Drainage, floods a concern with paving yard in Manoa

QUESTION: My mauka neighbor is building a BIG house, and now they are pouring concrete in the entire front yard. Is this legal? Already it’s hot here, and they cut down all the trees, too.

What we worry about is where will all that water go when it rains, when it floods? We may get flooded from our neighbors. It should go down into our aquifer, but with all that concrete it will run off and flood us, or flood lower down, or all that brown water will flood into the ocean.

Are there land use or zoning laws about this? We used to think that we had strong laws for nice old residential neighborhoods like ours.

What else could they and their contractors do to make this bad situation better? — K.N., Manoa

ANSWER: Permeable, porous concrete is the simple, "green" answer if your neighbors insist on paving over their land. This is a fairly new but aka­mai technology that is being used as architects and forward-thinking landscape architects realize that we all need to build smarter and with concern for trees and drainage.

This new porous paving was used at the Arizona Memorial, and it is working well. They are also using it at the University of Hawaii at Manoa at the new IT technology building. The concrete for sidewalks and the fire lane is porous so water will drain to our underground water supplies.

New buildings that go for GREEN or LEED certification are testing this simple technique. It looks like concrete being poured unless you watch carefully.

First, a thick base of chunky rock about 2 feet thick goes under the slab. Then the porous paving is poured over the top.

Trees are one of the best living infrastructures for preventing stormwater runoff. We always try to convince homeowners to keep their trees or plant new ones. The leaves slow down rainfall, and the leaves and roots help the rain sink into the ground. The roots get some of the water, and the rest recharges our aquifers, that amazing underground water source.

As we all learned in school, trees cool our islands, give us oxygen and make their own food with their leaves. In Hawaii, many of us favor native Hawaiian trees like ohia lehua, koa, lono­mea and loulu palms, or fruit trees like tangerine, breadfruit, mango and avocado. Not only do they feed and nurture us, they are also good for our aina and wai.

As for the permits, or legal questions about what your neighbors are doing, it’s worth calling the city permits department and stormwater branches to see if they are following the laws and getting all the proper building and drainage permits. These government agencies work for us via our taxes and water fees, and they have staff to address your concerns.

We all need to kokua and keep our islands and our planet cool and habitable for the future. A good number to call to find the right city department is 768-4381.

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Heidi Leianuenue Bornhorst is a sustainable landscape consultant specializing in native, xeric and edible gardens. Reach her at heidibornhorst@gmail.com.