The way water moves in cities has large effects on both water quantity and water quality.
Urbanization can result both in too much water and too little water reaching streams.
In many cities, subsurface flow has nearly been eliminated as a result of so many paved or other impervious surfaces. So, instead of water gradually entering streams through subsurface flow, much more water from a storm enters streams quickly through overland flow.
Water flowing off streets directly into streams causes water levels to rise quickly, making streams much more likely to flood during heavy rains. In periods without rain, in contrast, streams have a tendency to fall below normal levels because of reduced subsurface, or soil, flow. This type of behavior results in streams being described as “flashy”—meaning their water levels rise and fall quickly.
Below are some of the negative impacts flooding can have on both people and wildlife.
Economic costs to communities and individuals from flooding can be high. Floods may risk human life, damage property, and wreak havoc on daily routines in urban areas—causing additional economic costs to people and businesses. Flooding in the United States alone costs about $2 billion per year. In a 2002 report, the U.S. Environmental Protection Agency estimated that stormwater controls would save $14 million dollars annually.
Increased sedimentation carried by floodwaters leads to shallower streams that are more prone to flooding.
Stress to wildlife caused by changing stream dynamics reduces biodiversity in streams. In the figure at right (Source: Klein, R.D, 1979), an increasing percentage of impervious area due to urbanization correlates with reduced biodiversity in streams. Sedimentation, for example, results in shallower, warmer water with less available oxygen, which in turn creates a stressful environment for aquatic life. Sediments may also carry contaminants, including metals and organic chemicals, that are harmful to fish and other organisms.
Stream shape changes caused by flooding make streams more prone to flooding in the future.
Water washing off city streets, rooftops, eroded hillsides, and other urban surfaces will carry all sorts of things with it, including metals, dirt, and debris. Collectively termed contaminants, these materials are either dissolved in water or more commonly are attached to particles in water. Water-borne contaminants can be divided into a few basic categories.
- Plant nutrients, particularly nitrogen (N) and phosphorus (P)
- Metals, most commonly copper (Cu), zinc (Zn) and lead (Pb)
- Organic chemicals primarily related to gasoline products, but also pesticides and industrial products used in manufacturing and construction
- Pathogens from animal feces
- Trash and debris, especially plastics
A quick way to assess how contaminated water may be is by measuring the amount of total suspended solids (TSS). TSS is a measurement of how many particles are floating around in water. A turbidity meter is used to quantify the amount of particles in a water sample, based on how much light passes through the sample. Whether these contaminants are “bioavailable” (can be taken up by plants and animals) depends on the chemistry of the water and how the contaminants are attached to other particles.
Contaminants can have a negative impact on both humans and wildlife including:
Beach closures caused by contamination, particularly from pathogens, can result in loss of recreational opportunities and economic costs to businesses that depend on tourism.
Behavioral changes in fish and other harmful impacts on wildlife have been connected to metal exposure. For example, researchers in Washington have observed that metals are particularly harmful to spawning Coho salmon, and copper has been shown to inhibit salmons’ ability to detect predators.
Built up trash and debris can clog drains, threaten wildlife, and be aesthetically displeasing.
Eutrophication of water because of excess nutrients may lead to suffocation of fish and other aquatic biota, as well as water unfit for recreation.
How soil can help
Soil can play an important role in removing suspended solids and dissolved contaminants before they reach natural waters. By slowing the movement of water down, soil gives sediments time to settle out and allows chemicals to adsorb to soil surfaces.
The amount of contamination in stormwater will depend on surrounding land uses as well as the frequency and duration of storm events. Runoff from an industrial site, for example, may carry more metals, whereas runoff from a golf course or residential area may contain more pathogens and pesticides.
Contaminants, their sources, and potential concentrations in runoff are described in Table 1 below.
So the question is...
How do we get our cityscapes to manage water more like a forest?