Construction of the Kawai Nui flood control levee in 1966 cut off flow from the 800-acre Kawainui Marsh to the 135-acre Kawai Nui Stream, Hamakua Wetland, and Kaelepulu Pond and Wetland. The reduced flow allowed the stream mouth to be almost continually blocked by sand at Kailua beach and resulted in stagnation and poor water quality throughout the 135-acre estuary. In 2008 the Hawaii State Legislature allocated funds to DLNR to conduct a feasibility study of restoring flow from the Kawainui Marsh to Kawainui Stream. In 2012 DLNR contracted with a private consulting firm, Oceanit, to conduct a temporary flow restoration experiment, to conduct preliminary engineering studies to determine the most effective way to restore the flow without any increased risk of flooding, and to write an Environmental Assessment document disclosing the potential risks and benefits of permanent flow restoration to the community and surrounding ecosystems.
Flow Restoration Experiment Report
Efforts were made to understand the physical water quality dynamics of the system. From 2012 through 2015 processes involved in 30 stream mouth openings were documented, and physical water quality transects throughout the estuary (20 stations) were conducted on 23 occasions. Nine of the water quality transects were conducted between 5/15 and 9/4, 2015 during which period partial flow had been restored from the Marsh to the Estuary at a rate of about 2 cfs by means of a siphon installed across the Kawainui Flood Control Levee. Results showed that improved flow through the stream mouth could be attained by a high (~ 2-foot MLLW) water elevation in the estuary and the excavation of a narrow but deep channel through the sandbar coordinated with appropriate ocean tides. Flow restoration at 2 cfs proved to be sufficient to offset summer evaporation and raise the elevation of the estuary water surface by about ¼-inch per day. Freshwater restored by the flow appeared to spread across the surface resulting in a persistent stratification, particularly in the lower estuary. READ MORE (PDF)
Kawai Nui Engineering Report
The goal of the engineering study was to develop conceptual plans for a mechanism that would be capable of efficiently transferring at least 2 cfs flow from Kawainui Marsh to the Kawainui Stream without any increase in flood threat and by means that required as little human intervention as possible. A variety of methods were envisioned including pumps (electric and solar), siphons, penetrations through the levee, tunneling far below or around the levee, and located at a variety of places along the mile-long levee. US Army Corps of Engineers personnel cautioned that any device planned over, through, or under the levee structure would require extensive review through the Federal Section 408 process and would have a greatly reduced likelihood of success. The plan selected avoids the levee structure by taking water from the Marsh near Kailua Road through a 10-inch to 16-inch pipe around the southern end of the levee within the State Highways right of way and flowing into the “ITT” wetland near the entryway to Kailua Town. The pipe may be easily installed at grade through traditional trenching or by directional drilling and flow controlled by simple gate valves or weir board in a subsurface vault. READ MORE (PDF)
Kawai Nui Final Environmental Assessment
An Environmental Assessment was conducted that included most, but not all, of the requirements of State of Hawaii Environmental Policy Act. Portions of the process relating to archaeological impacts, social impacts, environmental justice, and complete public disclosure were not conducted due to the preliminary conceptual stage of the project and the desire, first, to understand the potential ecosystem and flooding impacts of the proposed project. A complete EA would need to be conducted to proceed with the final project.
Examination of hydrology studies conducted by the US Army Corps of Engineers (Corps) and calculations of maximum possible flow through the selected designed system indicate no measurable or significant threat to flooding to residents of Kailua town resulting from the project.
Extrapolation of results from the 3 ½ month flow restoration experiment indicated that the impacts of restored flow are largely positive. Under present conditions, evaporative losses in the lack of rainfall result in decreased water surface elevations (down to ~1.0 ft MLLW). At this level much of the Hamakua and Kawainui wetlands are dry, stagnation and eutrophic water quality are common problems, sometimes resulting in fish die-offs and potential botulism outbreaks in waterfowl that inhabit the system. Increased flows, particularly in the summer, would more than offset evaporative losses resulting in higher and more consistent water surface elevations. These higher water surface elevations (up to ~2.0 ft MLLW) improve nesting and foraging habitat in the wetland, improve general water quality within the estuary, and facilitate more effective openings of the stream mouth to ocean exchange. READ MORE (PDF)