Basic Hydrologic Science Course
Runoff Processes
Section Three: Basin Properties
Produced by The COMET® Program
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The physical properties of a drainage basin and its streams influence the amount and the timing of the runoff. These properties may come from natural and human factors.
In this section you will learn to:
Topics in this section include:
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Geomorphology is the study of the earth’s surface features, or landforms, and the processes that form them. An understanding of the geomorphology of a basin helps with anticipating runoff characteristics.
For a more detailed look at this topic see:
Fluvial Geomorphology Module
State University of New York College of Environmental Science and Forestry (SUNY ESF)
http://www.fgmorph.com/
The size of the contributing area of the rainfall in a basin has a direct influence on the total volume of runoff that drains from that basin.
It probably comes as no surprise that when rain falls in a uniform manner over a larger basin and a smaller basin, the larger basin produces more runoff volume. All things being equal, a drainage area that is twice as large can generate twice as much runoff volume as its smaller counterpart.
Of course many storms will cover only part of a basin. So for most situations, the runoff volume will be determined by the contributing area—that part of the basin covered by a storm—not the total size of a basin.
Likewise, consider two similarly shaped basins, with one larger than the other. Runoff traveling from most upstream point of the larger basin will travel a longer path, and therefore take longer to reach the basin outlet than runoff traveling from the farthest point in the smaller basin. In addition, a single thunderstorm will likely only impact a portion of the large basin at any given time, but it may envelope the entire small basin.
Click the image above to view animation.
Basin shape also has an influence on magnitude and timing of the peak flow at the basin outlet.
Consider two basins of equal area where one is long and narrow, and the other is more round. Then consider runoff traveling from the farthest point in each basin to their respective outlets. The runoff in the more round basin will arrive more quickly at the basin outlet.
In addition, water from multiple locations in this basin is more likely to arrive at the outlet at the same time, resulting in a greater peak flow. By contrast, in a longer, narrower basin, water from multiple locations is less likely to arrive at the same time.
Meanders in the stream channel add to the distance that water must travel from upstream to downstream.
Click the image above to view animation.
Consider a basin with meandering stream channels. If all of the meanders were straightened out, the distance that the water travels through the streams would decrease. This reduces the time it takes water to travel to the basin outlet. It also decreases the time over which water may infiltrate the ground through the bottom of the stream channel.
Meanders increase the travel time of runoff through the basin, and may reduce the overall runoff volume.
Click the image above to view animation.
The slope of a basin affects the amount and the timing of runoff.
As the slope of the land increases several factors come into play. The first is that water contact to the surface is no longer perpendicular. With the land sloping, gravity no longer pulls the water directly into the ground, so more water is likely to become surface runoff.
Another factor is the movement of water across the land surface. As the ground becomes increasingly steep, water will move faster and will have less time in contact with the ground surface, reducing the time during which it could infiltrate.
Also important is the amount of sediment carried by flowing water. Erosion occurs when water removes sediment from the ground surface. Although it is dependent on soil type and ground cover, erosion generally increases with increasing slope. With higher amounts of sediment in the water, the surface pores in the soil which the water might otherwise enter, can become plugged, reducing infiltration.
In general, the steeper the hill slope and the steeper the drainage channels, the quicker the flow response and the higher the peak discharges.
“Roughness” of a stream channel increases due to the presence of rocks, vegetation, and debris. Channelizing a stream by doing such things as removing vegetation and lining the stream bed with concrete will reduce the roughness. The roughness factor has a direct impact on how quickly the water will move in the channel and how high the peak stage will be. Manning’s equation is often used in hydrology to account for the roughness factor.
Click the image above to view animation.
The greater the roughness, the more turbulent the flow. More turbulent flow results in slower runoff and streamflow velocities. This allows more time for infiltration, and it also results in a broader flood wave with lower peak discharges than in rapid runoff situations.
Conversely, reducing channel roughness results in faster streamflow velocities and greater peak flows.
Refer to the Streamflow Routing module, Section Four: Stream Properties and Manning's Equation for more information on Manning’s Equation.
Images of actual natural channel conditions and associated roughness factors developed by the U.S. Geological Survey for selected streams in the Western U.S.:
http://wwwrcamnl.wr.usgs.gov/sws/fieldmethods/Indirects/nvalues/index.htm
The U.S. Army Corps of Engineers engineering manual discusses the Manning n roughness factor and how they predict values: http://www.rivers.gov.au/roughness/docs/MethodsforPredictingManningsn.pdf
Click the image above to view animation.
Stream density is the length of all channels within the basin divided by the area of the basin. Stream density is one of the most important characteristics for evaluating potential runoff.
A drainage basin with a large number of tributaries has a higher stream density than a basin with very few tributary streams. Higher stream density allows the landscape to drain more efficiently following a storm event. More efficient drainage means that water moves into streams and creeks faster, causing peak storm flows to be larger and to occur sooner.
A basin with a lower stream density usually indicates a deep, well-developed soil. In this case, water is more likely to infiltrate into the soil rather than become surface runoff and enter into the channel network.
Urbanization typically results in alterations to the natural ground surfaces and stream channels of a basin. Surface permeability, basin size, stream density, roughness, and channel length and slope can all be affected in a way that results in greater magnitude and speed of runoff.
In urban areas the greater coverage of pavement, buildings, and compacted soils prevents infiltration of rainfall and snowmelt compared to the natural ground surface. This can greatly increase the magnitude of runoff.
Urban features such as road embankments and berms can act to break down natural basins into smaller sub-basins. Smaller drainages react much more quickly to localized rainfall than larger basins.
Click the image above to view animation.
Road grids, ditches, and storm sewer systems act as a network of tributaries and effectively increase stream density. Higher stream density results in more rapid runoff to the stream channels.
Compared to a natural stream bed, road surfaces, culverts, and storm sewers have smooth surfaces. This decrease in surface roughness allows runoff to move much more quickly to the main stream channels than it would in a more natural setting.
Streams in urban areas often have vegetation removed and are sometimes lined with concrete in a process called “channelization.” This also decreases roughness and causes water velocity to increase.
Sometimes as part of channelization urban streams are straightened by having meanders removed.
This decreases the distance that water travels from the top to the bottom of the drainage basin. It also effectively increases the slope because the stream now experiences the same elevation drop but in a shorter distance. Decreasing the distance traveled and increasing slope will cause a more rapid flood response from the runoff.
Overall, an urban environment will result in faster runoff with more runoff reaching the streams than in a rural setting.
a) basin slope is increased
b) basin stream density is decreased
c) channel roughness is increased
d) stream meanders are removed
e) a basin has a long and narrow shape rather than a round shape
a) basin slope is increased
b) basin stream density is decreased
c) channel roughness is increased
d) stream meanders are removed
e) a basin has a long and narrow shape rather than a round shape
The correct answers are a) and d).
End of Section Three: Basin Properties