Waipuilani Wetlands

Natural and Ephemeral Wetlands Are Nature’s Stormwater Detention System

Along the coastline of Kīhei, natural and ephemeral wetlands form across the island’s broad coastal floodplain. These are not random low spots—they are part of a functioning system shaped over time to manage water moving from mauka to makai. The flat coastal areas are typically bordered by shoreline sand dunes, which act as natural barriers and form shallow detention basins behind them. When heavy rains occur upslope, floodwaters move downhill toward the coast and naturally collect in these low-lying areas. Instead of rushing directly into the ocean, the water spreads out laterally across the floodplain. This spreading action is critical. It slows the flow of stormwater—effectively de-energizing it—which reduces erosion and prevents high-velocity discharge into the nearshore environment. As the water slows, suspended sediment begins to settle out, reducing the amount of red silt and debris that would otherwise be carried into the ocean. At the same time, these wetlands allow water to gradually infiltrate into the ground. The sandy sub-soils common to this area act as a natural filter, allowing water to percolate downward while trapping additional sediment and pollutants. What eventually reaches the ocean has been slowed, filtered, and moderated by this natural system. These wetlands are not just passive features of the landscape—they are active, functioning infrastructure. When left intact, they reduce flood intensity, improve water quality, and help protect the fragile coastal ecosystem that depends on clean, balanced inputs from the land.

Ephemeral Expansion of a Perennial Wetland System

These storm events create what are often called “ephemeral” (temporary) wetlands, but in reality, they are the visible expansion of an existing system. During heavy rainfall, water spreads across the floodplain, enlarging the footprint of smaller perennial (permanent) wetland cores that persist year-round, often just below the surface or within limited visible areas. What appears to be temporary flooding is actually the natural expression of a long-established wetland system responding to increased water flow. These areas are known to be subject to flooding and have functioned as wetlands for hundreds of years, long before any urban development altered the landscape.

WAIPUILANI WETLAND: This area once contained many permanent freshwater lagoons that were used for bathing. This area also once supported a large fishing and agricultural community. Today much of this area is now a large shoreline reserve covered with a lawn made up of introduced foreign species of grass, but some of the areas in this reserve still have native plants, thanks to restoration and conservation efforts.

Floodwaters spread across the floodplain at Waipuilani Park. These open spaces on the margins of the wetlands are buffer zones that allow for the temporary expansion of wetlands during high rainfall events. As nature intended, this greatly expands the surface area and capacity of the wetland to hold vast amounts of floodwater. This lateral spreading and temporary storage helps slow the movement of water across the floodplain, reducing flow velocity and allowing suspended sediments to begin settling out before reaching the nearshore environment. Several hydrophilic plant species thrive in this environment including mangrove and other wetland species, adapted to salt tolerance and periodic flooding. These species are uniquely suited to survive in fluctuating water levels and play a role in stabilizing soils and maintaining the structure of the wetland system.

SHORELINE DUNES AND SANDBARS ARE STORMWATER FILTERS: At the shoreline, the sand dunes stop the water flowing directly into the ocean, and this results in the water sometimes backing up into surrounding areas. These natural formations—often incorrectly referred to as “sand plugs”—are in fact sandbars that act as critical filtration and retention barriers, allowing wetlands to function as true holding systems rather than simple flow-through channels. The county sometimes deems it necessary to breach these sand formations to allow water to drain out and lower water levels. However, this should only be done in cases of extreme flooding where life and property are at risk. Outside of those conditions, interfering with these natural barriers disrupts the wetland’s ability to store and filter stormwater and is generally not permitted. Sandbars occur naturally at the mouth of streams and help protect the ocean from land-based pollution by slowing the release of water, allowing sediment to settle, and filtering runoff before it enters the nearshore environment. These formations also function as a natural pressure-release system, where excess water will eventually breach the sandbar on its own after the wetland has expanded, slowed, and filtered stormwater across the floodplain as intended.