Mala Ua: A Hawaiian Rain Garden

At one time, rainfall percolated into our island aquifers or flowed relatively clean

Rain garden at He'eia State Park.

Rain garden at He’eia State Park.

into nearby water bodies as part of the water cycle.  As our lands have become developed, the installation of impervious surfaces, which prevent runoff from infiltrating into the ground, has changed the way water interacts with the environment.  As a result, less water is reaching our aquifers and an increased amount of polluted storm water is reaching our streams and ocean.

Following rain, storm water picks up pollutants such as fertilizers, trash, and sediment carrying these to storm drains which empty directly into our streams and near shore marine environments.  Hui o Koʻolaupoko (HOK) is working to address these issues by installing rain gardens and other low-impact retrofits throughout Ko’olaupoko.

A rain garden is a shallow, flat-bottom garden bed designed to serve as a collection and treatment site for storm water runoff from rooftops, driveways, walkways, streets, or parking lots.  Through the process of infiltration and phytoremediation, rain gardens can remove pollutants from runoff before water recharges aquifers or flows into our streams and ocean.

On March 25, 2011, a demonstration rain garden was constructed at Heʻeia State Park as a joint effort between HOK, Kamaʻāina Kids, University of Hawaiʻi Sea Grant Program and Oregon State University Sea Grant Program. With the help of a number of volunteers, the rain garden was constructed and planted in just one day. The site is open to the public and was constructed to provide an example what rain gardens look like and how they function.

Diagram: Cross section of rain garden

Diagram: Cross section of rain garden

There are several factors that need to be addressed before a rain garden is installed.  Typically, a rain garden is sized so it is between 10 and 20% of the impervious surface you are treating and between 6 to 24 inches deep. The size will depend on the area’s rainfall regime and soil conditions.  Additionally, rain gardens should be placed in areas with well draining soil so water drains within 48 hours to prevent mosquito breeding.

Plants installed in the garden should be chosen according to the climate of the region but also according to their ability to withstand periods of drought and inundation.  Plants in the basin of the rain garden and near the inflow point should be water loving as they will be wet for the longest periods of time.  Plants on the slope should be water tolerant and have deep or sprawling root system to help hold soils in place.  Plants on the berm should also have sprawling growth characteristics to hold the edges of the garden intact, but these plants should be slightly more drought tolerant.  During the period before the rain garden’s plants are mature, maintenance will consist of occasional watering, weeding and replacing dead vegetation.

HOK is developing the State of Hawaiʻi Rain Garden Manual for homeowners and landscape professionals to use as a guide when installing rain gardens.  HOK is funded under an EPA and State of Hawaii, Polluted Control Runoff 319 Grant to develop the Manual as well as the installation of several rain gardens in Koʻolaupoko. The program is a cost share for landowners to encourage them to install rain gardens to project streams and oceans.  Manual is due for completion in early 2012.

For more information about Hui o Koʻolaupoko visit www.huihawaii.org and for photos of the Heʻeia State Park rain garden installation visit http://www.facebook.com/huiokoolaupoko

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Kristen Nalani Mailheau is the Community Coordinator for Hui o Ko’olaupoko 

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About the author  ⁄ Chauncey Hirose-Hulbert

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