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Plan seeks more natural solutions to flood risks

Stabilized dunes, green flood walls among the proposals in Kihei plan

Muddy water runs toward South Kihei Road’s Kulanihakoi Bridge on Dec. 6. The road shoulder near the the bridge located near Kaonoulu Street and Kalepolepo Beach Park was washed away in the flooding and has not yet been repaired. South Maui residents and the county are seeking solutions to the frequent problems of flooding in Kihei during major storms. After a draft drainage plan proved unpopular with the community, the county sought a contractor to draw up an environmentally friendly alternative that seeks more nature-based solutions to fortify areas around the Kulanihakoi and Waipuilani gulches. The Maui News / MATTHEW THAYER photos

Reduced wetland areas due to development in South Maui, drought and degraded watersheds and intense storms are creating a “formula for disaster” and leading to “mud floods” that pollute the ocean as they did in a recent storm, a local water quality expert said.

However, Robin Knox said she’s optimistic and hopeful that flood prevention and ocean water quality protection is possible.

Between the proposed ecological alternatives to the Kihei Drainage Master Plan and the observations by Save The Wetlands Hui following the Dec. 5-6 storm, Knox said the answers are found in watershed restoration and management, wetland protection, community engagement and limitations on additional hardscape — solid materials like pathways and walls used in outdoor landscaping.

“The current wetland area is not sufficient to handle these storms and all of this sediment that is coming down,” said Knox, leader of the hui and president of Water Quality Consulting, during a Kihei Community Association meeting Tuesday night. “The trend is that we’re having these storms that are decreasing in frequency, which means longer droughts, and they’re increasing in intensity, so you’re getting a lot of rainfall in a very short time.”

Low-lying South Maui is often flooded during major storms, with runoff from rainy Upcountry making its way downhill, filling gulches and muddying shorelines. Drainage is a primary concern for residents, but after the latest 2017 draft Kihei Drainage Master Plan proved unfavorable to the community due to the proposed concrete channels, the county contracted Amanda Cording, an ecological designer and community watershed management adviser from EcoSolutions LLC, to provide an environmentally friendly alternative.

Water flows under Piilani Highway’s Kulanihakoi Bridge in Kihei on the morning of Dec. 6, the day after a major Kona storm dumped flooding rains to Maui.

With an emphasis on removing pollutants with low-impact development and green stormwater infrastructure, the updated proposed plan was released in 2020.

On Tuesday evening, Cording said that the proposal aims to address community concern, specifically the Waipuilani Gulch Diversion; convey the predetermined design and projected water flow of the 100-year, 24-hour storm for each watershed; meet or reduce the estimated cost of the project within the Kihei Drainage Master Plan; reduce ecological impacts; and identify areas where water quality improvements can be made or implemented.

The growing plan recommends nature-based and natural-area improvements, combined with some low flood walls, expanded culverts and elevated walking paths because “existing conditions” within the Waipuilani and Kulanihakoi drainage areas will not be able to withstand projections for a 100-year storm event.

Cording broke down the needs by area — for example, “Zone 5,” a section near Piilani Highway within Kulanihakoi Gulch, needs excavation to address the flooding, she suggested.

While some areas makai of the highway can hold the existing water flow, other areas would be underwater in a 100-year flooding event, Cording said. She proposed improvements like green flood walls — as opposed to concrete — to mitigate rising floodwaters, as well as increasing the size of the 4-foot culverts along South Kihei Road.

“These are highly undersized,” she said. “Current flood elevation is very, very high over the road — as we know, it’s 4 to 6 feet above the road — during a big storm event.”

At the bottom of the Kulanihakoi watershed, just reaching the beach, the current sand dunes barely hold the water and will likely need to be stabilized. Research for this natural technique is ongoing.

“The dunes and the culverts that are undersized across the road are pushing the water back, are holding the water back and creating flooding back upstream,” she noted.

Total project costs for Kulanihakoi Gulch are estimated at $6.5 million, much less than the original $57 million needed for the Kihei Drainage Master Plan, which proposed a diversion at Waipuilani Gulch that was not popular with the community.

For Waipuilani Gulch, Cording said they want water to flow from mauka to makai without constructing a diversion.

“There’s a lot of opportunity here for some good restoration work, but a challenging site with the narrow constraints,” she said.

The area just below Piilani Highway contains water “marvelously,” and Cording believes there is opportunity for vegetation improvement to increase water quality benefits.

Other improvements in the Waipuilani Gulch area could include widening channels, raising the road via bridges or removing or raising properties, which may require the county to purchase some property.

Makai of South Kihei Road, some options to mitigate the impacts of stormwater events include raising the roads or installing new culverts.

“One of the big questions is, if you want to avoid that big gulch diversion, there needs to be significant communication with these private landowners and with the county in terms of budget, to figure out the best option for getting the water to move from A to B,” Cording said.

In the area closest to the ocean containing Waipuilani Park, Cording said there are big opportunities to excavate and restore the natural wetland, which could be a historic fishpond at the outlet of the watershed.

She added that restoration work should develop in a “culturally sensitive historic direction” rather than just pushing water mauka to makai.

Nonprofit Maui Nui Marine Resource Council also announced at the meeting that it will be utilizing a National Fish and Wildlife Foundation and National Oceanic and Atmospheric Administration Coastal Resilience Fund grant to help with a coinciding project to address stormwater flooding and nearshore pollution in Kihei.

Jill Wirt, project and research coordinator with the marine council, said the grant is for $210,000. Consultants are Cording from EcoSolutions and Michael Reyes from Maui Environmental Consulting LLC to help develop assessments and nature-focused solutions to address recurring coastal flooding on two state parcels located makai and mauka of South Kihei Road, including Laie Wetland and the 12 parcels referred to as Keokea Wetland, respectively.

According to the marine council, some projected outcomes of the project include reduced flooding in coastal Kihei from Kulanihakoi, Waipuilani and Keokea gulches, and in neighborhoods near the Laie/Keokea wetlands; improved habitat for wetland wildlife; reduction of sediment transported by stormwater runoff into Kihei’s nearshore areas; and improved habitat for coral reef, fish and endangered marine wildlife.

* Dakota Grossman can be reached at dgrossman@mauinews.com.

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