Impacts to Watershed of the North South Collector Road Kihei:

This ongoing project has the potential to cause major changes to the South Maui Watershed. It has already caused massive impacts on natural systems and watershed features.

The north-south collector road cuts across several gulches streambeds, and flood plains. The north-south orientation connects traffic, but disconnects many streams and waterways in the process.

Here is how the northern part of the “collector road” site looks now, more or less, Image from Google Earth 2021.

WATERSHED ALTERATIONS: Grading around this roadway affects surface water flow. Compacting earth and excavation changes sub-surface flow, (compacted soils do not transmit water flow as easily as natural soils). Surface runoff is diverted, all of the minor tributaries and gullies are cut off, and effectively dammed. Where water is allowed to flow under the roadway, runoff is concentrated and redistributed unnaturally. Other streams are burdened with excess runoff diverted from streams, and the water generated by the impervious surfaces of the roadways and pavements are also dumped into streams.

Here is a satellite image from 2020, with streams in blue.
Here is a satellite image from 2020, with streams in blue.

LOSS OF STREAM FUNCTION: Bridge footings are dug deep into streambeds, often down to the bedrock, and cutting off any subsurface water courses, and construction activity such as backfilling and heavy equipment causes a hardening of the surrounding soils, reducing the soils ability to absorb and transport stormwater, or allow infiltration into the aquifer. Bridge construction usually involves hardening stream banks and stream beds upstream and down stream of the structure to prevent undermining during flood events, this effectively destroys the natural streambed functions for a large portion of the streambed. This results in a loss of natural stream function. Wherever bridges are built in narrow the gulches, they create choke points. The supports are usually placed within the gulch to reduce the span needed, and to save money, these support structures create a choke point. Where narrowing occurs, the natural flow patterns are altered. Narrowing a streambed result in an acceleration of stormwater runoff, which makes it more destructive to the watershed features downstream.

Proposed Collector Road, streams, and gulches.

POWER OF WATER: Accelerated runoff travels further and faster and has more energy to scour away soils and materials from streambeds, causing the runoff to carry more silt. Most of this silt will end up in the ocean if there is no catchment or natural floodplain. Downstream properties are not the only ones at risk. The roadway creates a dam across many smaller kahawai and low areas where overflow stormwater traditionally flowed, It causes downhill surface flows to back up and flow laterally into neighborhoods, creating new risks from flooding. Most all of the engineering on Maui is built to withstand a “50-year 1-hour” storm. However these roads, bridges, and culverts are designed to last 100 years or more, and will eventually encounter a 100-year storm which is many more times powerful that the 50-year storms. We are knowingly engineering flood hazards into our urban infrastructure because our building standards are based on short-term forecasts that use historical flow data. Another problems is that weather changes with climate.

CLIMATE CHANGE: And we know that we are in a phase of climate change, which will make all historical data obsolete. We should be building for future storms, larger than the ones we have experienced before. So why don’t we do that already? Building for larger storms is more expensive. Building a larger bridge, or bigger flood retention areas uses more resources, and land. Simply put, it is more expensive. To create the necessary infrastructure for future storms and bigger floods requires a significant capital investment, and long-range planning.

DRAINAGE ISSUES: The estimated drainage infrastructure upgrades needed for Kihei are estimated to cost around 85 million dollars including the land acquisition (that was in 2016).  So these costs could be considerably more considering the recent rise in the cost of real-estate. There is currently no money put aside for these sorely needed upgrades, and there are no plans to initiate these upgrades any time soon. Meanwhile more houses are being built, more condos, more hotels, and more roads. We are seeing our overburdened, inadequate drainage infrastructure being ignored, and overburdened with more and more urban runoff. And we are also seeing the systematic destruction of many natural watershed features that provide important watershed functions.

UNCHECKED DEVELOPMENT: Unchecked development is causing the destruction and degradation of streams, floodways, floodplains, wetlands, and open spaces. This creates increased risks to neighborhoods and the local population from flooding. And this means that our drainage problems on the land are passed through to the ocean. Pollution in runoff including, silt, mud, sewerage, chemically-contaminated runoff, is channeled into the ocean where it harms coral reefs, and fragile marine life.

Proposed Kihei Collector Road Diagram
Proposed (future) Kihei Collector Road Diagram
Kihei collector road diagram, Kaonoulu-to-Namauu
Kihei collector road diagram, Lokelani to Auhana