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Hanawi Stream near Nahiku, Maui, Hawaii.


Wailuku River Flood, September 13, 2016, Maui, Hawaiʻi

Intense, localized rainfall the evening of September 13, 2016 resulted in severe flooding, significant channel changes, and extensive property damage along Wailuku River in ʻĪao Valley, Maui. The U.S. Geological Survey (USGS) stream-gaging station Wailuku River at Kepaniwai Park (station 16604500) was destroyed by a landslide. The USGS crest-stage (flood-peak) gage Wailuku River at Wailuku (station 16607000) was severely damaged.

Crest-stage gage Wailuku River at Wailuku (station 16607000)

The USGS identified and surveyed high-water marks at crest-stage gage Wailuku River at Wailuku (station 16607000) and using a slope-area hydraulic model, computed a peak streamflow of 10,900 cubic feet per second, which is the largest streamflow at this site since the gage was installed in 1951. We estimated the annual exceedance probability corresponding to this particular flood at 0.2% (500-year recurrence). However, the uncertainty in the statistic is large, and, in general, a 66.7% confidence interval for the true annual exceedance probability of the largest flood in 66 years extends from 0.3% (330-year recurrence) to 2.7% (37-year recurrence). The estimated 0.2% annual exceedance for the September 13, 2016 flood is outside the 66.7% confidence interval.

Stream-gaging station Wailuku River at Kepaniwai Park (station 16604500)

The September 13, 2016 flood caused significant widening and filling of the stream channel at the gage site. Streamflow is no longer confined to a single, stable channel making accurate real-time streamflow monitoring currently impossible at the gage site. During the post-flood reconnaissance, the USGS did not identify any suitable alternative sites for immediate gage installation. Efforts by a private party are underway to align and stabilize the channel at the gage site. The USGS will install the stream-gaging station as soon as this channel-alteration work is completed.

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