pond, which until 1950 was much more wetland than water, was dredged in the 1950’s and transformed from a partial wetland into a private pond for the newly created community of Enchanted Lake. The pond was initially dredged to a depth of about 15 feet, was quite clear and supported prolific fisheries and oyster beds. Subsequent housing development of the surrounding hills and non-point source pollution from the urban neighborhoods has progressively silted in the pond (maximum depth presently –9 feet), lowered biological diversity, increased turbidity of the water, threatened already endangered waterfowl, and turned the pond into more of a liability than an amenity for the surrounding community.
The Kaelepulu Stream, above the wetland, is managed as a storm drain and retention basin by the City and County of Honolulu, as are the approximately 8.6 miles of storm drains (482 storm drain inlets) from the surrounding community that drain into the lake/pond system. Most of these drains receive inflow from the 482 drain inlets spread across 28.6 miles of paved public roads. Water drains from the lake through the City-owned and managed Kaelepulu Canal that joins with the Kawainui Canal Kailua town drainage system before the stream enters the ocean across Kailua Beach.
The Pond and Wetland acts as sediment trap for the City and County storm drain system serving the surrounding community. The wetland and pond serve as a collector to prevent movement of trash and sediments to the ocean where they would impact State owned public beaches and coral reef environments. There are no trash screens or collection devices at the storm drains as they enter the pond.
Oxygen, Salinity and Turbidity of the Lake and Canal
Each of the graphic on this page represents a cross section through Kaelepulu Stream and Lake. In each graphic the Kailua beach is on the far left and the lake is on the far right. The junction of the stream and the lake is at the shallow pointy spot (station 11). The yellow portion of each graph represents the bottom of the canal or pond. As can be seen, both the canal and the pond have a maximum depth of about 9 feet. These measurements were taken during the early morning when the sand berm at the ocean was closed and there were no recent rain events. The above graph shows how oxygen levels vary in the stream and lake with depth. Oxygen levels are typically lowest during the early morning hours (when this profile was taken). The graph shows that oxygen levels in the stream and lake are all quite high and able to support fish life in the pond.
Water quality improvements to the system, as mandated by the future TMDL, will likely require controls to the storm drain systems entering the lake and canal. However, there are about 500 inlets to the storm drain system, and only about 50 inlets from the drains to the pond. To effect controls, we need to determine if it is more efficient to catch pollutants at a large number of locations before they enter the storm drain system, or from fewer (but more difficult) locations as these drains enter the pond. This can be effective with a comparative study of drainages fit with curb drains against others fitted only with filters at inlets to the pond. At some point in the near future we will attempt to collaborate on such a study with the City, hopefully in coordination with the DOH and KBAC.
Water quality improvement in the Kailua Waterways is also a function of residence time and flushing. Residence time of water in the system and improved flushing action will be sought through three separate mechanisms. We are working with the Army Corps of Engineers to explore the possibility of diverting water from the Kawainui Marsh into the distal end of the Hamakua canal (a.k.a. Kawainui Stream). This is the historical outlet for Kawainui Marsh and will serve to flush anaerobic waters from this end of the system. See Figure , top for a water chemistry cross section of this canal showing very low oxygen levels and high turbidity levels.
The above graph shows a cross section of the water salinity in Kaelepulu Stream and pond during the early morning. There are two important facts displayed by this graphic. The lake is highly stratified with the low density fresh water floating above, but not mixing with the higher salinity water below. At the junction of the stream and pond (about section 11 on the graph) the water is very shallow. This shallow point keeps the very dense seawater (salinity >30) from entering the lake where the salinity maximum is about 20ppt at the bottom.
The above graph shows the temperature distribution during the early morning hours in the Kaelepulu Stream and the pond. Note that during the morning the waters deep in the pond are warmer than the surface layers. In the afternoon on a sunny day, the surface waters will be warmer, but the waters down deep will remain about the same temperature.
The above graph shows the turbidity of Kaelepulu stream and pond during early morning hours. Note that the deep waters of the lake and canal have a relatively high turbidity and the surface water are clearer. During periods with little or no runoff, any particles in the water gradually settle to the bottom where they may be re-suspended by bottom dwelling organisms thereby keeping the water near the bottom more turbid.
A second method to improve flushing will be to coordinate with City crews who are responsible for opening the and plug at the Kaelepulu Stream mouth on a monthly basis. If timed correctly with the tides so that the hydraulic forces work with the excavator, this activity can reasonable be expected to exchange about 50 acre-feet of water twice a day for two to three days every month. This will also have the effect of increasing the salinity gradient within the estuary system and thereby promoting the growth of a benthic filter feeding community (oysters) to help clear the waters.
The third activity to improve flushing involves dredging a portion of the Kaelepulu canal at its junction with the pond. Makai of this location a major drainage canal enters the system from residential areas in Keolu hills. During storm flows the initial flow direction from this side channel is towards the lake, as the ocean channel is not typically open. As the channel widens towards the lake and flow velocities decrease, all of the sediments and trash from the side channel tend to settle in the main channel. Over the years this section of channel has decreased in depth from about 8 feet to only about 6″ to 2′ today (See Figure b) This partial plug serves to block the flow of the clean but denser salt water as it enters from the ocean. If this plug were not present, the salt water entering on the rising tide would reach and “fall” into the deeper lake, and the lighter surface fresh water would tend to flow out on a falling tide. This would effect a more complete exchange. In the present partially blocked condition, the canal fills up with the denser salt water on incoming tide forcing the surface fresh water back into the lake. On the falling tide it is predominantly the sea water that just came into the canal that flows back into the ocean. This has the long term effect of accumulating nutrients and sediments within the Kaelepulu canal and pond until major flood events (typically 2-3 times per year) flush large quantities of built up material out onto the beach and reef. The secondary effect of this lack of seawater exchange, is that the pond salinity has become too low to support the once plentiful oyster beds. The primary filter feeder in the lake now is Cassiopeia andromeda, the upside-down jellyfish. In many locations this animal completely coats the bottom of the pond with a living, pulsing, mat of zooist protoplasm.
Once the mangroves have been eliminated, freshwater and saltwater exchange re¬established, and hydraulic blockages removed we will be ready to attempt to modify the trophic level of the pond ecosystem. Presently the pond is dominated by alien benthic algae (Acanthophora spicifera is currently the most dominant), open water phytoplankton (secci disk readings typically 2-5 ft), and benthic jellyfish filterfeeders. Dominant fish species are tilapia, milkfish, and mullet, all of which proliferate in plankton dense nutrified aquatic systems. With improved water flow and increased salinity we hope to be able to re-establish the prolific oyster beds that once lined the lake. This may require additional removal of soft sediments to expose hard substrate that the oysters can colonize. The re-establishment of oyster beds will be followed by decreased phytoplankton populations, increased water clarity, and probable use of the system by a variety of juvenile and adult marine and brackish water fish.