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Flood Mitigation Strategies for Kihei

February 16, 2012
Michael Howden · Permaculture Designer · Olinda , The Maui Weekly

With more than 90 percent of the acknowledged (mapped) Kihei wetlands densely developed for housing and commercial use, it is not surprising that in flood events, Kihei is particularly vulnerable.

The design of Pi'ilani Highway funnels storm water flow, and the attendant development mauka of that highway has created concentrations of water detrimental to much of the environment makai of the highway.

And much more is planned.

Mitigation of excessive stream flow in storm events begins at the outlet, the ocean. But given that Kihei is largely a built environment, and that rights of access and ownership are complex, it may make more sense to begin designing a water mitigation plan mauka of Kihei town.

The purpose of the following strategies is to capture water in the mauka (dryland) landscape, and to spread and hold it to support reforestation, and thus reduce erosion. This is known in principle as the Keyline System developed by P.A. Yeomens of Australia.

The design would focus on interceptor banks moving water along the contour through deflector or spreader banks/channels to retention basins. The deflector banks are essentially wide swales, the down sides of which can be planted, given the moisture held by the swales and the organic matter they capture.

Much of the Upcountry soils are hydrophobic, possibly due to deflocculation, and tend to powder when they are dry. Thus, a wash forms with sheets of water running across the landscape.

Using Keyline principles, swales with a makai-side berm running along the contour of the landscape and retention basins would help hold moisture, sediment and organic matter that would otherwise run into the ocean, killing our corals and polluting our near-shore waters.

There are numerous strategies applicable to Upcountry ranching and farming, such as timely adherence to rotational grazing (in general), contour planting in agriculture and use of earthforms to help retain moisture on the farms themselves.

In addition to ephemeral retention basins, reforestation based on natives as its core is foundational in designing a landscape that can be a nourishing habitat. Gradual reforestation will help expand treed perimeters, also using bushes, vines and other groundcovers. Vetiver is known for use in erosion control, and pili grass would also be appropriate.

The details, such as species assemblies, can be complex and open to discussion. Using appropriate guilds, certain plants help nourish their neighbors and can be used both to shelter and feed native perennials such as lama, koa'ia, ohia, 'iliahi, wili wili and others.

Nothing enduring can be accomplished without the cooperation and contribution of the large landowners, as well as all stakeholders, especially our county and state administrations.

The state administration's new initiative, "The Rain Follows the Forest," is just such an initiative. Closer to home, we need leadership by our county in addressing the needs of our South Maui community.

I would recommend Section 11.13, "Desertification and the Salting of Soils," in Bill Mollison's "Permaculture Designers Manual" as a foundational and essential reference in this endeavour.

Michael Howden can be reached at michael@permaculturemaui.com.

 
 
 

 

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