When Streams Become Stormwater Drains

During the process of watershed urbanization, streams are altered and adapted to suit human activity.

Roads, Bridges, Culverts, and Drains: Roads are built across streams, and either a bridge is built or a culvert or with smaller streams often a stormwater drain. Streams are often used for their ability to provide water, but also for their ability to dispose of unwanted rainwater. In South Maui, there are relatively few stormwater systems that flow all the way to the sea.

Stream Dumping: Most of the urban stormwater system relies on natural streams, and also cryptorheic basins. As more housing and development is built, the surface of the land becomes covered with rooftops and concrete pavements and roadways and driveways, parking lots etc. These surfaces are impervious to rainwater, so they cannot absorb water like natural soil surfaces can.

Keokea Stream forced underground through Kauhaa Road culverts

Impervious Surfaces: Every square foot of earth/soil that is covered increases the runoff of rainwater, and increases stormwater production. Every development increases the net amount of stormwater produced, so they have to find a way to dispose of the additional stormwater. Typically they dump it into the nearest stream. Developers in South Maui have very little regulation as to where they can dispose of their stormwater, so they also will often dump it onto a neighboring property. This type of action creates a daisy-chain of properties dumping stormwater onto their neighbors, which increases the stormwater loads for every downslope property.

Surface Flows vs. Groundwater: In a natural watershed, a great deal of water travels through groundwater. On the slopes of leeward Haleakala in the moku of Kula, there are approximately 50 square miles of land that are tilted at a fairly steep angle toward the ocean.  This is a huge catchment area for any rain that falls on this side of the mountain. This land is made up of various materials including a large amount of porous lava and various soils. This porous material allows for a large amount of the rainwater falling on the surface to infiltrate down into the ground, where it travels downslope as groundwater. When the groundwater gets close to the surface springs appear and can form into pools or streams. These streams may travel overland for a while before moving underground again. All streams have a groundwater component.

Hardened surfaces increase surface flow: When surfaces are hardened, or when soils are degraded their ability to absorb and infiltrate water is reduced, and more rainwater falling on the surface will travel overland as surface flow.  Surface water picks up loose dirt and debris, and carries it along as suspended solids. Sheet flows, wide areas such as farmer’s fields and ranchlands, will often sheet off their rainwater. Sheetflow from agricultural property is a primary source of “non-point source pollution. This is where the infamous South Maui “Mud Floods” get their mud. Sheet flow from agricultural land including ranchlands, can also contribute various contaminants such as chemicals and other pollutants.  Surface sheet flow can pick up agricultural chemicals as well as fecal materials from livestock and feral animals as well. This process tends to contaminate the stormwater runoff.   Read more about Mud Floods

Natural Streams: Natural streams are created by the physics of moving water acting on the surface of the landscape. Softer materials such as sand and soil are easily eroded and streams carve their stream beds along the path of least resistance. They follow the fall line and gravity determines the shortest route to the lower elevations.  Streams will detour around rocky outcroppings, and rises in the topography, they will become narrow between boulders and spread wide on flat surfaces.

Sediment Deposition: Streams in addition to eroding materials will also deposit sediments and rocks and debris, where the water slows. The speed, course, and velocity of a natural stream are able to change according to its needs. And the stream also changes according to how much debris and sediment is deposited. Natural streams change their courses over time as sediment deposits build up. Natural streams also change depending on the amount of water flowing through them. They can have multiple alternate streams that are active during high flow, and dormant at other times. Read more about Sediment Deposition

Overflow, flood plains: Streams can often overflow their banks and spread out into a wider version of themselves. and they can overflow into flood plains that are only needed when flood-level events occur.  Finally, when the flow of water in a stream is less than the ability of the soil to absorb water, the stream will infiltrate into the ground and travel as groundwater. When a stream travels underground it will descend down through the porous materials until it finds a less porous layer like bedrock.

Stream Crossings: In the urban setting, stream crossings can take many forms. From a dirt road built across a stream bed. Dirt roads change the surface and create loose soil that can wash into streams. Concrete surfaced stream crossings, also change the stream flow, and reduce the porosity of the streambed. Also, stream crossings usually involve changing the profile of the streambed, to make it passable by vehicles. This can involve a flattening of the stream’s profile, or a filling in of the deepest part of the streambed. And also alterations to the approach sides of the crossing can include raised roadways, onramps, etc. Raised road beds alter the stream bed and also the stream banks, and watershed. These earthworks change the flow of not just the stream but also the water entering the stream.  A hardened stream crossing made of concrete or solid materials limited the position of the stream, and usually also limits the scale of the stream. Read more about Streamway Obstructions

Waimahaihai Stream

Channelizing Streams: Whenever you limit the width and or depth of a stream, by hardening the surfaces it is called “channelizing”. This can be just digging in the streambed, straightening the stream, or building up the banks. When you channelize a stream you limit its options. A channelized stream will change its behavior according to the limitations of its now-channelized streambed.

