The Waipuilani Gulch in Kihei is a prime example of what happens when water flows mauka to makai. Located in the Moku of Kula (Kihei), in the watershed of Hapapapa, and the Ahupua’a of Waiohuli. Waipuilani Gulch flows through neighborhoods has its outlet at Waipuilani park at the Waipuilani Kai wetlands. These wetlands and riparian areas are located on the Kihei floodplain and shares the aquifer with multiple muliwai (wetlands). The closest neighbors are the Waiohuli kai wetland, and the Kulanihakoi wetlands.
Wetlands in the Watershed Context: When you look at a wetland you have to view it in the context of its entire surrounding watershed. The wetland itself is just one feature of that larger watershed. It is one link in the chain of features that manage the flow of water.
The combined Waipuilani-Waiohuli gulch services an extremely large watershed area. The majority of Waipulani gulch’s catchment area, aka “Piilani Basin”, is mauka of Piilani highway. All of that rainwater falling upcountry flows downslope towards the ocean descends rapidly, gaining energy.
Rainfall becomes stormwater: This rainwater flows across the landscape, picking up debris, dirt, silt, and other contaminants along the way. This rainwater, now becomes contaminated stormwater, is progressively diverted into the narrow gulch. This stormwater then flows under the highway through a 100-foot wide concrete culvert where it is further accelerated.
The Waipuilani Gulch Bridge at Piilani Highway is a 100-foot wide bridge that is designed to withstand the 100-year flood levels. Its engineered design capacity is greater than 10,000 cfs (cubic feet per second). The Kihei Drainage Master Plan (KDMP) shows the Q-100 24hr flow at Waipuilani Bridge at 8089 cfs (cubic feet per second). By comparison, the Average Daily Value of the Colorado River is: 7,750 cfs.
Stormwater becomes a flood: High flow rates reported here can be equal to some significant rivers when it rains. The 100-year storm levels (Q100) are estimated to be approximately *8,719 cfs (cubic feet per second). This is no problem for the 100-foot wide bridge at Piilani highway (with a max capacity of 11,200 cfs), but it totally overwhelms the 10×2 foot culvert at South Kihei Road. Flow rates for Q100 storm are *8,120 cfs at the South Kihei Road Culvert. *Source: Kihei-DMP-Pre-Final-Report-Complete-11-04-16.pdf
This is called a flash flood. Flash floods can come quickly from remote rainfall and can suddenly appear on the lower slopes, and can pack a punch and catch people off guard. When this fast-moving floodwater flowing down the gulch hits the floodplain, it slows down, loses energy, and spreads out in the widening part of the streambed. This wide area of the lower gulch is an areas known locally as Fong’s gulch.
This area regularly becomes inundated and during rains can become covered in a foot or more of surface water. This part of the gulch is the widest part of the riparian area of the streambed on the floodplain between Piilani Highway and South Kihei Road. This wide part of the gulch acts as the area’s de facto drainage detention basin.
Detention Basin: The total estimated capacity of the stormwater detained in a 12.8-acre site like this could be in the range of 4 million gallons, even if it is just 1 foot deep. This stormwater usually contains many tons of silt and soil coming in stormwater from upcountry. This stormwater also contains many other unhealthy contaminants as well. And if the flow of the stormwater is allowed to slow down enough, in this de facto detention basin, the silt and soil suspended in the stormwater will settle out onto the ground. [CONTENT REMOVED]
Porous Soils: The soils here are particularly adept at absorbing vast quantities of water, down into the aquifer. However, if the amount of stormwater exceeds a certain limit, excess water will continue to flow seaward overland and push through a narrow opening at the base of the gulch.
Canyonized Streambed: This narrowing of the gulch [CONTENT REMOVED]. The stormwater flowing down this narrowed “canyonized” streambed will then flow under South Kihei road through a single narrow “con-span” culvert. This is culvert one of the most notorious bottlenecks in the Waipuilani Gulch watershed.
This constriction has been the cause of many flooding events in recent times and has caused problems for residents and for motorists as stormwater floods nearby roadways.
The Waipuilani Gulch wetlands and riparian areas are doing their best to handle floodwaters, but they are increasingly under pressure to handle more stormwater as nearby developments surrounding it created more impervious surfaces that increase surface runoff and stormwater production. In addition to the increasing amount of stormwater, the massive amounts of silt transmitted downslope tend to fill up the gulch, and reduce the wetland’s holding capacity.
Development is planned in this Gulch. [CONTENT REMOVED]
Cultural Sites: Adjacent to this wetland is the historical David Malo church that was said to be built on the banks of the perennial freshwater pools here. Excavations near this site revealed a long cycle of inland freshwater pools punctuated by flooding events. Hawaiian settlements are believed to have been located along this gulch to take advantage of the favorable conditions of the wetland. The foreshore area also supports a large settlement as well as being the site for another Royal fishpond at the mouth of this gulch. The Waiohuli Kai fishpond stands at the mouth of this stream, and was the one of major engineering infrastructure accomplishments of the Hawaiian settlements here.
Floodwaters threaten our marine live, coral reefs and our cultural sites: Brownwater contains many contaminants and sediments that are harmful to sea life and especially to coral reef. The silt harms coral polyps and stays in the ocean for extended periods, and it can be stirred up every time there are waves. This turbidity bocks sunlight necessary for photosynthesis, and suffocates delicate organisms under a layer of mud.
The harmful effects from even a single storm, can continue on for weeks or even months afterward. It is best to mitigate flood runoff, capture it on land, and prevent stormwater from entering the ocean. Some runoff is natural, but massive amounts of runoff that we see these days, are very destructive and are degrading the water quality, and creating unhealthy ocean conditions for sea life and for humans as well.