Project tackling Big Island mud
By Jan TenBruggencate
Advertiser Science Writer
The mud started settling on the bottom of Pelekane Bay, next to the Big Island's Kawaihae Harbor in Kohala, shortly after the harbor was built in 1962.
Air Survey Hawaii photo
Forty years later, after more changes in the shape of the coastline, that muddy sediment covers the rocky bottom to a depth of 6 to 9 feet in places. It has killed coral and seaweed and is believed to have covered an underwater shark heiau.
Runoff from two gulches above Kawaihae have dumped sediment into little Pelekane Bay, at the upper right of the Kawaihae Harbor coral fill area. State, federal and private efforts are aimed at restoring the environment.
Experts believe the harbor blocked normal currents. The other problem is that land-use changes upslope increased the amount of sediment that washed down two major gullies that flow into Pelekane.
Federal and state officials are looking for ways to fix the problem.
"At the request of the Mauna Kea Soil and Water Conservation District, the Corps is undertaking a feasibility study, which will determine whether the Corps of Engineers can assist in restoring the ecosystem for marine habitat life in Pelekane Bay," said Joe Bonfiglio, chief of public affairs for the Corps Honolulu District.
The Corps is working with Group 70 Architects on the project. Environmental planner Jeff Overton of Group 70 said one of the major programs is likely to be changes on the slopes above Kawaihae — including planting vegetation in areas denuded by cattle and goat grazing and building sediment traps or check dams along Makahuna and Makeahua gulches. That should reduce the amount of mud flowing into the bay.
Another program may include pumping sediment off the reef in Pelekane Bay — probably to a sediment-retention basin on filled land next to the harbor. That should reduce decades of trapped sediment on the bay floor.
While reducing mud flow and removing mud will help, "I think we're always going to have some sediment transport," Overton said. So another plan is to establish a means for the bay to clean itself in the future.
Bonfiglio said that could include "installing a circulation culvert or channel connecting Kawaihae Harbor with the bay," he said.
Overton said such a waterway might need to have a one-way valve system — something he said the Corps has done successfully elsewhere — so flushing water can flow from the harbor to the bay, but sediment will not flow from the bay back into the harbor.
Among the other options is to build on a series of experiments conducted in the area, transplanting coral to the bottom around Kawaihae. Research shows that transplanting works.
At Pelekane, Bonfiglio said the Corps might try to create an artificial reef area that would support transplanted coral, then to seed or transplant corals onto the new substrate.
He said federal money would pay for the feasibility study.
Reach Jan TenBruggencate at jant@honoluluadvertiser.com or (808) 245-3074.
