How do natural watersheds become urbanized?

What is a watershed? A natural watershed is a land area that drains all the water that falls within it, either to a river, stream, lake, or ocean. When an area is urbanized, it means that it is developed for human use, often including the construction of buildings, roads, and other infrastructure. As a result of this development, the land is often altered in such a way that it is no longer able to absorb and filter water in the same way as it did before.

What are Impervious Surfaces? When natural watersheds become urbanized, the impervious surfaces (such as buildings, roads, and parking lots) that are created prevent rainwater from soaking into the ground. Instead, the water runs off these surfaces and into nearby streams, rivers, or other bodies of water. Parking lots and paved areas become the new catchment areas and roads and driveways can turn into artificial streams and rivers when it rains. This can lead to increased flooding in neighborhoods, as there is no longer enough natural land to absorb and filter the water.

Wetlands naturally hold water: Wetlands are areas that are characterized by saturated soils and the presence of water-loving plants. These areas are important for a variety of reasons, including the fact that they help to filter and clean water, reduce erosion and flooding, and provide a habitat for a variety of plants and animals. Despite their importance, wetlands are often drained and filled in to make way for development.

What happens when you build on top of wetlands? When wetlands are developed and built over, the underlying wetland hydrology can persist, meaning that the water-saturated soils and groundwater continue to exist beneath the surface. However, these features may not be visible or able to perform their natural functions due to the development that has taken place on top of them.

What happens when you build in Flood Zones? Building housing in flood zones, wetlands, or other flood-prone areas is generally a bad idea because of the risk of future flooding. These areas are prone to flooding because they are low-lying and have soils that are unable to absorb large amounts of water. When it rains heavily or when there are large amounts of snow melt, these areas are at risk of being inundated with water. Building in these areas puts people and their homes at risk of being damaged by flooding, and can also lead to increased flood risks for the surrounding areas.

Kihei Flooding damage, mud and debris on South Kihei Road near Waipuilani Gulch
Urbanized watersheds and neighborhood flooding are the results of building in a wetland flood zone.