GEOL105 Chapter Notes - Chapter 6: Downcutting, Overbank, Outburst Flood
31 May 2018
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CH 6: Flooding
6.1 An Introduction to Rivers
● Streams and rivers only differ in size
○ Streams are small rivers
● Use the term stream for any body of water that flows in a channel
● Drainage basin: the region drained by a single stream or river (also called watershed,
river basin, or catchment)
○ Each stream has its own drainage basin or watershed that collects rain and other
precipitation
● Gradient: the slope of the land over which the river flows
○ Determined by calculating the vertical drop in elevation of the channel over some
horizontal distance
● Base level: the lowest elevation to which the river may erode
● Total load: visible and invisible material transported by rivers
○ Bed load: makes up less than 10% of total load, mostly sand and gravel particles
that slide, roll, and bounce along the channel bottom in rapidly moving water
○ Suspended load: mostly small silt and clay particles that are carried above the
streambed by flowing water, accounts for nearly 90% of total load, makes rivers
look muddy
○ Dissolved load: consists of electrically charged atoms or molecules, called ions,
which are carried in chemical solution, most dissolved load is derived from
chemical weathering of earth materials in the drainage basin
● Rivers are the basic transportation system of that part of the rock cycle involving erosion
and deposition of sediment
● Discharge (Q): the volume of water moving through a cross section of a river per unit
time (AxV)
● Alluvial fan: fan shaped deposit composed of coarse sediment that is dropped by a
stream as it emerge from a mountain front onto flatter terrain; typically made of a
variable proportion of stream and debris flow deposits. The fan takes the shape of a
segment of a cone
● Delta: low, nearly flat area of land formed near the mouth of a stream where it enters a
lake or the ocean; commonly triangular or fan-shaped and crossed by branching
distributary channels of the stream that created it
Channel Patterns and Floodplain Formation
● Most physical features of rivers and floodplains result form the interaction of flowing
water and moving sediment
● Channel patterns: the shape of a flowing stream as viewed from above
○ Most common- straight, meandering, and braided
● Meandering: a stream channel pattern that is sinuous and is characterized by gentle
bends that migrate back and forth across a floodplain
○ Floodplain: relatively flat land adjacent to a river that is produced by river
processes
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○ Fast moving water erodes the riverbank on the outside of the bend to form a
steep or near vertical slope known as a cutbank
○ Slower water on the inside of the meander bend deposits sand and sometimes
gravel to form a point bar
○ Continual erosion of the cutbank and deposition on the point bar cause each
meander bend to migrate laterally in a different direction
○ Overbank flow: a condition that develops when rising water spills over the river
bank onto the floodplain
○ Often contain a series of regularly spaced pools and riffles
■ Pools: deep areas produced by scour, or erosion, at high flow
■ Riffles: shallow areas formed by sediment deposited at high flow
○ Changes in water depth and velocity along a stream create different habitats
● Braided pattern: with respect to a river or stream, a channel pattern with numerous
islands and sand and gravel bars that continually divide and subdivide the flow of water.
This pattern is most evident during low and moderate stream flow
○ Tend to be wide and shallow
○ Steep gradient and abundant, coarse bedload sediment
○ Often found in areas where tectonic processes are rapidly uplifting the land
surface and where rivers receive water and sediment from melting glaciers
● River systems: consists of three zones from headwater to river mouth known
respectively as the zone of sediment and water production, zone of transport of water
and sediment, and zone of sediment deposition
○ Zone 1: the zone of water and sediment production (also called the zone of
production)
■ Upper parts of the system where topography is steeper and more
precipitation falls
■ Water velocity is fast and downcutting and erosion occur
■ Channels may be steep sided, v shaped valleys with waterfalls and rapids
controlled by hard rocks
○ Zone 2: the zone of transport
■ Where water and sediment are conveyed by a river with a broad valley
and floodplain, created as the river moves laterally rather than
downcutting a steep valley
■ Sediment is frequently deposited in river bars or on the floodplain, but
such deposition is temporary in the history of a river
■ The channel pattern may be braided, meandering, or a combo of both
○ Zone 3: the zone of deposition
■ Where water velocity slows near base level and sediment is deposited
■ May be an alluvial fan, lake, or a delta
○ Avulsion: the process by which all or part of a river or distributary channel is
abandoned in favor of a new channel
6.2 Flooding
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● Flooding: the natural process of overbank flow
○ Related to the amt and distribution of precipitation in the drainage basin, the rate
at which the precipitation infiltrates into the earth, and how quickly surface runoff
from that precipitation reaches the river
○ Flood discharge: the discharge of the stream at the point where water overflows
the channel banks
○ Stage: the height of the river
○ Hydrograph: a graph showing changes in stream discharge, water depth, or
stage over time
○ Flood stage: used to indicate that the elevation of the water surface has reached
a level likely to cause damage to personal property
■ Based on human perception
● All flow events can be measured or estimated from stream-gauging stations
● R = (N+1) / M
○ R is a recurrence interval in years
○ N is the number of years of record
○ M is the rank of the individual flow within the recorded years
● Bankfull discharge: a flow with a recurrence interval of 1.5 to 2 years
○ Bankfull is the flow that just fills the channel
● As flow records are collected, we can more accurately predict floods
● Flash floods: overbank flow that results from a rapid increase in stream discharge;
commonly occurs in the upstream part of a drainage basin and in small tributaries
downstream
○ Typically occur in zone 1
○ Generally produced by intense rainfall of short duration over a relatively small
area
○ Do not generally cause flooding in the larger streams, but they can join
downstream and may be quite severe locally
○ Most common in arid and semiarid environments, in areas with steep topography
or little vegetation, and following breaks of dams, levees, and ice jams
● Downstream floods: condition in which surface runoff from a relatively wide area has
caused a stream to overflow its banks-more common in the lower part of a drainage
basin where tributary streams have increased the discharge of the overflowing stream
○ Zone 2
○ Often associated with the inundation of the floodplain
○ May cover a wide area and are typically produced by storms of long duration that
saturate the soil and produce increased runoff
○ The combined runoff from thousands of tributary bains produces a large flood
downstream
■ Characterized by the downstream movement of the flood crest with a
large rise and fall of discharge at a particular location
● Downstream floods of zone 3-alluvial fans and deltas
○ Are particularly hazardous because of uncertain and changing flow paths
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Document Summary
Streams and rivers only differ in size. Use the term stream for any body of water that flows in a channel. Drainage basin: the region drained by a single stream or river (also called watershed, river basin, or catchment) Each stream has its own drainage basin or watershed that collects rain and other precipitation. Gradient: the slope of the land over which the river flows. Determined by calculating the vertical drop in elevation of the channel over some horizontal distance. Base level: the lowest elevation to which the river may erode. Total load: visible and invisible material transported by rivers. Bed load: makes up less than 10% of total load, mostly sand and gravel particles that slide, roll, and bounce along the channel bottom in rapidly moving water. Suspended load: mostly small silt and clay particles that are carried above the streambed by flowing water, accounts for nearly 90% of total load, makes rivers look muddy.
