Stories From The Field

Identifying Areas for Water Quality Improvement Projects in Oahu, Hawaii

Abstract background image with blue overlay

Issue

The urban areas of Ko‘olaupoko (Makapuʻu to Kualoa in Windward Oahu) have subwatersheds that are covered with impervious surfaces ranging from 1 percent to nearly 60 percent. Research suggests that as impervious surfaces increase, associated water quality problems also increase, and water bodies have less ability to support aquatic functions. The Center for Watershed Protection estimates that watersheds covered with 10 to 25 percent impervious surfaces have impacted watershed health, those with 25 to 40 percent impervious surfaces have non-supporting watersheds, and those with over 60 percent coverage are considered urban drainages. The watersheds of Ko‘olaupoko streams with significant urban surroundings have a suite of water quality problems that include nutrients, phosphorus, total suspended solids, and other pollutants associated with nonpoint sources.

Process

GIS digital investigations of the Ko‘olaupoko region were conducted using various data and conservation tools to identify potential project sites with indicators that directly or indirectly affect water quality. Recent lidar-derived elevation data and high-resolution Coastal Change Analysis Program (C-CAP) land-cover and impervious-surface data (developed in partnership with NOAA and the Hawai’i State Office of Planning) were key framework data sets used in the GIS analysis. The Nonpoint-Source Pollution and Erosion Comparison Tool (N-SPECT) was used to generate depictions of sediment sources that were aggregated at watershed and sub-watershed scales and applied as coarse filters in the site-identification process. The Habitat Priority Planner was configured to include percent imperviousness (computed from the C-CAP data), parcel size, and zoning district classifications to identify potential project sites, and to model how sites that currently address nonpoint source pollution by capturing and treating stormwater might impact the watershed. The resulting sites were subsequently ground-truthed to provide further detail, such as location of storm drains and building downspouts, current post-construction stormwater best management practices, and potential retrofit feasibility. Additionally, sites were assessed for conflicts such as utilities, right-of-way, and simply a lack of open space or landscaped areas to implement retrofits.

Impact

Analyses in Ko‘olaupoko have highlighted areas that can be retrofitted to capture and infiltrate storm water, ultimately reducing the amount of pollutants that enter storm drains and flow into receiving waters. Two-hundred twenty sites were identified and ground-truthed, resulting in a total of 60 sites being ranked candidates for low-impact retrofitting. If all ranked projects were implemented, it is estimated that the following pollution reduction could be achieved annually:

Total suspended solids: 6,187 pounds
Total phosphorus: 53.68 pounds
Total nitrogen: 191.69 pounds
Annual runoff reduction: 25,257 inches

Since completion of the Ko’olaupoko Sub-basin Action Plan, Hui o Ko’olaupoko has secured funding to move forward with implementation of a low-impact retrofitting project at Windward Community College. Also, this project has been recognized with an Environmental and Preservation Award from the American Planning Association's Hawaii Chapter.

img-infographic
Tell us how you use this resource.

Help us build better tools and services by sharing your story with us.