Channelized-stream. Kihei High School. Photo: D&S.Dorn

When a channelized stream receives more stormwater, it cannot spread or meander, and overflow as it might in the natural state. Instead, it has to flow faster to conduct the increased volume of water. This is where problems really begin. When water travels faster it carries far more energy than slower-moving water. Fast-flowing water has a greater ability to erode stream banks and carry away soils, and even move rocks and boulders.

Stream turned into an Open Ditch for stormwater runs through a Kihei neighborhood.

When streams are narrowed at road crossings: Stream crossings are usually obstructions to streams. At road crossings (weirs, culverts, bridges) the stream is typically narrowed, and this has two effects: 1) it creates a blockage at the upstream side of the obstruction, and, 2) a constriction through the obstruction.  This has two effects on the velocity of the stream flow. The stream is slowed on the upstream side of the crossing, which causes it to drop out suspended sediments and deposit debris. This can cause the rapid build-up of mud, dirt boulders, and debris, restricting flow. And the construction causes the water to travel faster which creates highly energized (fast-moving) water to exit on the downstream side. This swift water will cause greater erosion to the streambed, unless the outflow side is engineered and landscaped to reduce the stream’s velocity.

Bridges: bridges are able to span wide streams and gulches, they also allow for the largest amount of water to flow underneath. The road bed is elevated above the highest anticipated flow levels (head, crest). bridges also require hardened supports driven into the ground, as well as usually have hardened stream banks on the approaches to reduce the stream from undermining the supports. bridge-type stream crossings are almost always narrower than the natural gulch/stream bed width, so they form a streamway alteration and a streambed constriction.

“Mud-flood” in Kulanihakoi Gulch transports upcountry soil from mauka to makai.

Low Bridges/Causeways: Low-slung bridges are designed to get vehicles across streams, but are designed to be overtopped during floods. For example, the Waiakoa Bridge on South Kihei Road, was built to handle flooding better than it’s predecessor but still is massively undersized for that required task.

Waiakoa Stream Bridge underwater. Photo: D.Dorn

Bad Bridge Design: Unfortunately this particular low-slung bridge design never accounted for all the debris that accompanies the floodwaters. Degraded ranchlands upstream of the bridge shed thousands of tons of dirt and pieces of debris for example, “deer-chewed kiawe branches”. In this photo, you can see the debris-dam on the roadway that was formed during a flash flood, that cause the entire neighborhood to flood. The designers only anticipated that free-flowing water would pass through the bridge, and did not account for the accompanying suspended sediments (mud), and masses of debris.

Debris cleanup at Waiakoa bridge South Kihei Road-December 2021
Post-storm debris cleanup at Waiakoa bridge South Kihei Road-December 2021

Raised causeway: This causeway-bridge is built across the Keokea gulch stream way. his road is raised above the wetlands that surround it. This culverted-style bridge is constructed using “Conspan” units. These are cheaper to build than single-span bridges, as they consist of a few concrete or steel tunnels overtopped with dirt.  This style of culvert stream crossing, is highly susceptible to clogging and can trap debris and become filled with mud. These bridges narrow the stream beds at the approaches and reduce the flow velocity of the stream, causing flooding and sedimentation on the upstream sides.

Kauhaa Street Culvert Causeway crossing the Keokea Gulch Stream at Laie Mauka Wetlands

Submerged Causeway: In Kihei, there is an example of a submerged causeway, that connects two wetlands together. More accurately the causeway allows the road to pass over the complete wetlands that used to be joined together. The La’ie Mauka wetlands and La’ie Makai Wetlands are the same wetlands. they just got separated by the construction of South Kihei road.

Engineered "conspan" bridge under South Kihei Road at the La'ie Wetlands
Engineered “conspan” bridge under South Kihei Road at the La’ie Wetlands

Culverts:  Culverts are used where a road crosses a stream or where a stream crosses a road.  They can be any size, but are often made from prefabricated concrete sections and dropped in place, and the roadway is built over the top.  Culverts are still streams, but they are taking on a man-made appearance, and the stream starts started to lose its natural identity.

Waiohuli Stream at Liloa Drive, Photo: D&S.Dorn

This Culvert is the infamous Waipuilani Gulch (stream) Culvert at South Kihei Road, and is the cause of many floods.

Waipuilani Stream Culvert under South Kihei Road

Stormwater Drains: Smaller than culverts are the common variety of stormwater drains. These are often where streams are forced underground, or placed so that building can cover over that original stream. So many streams that are forced underground in this way are usually forgotten.  Most big cities have drives and streams underground because the city was built up over the top of the streams that originally gave them life.

Stormwater drains like this force streams underneath developments and urban areas. This stormwater drain passes under Paradise Gardens condominiums in Kihei

Streams beneath our Feet: Whenever you walk down the street pay attention to the manhole covers that read storm drain. Chances are that there is a stream flowing beneath your feet. Now try to imagine what that stream looked like before any people were here, and before any buildings, and roads were put in. Now ask yourself where did that stream come from, and where is it flowing to. Maybe there is a wetland downstream of where you are standing.

Waimahaihai Stream storm drain. Photo: D&S.Dorn