Paikō Lagoon

Mālama Maunalua studies effects of severe April flooding on Maunalua Bay

On Friday, April 13, record-breaking rainfall produced severe localized flooding from Aina Haina to Hawai’i Kai along the leeward slopes of O’ahu, according to the National Oceanic and Atmospheric Administration (NOAA).

In the Maunalua Bay region, NOAA monitors rainfall only in Niu Valley, which recorded 5.64 inches during the 48-hour period ending at 6 p.m. HST on April 15. The U.S. Geological Survey (USGS) stream gauge on Kuli’ou’ou Stream measured peak flow at 1,800 cubic feet per second. This is the second highest flow since 1970 after the 1987 New Year’s Eve storm.  Plumes laden with terrestrial sediment extended from the streams’ mouths across the region into the bay. Department of Health “brown water advisories” were in effect for much of the island of O’ahu. After a week, the waters in Maunalua Bay were still noticeably brown, with abundant plant debris and other rubbish floating in the water.

Immediately after the storm, Mālama Maunalua visited the Bay. At Paikō, numerous dead fish and crabs were observed in the water and along the beach. We also saw discolored marine algae, particularly Halimeda discoidea, known for producing new sand as its blades release stored calcium carbonate when they die.  We suspect the casualties may have been caused by osmotic stress from the sudden freshwater outflow, and/or high concentrations of sediment in the water.

Maunalua Bay Beach Park

Another storm casualty: We found a large lay net, about 500 yards outside Kulio’uo’u, three days after the storm…apparently swept away by the weather. It contained more than 60 fish, mullet and others, all dead and entangled in the net. They ranged from 1-2 feet long.  Some were fresh – others badly decayed. We notified the enforcement branch of the Division of Aquatic Resources, and they responded by retrieving the net the same afternoon, before this “ghost net” could kill even more fish.

Mālama Maunalua was also interested in understanding the dynamics of the sediment brought into the bay by this storm on the nearshore fringing reef. Mālama Maunalua Core Volunteer Scientist Ralph Dykes has been monitoring the Paikō Restoration Area (26+ acres of reef flat off Paikō Drive) for the past five years, mainly to determine the effectiveness of the Mālama Maunalua organized community huki events when volunteers remove invasive alien algae (IAA) at designated areas along Maunalua Bay.

With pre-storm sediment-depth data available, he and MM intern Nolan Pho measured the sediment depth again at 37 locations post-storm. The team found the sediment did not appear to accumulate – there was no statistical difference in sediment depth before and after the storm. It is too early to state whether this finding of low sediment accumulation is due to MM’s community effort to remove IAA (especially leathery mudweed, Avrainvillea amadelpha, known for trapping fine sediment).

A few years ago graduate student Sean Macduff working with Dr. Eric Wolanski (James Cook University) and Dr. Robert Richmond (Kewalo Marine Laboratory) studied fine sediment[1] flushing of previously cleared and uncleared plots at Paikō Restoration Area. His team found that fine sediments were not accumulating in areas cleared of IAA due to huki events).

In addition, Macduff et al. estimated flushing of fine sediment out of Paikō reef. He found significantly less re-suspendable fine sediment in areas cleared of IAA. Deep and shallow waters were flushed approximately 3.67 and 5.63 years, respectively.  Though how long the sediment from this storm may be trapped and circulating in the nearshore reef flat is a problem, MacDuff et al.’s findings show that MM’s huki events will be a positive activity that may help to reduce the retention time of the fine sediment.

MM is still concerned with the effect of the “brown-out” conditions.  According to a USGS study that monitored turbidity after the 1987 New Year’s Storm in the Maunalua region (see Storlazzi et al. 2010, available in the Resourced Library on the MM website), the extremely elevated turbidity levels after that event (> 35 NTU) likely resulted in no sunlight reaching the seabed, thus a complete shut-down of photosynthesis.  Mālama Maunalua will continue to monitor the fine sediment.

In addition, Mālama Maunalua will be working with partners in designing a water quality plan to effectively monitor problems, including on human health, over the upcoming months. is saddened by the damage to people’s property and lives. By better understanding the reasons for the damage, we can work together to address them in a way that minimizes the likelihood of similar events happening in the future. Climate change projections indicate the amount of rainfall experienced in mid-April will be more common. By making the right decisions now, we hope to minimize the severity of impacts to people, places, and nature in the future.

 

[1] The focus was on fine sediments because these sediments are usually terrigenous or derived from land.