ENVSCI 201: Lecture 2 - The Urban Stream Syndrome

Urban Stream Syndrome

Describes the consistently observed ecological degradation of streams draining urban land.

How the precense of urban landscapes affect and changes the hydrology, connectivity, chemistry, and complexity of streams.

Basic Hydrology Cycle

The main input is precipitation. The main outputs are runoff and evapotranspiration. There's many fluxes, the movement of water, between other bodies of water, the atmosphere, the biosphere, and so on.

Infiltration

The flux of water from runoff and water on the ground moving underground

Overland Flow

The flux of water accross the land either as rainfall (runoff) or flood water from streams

Discharge to Surface

The flux of water underground leaching into bodies of water

Runoff

When precipitation falls onto and flows over land. Typically most will infiltrate into the ground under natural permeable landscapes.

However, in urban systems, the impermeable surfaces prevent runoff from infiltrating. There are some permeable spaces in urban areas like green spaces and gardens.

Urban Stream Syndrome Effects on Hydrology

Higher rates of runoff with almost no rates of infiltration and some evapotranspiration.

All the excess runoff typically flows into and overloads nearby streams. Sometimes the runoff is collected and goes through subsurface routing into streams.

All of this means a higher magnitude and faster rate of change of discharge. This leads to situations such as floods and the faster high intensity water also causes more erosion.

• Higher magnitudes of flow

• More frequency of overland and erosive flow

• Less lag time between peak flows

• Higher and lower rises and falls of storm hydrograph

Baseflow magnitude (magnitude when there's no rain) tends to be inconsistent as well.

Urban Stream Syndrome Effects on Morphology

The physicality of the streams are modified to better suit human needs. Channels are straightened, made wider and deeper, and less complex. The flashier flows also tends to make more scour at the bottom. They are also typically burried and put underground and diverted elsewhere.

The levels of sediments entering also tends change too, however more inconsistently. Initially with all the developments, sediments will skyrocket but after things settle down, there tends to be little to no sediments.

Naturally streams would have diverse intact vegetation (riparian zone), natural channels with curves, and morphology to support variable flows, pool sizes, sediments, and ripples.

• Wider channels

• Deeper streams

• More scour

• Less complexity

Urban Stream Syndrome Effects on Chemistry

There is a higher inputs of natural sediments and novel pollutants typically being carried by runoff the goes over urban landscapes. For example, road salinisation for snow results with higher salinisation in bodies of water.

Streams are being overloaded with nutrients such as nitrogen and phosphorus typically from argicultural practises (fertilise-use and animals). Some may even come from urban runoff from the combustion nitric oxide.

Temperatures are higher due to the urban heat island effect, the lack of riparian shading, and hot incoming runoff.

Similarly, sediments within the stream also tends to be inconsistent, depending on the stages of development, policies invovled, and so on.

Urban Heat Island Effect

The phenomenon in which urban areas are warmer than the surrounding countryside due to pavement, dark surfaces, closed-in spaces, and high energy use