Honouliuli National Historic Site Preliminary Hydrology Assessment (STAR #27864)

Leonard, Christy; Schook, Derek; Dunn, Doug

Honouliuli National Historic Site (HONO) was founded in 2015 to “conserve and interpret the largest and longest-used World War II incarceration site in the Hawaiian Islands.” At the time of this report, the park is planning how the site will be accessed and facilitates need to support park resources and visitor experience. The park is not yet open to the public. Central to HONO planning needs is information on Honouliuli Tributary streamflow, which runs through Honouliuli Gluch in the center of HONO. This report presents a preliminary hydrologic assessment of Honouliuli Tributary using existing data from stream gages operated by the U.S. Geological Survey. The report also summarizes information from groundwater wells located within and adjacent to HONO. Honouliuli Tributary is a moderately steep, ephemeral stream that flows naturally during rain events. At the upstream boundary of HONO, trans-basin diversion and subsequent irrigation releases from Waiahole Ditch augment low flow conditions in Honouliuli Tributary through HONO, providing more consistent baseflow through HONO than would otherwise occur. Waiahole ditch contributions are relatively small compared to natural flow that occurs during rainfall events and have little effect on flood flow conditions. An initial evaluation of flood risk at the site was conducted using regional regression relations through the USGS StreamStats program and a simplified modeling method to map flood inundation extent. The deep, steep gulch that Honouliuli Tributary flows through has little space for flood flows to expand laterally, and the inundation extent for the 0.2%-annual chance flood (i.e., 500-year flood) remains near the channel in the valley bottom. A more robust assessment of flood hazards should be conducted if infrastructure is planned in or near the 0.2%-annual change floodplain identified in this study.